Assistant Professor University of North Carolina at Greensboro Greensboro, United States
Abstract: Long-lasting debates exist on determinants of the food chain length (FCL), i.e., a vertical dimension of biodiversity. Previous studies propose three environmental factors as putative controls of FCL: resource availability, disturbances, and ecosystem sizes. The theoretical foundation for these hypotheses is derived from relatively simple food webs, often including only three or four species. Early experimental investigations found firm support for some of the hypotheses; however, results are controversial in field studies with stable isotope approaches, which approximate FCL without illuminating the entire food webs. These inconsistent findings imply the existence of missing links between the environmental drivers and FCL.
Here, we explore the role of the internal structure of food webs in mediating the relationship between the environmental drivers and FCL. We hypothesized that species richness and motifs (i.e., 3-species subgraphs in a food web) were internal modulators of FCL. An empirical food web database showed that FCL positively correlated with the 3-species chain motif and species richness. Although species richness more strongly affected FCL than the chain motif, FCL disproportionately changed at low species with high species richness. Our mathematical model of N-species food webs highlighted that disturbance and ecosystem size produced clear gradients of species richness and FCL. In the meantime, resource availability only slightly affected species richness with limited influence on FCL. However, an increase in species richness due to less frequent disturbances or a larger ecosystem did not always result in enhancing FCL; FCL was almost constant when species richness exceeded 15.
To conclude, species richness modulates the relationship between the environmental drivers and FCL. Because of the non-linear relationship between species richness and FCL, our findings suggest that the environmental drivers affect FCL when species richness is low while the environmental effects on FCL can be masked at high species richness. This emergent perspective would explain inconsistent relationships between the three environmental drivers and FCL in previous field studies.
