Supplement B. Here, we provide the results from using the methodology on the same
food webs used in two seminal papers on compartments in empirical food webs [Pimm,
S. L. & Lawton, J. H. Are food webs divided into compartments? J. Anim. Ecol. 49,
879-898 (1980); Raffaelli, D. & Hall, S. J. Compartments and predation in an estuarine
food web. J. Anim. Ecol. 61, 551-560 (1992)]
n
Odds
ratio
P-value
IC
Askew (1961)a
62
5.63
<0.996
0.05
Bird (1930) Prairiea
15
5.21
<0.819
0.13
Bird (1930) Willowa
12
2.41
<0.940
0.14
Jones (1949)a
12
2.77
<0.922
0.20
Koepcke & Koepcke (1952)a
46
11.02
<0.967
0.04
Menge & Mauzey (1978)b
22
2.81
<0.998
0.11
Milne & Dunnet (1972) Mudflata
12
3.89
<0.945
0.14
Milne & Dunnet (1972) Mussel beda
10
5.93
<0.814
0.18
Minshall (1967)a
13
1.60
<0.991
0.23
Niering (1963)a
27
8.21
<0.544
0.06
Paine (1966)
13
2.11
<0.990
0.18
Summerhayes & Elton (1923)c
29
18.09
<0.001*
0.06
Tilly (1968)a
11
3.83
<0.820
0.18
Zaret & Paine (1974)a
13
9.03
<0.519
0.10
Name
a
These webs were taken from: Cohen, J. E. (compiler). 1989. Ecologists' Co-Operative Web Bank.
Version 1.00. Machine-readable data base of food webs. New York: The Rockefeller University.
b
This web was taken from: Cohen, J.E. 1978. Food Webs and Niche Space. Princeton University Press,
Princeton, New Jersey.
c
This web was taken from: Pimm, S.L. and J.H. Lawton. 1980. Are food webs divided into compartments?
J. of Animal Ecology. 49:879-898
Our results for nine of these food webs sustained the same conclusion of Pimm and
Lawton (1980) and Raffaelli and Hall (1992). That is, these nine food webs did not have
odds ratios higher than what was expected by chance, and therefore they were not
compartmentalized. The lack of compartmentalization is likely due to how these food
webs were constructed rather than a lack of compartments in the actual food webs or
limitations in the method. The interactions in all of these webs were unweighted and our
analysis demonstrated the difficulty in identifying compartments in unweighted food
webs. In addition, these food webs likely represented a fraction of the species within the
actual food web or aggregated species in a way that might obscure compartments. For
example, two species of fish may interact with ten species of zooplankton for a total of
ten realized interactions. If the fish are aggregated into one taxanomic group and the
zooplankton are aggregated into another, then the ten interactions are reduced to one
interaction. The method would no longer have the same information for identifying
compartments. Weighting the one interaction would help to alleviate this problem.
Our conclusions diverge from the previous studies with the Menge & Mauzey (1978) and
Paine (1966) food webs. Both of these food webs were found to be significantly
compartmentalized with 2 compartments. In our analysis, these food webs did not have
odds ratios higher than expected by chance. A similarity index was used in both of the
Pimm and Lawton (1980) and Raffaelli and Hall (1992) analyses and similarity indices
have been used to identify trophic levels. Thus these analyses may have picked up on
trophic levels.
Koepcke & Koepcke (1952), Niering (1963), and Summerhayes & Elton (1923) were
food webs where Pimm and Lawton (1980) were a priori defined compartments. They
compared realized interactions between two compartments against an estimate (based on
the average number of interactions per species and number of species within each
compartment) using a chi-square distribution. They found that all of these food webs had
lower than expected realized interactions between compartments. Our results indicate
that there are only significant compartments than by chance alone for Summerhayes &
Elton (1923). The Askew (1961) food web was published in the Pimm and Lawton
(1980) paper but Pimm and Lawton were unable to analyze it due to lack of computing
power. The Summerhayes and Elton (1923) food web had an interactive connectance
within compartments = 0.12 and interactive connectance between compartments =
0.0072. This food web was found to have two compartments (see table below).
Pimm and Lawton (1980) reported a priori placement of the taxa from the Summerhayes
and Elton food web into three separate compartments. In their study, one compartment
contained marine taxa, a second contained terrestrial taxa, and a third contained
freshwater taxa. In our arrangement, all of the marine taxa, four terrestrial taxa, and six
freshwater taxa are in compartment A and thirteen terrestrial taxa and three freshwater
taxa are in compartment B. Only three interactions are shared between the two
compartments of the potential 416 interactions which produce a very low between
interactive connectance.
The density of interactions between and within our
compartments produced an odds ratio lower than the Pimm and Lawton’s a priori
assignments. In fact, the odds ratio of the original analysis was smaller than 983 of the
simulated food webs of this web and therefore would not have been significant in our
analysis.
Summerhayes and Elton (1923) food web divided into compartments based on our
analysis
Compartment A
Compartment B
marine plankton
Plants
marine animals
Worms
Seals
Geese
Sea-birds
Colembola
ducks and divers
Diptera (terrestrial)
skua and glaucous gull
Mites
benthic algae (freshwater)
Hymenoptera
Protozoa
snow-bunting
Diptera (freshwater)
purple sandpiper
other invertebrates
Ptarmigan
Lepidurus
Spiders
polar bear
arctic fox
decaying matter
planktonic algae (freshwater)
Protozoa
invertebrates (freshwater)
dead plants
Original Citations for the food webs:
Askew, R. R.. On the biology of the inhabitants of oak galls of Cynipidae
(Hymenoptera) in Britain. Trans. Soc. Brit. Entomol. 14:237-268 (1961)
Bird, R. D. Biotic communities of the Aspen Parkland of central Canada,
Ecology, 11:356-442 (1930).
Bird, R. D. Biotic communities of the Aspen Parkland of central Canada,
Ecology, 11:356-442 (1930).
Jones, J. R. E. A further ecological study of calcareous streams in the "Black
Mountain" district of South Wales, J. Anim. Ecol. 18:142-159 (1949).
Koepcke, H. W. and M. Koepcke, Sobre el proceso de transformacion de la materia
organica en las playas arenosas marinas del Peru, Zoologie Serie A, No. 8, Publ. Univ.
Nac. Mayer, San Marcos (1952).
Milne H. and G. M. Dunnet, Standing crop, productivity and trophic relations of
the fauna of the Ythan estuary. In: The Estuarine Environment, R. S. K.
Barnes and J. Green, Eds., Applied Science Publications, Edinburgh,
Scotland (1972)
Minshall, G. W. Role of allochthonous detritus in the trophic structure of a
woodland springbrook community, Ecology 48(1):139-149 (1967).
Niering, W. A. Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands,
Ecol. Monogr. 33(2):131-160 (1963).
Paine, R. Food web complexity and species diversity. Am. Nat. 100:65-75 (1966)
Summerhayes, V.S. and C.S. Elton. Contributions to the ecology of Spitsbergen and
Bear Island. J. Ecology. 11:214-286 (1923)
Tilly, L. J. The structure and dynamics of Cone Spring. Ecol. Monogr.
38(2):169-197 (1968).
Zaret, T. M. and R. T. Paine, Species introduction in a tropical lake, Science
182:449-455 (1973)