Do Communities Evolve? A Major Question
in Evolutionary Paleoecology
RICHARDK. BAMBACH
and J BRETBENNINGTON
Chapter 7 of Jim Valentine's classic book, Evolutionary Paleoecology of the Marine Biosphere, is titled "Community Ecology and Evolution" (Valentine 1973,
269-336). The first sixty-two pages of the chapter are about various general
properties of community ecology and paleoecology, the next three and one-half
pages concern the evolution of species within communities, and only the last
two and one-half pages focus on communities through geological time. Valentine headed this short concluding section, "Many More Data Are Needed On
Fossil Communities." Two decades later the data exist to deal more explicitly
with the question of community evolution (103 of the 122 references used in this
chapter postdate the writing of Valentine's book). We hope to use these data to
answer the question, do communities evolve? The focus in this chapter is primarily on marine paleoecology, in parallel with Valentine's original subject.
The first questions asked in discussions of theory about communities are (1)
what are communities, and (2) do they exist as identifiable entities? Numerous
concepts and definitions of communities exist, ranging from the idea of the
community as a superorganism to the view of the community as just a random
aggregation of populations in a local habitat (Boucot 1981, 177-78; Schoener
1986; Underwood 1986; Southwood 1987; Giller and Gee 1987). However, all
community definitions are ultimately rooted in the fact that populations of individuals of a variety of species live together in local geographic areas. Whether
these populations are linked together tightly in a coevolved structure or not,
a variety of interactions (ranging from trophic activity to habitat modification)
take place among these organisms, either contemporaneously or over time. The
reality of coexistence and interaction justifies the recognition of community as
a level in the ecological hierarchy for associations of populations of nonconspecific organisms (Eldredge 1985, 162-73; Salthe 1985; Eldredge and ~ i e n e
1992).
One reason why agreement on what is meant by terms such as community has
never been achieved is that both ecological interactions and species occurrence
patterns take place at a variety of scales. One worker may focus on a local patch,
another on a widespread environment. Each may consider that the fauna and
flora in the geographic region studied comprises a community, yet they may not
be dealing with comparable entities. However, by using the concept of an eco-
logical hierarchy, as advocated originally by Valentine (1968. 1973) and more
recently by Miller (1986. 1990b. 1991. 1993a.b) and by Eldredge 2nd his colleagues (Eldredge and Salthe 1984; Eldredge 1985, 1989: Eldredge and Grene
1992). it is possible to formulate several community-related concepts to which
many of the varied ideas scattered through the literature can be related in a consistent fashion.
Community-Related Entities in the Ecological Hierarchy
Because the potential for interaction is inherent in neontological community
concepts, contemporaneity and spatial proximity are both useful criteria for defining the smallest subdivision of the ecosystem level in the neoecological hierarchy. We suggest the term local community for this fundamental group entity, a
term parallel to local ecosystem as used by Miller (1986, 1990b) and parch as
used by Jackson, Budd, and Pandolfi (chapter 5, this volume). A local community is the set of avatars that exist together in one local area ant1 can interact
directly in space and time. Avatar, a term coined by Damuth (1985) for the group
of individuals from a single species that actually participate in the "economy"
of a community or local ecosystem ("the 'embodiment' or 'representation' of
the species in the local community": Damuth 1985, 1137). is more ecologically
precise than the term population, which also carries the connotation of a deme,
the group of individuals from a species that forms a local I-eprotiucingpopulation. Eldredge (1985, 1989) distinguished between coti1tt1ut1itj:the living association of avatars, and ecosystem, the organisms plus their environment. In later
studies on economic systems he has preferred the term ecosystetn, but in this
chapter we are referring only to organisms and the remains of organisms in fossil
assemblages. and not to environmental properties. interactions of food webs, and
so forth. so we use the term community rather than eco.sj:.sretir.
Because of time averaging, however, it is not possible to identify exactly contemporaneous individuals, or groups of individuals, in a fossil assemblage. As
recent studies by Karl Flessa and his colleagues (Flessa. Cutler. and Meldahl
1993; Flessa 1993; Flessa and Kowalewski 1994), as well as other work by Powell and Davies ( 1 990) and Murray-Wallace and Belperio ( 1 994). indicate, the
shells being incorporated into single shell beds today have median ages of hundreds of years, with some ranging from 3,000 to over 9,000 years in age. Therefore, even a single fossil assemblage collected from a single horizon at one spot
is not temporally the same as any neontological collection. Avatars are simply
not identifiable in fossil collections, nor is it possible to observe the area over
which those interacting populations would have ranged at any one time. We
cannot determine the full range over which the organisms that lived in a local
community in the past interacted, nor can we determine the full set of interactions that occurred in an ancient local community. Therefore, the only way to
recognize a paleoecological unit that might be roughly comparable to a local
community is to restrict the suite of fossil remains included in a local paleo-
community to those that occur at a spatial scale over which we can be sure that
they would have been derived from the avatars that did live i n a local setting
during the time interval represented by the fc~ssiliferoushorizon. A local paleocommunity, then. is defined as the assemblage collectible from a single bed at
one outcrop, assuming that sedimentologic and taphoriomic interpretation indicate that the fossil deposit is generally untransported.
While it is tempting to think ot'a community as a set of interacting. or potentially interacting. organisms, this is only spatially practical on the relatively
small scale of the local community. But if one fi nds the same species in the same
abundance pattern at several places, then the form and structure of a local community has recurred, and it seems only reasonable that both places should be
regarded as having the same ecological entity present. In our hierarchical ecological scheme we define a communiry as the aggregate of the local communities
arrayed in space and time that are sufficiently similar to one another that they
cannot be shown to be significantly different statistically. Thus. the community
concept becomes one of recurrence beyond the scale of the local community. A
local community is a real community, but if it is a unique entity. there is little to
generalize about. whereas recurrence opens the way to further ecological analysis. Satisfying this definition of community requires statistical evaluation, not
simply the recurrence of many of the same species in similar, but not statistically
identical, assemblages. We have discussed this concept in detail elsewhere (Bennington and Barnbach 1996).
A paleocommunity, then. is defined as the aggregate of local paleocommunities that are not statistically significantly different from one another. Note that
paleocommunities are based on local paleocommunities, which, although at a
position i n the paleoecological hierarchy equivalent to that of local communities
in the neoecological hierarchy. are not spatially or temporally comparable entities (as noted above). Therefore, paleocommunities are not exactly comparable
to communities. either, although paleocommunities are at the level in the paleoecological hierarchy equivalent to that of communities in the neoecological
hierarchy.
There are a number of reasons why several local communities might be similar, but might not be demonstrably members of the same community. We propose the more general concept of community type for the entity that combines
such apparently allied assemblages. The community type is the aggregate of
local communities and communities that have similar. but not identical, taxonomic membership and occur in similar, but not necessarily the same, environments. This is a level in the ecological hierarchy similar to the one that Hanski
and Gilpin (199 1 ) and Jackson, Budd, and Pandolfi (chapter 5, this volume) call
the metacommunity. We prefer the term conttniinity ppe because Hanski and
Gilpin (1991, 9) require that species be "confined to the same habitat patches"
to form a metacommunity, but it is common for many marine species to exist, at
least in small numbers, in many different habitat settings and not be mutually
confined to particular environmental conditions. Similarly, the groups (often
126
Richard K. Bambach and J Brer Bentringrot]
identified using analytic techniques such as cluster analysis) of similar paleocommunities and local paleocommunities that occur in rocks interpreted as representing similar depositional environments are termed paleocommunity types.
Because of the nested nature of the ecological hierarchy, comnlunity type and
paleocommunity type are inclusive of similar local communities and local paleocommunities respectively, whether they can be statistically grouped into communities/paleocommunities or not. Therefore, if statistical analysis has not been
done, the best term to use in designating an aggregate of similar local communities or local paleocommunities is community type or pcrleocornniutliry type.
Valentine defined communities loosely in 1973 (256) "as collections of populations that are associated in time and space." Earlier, he had made the point
that ecological units are polythetic in nature-"they cannot be defined precisely
on the basis of any particular combination of components or even of any single
one of their components" (1973, 85-86). We believe that the three levels we
propose for subdividing the ecosystem level of the ecological hierarchy-local
communities, communities, and community types-can sharpen our understanding of scale in generalizing about community-related phenomena. We further suggest that the difficulty in precise definition noted by Valentine is not
necessarily related to taxonomic variability (which can be dealt with statistically), but rather is the problem of defining scale. Yet if we are to examine synecological relations through time. we must define scale, and we think our method
of subdividing the ecological hierarchy permits us to do that.
