Area effects are more prominent than productivity effects for higher trophic levels

Background/Question/Methods

Net primary productivity and habitat size can have major effects on community structure and species diversity. The productive-space hypothesis predicts longer food-chains from more productive and larger habitats, while island biogeography theory predicts larger habitats to have greater diversity.I tested these predictions by examining how plot size (1 m² and 10 m²) and productivity (fertilizer added or not) affected the assembly of insect communities in experimental field plots. In spring 2003 I sampled insect colonists four times from plots of California native annuals planted in a UC Davis field. Over 90,000 individuals of Coleoptera, Heteroptera, Homoptera, Hymenoptera and Neuroptera were keyed to family and assigned to morphospecies via a study-specific voucher collection.

Results/Conclusions

Consistent with other experiments, habitat size influenced community properties more so than productivity, particularly at higher trophic levels. Large plots supported higher predator-prey ratios but lower total abundance per unit area, suggesting predators require more area to find sufficient amounts of prey in response to energetic constraints imposed by diminished energy transfer up the food-chain. Unexpected higher overall densities on small plots could indicate decreased predation pressure from edge effects, spatial effects, less predator suppression, or subsampling/disturbance effects on the large plots. Strong date effects and other size and fertilizer trends were also detected. Lack of diversity trends on these “terrestrial islands” suggests that other ecological factors may predominate over area, degree of isolation, and productivity effects to influence diversity.