The role of temporal and spatial heterogeneity along a habitat duration gradient: influences on food-chain length in aquatic food webs

Background/Question/Methods

Food webs depict consumer-resource interactions among species or individuals in a habitat.  Food-chain length, the number of trophic transfers from the base to the top of a food web, is an important metric to describe food web structure because it influences ecosystem functions, community composition, and species diversity. Two widely-cited hypotheses to explain variation in food-chain length: dynamic stability hypothesis and the ecosystem size hypothesis predict habitats with frequent disturbance will have shorter food chains while larger ecosystems support food webs with longer food chains. To date, most studies have utilized lakes, rivers, or artificial ecosystems to test these hypotheses; however there is a need to use diverse natural ecosystems to disentangle the mechanisms driving food-chain length. Intermittent ponds are highly dynamic, short-lived ecosystems that may challenge the static view of food webs therefore making them excellent systems to study. We tested the dynamic constraints and ecosystem size hypotheses using carbon and nitrogen stable isotope ratios of basal resources and aquatic consumers in nine ponds spanning a natural gradient of habitat duration and size over the ice-free period. In addition, we identified potential proximate mechanisms influencing differences in food-chain length within and among pond food webs.

Results/Conclusions

We found food-chain length varied by more than a half trophic level across ponds (2.83 to 3.53) and by about one trophic level within ponds (e.g., 2.64 to 3.53) over time. We found that our data do not follow the predicted patterns of the either the dynamic stability or ecosystem size hypotheses; instead food-chain length was not static but elongated and contracted depending on larval amphibian species composition and phase in hydroperiod. Ponds with intermediate hydroperiod had the longest food-chains (3.25) compared to ponds of shorter and longer durations. The results of our study highlight the importance of using naturally occurring gradients to reveal the underlying mechanisms controlling food web structure.