Miller (1986, 1990b, 1993a,b) has rightly established that the local ecosystem
(local community and local paleocommunity in our terminology) is the "basic
descriptive-interpretive unit of paleosynecology" (1990b. 3 1). We have tried to
build on Miller's perceptive analysis by expanding the series of hierarchical levels to more fully encompass the entities that are actually dealt with in ecological
and paleoecological analyses. We believe that this will enhance understanding
of the position in the hierarchy at which observations are actually made and to
which theoretical arguments are directed. We also feel that the realization of
noncomparability between neoecological and paleoecological entities is crucial.
This recognition permits a response to Miller's question (1990b, 3 I ) , "Do patterns at any of these nested levels of variation-patches within shell beds, shell
beds within biofacies, and so on-represent the elusive original community of
organisms?" In many instances they are derived from those original communities, but in no instance do they directly correspond to them.
The authors cited in the following discussion have used many different temporal and spatial scales. However, they all either refer to entities that they believe
are approximately at our community level (but which, in many instances, actually may be more comparable to community/paleocommunity types) or deal with
intervals of time over which their arguments for stability imply some measure of
stability for the community systems that existed within those intervals. We shall
try to make these distinctions of scale clear at each point i n the discussion.
Do Cnmmuniries Evolve?
127
The Scale and Degree of Community Change over Time
The Phanerozoic as a Whole
Because the fauna of the marine biosphere has evolved and changed radically in
terms of dominant taxa, diversity, and ecospace use throughout the Phanerozoic,
marine benthic communities must also have changed through time. Sepkoski's
. three evolutionary faunas were characterized by different dominant classes (Sepkoski 1981). and so, necessarily, were the communities present during the time
intervals characterized by those evolutionary faunas. Trilobites and inarticulate
brachiopods dominated the skeletal fauna in the Cambrian, and much of the rest
of the diversity of that fauna consisted of now long extinct taxa, many of uncertain affinity with living organisms (Whittington 1985; Gould 1989; Dzik 1993;
'
Briggs, Erwin, and Collier 1994). Brachiopods, stenolaemate bryozoans, various
stalked echinoderms, cephalopods, and tabulate and rugose corals, as well as
archaeogastropods, nonsiphonate bivalves, and trilobites, characterized the postCambrian Paleozoic fauna. Teleost fishes, echinoids. caenogastropods, siphonate bivalves, malacostracans, and scleractinian corals rose to dominance during
the Mesozoic and Cenozoic.
Each succeeding evolutionary fauna utilized a greater range of ecospace, and
diversity and the number of guilds (groups of species in a community that share
common modes of life, physiological constraints, and limiting resources, as described by Bambach 1983) represented within conlmunities increased from the
time dominated by one evolutionary fauna to the next. As documented by Bambach (1983, 1985). only nine of twenty possible general modes of life (defined
by life site, activity, and gross feeding type) were utilized by diverse classes in
:
the Cambrian, and five were still relatively little utilized in the middle and late
Paleozoic, but all were fully utilized by the Cenozoic, and many became utilized
by representatives of several different classes. As the utilization of ecospace increased, diversity within habitats also increased (Bambach 1977). as demonstrated by the fact that for species with high preservation potential, a sample of
!
59 Cambro-Ordovician benthic paleocommunity types from open shelf environ; ments had a median species richness of about 20, 88 comparable middle and late
Paleozoic paleocommunity types had a median species richness of 30, and 56
1 Cenozoic paleocommunity types had a median species richness of 61.5. The
median number of guilds recognized in benthic paleocommunity types of the
more species-rich Cenozoic (18) is nearly twice that for environmentally comparable Paleozoic paleocommunity types (I I ) (Bambach 1983). Not only has
!
I
diversity within community types changed as the number of modes of life has
increased, but fundamental properties of the biosphere, such as tiering into
the third dimension (Bottjer and Ausich 1986). rates of bioturbation by
infauna (Thayer 1983), and productivity and biomass (Bambach 19931,
have also changed over time. and each of these would have had an effect on
i wmuni[ies.
'
/
I
i
r
b
Richard K. Bantbach attd J Brer Ber~rtitt~ro~t
128
Ecological-Evolutionary Units (EEUs)
Within each of the intervals characterized by Sepkoski's three evolutionary
faunas. Boucot recognizes several intervals of time that he calls Ecolosic-Evolutionary Units (EEUs), each of which he claims is characterized by different
community types and distribution patterns (Boucot 1983. 1990b.c.d). Boucot
recognizes twelve EEUs. three subdividing the interval dominated by the Cambrian fauna. five subdividing the interval dominated by the Paleozoic fauna, and
a sequence of four during the Mesozoic and Cenozoic (fig. 6.1 ). The short transitions between the EEUs are usually major events such as mass extinctions and
major radiations, resulting in decreases and increases in diversity respectively.
Whereas Boucot has argued that communities are stable during each of the
twelve EEUs, Sheehan (Sheehan 1985. 1992, 1994) has argued that the Phanerozoic might better be partitioned into nine major EEU intervals (rather than
twelve), and he points out that, at least for the last six (mid-Ordovician through
M~il<ons
01 Years AGO
Geologic
Time Scale
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sw
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Paleozoic
Fauna
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Fauna
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Ecologic-Evolutionary Units
- - Sheehan's Revision of Boucot's
Ecologic-Evolutionary
- -Units
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Marine Units
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...-
Helderberg
CmY
equivalen~r
P F 1 3
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6-'my
:
' 1 3 1 ~l l 3 ~ ~ b k l 3
Fig. 6.1. Comparison of the various temporal scales at which signifcant change in
communities is documented. The time spans of Sepkoski's three evolutionary faunas,
Boucot's Ecologic-Evolutionary Units. and Sheehan's (1992) moditication of Boucot's
EEUs are diagrammed at the same scale. The time span for the set of Brett and Baird's
subdivisions of EEU V1 in the Appalachian Basin is shown expanded from the EEU
time line by long dashed lines. The location in time of the Breathitr Formation is shown
by arrows from the EEU time line. and the estimated tetnporal spacing of the four marine units in the Breathitt Formation reported in this chapter, plus the other sedimentary
cycles in the Breathitt. compared with one of Brett and Baird's subdivisions is shown by
dotted lines.
the Cenozoic). a short "Reorganizational-EEU" begins each interval, followed
by a temporally longer "Stasis-EEU" (fig. 6.1 ). Boucot's discussions of EEUs
emphasize the idea that little community change occurs during an EEU, other
than phyletic evolution within lineages. and that during an EEU patterns of community organization and distribution remain relatively stable. Sheehan has altered this view by identifying Reorganizational-EEUs as times during which
considerable change actually occurs in conln~unitystructure and pattern and by
noting that the Stasis-EEUs appear to exhibit stable patterns of community occurrence and structure.
Sheehan does not recognize "Reorganizational" intervals at the beginning of
each of the three EEUs dominated by the Cambrian fauna, and he suggests that
they may be different from later EEUs (Sheehan 1992. 112). As Sheehan notes,
these ecological-evolutionary units really represent the initial Cambrian radiation (Early Cambrian). the succeeding interval when the Cambrian fauna was
dominant and global diversity changed only slightly (Middle and Late Cambrian), and the interval in the Early Ordovician during which the rapid diversification of the Paleozoic fauna was getting under way and the Cambrian fauna,
while still diverse, was beginning to decline. It very well may be that the more
stable features of the global ecosystem that Sheehan believes characterize StasisEEUs were not fully developed in the biosphere until the majority of general
modes of life were finally occupied extensively. and this happened only with the
diversification of the post-Cambrian Paleozoic fauna. The first three EEUs in
Sheehan's and Boucot's schemes may simply represent phases in the initial filling
of ecospace by metazoans. In this case, EEU I, the Cambrian "explosion." could
be regarded as a "Diversification-EEU"; EEU I1 as a primitive Stasis-EEU. but
without ecospace being fully occupied; and EEU I11 as a second "Diversification-EEU" during which ecospace became more fully utilized as the Paleozoic
fauna diversified. EEU IV would then be the first full-scale Stasis-EEU (with its
initial "diversification" phase recording the completion of the diversification of
the Paleozoic fauna). Subsequently, mass extinctions mark the end of each Stasis-EEU, and short Diversification-EEUs follow as diversity recovers, leading to
another Stasis-EEU in Sheehan's scheme.
