Community Change
• Species turnover
• Succession
–Replacement of one type of
community by another
–Nonseasonal directional pattern of
colonization & extinction of species
Succession
• Apparently orderly change in community
composition through time
• venerable subject in community ecology
• mechanisms that drive succession?
Modern hypotheses
• Summarized by Connell & Slatyer (1977)
• Three mechanisms drive species
replacement
– Facilitation
• site modification
– Tolerance
• interspecific competition
– Inhibition
• priority effects, disturbance
• Null hypothesis
– Random colonization & extinction
Facilitation hypothesis
• Early species make site more suitable for
later species
• Early species only are capable of
colonizing barren sites
– specialists on disturbed sites
• Climax species facilitate their own offspring
• Primary process: Site modification (soil)
Tolerance hypothesis
• Later species outcompete early species
• Adults of any species could grow in a site
• Which species starts succession
– Chance
– Dispersal ability
•
•
•
•
Early species have no effect on later species
Later species replace early species by competition
Climax species are the best competitors
Primary process: Interspecific competition
Inhibition hypothesis
• Adults of any species could live at a site
• Which species starts succession
– Chance
– Dispersal ability
• Early species inhibit (out compete) later species
– Persist until disturbed
• Later species replace early species after
disturbance
• Climax species are most resistant to disturbance
• Primary process: Priority effects
Random colonization hypothesis
•
•
•
•
Nothing but chance determines succession
No competition, no facilitation, no inhibition
Colonists arrive at random
Species in the community go extinct at
random
Resource ratios and succession
• Based on Tilman & Wedin 1991a, 1991b
• As secondary succession procedes:
– soil N increases over time
– light at soil surface decreases over time
• Consider light and soil resource (N) as two
essential resources
• Successional sequence of species may
result from changing resource ratios
Resource
ratio
hypothesis
of
succession
LATE
N
2
13
24
3
EARLY
Light
Resource ratio
hypothesis of succession
•
•
•
•
Early species (3, 4) are good competitors for N
Late species (1, 2) are good competitors for light
Resource competition drives succession
Alternative succession hypotheses e.g.,
colonization-competition hypothesis
– early - good dispersers, poor competitors
– late - good competitors, poor dispersers
– most similar to tolerance hypotheses
Experimental tests of the resource
ratio hypothesis of succession
• Test resource competition theory for this system
– determine R* for N a set of species
– determine whether R* values predict the competitive
winners: Low R*  high competitive ability
• Test resource ratio hypothesis of succession
– determine R* for N for a set of species
– test prediction that R* is low for early species
– test prediction that early species win in competition
• possible to refute one or both
Old field successional grasses
• 5 species studied
• Agrostis scabra (As)
• Agropyron repens (Ar)
Introd.
• Poa pratensis (Pp)
• Schizachyrium scoparium (Ss)
• Andropogon gerardi (Ag)
Early Native
Early
Mid Introd.
Late Native
Late Native
% cover during succession
Predictions based on RR
hypothesis for succession
• R* for N
• As < Ar < Pp < Ss < Ag
• In competition for N
– As best
– Ag worst
Experimental gardens
• Bulldoze to 60 - 80 cm … bare
sand
– N = 90 mg/kg
• Add topsoil (0% to 100%) & mix
• 4 N levels
1
0 - 20%
180 mg/kg
2
40 - 55%
460 mg/kg
3
70-100%
800 mg/kg
4
100%+ 6.55 g/m2/yr
1000 mg/kg
Determining R*
• Raise each species in monoculture
• After 3 yr. determine Soil N (R*)
• Also determine:
– Root mass
– Shoot mass
– Root:Shoot
– Reproductive mass
– Viable seed production
Measured R* values
• Soil NO3
–As > Ar, Pp > Ss, Ag
• Soil NH4
–As, Ar > Pp, Ss, Ag
• Does not support RR
hypothesis of succession
Root masses at all N levels
• Ag, Ss > Pp > Ar, As
• Root mass predicts R*
– accounts for 73% of variation in R* (NO3)
– N uptake + related to root mass
log(R*) As
Pp
Ag
log(root)
Reproductive traits
•
•
•
•
•
•
Reproductive mass: As > Pp, Ar, Ss, Ag
seeds / m2 : As > Pp, Ss > Ag, Ar
Rhizome mass: Ar >> Pp, As, Ag, Ss
Early species invest most in reproduction
suggests colonization advantage
consistent with colonization- competition
hypothesis
Colonization-Competition
• Premises
– Trade-off of colonization vs. competition
– Strict competitive hierarchy
– No priority effects
– Metacommunity structure
Does R* = competitive ability?
