Identifying the ecological mechanisms that determine foodweb structure is critical for understanding the causes and consequences of diversity. Food-chain length (FCL) is a product of the biotic interactions within a community and the environment, but how environmental variation affects FCL is not well understood. We examined how gradients of ecosystem size and environmental variation in hydroperiod affected the FCL of ponds. Using C and N stable isotopes, we found that average FCL was 3.3 and varied by 3 trophic levels across ponds and years. We showed that ponds with shorter hydroperiods have shorter food chains, and that FCL is not strongly influenced by ecosystem size. These results demonstrate support for the dynamic constraints hypothesis, which states that less-predictable environments should have shorter food chains. Our data are not consistent with the prediction of the ecosystem size hypothesis that larger ecosystems have longer food chains. Insect and amphibian richness increased with increasing pond size and hydroperiod, results indicating that insertion of new species into the pond communities is a driving mechanism causing variation in FCL. Omnivory could explain variation in FCL, but our results show that the incidence of omnivory was similar across the environmental gradients. Ours is one of the few empirical studies to link structural changes in the food web with variability in FCL. We showed that temporal variation and species composition shape pond communities and influence foodweb structure.