Subunits within Ecological-Evolutionary Units
In just one geographic region within just one of Boucot's Ecologic-Evolutionary
Units, Brett and Baird have described multiple intervals during which they claim
communities remained "stable," each bounded in time by brief intervals of
marked community change (Brett, Miller, and Baird 1990; Brett and Baird 1995).
The EEU in question is a Stasis-EEU in Sheehan's scheme (EEU VI of Boucot,
covering the 45-million-year time span from the Llandovery through Givetian).
Brett and Baird identify ten stratigraphic intervals in the Appalachian Basin that
they designate as ecological-evolutionary subunits, each lasting from 2 or 3 to 7
or 8 million years (fig. 6.1 ). Each subunit is characterized by reduced faunal turnover, with an average of 75% (range 66-80%) of the species persisting through
130
Richard K. Bambach und J Brer Benningron
most intervals. Great faunal change is concentrated at subunit bound-aries, at
which approximately 80% (range 66-92%) of the species are replaced and at
which organizational and distributional patterns of paleocommunity rypes also
change. Because paleocommunity types persist throughout each subunit, asdo the
distribution patterns of the paleocommunity types, Brett and Baird characterize
the subunits as intervals of "coordinated stasis." However, the 20-33% turnover
noted within each subunit suggests that stasis within paleocornmunity-relatedentities is not as constant as the term "coordinated stasis" implies on first impression. The 20-30% variability in species throughout an interval of "coordinated
stasis" affects statistical comparisons of paleocommunity-related patterns within
such an interval, and is one feature responsible for our conclusion (Bennington
and Bambach 1996) that paleocommunity type (not paleocommunity) is the appropriate level at which stability is observed in intervals of coordinated stasis.
The cause of the rapid faunal turnover associated with the change from one interval of coordinated stasis to the next is an intriguing problem. A common factor
may link the episodes of ecological reorganization that bound intervals of coordinated stasis and the episodic nature of extinctions observed by Raup (chapter 16,
this volume). However, the episodes of faunal turnover at transitions from one interval of coordinated stasis to another are not simply extinctionlradiation couplets, although in at least some instances they coincide with worldwide events.
Brett and Baird (1995, 306) note that the Onondaga-Hamilton faunal change is
coincident with the Kacak or Otamari event observed on several continents besides North America. Brett and Baird ( 1 995,300) point out. however. that biogeographic changes, not just extinction and origination, occur at this time, as "about
half the Hamilton fauna appears to record immigrant taxa."
Boucot ( 1975. 1982. 1990e) has also recognized changes during this interval
(EEU VI). but he has regarded them as minor compared with the changes that
take place between EEUs. Because the better-known EEUs are bounded by the
great mass extinctions, more faunal change occurs between than within EEUs;
however, this does not mean that the internal changes are unusually small, only
that the changes at the boundaries are unusually large. The combination of
change at EEU boundaries and at the subunit boundaries recognized by Brett
and Baird emphasizes the developing view of modern paleontology that the history of life is episodic. The proposal of coordinated stasis as a characteristic of
the subunits recognized by Brett and Baird has generated renewed interest in the
factors that control or influence community stability and change, and considerable attention is now being paid to "community evolution."
Patterns below the EEU Subunit Level
The question of what goes on at shorter time scales is addressed by some of our
own ongoing research. We have been working on the question of stability and
change in marine paleocommunities and on the problem of whether communityrelated patterns can be reestablished when habitats recur after intervals of
changed conditions. Our study examines recurrence of local paleocommunities
Fig. 6.2. Breathitt Formation stratigraphy. Age assignments are given in the two left
columns; formation nomenclature in the three left-center columns; named members and
local units (with the four studied marine units in boldface) and positions of all coals and
marine-influenced horizons (labeled m) in the right-center column (details from Chesnut
j 1991); and a diagrammatic lithologic section at right (note the overall coarseningupward sequence.)
I
!
during an interval of time equivalent to one of Brett and Baird's subunits
i (fig. 6.1). Our target has been four different marlne unlts, in ascending strati-
1
1
r
graphic order, the Elklns Fork, Kendrick, Magoffin, and Stoney Fork members
of the primarily nonmarine Breathitt Formation of Pennsylvan~anage in eastern
Kentucky (Bennington and Bambach 1994) (fig 6.2). These four marine units
(selected from 28 horizons with marine characterlstlcs In the Breathitt Formation) represent the largest transgressive events in a series of cyclothemic para-
sequences generated by sea level fluctuations associated with Southern Hemisphere glaciation and Alleghanian tectonic activity (Heckel 1986; Ross and Ross
1988; Chesnut 199 I ) . which resulted in cycles that are believed to be in the range
of 100.000 to 400,000 years. In each cycle marine habitats shifted away from
eastern Kentucky by up to 1,000 km during sea level 1owst;inds. eliminating the
local marine communities. The intervals between the recurrent fully marine units
we sampled lasted approximately 400.000 years for the Elkins Fork to Kendrick
interval and. because there are four or five cycles includinf less fossiliferous
marine tongues between each of the succeeding sampled units. approximately
2-2.5 million years for the Kendrick to Magoffin and Magoftin to Stoney Fork
intervals (Chesnut and Cobb 1989).
We have identified five different associations of samples of macrofossils by
Q-mode cluster analysis using relative abundance data (tig. 6.3). Four of these
associations recur in several different marine units. Each association has adistinct
ecological structure, as shown by comparing clusters based on species abun-
Main Cluster Groups for Mode of Life Cluster Analysis
I
Sus~enslonFeedlrlg
Eplfaunal Hlgh
Eplfaunal Low Anached
Eplfaunal Recllnlng
Semi-lnfaunal
lnfaunal Shallow A d ~ v e
1
"Small
Mollusk"
1 -
"Chonetid
Mollusk"
"Productid
Chonetid"
"Spiriferid"
"Productid"
Fig. 6.3. Q-mode cluster analysis, using relative abundance data for species. of the 74
largest samples (200-specimen average) from the four studied marine units in the
Breathitt Formation. Five major clusters, chosen using similarity coefficients of 0.4 or
greater, are named for their dominant taxa. Marine units for each sample are identified
by capital letters at the base of the cluster diagram: E. Elkins Fork; K. Kcntfrick; M,
Magoffin; S. Stoney Fork.
I
-
Eplfaunal Mob~le
lnfaunal Shallow Actlve
Fig. 6.4. Two-way comparison of quantitative Q-mode clusters (as in fig. 6.3) (vertical
axis) and Q-mode clusters obtained by pooling species with similar modes of life
(horizontal axis; pie diagrams show proportions of modes of life in each cluster).
Intersecting lines show boundaries between major clusters in each analysis. Letters in
fields indicate marine units for each sample (letters as in ti g. 6.3). Note that the majority
of the samples in each quantitative cluster fall within a single mode of life cluster,
indicating a predominant ecological structure for each quantitative cluster.
dances with clusters formed by grouping samples on the basis of relative abundance of specimens pooled into categories representing different modes of life
(fig. 6.4). Each cluster of samples also has a different mix of dominant species
(table 6. I). The Productid cluster is dominated by spiny productid brachiopods
that lived partially buried in the sediment. "rooted" by their spines. The Small
Richard K. Batnbach u n d J Brer Benr~it~grotr
134
TABLE 6.1 Abundances (%) of important taxa in each quantitative cluster
Cluster
Taxon
Desmoinesia
Linoproducrus
Juresanio
Attriquiror~ia
Chonetids
P1icochonere.s
Derbyia
A~trhracospirifer
Puncrospirifer
Crurirhyris
Husredia
Composira
Orbiculoidea
Seprimyali~ta
Aviculopecten
Parallelodon
Asmrrella
N~cculoidea
Nuculopsi
Phesriu
Rellerophon
Euphetnires
Glabrocingul~mt
Trepospira
Ananais
Productid
Spiriferid
50.75
13.08
5.30
2.08
7.13
4.41
1.59
0.48
9.86
3.72
0.14
8.00
35.86
1.59
3.49
1.90
1.43
0.39
0.42
1.32
0.65
0.49
1.18
1.55
1.1 1
0.09
0.35
1.06
0.37
0.05
1.37
0.07
-
0.07
15.17
0.2 1
5.24
2.28
2.34
2.41
0.2 1
0.07
0.14
0.48
0.07
-
ChonetidProductid
Cho~ietidMollusk
Small
Mollusk
18.95
0.80
0.52
3.94
13.80
10.03
6.80
7.00
5.58
0.47
4.08
4.79
0.19
0.39
0.06
4.82
3.36
0.58
0.96
0.83
0.17
0.25
0.14
0.14
0.08
16.95
0.59
I .0J
0.52
34.42
0.36
0.02
-
0.97
1.12
1.41
2.75
0.97
1.12
3.35
0.15
1.12
0.97
3.42
0.89
1.19
1.26
2.53
0.50
3.20
8.25
-
5.75
0.02
0.34
0.02
0.14
2.01
0.05
0.05
0.67
-
0.19
0.02
0.12
23.81
13.30
14.40
0.4 1
1.63
11.93
10.06
12.46
Nore: All taxn listed are among the ten most abundant in at least one cluster (shown by boldface tiurnben).