• If low R*  competitive ability:
– resource competition theory is incorrect
– succession may still be driven by resource
ratios
• If low R* = competitive ability:
– resource competition theory is correct
– resource ratio hypothesis is refuted
• 3 pairwise competition experiments
Competition experiments
• Schizachyrium scoparium vs. Agrostis
scaber
• Andropogon gerardi vs. Agrostis scaber
• Agropyron repens vs. Agrostis scaber
• Based on R*, predict As loses
– As and Ar closest, longest time to exclusion
• seedling ratios 80:20, 50:50, 20:80
• 3 soil N levels (1, 2, 3)
A. scaber excluded by late
spp.
As (dashed) + 20% ,
50%,  80%,  monoculture
Ss or Ag (solid) 20% , 50%,  80%,  monoculture
A. scaber & A. repens - 3 yr.
As (dashed)
Ar (solid)
+ 20% ,
50%,  80%,  monoculture
20% , 50%,  80%,  monoculture
Measured R* values
• Soil NO3
–As > Ar, Pp > Ss, Ag
• Soil NH4
–As, Ar > Pp, Ss, Ag
• Does not support RR
hypothesis of succession
A. scaber & A. repens - 5 yr.
As(dashed) + 20% ,
50%,
 80%
Ar(solid)  20% ,
50%,
 80%
Overall conclusions
• Resource competition theory supported
– R* accurately predicts competitive ability
• Resource ratio hypothesis of succession
refuted
– early species are the worst competitors for N
• Colonization-competition hypothesis of
succession consistent with results
MetaCommunities
(Leibold 2004 Ecol. Lett. 7:601-613)
• set of local communities linked by
dispersal of >1 potentially interacting
species
• two levels of community organization
– local level
– regional level
• Patterns of regional persistence of species
depend on local interactions and dispersal
Spatial dynamics (regional)
• Mass effect : net flow of individuals
created by differences in population size
(or density) in different patches
• Rescue effect: prevention of local
extinction by immigration
• Source–sink effects: enhancement of local
populations by immigration into sinks, from
sources
Balance between regional & local
• What determines local and regional
species persistence?
– Strengths of local interactions
– Dispersal among locations
– Patterns of spatial dynamics
Metacommunity
paradigms
• Patch dynamics
• Species-sorting
• Mass-effect
• Neutral
Metacommunity paradigms
• Patch dynamics
– patches are identical & capable of
containing populations
– patches occupied or unoccupied.
– local diversity is limited by dispersal.
– spatial dynamics dominated by local
extinction and colonization
– Similar ideas to colonization-competition
hypothesis
Metacommunity paradigms
• Species-sorting
– resource gradients or patch type
heterogeneity cause differences in
outcomes of local species interactions
– patch type partly determines local
community composition.
– spatial niche separation
– dispersal allows compositional
changes to track changes in local
environmental conditions
Metacommunity paradigms
• Mass-effect
– immigration and emigration dominate
local population dynamics.
– species rescued from local
competitive exclusion in communities
where they are bad competitors via
immigration from communities where
they are good competitors
Metacommunity paradigms
• Neutral
– all species are similar in their competitive
ability, movement, and fitness
– population interactions consist of random
walks that alter relative frequencies of
species
– dynamics of diversity depend on
equilibrium between species loss
(extinction, emigration) and gain
(immigration, speciation).
MetaCommunities
• Leibold et al. 04 Ecol. Lett.
