Variance in ecological consumer-resource interactions

Food-web models use the effect size of trophic interactions to predict cons umer-resource dynamics(1-3). These models anticipate that strong effects of consumers increase spatial and temporal variability in abundance of specie s, whereas weak effects dampen fluctuations(4-6). Empirical evidence indica tes that opposite patterns may occur in natural assemblages(7). Here I show that spatial variance in the distribution of resource populations is sensi tive to changes in the variance of the trophic interaction, in addition to the mean effect of consumers, relative to other causes of spatial variabili ty. Simulations indicate that both strong and weak direct effects of consum ers can promote spatial variability in abundance of resources, but only tro phic interactions with a large mean effect size can reduce variation. Predi ctions of the model agree with the results of repeated field experiments an d are consistent with data from published consumer-resource interactions, p roving to be robust across widely varying environmental conditions and spec ies' life histories. Thus, food-web models that embody variance in trophic interactions may have increased capacity to explain the wide range of effec ts of consumers documented in empirical studies.