Dashes indicate that a taxon does not occur in samples grouped in that cluster. Note that 17 of [he 15 taxa occur
in all five clusters.
Mollusk cluster is dominated by detritus- and deposit-feeding bivalves and gastropods. The Chonetid-Mollusk cluster has a rather evenly distributed variety of
modes of life. The Spiriferid and Chonetid-Productid clusters each have a similar
mix of modes of life, but are distinctly different in the morphology of their dominant species and in their species dominance-diversity patterns (table 6.1).
Samples representing each of these five associations occur in comparable lithologies and at comparable stratigraphic positions within each transgressive1
regressive unit, demonstrating that the clusters are closely associated with particular environmental conditions. For example, samples grouped in the Small
Mollusk cluster occur only in the Magoffin, the unit representing by far the thickest, most widespread transgressive event in the sequence. The Small Mollusk
samples all occur at a single stratigraphic position in fine-grained dark gray mudstones just above the maximum flooding surface in the Magoffin. This is the
position of the deepest, quietest water environment in the Magoffin. Because
deep, quiet water provides an environment where organic matter of low specific
gravity can settle from suspension and form a significant detrital food supply in
the sediment, it is logical that deposit feeding is the dominant mode of life in the
Small Mollusk samples. None of the other marine units were produced by transgressions with water quiet enough to form the appropriate environment for
deposit feeders to predominate. On the other hand, the Productid cluster is composed of samples from all four units. It is overwhelmingly dominated by specimens of productids (70%) and includes few deposit feeders (about 3%). Because
overdominance is common in stressed environments and little organic matter to
serve as food for deposit feeders accumulates in high-energy settings, the Productid cluster apparently represents assemblages formed in a stressed, relatively
high-energy environment. This interpretation is also supported by two stratigraphic relationships: first, samples of the Productid cluster constitute the only
marine fauna in a small transgression (the Elkins Fork), which would have contained only variable, shallow-water, relatively high-energy habitats, and, second, samples of the Productid cluster are the last fauna to occur in shoaling,
prograding packages, as seen in the upper portions of the Kendrick, Magoffin,
and Stoney Fork.
Each of the major clusters (see fig. 6.3) represents an association of samples
with similar taxonomic and relative abundance makeup, a similar balance in
abundance of individuals with particular modes of life, and similar environmental affinities. Sedimentologic and taphonomic analyses also demonstrate that
these samples are formed from untransported (although often disturbed) specimens that accumulated in their habitat of life. These associations of recurrent
fossil assemblages represent well-defined paleocommunity types. These five
paleocommunity types recur laterally in space within each marine unit and, in
many instances, also recur in time (four of the five paleocommunity types are
represented in more than one of the superposed marine units). Therefore, we
can use them to test for recurrence of local paleocommunities within both a spatial scale and a time frame equivalent to one of Brett and Baird's ecologicalevolutionary subunits. If the same local paleocommunities recur regularly, the
contention that paleocommunities remain stable during an interval of coordinated stasis is supported. If the same local paleocommunities do not recur regularly, then the phenomenon reported as coordinated stasis may not record unusua! stability of paleocommunities as entities, although it may describe the
recurrence of paleocommunity types, which does occur, as our analysis already
demonstrates.
Because it provides the best opportunity for a comprehensive test of paleocommunity stability within an interval equivalent to one of Brett and Baird's
ecological-evolutionary subunits, we examined in detail the paleocommunity
type that occurs in all four marine tongues: the Productid cluster. This paleocommunity type is found in each marine unit and recurs widely within three of
the four units, and the same suite of species is present or available in each unit,
with many of the same species dominant in the samples of this paleocommunity
Richard K. Batnhc~c.hcltid J Bret B e t i ~ i i ~ ~ g ~ o ~ i
136
T A B L E 6.2 Comparison of
f r o m each marine unit
Elkins Fork
the top ten most abundant taxa in
Do Comnulniries Evolve?
P r o d u c t i d cluster s;l~iiples
.-
Kendrick
Magoffin
Stoncy Fork
Dc.v~~roi~tc.sicr
Flat c h o n e t i d s
Desmoinesia
Desmoinesia
Desmoi~~esia
Present ( 2 8 )
Flat c h o n e t i d s
Flat chonetids
Anrhracospirjfer
Littonroducrus
J~tresania
Anriquaro~tia
Comnosira
At~rhrncospirifer
Linoproducrrrs
Juresatlia
Anriquaronia
Present (15)
Hirsredia
Atlrhracospirjfer
Linoproducfus
Juresnnia
A~lriquaronia
Composita
A~i~/~~-cico.sl~rrr/~~r-
Available
Available
Present ( 2 4 )
Present ( 14)
Present ( 2 6 )
Drrb?itr
Puncrospiri'fer
Derl>?.irr
Piit~crospir!/er
O~~l~ic~iloitlc~o
Aviculopecren
Phesria
Acanrhopecren
Present ( 2 1)
Aviculopecren
Phesria
Present ( 1 3 )
Present ( I?)
Asrarrella
Present ( 13)
Available
Available
Available
Available
Phyniaropleura
Present ( 3 7 )
Available
Preent (43)
Present ( 3 4 )
Glabrocin~ulltm
Present ( 2 7 )
Availnble
Present ( 3 0 )
Present ( 16)
Present ( l I )
Present ( 1 7 )
Available
1~i11oproducru.s
Pre.;c.nt (2 I )
Present ( I I )
Composita
Present ( 4 7 )
Present ( 1 2)
Present ( 3 0 )
4.sriir-rrlla
Parollelodon
-Nore: Diagrammatic pictures and dividing lines identify brachiopods (top). h~valvt\[centcrl. i~ndgastropods
(bottom). The name of a taxon appears in the column for a marine unit when that taxon 15 one of thc ten most
;~bundnnrin [he samples (as a group) assigned to the Productid cluster from that rn;lrlnc un~t.Thc word "Present"
(ollowcd by a nuniher In parentheses indicates that the taxon i \ prescnt in thc s ; ~ ~ n p li!\\lpncd
c\
to lllc I'r<~ductid
cluster from that marine unit, but is not one of the ten most abundan[ in that group of \:~nlplea:thc number is the
rank abundance position of the taxon in those samples. The word "Available" mean5 rhill thc taxon i \ known
from samples assigned to other clusters in that marine unit. but does not occur in the Producud clusrer samples in
[hat unit. A blank cell indicates that the taxon is not recorded in that marine unit. Several speclcs change within
genera between marlne units, as shown when a taxon is listed in boldface, but the functional morphologies of the
different specles are very similar.
type in each unit (table 6.2). In spite of the overall similarity of the fossil assemblages grouped in the Productid cluster. quantitative analyses of the data show
that there are identifiable differences in the Productid paleocommunity type from
marine unit to marine unit. Review of the most abundant species in the Productid
cluster samples from each marine unit (table 6.2) reveals that seventeen different
species appear in the top ten in abundance in one unit or another. Only three
species are among the top ten in abundance in every marine unit. three are in the
top ten in three of the four units, and eleven are in the top ten in only one or two
units. A principal components analysis (PCA) of the Productid cluster segregates
samples from each marine unit into different regions of space defined by the
principal component axes (fig. 6.5). A similar conclusion of variability from
marine unit to marine unit can be reached for several of the other Breathitt
paleocommunity types as well. If the results of the Q-mode cluster analysis using
relative abundance data are combined with the results of a Q-mode cl~lsteranaly-
137
sis using only presencelabsence data (fig. 6.6), the resulting sets of samples that
are similar, both in abundance of common species and overall species composition, and which therefore group within boxes formed by intersecting cluster
boundaries, are mostly composed of samples from single marine units. Although
the same general environmental conditions (as determined by the sedimentologic analysis discussed in Bennington's Ph.D. dissertation at Virginia Tech
[I9951 and, as noted above, apparent from the consistent association of samples
from particular Q-mode clusters with particular stratigraphic settings in each
marine tongue as well as their consistent association with particular lithologies)
recur in the different marine units, and the general aspect of each paleocommunity type recurs as well, changes in the fauna inhabiting the same general environment occur from one marine unit to the next.