• Ellis et al. 06. Ecology.
– tested data on mosquito assemblages in
Florida tree holes for consistency with the 4
paradigms
– 15 tree holes censused every 2 wk. from 1978
to 2003
– mosquito species enumerated
Ellis et al.
Ecological Niche
• Grinnell emphasized abiotic variables
• Elton emphasized biotic interactions
• Slightly later (1920’s & 30’s)
– Gause, Park lab experiments on competition
– competitive exclusion principle
– “Two species cannot occupy the same niche”
Ecological Niche
• Quantitative approaches to ecology (1960’s)
• G. E. Hutchinson
fitness
– relate fitness or reproductive success (performance) to
quantitative variables related to resources, space, etc.
resource
More axes (dimensions)
Fitness
C
B
B
A
A
In multiple dimensions…
• multidimensional space describing
resource use
• N-dimensional hypervolume,
expressing species response to all
possible biotic & abiotic variables
• You can quantify
– Niche breadth
– Niche overlap
Web height
Simplified Niches of Argiope
A. aurantia
A. trifasciata
Prey size
Intertidal height
Simplified Niches of barnacles
Balanus
Chthamalus
Particle size
Niche overlap
• Literature on niche
– overlap = competition (e.g., Culver 1970)
– overlap = lack of competition (e.g., Pianka
1972)
Chase-Leibold Approach
• Niche axes are quantitative measures of factors in
the environment
• Niche defined by
– Requirements (isoclines – amount needed for ZPG)
– Impacts (vectors – effects on a factor)
• Trade-offs required for coexistence
Niche
• What was the question?
– Diversity
– Coexistence / Lack of coexistence
– Hypotheses?
• Niche overlap/Niche breadth
– Does not yield testable hypotheses
• Chase-Leibold
– Testable hypotheses about requirements and impacts
Neutral theory of biodiversity
• Hubbell, SP 2001. The unified neutral
theory of biodiversity and biogeography.
Princeton Univ. Press.
• see also Chase & Leibold ch. 11
• Reading: Adler et al. Ecol. Lett. 10:95–
104.
Understanding species diversity
• Hubbell is interested in biodiversity in
the narrow sense
– biodiversity = species diversity
– S, E
– Conservation biology and policy oriented
discussions use a broader definition
• Hubbell specifically considers diversity
within a tropic level
– e.g., trees, or other primary producers
Neutrality
• Does not mean that species
interactions are absent or unimportant
• Neutrality: all individuals and species
are the same in all relevant properties
– hence random processes are what govern
community dynamics
– differs from "neutral models" used to test
statistically for presence of ecological
interactions
Understanding diversity
• Niche assembly perspective
– diversity is a result of interspecific differences
– trade-offs -- that enable species to coexist
despite the diversity-eroding effects of
competition
– assembly of communities governed by rules
about which species can coexist
– typically tied to equilibrium conditions
Understanding diversity
• Dispersal assembly perspective
– diversity is a result of chance and history, and
the balance between species arrival and loss
– arrival = colonization, speciation
– loss = extinction
– assembly of communities governed chance
– despite name, need not be based on
dispersal
– species all equivalent, hence there are no
rules about coexistence
• equilibrium theory
of island
biogeography
• neutral model
– species pool, all
equivalent
– includes effects of
competition
rate (colonization or extinction)
MacArthur & Wilson
S
MacArthur & Wilson
• accounts for variation in S
• does not account for variation in E
– species abundances
• Hubbell's theories explicitly seek to
explain species abundance patterns
Neutral theory:
important premises
• Numbers of individuals in a community
must be limited
– J = r A, where…
– J = number of individuals (e.g. trees)
– A = Area
– r = density of individuals
• and, in any large area communities are
saturated with individuals
• No unused space
Zero-sum game
• Dynamics of the community are thus a
zero-sum game
• for one species to increase in
abundance another must decrease
• extinctions associated with changes in
abundance of others
• inter- and intraspecific competition
Rules for zero-sum game
•
•
•
•
•
•
J individuals
each individual occupies one space unit
resists displacement by other individuals
[think of trees]
