Scale Invariance in Food Web Properties: An Important Ecological Theme Missed

1. Scale Invariance in Food Web Properties: An Important Ecological Theme Missed? Jonathan Bowers

2. Scale Invariance in Food Web Properties George Sugihara, Kenneth Schoenly, Alan Trombla

3. Dr. George Sugihara B.S., University of MichiganM.S., Ph.D., Princeton University, New Jersey Dr. Sugihara is a theoretician who has worked across a diverse variety of fields including algebraic topology, algal physiology and paleoecology; more recently, ecology, neurobiology, atmospheric science, and finance. 

4. Dr. Kenneth Schoenly Bug Hunting with the Kids Entomology and Crime Investigations Insects and the Environment   An acknowledged expert on entomology and its use in the investigation of crime scenes, Schoenly has conducted research on his specialty all over the world. He co-authored a 2001 research paper titled “Impact of Insecticides on Rice Ecosystems” that addressed the issue of reducing insecticide use in Asian rice fields to reduce health risks for rice farmers and their families.

5. Food Webs A food web is defined as a complex network of many interconnected food chains and feeding interactions These are tools used in depicting a number of trophic and species interactions, ranging from predator-prey, parasite-host, plant-herbivore as well as some indirect underlying interactions including mutualism, competition, apparent competition, and commensalism

6. Properties of Food Webs There are five properties typical of food webs that were focused on for the purposes of this study Property of Chain Length Property of Network Connectance Property Predator/Prey Ratio Property of Top, Intermediate, and Bottom Trophic Species Property of Rigid Circuitry

7. Chain Length in Food Webs Chain length is defined as the simplest direct pathway of nutrient flow Producer ? Primary Consumer ? Secondary Consumer ? Tertiary Consumer ? Top Predator, e.g. P1 ? Chain length within food webs is typically short and reaches no more than 5 trophic levels

8. Connectance Connectance is defined as the total number of observed links in a community divided by the total possible links (including cannibalistic links) C = Lobs / Lpos P2 ? Connectance declines at a rather constant rate relative to total species number i.e. C * S = x Where S is community species (or guild) number and x is a constant

9. Predator/Prey Ratio With respect to biomass these ratios can vary depending on overall community structure (with prey and bottom trophic level species having greater biomass) However, P3 ? the ratio of these species in a web is roughly constant (1-3) This figure is independent of community species (guild) number

10. Trophic Level Species Composition Species within a food web are associated with a trophic level at which they typically feed, although species can acquire a food source from multiple trophic levels (omnivory) P4 ? The number of top-level predators (with no observable predation risk), intermediate species, and bottom-level producers is independent of community species (guild) number

11. Rigid Circuitry Rigid Circuitry is defined as a triangular pattern observed in food webs in which three predators in series are indirectly related on the basis of common prey. P5 ? Niches are closely packed in food webs, hence, rigid circuitry is common in food webs Side Note: These fit with Patten closely connected compartments as central to energy cycling

12. Research of Scale Effects Does scaling (clustering of trophic levels for independent study, aggregation of trophic clusters [guild grouping], etc.) have an effect on these properties? Reason for conducting this research is to observe if properties remain constant even when considering sub-communities or trophic clusters rather than individual taxa

13. Problems and Criticisms of Food Web Research By their nature, food web assemblages have been the subject of major criticism in the literature Incompleteness of food webs (Cohen et. al. 2002) Inaccurate reports of observable links (Cohen et. al.) Food webs are typically assembled (when community composition is understood) based on gut-tract analysis and previous understanding of species diet cross-matching available prey items

14. Methods of Research 60 food webs of varying species number were pulled from the literature These webs are highly variable with respect to habitat size (macro- and micro-habitats), species diversity, and type of interaction (plant-herbivore, parasite-host, etc.) Webs are invertebrate dominated

15. Results 41 of 60 collected food webs were found to have the all five of the listed properties of food webs Briand and Cohen (1984) collected food webs, and nearly all showed the five properties as well The results then show that independent of scale, these properties of food webs tend to hold true and are statistically significantly consistent over a wide range of food webs One exception was a predictable proportion of basal (bottom-trophic) species There were more basal species in less diverse webs, although not to a statistically significant or normalized pattern

16. Arguments of Food Webs Seasonal variation in species composition Incompleteness of Food Webs as a whole Both in number of taxa present as well as an accurate measure of feeding links Most parasite interactions are not shown in documented food webs, and cannibalism is not shown very often One node-loop interactions (cannibalism and storage) are thought to be relatively common in community food webs Life-history omnivory may not be documented in an acute study

17. Criticisms Of The Work Although work includes suggestions of scale invariance in rigid circuitry (of which the concept of pathway proliferation is a subset), this differs with Borrett and Patten. Found that cycled flow was most important factor to variation in indirect flow through linear regression modeling Also found that indirect pathways or effects typically outweigh and out-number direct interactions Work does not take into account indirect pathway and consequences of connectance and circuitry on the variation in such Food webs vary in number of compartments, with as few as 2 nodes in a food web Top predator in one of the food webs is a species of mite!!!

18. Publications 11. Zhang, W.J. and K.G. Schoenly. 2001. A randomization test and software to compare ecological communities. International Rice Research Notes 26(2): 48-49. Schoenly, K., J.E. Cohen, K.L. Heong, G. Arida, A.T. Barrion and J.A. Litsinger. 1996. Quantifying the impact of insecticides on food web structure of rice-arthropod populations in a Philippine farmer’s irrigated field: a case study. Pages 343-351 in G. Polis and K. Winemiller, editors. Food Webs: Integration of Patterns and Dynamics. Chapman and Hall, New York. 14. Schoenly, K. and J.E. Cohen. 1991. Temporal variation in food web structure: 16 empirical cases. Ecological Monographs 61: 267-298. 19. Schoenly, K., J.E. Cohen, K.L. Heong, J.A. Litsinger, G.B. Aquino, A.T. Barrion, and G. Arida. 1996. Food web dynamics of irrigated Philippine rice at five elevations in Luzon, Philippines. Bulletin of Entomological Research 86: 451-466.18. Cohen, J.E., K. Schoenly, K.L. Heong, H. Justo, G. Arida, A.T. Barrion and J.A. Litsinger. 1994. A food-web approach to evaluating the impact of insecticide spraying on insect pest population dynamics in a Philippine irrigated rice ecosystem. Journal of Applied Ecology 31: 747-763. 21. Sugihara, G., L.F. Bersier and K. Schoenly. 1997. The effects of taxonomic and trophic aggregation on food web properties. Oecologia (Berlin)112: 272-284.