To test statistically for differences in the Productid paleocommunity type
within, as well as between, different marine units, we took three replicate
samples (separated laterally by 1-3 m) from the same bed at two separate localities in the Elkins Fork and Kendrick, at the one locality representing the
paleocommunity type in the Magofin, and at three localities in the Stoney Fork.
Analysis of variance (ANOVA) of the relative abundances in the replicate
samples for eight of the nine most abundant species in the total sample set revealed that in only one case (Anthracospirifer) were the abundances similar
enough among localities within each marine unit to accept the null hypothesis
that there was no difference in abundance between localities, even when the local
+ Stoney
Fork
0 Magoffin
+ Kendrick
Productid Cluster
Correlation PCA
Elkins Fork
Kg.6.5. Stereo-pair of Productid cluster samples (see fig. 6.3) ordinated on first three
principal component axes. Note that samples from the same marine unit tend to occupy
discrete regions of the ordination volume, showing greater sample similarity within
marine units than between marine units. Correlation principal components analysis was
performed using SAS PROC PRINCOMP.
R i c h u r d K. Bantbach a n d J B r e t Benningrotl
Main Cluster Groupings for Quantitative Cluster Analysis
'Productid'
'Spiriierid'
'Chonetl
Produdid'
'Chonelid
Mollusk'
'Small
Mollusk'
that can be grouped to form the Productid cluster are not statistically similar
enough to be recognized as forming a paleocommunity (the two Elkins Fork
local paleocommunities could not be distinguished by [MIANOVA, whereas the
two Stoney Fork local paleocommunities are statistically different: see Bennington and Bambach 1996). The Productid paleocommunity type was not a stable,
unchanging entity. The paleocommunity type not only could vary significantly
from place to place at any one time (in a single marine unit). but also varied
significantly from one time to another, even when compared with the variability
known to be present at any one time.
The conclusion from this analysis is that statistically significant variation in
local paleocommunities does occur within a time interval equivalent to one of
Brett and Baird's intervals of coordinated stasis. These results, coupled to the
patterns observed by Brett and Baird (1995), Boucot (1983, 1 9 9 0 ~ ) Sheehan
.
(1992), Sepkoski ( I98 l), and Bambach (1 977, 1983) noted above, demonstrate
that marine benthic communities, regarded either as local paleocommunities or
as aggregates of local paleocommunities, are never truly stable, but change significantly through time at all scales of temporal resolution from the whole Phanerozoic down to the parasequence level (see fig. 6. I).
Jackson. Budd, and Pandolfi (chapter 5, this volume) reach an apparently dif-
E%7
s
Fig. 6.6. Two-way comparison of Q-mode clusters assigned using presencelabsence
data only (vertical axis) with quantitative Q-mode clusters (as in fig. 6.3; horizontal
axis). Intersecting lines show boundaries between major clusters in each analysis. Note
that the second large presencelabsence cluster from the bottom is divided into three
fields by the quantitative analysis, two of which group quantitatively with samples from
other presencelabsence clusters, and that the Chonetid-Mollusk quantitative cluster
unifies samples scattered among many presencelabsence clusters.
variability found in replicate samples was taken into account by factoring it out
of the test (fig. 6.7). Abundance was significantly different between localitiesfor
all other species, indicating that, in general, although these species are always
fairly abundant in the samples, they are patchy, not regular, in distribution within
a paleocommunity type during any one time interval (marine unit). ANOVA
comparison of abundances of species between marine units always rejected the
null hypothesis of no difference in abundance (fig. 6.7). When the analysis was
extended to multivariate analysis of variance (MANOVA). significant differences were found in the composition of the paleocommunity type both between
localities and between units, even when both sampling and geographic variability were factored out of the test (fig. 6.7). Although the samples assigned to
the Productid cluster are similar enough to be regarded as representing a paleocommunity type, they are not statistically the same. The local paleocommunities
Breathitt Formation
12 species
9781 specimens
ANOVA
!.,gcality
Desmoinesia
Chonetids
Linopr~uclus
Derbyia
Juresania
Anlhracospirifer
Punctospirifer
Parallebdon
Lpcalitv: H,
:
H,
-
-
,0002
.0001
.0001
.0001
.0001
.23
,0001
.0001
,0005
,0001
.0001
.0001
.0001
,001
,0001
.0001
Wilks
Pillai
Hotelling
Roy
.0001
.0001
.0001
.0001
.0001
.0001
.0001
.0001
no significant differences between localities within marine units
no significant differences between marine units
Bold number = accept null hypothesis of ~lpditference.
Fig. 6.7. Analysis of variance (ANOVA) and multivariate analysis of variance
(MANOVA) data for eight localities representing the Productid cluster from which
replicate samples were taken. The stratigraphic positions of the localities shown at
upper left; sample numbers from Bennington 1995 and thickness of sampled interval at
upper center; information on data used at upper right. ANOVA results are given at lower
left; MANOVA results at lower right. The analysis was performed using SAS PROC
GLM,data transformed to arcsine Freeman-Tukey variates to stabilize variances, and
observations weighted by total sample size.
142
Richard
K. Bambach and J
Bret Benningron
is, do communities incorporate the features needed for us to say that the changes
in communities over time are produced by a process of community evolution
akin to the process of the evolution of species?
Communities Are Different from Genealogical Entities
Immigratiorl Forms New Local Communities
New local communities developing in newly formed or recently vacated habitats
arise by recruitment in a more or less open system. not by reproduction. Avatars
are acquired from the species living in any location with geographic access to
the area occupied by the new community (Buzas and Culver 1994). This can be
seen on all scales, from the recolonization of small mangrove islands by arthropods after fumigation, to the return of flora and fauna to the area devastated by
the explosive eruption of Krakatau in 1883, to the migration of the biota into
recently deglaciated regions or regions where climatic change has forced major
change in the ecosystem. All of these examples at different scales involve the
formation de novo of local communities by immigration of the founding members of their avatars. Comparable examples in the marine realm range from the
colonization of barren dredge spoil to the influence of sea level change on shelf
regions.
In Simberloff and Wilson's classic study (Wilson and Simberloff 1969; Simberloff and Wilson 1969) of recolonization of small mangrove islands (small
enough to cover and fumigate to kill all the resident arthropods), diversity had
fully recovered through immigration after 1-2 years when the defaunated islands were in close proximity to unaffected islands (2- 170 m), although only
about 40% of the species originally present had found their way back in that
time. On more remote islands, the pace of immigration was slower than on those
closer to sources for recolonization. The most remote spot (533 m from an unfumigated mangrove island) had recovered only 55% of its initial diversity and
only 10% of its original species in 1 year; the second most remote island (379 m
from others) had 76% of its initial diversity and 32% of its original species back
after a year. To establish these local communities, all species had to arrive by
immigration.
Elton (1958.77) notes that a "rich and maturing jungle of forest inhabited by
epiphytic plants and many kinds of animals" was present 50 years after the eruption that destroyed all life on the island of Krakatau in 1883. The fauna included
720 species of insects, 30 bird species, and several reptiles and mammals, although no amphibians were recorded. All plants and animals had to arrive by
immigration from the adjacent Indonesian islands of Java and Sumatra, each
about 40 km distant.
Pielou (1991) documents changes in the distribution of flora and fauna after
the last Ice Age as the biota moved back into formerly glaciated territory. AS
D o Communiries Evolve?
143
will be noted for some examples below, the new local communities were usually
formed by the migration of extant taxa as changing conditions permitted them
to change their geographic range. rather than by evolution of new forms. Nonetheless, new faunal associations developed during this major, long-term climatic
transition. Combourieu-Nebout (1993) presents a particularly clear example of
repeated shifts in community types related to recurrent environmental conditions
during Pliocene climatic cycles in Italy, related to the onset of Northern Hemisphere glaciation. She documents four different floral groups that repeatedly recur in sequence, with similar, but not identical, plant abundances returning in
each recurrent phase as the climates cycle from warm and humid to cold and dry
and back again.