individuals die with probability m
replacing individual
– probability it is species i is proportional to
species abundance of i
Probabilities of species'
population change
• decrease:
Pr{Ni-1|Ni} = m Ni (J – Ni)/J(J-1)
• no change:
Pr{Ni|Ni} = 1 – 2m Ni(J – Ni)/J(J-1)
• increase:
Pr{Ni+1|Ni} = m Ni (J – Ni)/J(J-1)
– Note: Pr(increase) = Pr(decrease)
–
Ni = abundance of the ith species
Ecological drift
• all species equal competitors on a per capita
basis
• all species have average rate of increase r = 0
• dynamics of any species' population is a random
walk
• species' abundances may increase to J or
decrease to 0
– absorbing boundary
– time to extinction via ecological drift can be long
Consequences
• zero-sum game plus ecological drift
– relative abundances approximately log normal
• but with excess rare species
• referred to as "zero-sum multinomial"
• if instead the zero-sum game occurs with
frequency or density dependent transitions
– relative abundances do not approximate log
normal
Significance
Number of species
• Log normal has been argued to be the
most widespread species abundance
pattern in communities
0
1 2 3 4
5 6
7 8 9 10 11
log2(individuals/species)
analogy to genetics
• allele frequencies
• what maintains genetic diversity in the
face of tendency for selection to erode
genetic diversity?
• selective neutrality
– allele frequencies change at random
– random walk to extinction
Problem: absorbing boundaries
• end of the process for any species is always
either extinction or complete dominance
• time to extinction increases with J
• expected abundance depends on J, but not
immigration rate
• variance of abundance depends on
immigration rate
• metacommunity dynamics - recolonization
Metacommunity
• All trophically similar individuals and
species in a region
– multiple connected local communities.
– JM = metacommunity size
Speciation
• Ultimately species replaced by speciation
•  = speciation rate
Fundamental biodiversity number
•  = 2 JM 
•  is the fundamental biodiversity
number
• controls equilibrium S and relative
abundances (E )
• controls shape of dominance-diversity
plot
log(relative abundance)
Effects of  on metacommunity
 = 100
 = 20
 = 0.1
=5
Rank abundance
 = 50
Outcome
• Postulate
– species saturation of the community
– random processes of species replacement
– metacommunity
• Yield: wide array of possible species
abundance distributions
• "Niche differentiation" and coexistence
mechanisms are not necessary for
diversity
Unified Neutral Theory (UNT)
• many community ecologists resistant to UNT
– careers invested in research on coexistence
mechanisms; rendered irrelevant in UNT
– data show that coexistence mechanisms are often a
prominent feature of species' biologies
– UNT disconnects behavioral, physiological, and
population ecology from community ecology
– renders studies of ecology of individual species
largely trivial (from community perspective)
Where does UNT get us?
• UNT and "niche based" coexistence mechanisms are
not necessarily mutually exclusive in the large sense
• some kinds of organisms or communities may be
governed by one, some by the other
• What is the domain of applicability of each?
• Even if UNT explains broad diversity patterns of
whole communities, "niche based" coexistence
mechanisms may be vital to understand dynamics of
small sets of strongly interacting species.
Reconciling UNT and Niche based
community ecology
• Under what circumstances is ecological
drift quantitatively important?
– Hubbell: ecological drift is always present;
when does it matter?
– much like genetic drift
• UNT designed to explain species number
and evenness at the whole community
level
Hubbell 2005.
Functional Ecology 19:166-172
• “Probably no ecologist in the world with
even a modicum of field experience would
seriously question the existence of niche
differences among competing species on
the same trophic level. The real question,
however is … what niche differences, if
any, matter to the assembly of ecological
communities.”
Hubbell 2005
• functional equivalence of species
– seems to fit tropical trees well
– suspect it will be less likely for mobile animals
• still, neutral theory predicts species
number and relative abundance well
• neutral theory captures “aggregate
statistical behavior” of biodiversity