A potential mechanism underlying the ecosystem size food chain length trend: Shallow lakes as model ecosystems for addressing the predator prey interaction hypothesis

Background/Question/Methods

Food chain length, a widely studied ecological metric, has numerous important implications ranging from ecosystem functioning, ecosystem services and contaminant bioaccumulation. There is some support for the long-standing hypothesis that food chain lengths are energy limited. However, the importance of ecosystem size has been recognized more recently. The mechanism that underlies the ecosystem size trend has yet to be determined. One hypothesis is stabilization of predator prey interactions within larger ecosystems, thereby, leading to predator prey co-existence.  Similarly, macrophytes have indirect effects on prey species within lakes by providing refuge from predators. To determine if stabilization of predator prey dynamics increase food chain length, we constructed a simulation model and are conducting an empirical study.  Twenty shallow lakes within South Eastern Quebec that exist along a macrophyte abundance gradient were sampled to parameterize our simulation model.  Stable isotope analysis will be used to quantify food chain length for the empirical portion of this study and will supplement our modeling results.

Results/Conclusions

Results from our simulation model show that when predation pressure was high, corresponding to low habitat structure, predation pressure was the major driver of food chain length. Conversely, when predation pressure was low, or non-existent, food chain length was limited by basal resources and transfer efficiency. Therefore, dampening of predator prey interactions, as one would expect to occur with increased structural complexity of habitats and increased ecosystem size (when unrelated to productivity), resulted in a higher probability of longer food chain lengths. Further, by decreasing the importance of predator prey interactions, the importance of energy in determining food chain length was increased.  These results may explain why both ecosystem size and productivity have been found to be significant predictors of food chain length within the literature. This study demonstrates that predator prey interactions are likely important drivers of food chain length, which is of great importance for basic ecological theory and conservation efforts that seek to maintain ecosystem integrity.