CommunitiesAre Not Unijied Individuals
Single species are not limited to membership in only one community type, but
frequently occur in several. Elton (1958) has documented the invasions of numerous species, primarily transported by humans, into areas remote from their
natural distributions. The economic impact and local community change these
invasions have caused demonstrates that species may fit into more than one community type without any need, or time, for evolutionary modification. The invading species persist in their original settings, but become important constituents of others as well.
Because of the ability of their constituent species to fit in wherever environmental conditions (including the biota) are adequate for their survival, local
communities, communities, and community types do not necessarily maintain
coherence when subjected to environmental change. This is now well known
from floral and faunal changes associated with major climatic changes, especially as documented for the glacial/interglacial shifts in the Quaternary, and a
considerable literature has developed on "disharmonious faunas" and "community disequilibrium" (Wright 1984; Davis 1986; Bennett 1990). The terms
"disharmonious" and "disequilibrium" apply only in the narrow sense of referring to past distributions and associations as compared with present community
affiliations, as if the modern world were the "normal" one. In fact, the regularity
of climatic change due to Milankovitch cycles, even though not always as extreme as in the Pleistocene, is now well documented for much of the geological
record (Wilgus et al. 1988; Bennett 1990). and avatars must constantly accommodate to that constant long-term fluctuation of physical conditions. After all,
species cannot survive in environments to which they are not adapted.
Changes in the habitat associations of mammals between the Pleistocene and
the Holocene have been described by Graham (1986), who prefers the term
"intermingled" rather than "disharmonious" for Pleistocene paleocommunity
types without modern analogues because he feels they "were undoubtedly in
harmony with prevailing environments" (Graham 1986. 306). Among the examples of intermingled Pleistocene paleocommunity types that Graham cites
144
Richard K. Bambacl~clnd J Brer Ber~tii~l,qroll
is one from Bedford County, Pennsylvania. containing three m:lrllln~~lspecies
found separately today in the Appalachians. the Great Plains. and arctic Canada,
and another from Nuevo Lebn, Mexico, containing one species now confined
~~~i~~ and another that ranges from Alaska to Newfoundland. extending no
further south today than Tennessee and Colorado.
craham (1986) also mentions that intermingled floras. as well as intermingled insect, terrestrial mollusk. bird, and amphibian and reptile faunas. have
been described, Intermingled insect assemblages. reflecting distinct climatic regimes, have been described from several European localities by Coupe (1987)
and from Kitchener. Ontario, by Schwert et al. (1985). Coopt: points out that the
species of insects living today that are found as Pleistocene fossils have not undergone any apparent evolutionary change for hundreds of thousands of Senerations; hence the shifts in geography and community association were forced On
these stable taxa as climates changed. Rousseau (1992) documents the
separation since the Pleistocene of formerly overlapping ranses of some EurOpean freshwater and land snails. For instance, two species that co-occurred in
East Anglia in England in the Pleistocene are now found only on the
mainland,one in Germany and Poland, the other in a small area i n the western
pyrenees near Bayonne, France. Two other species that once co-occurred in
northern France have each shifted geographic range, one to the north- where it
occurs only in the British Isles, and the other to the south. \\'here i t is found
primarily in southern France.
In the marine realm, species have also changed community membership Over
various time intervals as marine climate has changed. This occurred along the
coast of northern Europe at the end of the last glaciation. Thomsen and VOrren
(1986) record variation within faunas of various ages, and through time, On the
Norwegian shelf during the transition from the Pleistocene to the Holocene. Peacock (1989) documents changes in molluscan distributions along the northern
accompanying changes in temperature and sediment deposition.
European
~l~~~ the ~ a l i f o r n i acoast, molluscan provincial boundaries shifted during the
pleistoc-ne (Valentine 1961; Addicott 1966), as well as earlier in the Cenozoic
(,?,ddicott 1970), and formerly co-occurring species are now found. at least in
the more extended parts of their ranges, in entirely different associations than
they inhabited at times in the past. Similar C O ~ C ~ U can
S ~ O
be~ drawn
~
for IndoPacific reef-building corals as they responded to changing shelf areas and water
depths as sea level fluctuated during the Pleistocene (Potts 1984). Because the
variation in generic longevities found in hermatypic coral faunas of both the
Caribbean and Pacific, where 10-20% of the genera appear to be newly
25
(have no fossil record) but the average generic age in the faunas ranges
years
($tehli
and
Wells
197
I),
it
is
clear
that
associations
in
loca1
to 60
reef communities changed as new genera (and species) evolved while the
taxa persisted. Valentine and Jablonski (1993) have recently reviewed the variation in makeup of marine communities through the Pleistocene.
Do Comtn~rrliriesEvoh~e?
145
However. the lack of coherence of communities as clin1ates change does not
mean that similar communities do not recur. Recurrence of similar species associations has been observed at many scales. Similar local paleocommunitiesdo
recur in the fossil record (Bayer 1967; Bretsky and Bretsky 1975). Even though
communities seem to split up and species reorganize at different rates, similar
Communitytypes recur when similar environmental systems recur after long intervals, as exemplified by the long-term recurrence of particular vegetation patterns in the Pliocene of Italy (Combourieu-Nebout 1993) and the recurrence of
the BKathitt Formation paleocommunity types documented above. This recurrence, howe\'er, may simply result from the tendency for organisms, when they
have the o ~ ~ o r t u n i tto
y ,colonize and succeed i n environments to which they are
well adapted, not from a lockstep integration and migration of communities as
units. This tendency could account for apparent "faunal tracking" (Brett, ill^^,
and J3aii-d 1990) as habitats shift position as environments change. ~ l t h ~ ~ ~ h
faunal tracking has not been proven as the mechanism that produces the apparent
fama1 stability in intervals of coordinated stasis, faunal tracking would have
been possible when environmental conditions shifted gradually enough so that
continuous change in the geographic ranges of all community constituents
occur simultaneously, and when the necessary environmental conditions to support the faunal association also migrated across the distance over which the initial environment was being displaced. It would be possible to test for faunal
tracking versus environmental recruitment of appropriately adapted species if
one could follow faunal changes continuously in a widespread transgressive systerns tract or prograding high-stand systems tract.
The breaching of biogeographic barriers is another way in which species distribution Patterns can be changed and communities in the affected area can be
modified, Yet little evolution may occur as this takes place. The formation of the
Central American isthmus permitted migration of terrestrial mammals between
theNorth and South American continents. Many of the migrant taxa entered new
associations, and although much species evolution occurred, families and often
genera persisted without significant evolutionary modification (Marshall 1985;
Webb 1985, 199 1). Ground ~ 1 0 t ranged
h~
from South America up to North Careh a - and North American mastodons reached central Chile and Argentina, while
many of the native m m m a l s persisted in their original settings as the immigrant
taxa came in and modified the makeup of local communities. A major marine
interchange took place in the Pliocene when the Bering Strait opened, permitting
exchange between the Pacific and the Arctic-Atlantic (Vermeij 1991). Vermeij
notes that "in a very large number of trans-Arctic genera. no morphological
s~cies-leveldivergence has taken place" (Vermeij 199 1, 297), and he is -im-Pressed with the evolutionary conservatism of invaders" (Vermeij 199 1, 298).
Yet these invading species entered into entirely new community contexts in the
k m t o r ~they invaded. In a review of bipolar distributions of marine species in
the Pacific, Lindberg ( 199 1 ) argues that most antitropical species distributions
146
Richard K. Bambach and J Brer Ber~r~it~grori
Do Communities Evolve?
result from biotic interchange (breaching the tropical barrier) rather than vicariance. Lindberg also points out that "interchange and endemic taxa coexist"
(Lindberg 1991, 308). and he concludes that in local communities with interchange species, "only the taxa have immigrated; the linkages and interactions
appear to be independent and locally derived" so that "each temperate community is a mosaic consisting of cosmopolitan species, species that share common
ancestry, and species having independent origins within each region" (Lindberg
1991, 320). The alteration of circulation due to the closing of the connection
between the Pacific and the Caribbean by the Isthmus of Panama, along with
change in the marine climate from the Pliocene to the Holocene, also changed
the distributions of marine species and their biogeographic associations in the
Caribbean and Gulf of Mexico (Petuch 1982). This left several "anachronistic
subregions" and "relict pockets" in which avatars of many species found themselves in community contexts quite different from those in other parts of the
geographic range of those species.
From these examples it is clear that many species are not limited to membership in a single local community, community, or community type. but frequently :
occur in several community contexts. Our work also shows this for Paleozoic
settings. Although each of the clusters of samples in our study of Pennsylvanian
paleocommunity types has a distinctly different mix of guilds. indicating the
association of each paleocommunity type with specific environmental conditions and resource availability (see fig. 6.4), most of the dominant species in any
one paleocommunity type are also present in the others, although making up a
smaller proportion of the fauna (see table 6.1). Similar broader distributions of
species, in which their relative abundance, not simple presence or absence, is
important in differentiating paleocommunity types, characterize Ordovician
(Springer and Bambach 1985), Silurian (Levinton and Bambach 1975; Calef and
Hancock 1974), and Devonian (McGhee 198 1; McGhee and Sutton 198 1,1983)
assemblages as well.
0
,
Communities Are Segments of Gradients, Not Discrete Entities
In fact, local communities are not precisely bounded entities, despite the presence of interactions between some of their constituent taxa. This fact has been
repeatedly demonstrated by ordination techniques (Whittaker 1973; Digby and
Kempton 1987; Ludwig and Reynolds 1988). Local communities simply represent the distribution of organisms within segments of environmental gradients
(Whittaker 1975; Pearson and Rosenberg 1987).
Although we commonly use cluster analysis to group samples as a way of
designating community types, it must be remembered that the technique is set
up to create discrete clusters, even from a continuous distribution of data. The
only meaning of the cluster groupings is that members of a cluster have a greater
similarity to one another than they do to other samples or clusters. For example,
the sequence of six Q-mode clusters of samples in Miller's (1988) study of dead
shell assemblages in Smuggler's Cove. St. Croix, simply subdivided a transect
147
100 200
Pacific Ocean
Fig. 6.8. Sketch map of the eastern portion of Mugu Lagoon, California.as i t was in the
late 1960s. Numbers and shaded areas illustrate the gradient array of Q-mode clusters,
based on abundance data, for samples collected by John Warme (Warme 197 1, table 1).
The gradient extends from the inlet through the barrier beach eastward to the isolated
end of the long tidal channel and landward (northward)from the tidal channel onto
he tidal flats. Five subdivisions of the gradient are apparent from the distributionof
samples in various clusters: chster 1 (dense stippled area) in the sand channel near the
inlet; cluster 2 (lightly stipple area) in the belt just shoreward and up-channel from
cluster 1; cluster 3 (upper left-lower right cross-hatched area) in the first large pond in
he 10%tidal channel; cluster 4 (unshaded) in tidal channels and ponds landward of the
first three clusters and in the more remote eastern ponds of the long tidal channel; and
cluster 5 (upper right-lower left cross-hatched area) i n the isolated area on the landward
side of the far eastern end of the long tidal channel.
along a continuous gradient in the breakup and loss of vegetation cover. Each
cluster contained geographically adjacent or nearly adjacent samples, with one
end-member cluster composed of samples from an area completely covered with
j"halassia and the opposite end-member cluster composed of samples from biom a t e d sand lacking vegetation.
The same situation holds for the data compiled by Warme (1969, 1971) from
dead shell assemblages in Mugu Lagoon, California. Using presencefabsence
infomation. Warme identified only two general groupings of samples, those
from a sandy channel and those from muddy conditions in tidal channels,
Ponds, and tidal flats. We reprocessed Warme's data, published in 197 1, using
quantitative abundance information in his table 1. We found eight clusters
subclusters of samples, which fall along a combined salinity, topographic,
vegetation gradient. This gradient extends from the inlet through the barrier
island, which connects the lagoon with the open sea, to an area isolated from
h e exchange with the open sea at the landward end of a long tidal channel
(fig. 6.8). The five major clusters in the cluster analysis clearly relate to [he gra-
Do Cot71muniries Evolve?
dient; the other clusters are small (one to three aberrant samples) iund attach to
the five larger clusters.
Each major cluster identifies a particular portion of the gradient (tig. 6.8).
One cluster contains samples from the sandy channel within 400 ni ot'the inlet
that leads to the open sea (the same group of samples identified by Warme as
representing the sand-channel in his original presencelabsence analysis). A second cluster contains samples from sandy muds upstream in the channel, 400800 m from the inlet. The third cluster contains samples from the eelgrass-covered bottoni of a subtidal pond located along the channel 800- 1.400 rri from the
inlet. The fourth major cluster contains samples from farther up the large channel, samples from the landward side of the channel, and those from small channels through the marsh in the low intertidal zone. The fifth cluster- contains
samples from an isolated area farthest from the inlet. at the landward end of the
long tidal creek. Examples such as this and the results of Miller's study in St.
Croix are convincing evidence that most time-averaged dead shell accumulations derived from marine local communities actually represent segments of continuous gradients, not discrete, sharply bounded entities.
A distribution along gradients is also the common situation for marine local
paleocommunities recognized in the fossil record. The following examples are
all from the Silurian. In the Homerian Stage of the Wenlock. 422.5 to 420 million years ago, continuous change from local paleocommunities doniinated by
nonsiphonate to local paleocommunities dominated by siphonate deposit-feeding bivalves has been recorded in a shoaling sequence in Nova Scotia (Levinton
and Bambach 1975). Cocks and McKerrow (1984) have demonstrated the intergrading of abundances in the samples of Ziegler's original five-community
scheme for the distribution of organisms from the shoreline to the outer shelf in
the Llandoverian (Ziegler 1965; Ziegler, Cocks, and Barnbach 1968; Ziegler,
Cocks, and McKerrow 1968). Calef and Hancock (1974) recognize that during
the Wenlock and Ludlow, paleocommunity types dominated by brachiopods
were actually arrayed along environmental gradients, and they comment on "the
gradual changes from one community to the next" (Calef and Hancock 1974,
797). Watkins (1979) also demonstrates a strong control by environmental gradients in the distribution of benthic local paleocommunities in the Ludlow of the
Welsh Borderland.
Arbitrary Recognition of Paleocommunity Types
In fact. the designation of particular fossil community types normally can be
done only by subdividing a continuum. In practice, even when samples are
grouped objectively by cluster or factor analysis (Macdonald 1975; Cisne and
Rabe 1978; Springer and Bambach 1985), paleocommunity types are arbitrarily
designated because most local paleocommunities really represent a segment of
the distribution of organisms as they were arrayed along an environmental gradient. Consider Miller's (1988, 91. fig. 4) contrasting Q-mode and R-mode
149
analyses. Are there six community types. as a direct acceptance of the six
Q-mode clusters of samples would suggest. or three, as suggested by the three
R-mode clusters of species? Or two, one of vegetation-associated species and
the other of open-sand associates, with the two intergrading in the time-averaged
samples of dead shells where there are adjacent patches of vegetation and sand?
This could be rather easily determined by direct observation of the distribution
of living individuals in the Recent-but i t is not a trivial question in the fossil
record, in which we have only the time-averaged fossil assemblages and no direct information on detailed microenvironmental distributions or contemporaneity in life. Also, the fossil record seldom permits continuous sampling from a
single horizon for hundreds of meters (Miller's transect was 360 m long), and
the sediments seldom preserve information that can be used to detail subtle environmental differences from place to place, such as changes in the relative sizes
of patches of vegetation and bare sediment.
The arbitrary nature of paleocommunity designations is also demonstrated in
the case of our Pennsylvanian samples. Analysis of presencelabsence data
(which many workers use for recognizing paleocommunity types: see Macdonald 1975) yields four major sample groupings, but analysis of quantitative data
reveals five (see fig. 6.6). The four presencelabsence clusters are not simply
the same as the five quantitative clusters with two quantitative clusters undifferentiated by presencelabsence data. The quantitative clustering splits one of
the presencelabsence clusters among three quantitative clusters. Some of the
samples from the Kendrick, Magoffin, and Stoney Fork found in this particular
presencelabsence cluster form the Productid quantitative cluster together with
samples from the Elkins Fork that come from another presencelabsence cluster;
the Spiriferid cluster in the quantitative analysis is formed from other Magoffin
and Kendrick samples from the subdivided presencelabsence cluster; and the
remaining samples from the Kendrick and Stoney Fork in that presencelabsence
cluster appear in the Chonetid-Productid quantitative cluster along with samples
from the Magoffin drawn from yet another of the presencelabsence clusters.
Conversely, another of the quantitative clusters, the Chonetid-Mollusk cluster,
unifies a set of samples that appear dissimilar based on presencelabsence criteria
alone (the samples appear in seven different presencelabsence clusters). The difference between presencelabsence clustering and quantitative clustering of the
Breathitt samples illustrates the fact that both methods are clustering samples
from an environmental continuum without discrete spatial boundaries or coordinated changes in species abundance and composition. Depending on whether
one emphasizes species abundances (quantitative) or species composition (presencelabsence), different groupings of samples result from the analyses, representing different approaches to subdividing the same continuum. There are no
natural seams to cut unambiguously in these cases.
One might argue for other methods of grouping samples ecologically, but
because fossil assemblages are composed of time-averaged accumulations and.
150
Richurd K. Barnbuch and J Brer Bentlingrotl
due to taphonomic losses, are incomplete samples of the biota at best, it is not
possible (as noted earlier in this chapter) to construct meaningful groupings using such criteria as interaction on the basis of fossil assemblages. The interactive
network of an ecosystem cannot be observed in the fossil record; it can only be
inferred, and then without any knowledge of the contemporaneity of the constituents. And even if it could be determined for fossil assemblages. the interactive network is a web with connections of various strengths that extends from
the local ecosystem out to eventually encompass the global biota. There are no
unambiguous seams along which to cut in the interactive network, either, only
regions with less dense connectivity.
Communities and the Criteria for Evolution
So, where does this leave us? Do the properties of communities meet the criteria
for evolution to be the mechanism by which communities change?
Local communities vary, and so do the entities at higher levels of the ecological hierarchy. All vary at all time scales, as noted in this chapter, but that is
only one criterion needed for the process of evolution to operate. Are local communities, communities, or community types individuals capable of reproduction,
do they have heritability, and are they capable of undergoing selection?
In his essay on individuality and selection, David Hull (1980,3 13) argues that
"anything that has the characteristics necessary to be selected in the same sense
in which organisms are selected has the characteristics necessary to count as an
individual and not a group. Not all individuals can function as units of selection,
but only individuals can be selected." He then points out that "individuals are
spatiotemporally localized entities that have reasonably sharp beginnings and
endings in time," and specifies that "the entity must exist continuously through
time and maintain its internal organization." And in referring to ecological communities, he states (1980, 326-27) that "the major stumbling block in the path
of treating such systems as interactors is the independence of t h e ~ rconstituent
replicators. The organisms that comprise an ecological community may interact
with the environment of the community as a cohesive whole, but the effects of
these interactions on their constituent replicators are not unitary."
Because species are not restricted to membership in single local communities
or even to membership in more encompassing community types. and because
community-related entities of all sorts are simply segments of distributions along
environmental gradients, local communities, communities, and community
types, although definable as entities, do not have the requisite bounding conditions to be regarded as unified individuals. Even though interactions occur in
local communities, those interactions are altered as avatar membership changes,
which occurs frequently, as documented above, so that the interactive network
is not a fixed system in most instances, either. Because community-related entities are not unified individuals, they are not capable of reproduction. Because
Do Communiries Evolve?
1.51
community-related entities are not unified individuals capable of reproduction, community-related entities cannot be characterized by heritability. There is
no heritability in local communities, communities, or community types, only in
their constituent taxa. Because community-related entities are not reproducing
individuals with heritability, selection cannot act on local communities, communities, or community types as units. Therefore. community-related entities do
not possess the requisite properties and characteristics to support a process comparable to the evolution of species at any point within the ecological hierarchy
above the level of avatar.
Valentine sensed that community evolution comparable to evolution in the
genealogical hierarchy was not likely. In referring to the origin of regulation of
stability within communities, he said (1973,287), "thus, community stability is
adapted to a given regime but is presumably not evolved as an independent community-level entity. Indeed, it is difficult to see how this would be possible by
natural selection." He also commented (1973, 287) that "although the evidence
available is far from conclusive, communities cannot at present be said to have
qualities that transcend the qualities of their component populations." From the
information that has accumulated since he wrote, we now think it is possible
simply to state that communities do not evolve.
But community-related entities do change over time. For instance, Miller
(1986, 1990a, 199 1, 1993a,b) has written extensively and perceptively on the
various manifestations of local community and local paleocommunity change.
How are these changes brought about? Changes in local communities arise from
(I) stochastic variation, (2) environmental change, (3) the immigration and emigration of taxa, (4) the evolution or extinction of species, (5) the adjustment in
abundances caused by interactions after a taxonomic change, and (6) coevolutionary responses (such as character displacement) stimulated by change in one
or more avatars. Factors 1-3 and 5 are not evolutionary processes. Only factors 4 and 6 are strictly evolutionary, and they affect only a portion of the local
community.
Evolution does occur within local communities, as happened in the GalApagos after the initial taxa had arrived from other places (Lack 1961), and this does
change community-related entities. Species also evolve in concert and in response to interactions between them (coevolution) (Futuyma and Slatkin 1983;
Futuyma 1986a; Thompson 1994), and this is truly evolution within and as part
of local communities. Coevolution may markedly change or integrate parts of
local communities, but it is evolution in only part of the ecological entity, not
evolution by the entity as a whole. Conversely, many species persist through
disruptions of community types to regroup into new community types and local
communities. It appears that change is a ubiquitous feature of the Earth's ecosystems as environments change through time, but one that does not always have
a great impact on the tempo of evolution and extinction.
Both evolutionary and nonevolutionary events combine to produce the
152
Richard K. Bambach and J Brer Bermingrot1
Do Cotntnuniries Eilolve?
changes we see at all levels of the ecological hierarchy. But evolution itself is a
process restricted to the genealogical hierarchy. As Eldredge points out (Eldredge 1985. 170), the genealogical and ecological hierarchies are "worlds
apart." He goes on to say, "It is at the blatantly cross-genealogical level of communities that the independence of these two process hierarchies becomes utterly
clear and justifies the recognition of two separate hierarchies." The reason for
this is that "more-making, the general process that provides the temporal cohesion for next larger genealogical entities, does not enter into consideration of
cohesion of the elements of the ecological hierarchy" (Eldredge 1985, 168).
Eldredge and Grene (1992, 125) point out that the evolution of local community
members results in the accumulation of phylogenetic pedigrees of the component avatars, but that "there is no process of information storage in economic
systems that is comparable to phylogenetic systems." We agree with Eldredge
(1989, 189) that "if we restrict the term 'evolution' basically to the fate of genetically based information, evolution is probably best thought of strictly in connection with the genealogical hierarchy." Community-related entities in the ecological hierarchy just change-they don't evolve.
worked in the past. The more we learn about the history of community-related
change, the more clues we will have to the processes that drive change in the
Earth's biota through time.
Afterword: "Many More Data Are Needed
on Fossil Communities" (Valentine 1973)
Although we have learned enough about fossil communities since Valentine
wrote the concluding section of his chapter on community ecology and evolution
in 1973 to know that communities do not evolve, his words are still true. The
patterns and processes involved in community assembly, breakup. and change
still require explanation. The intervals of relative stasis contrasting with intervals
of rapid change seen at the levels of the Ecologic-Evolutionary Unit and subunit
are particularly intriguing. Why is it that large numbers of species persist with
little evolution and extinction through all manner of cyclic environmental
changes, be they glacial/interglacial climatic cycles or transgressive/regressive
sea level cycles, both in the Pleistocene-Holocene and in the Paleozoic, while
major changes in taxa and in patterns of distribution of paleocomniunity types
occur in some short stratigraphic intervals apparently little different than others?
Does coevolution have a major role in local community organization even
though local communities do not seem to be tightly structured or discretely
bounded entities? Is recovery from mass extinction the only trigger for the
more profound changes in community-related entities that occur during Reorganizational EEUs? Under what conditions do major evolutionary innovations
form, leading to new adaptive types and thus the higher taxa responsible for the
fundamental changes in the utilization of ecospace that alter the structure of
ecosystems? The data required to answer these questions still need to be compiled. The fossil record of paleocommunity-related entities will always be our
only route to understanding the context in which evolution and extinction have
153
Acknowledgments
R. K. B. thanks Jim Valentine for 25 years of intellectual inspiration. as well as
friendship. We appreciate the invitation from Doug Erwin to participate in the
Paleontological Society Symposium in October 1994 that led to this chapter.
:
William Miller 111 provided a detailed and challenging review that stimulated us
! to be more precise and helped us to sharpen the focus of the chapter. The original
! work on Breathitt Formation communities was supported by grant EAR
9105469 from the National Science Foundation. That work also is part of the
dissertation work of J Bret Bennington in partial fulfillment of requirements for
. the Ph.D. at Virginia Polytechnic Institute and State University.
,
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