Food web properties persist following amphibian extirpations in a Neotropical stream
As species are extirpated, ecosystem productivity will likely decline with concomitant declines in food web structure as linkages are removed. Catastrophic amphibian extirpations in Central America can affect ecosystem function, but have unknown consequences on food web structure. We empirically assessed periphyton-insect food webs, both before and after (5 yrs) a disease-driven amphibian extirpation in a highland Panamanian stream. Food webs were constructed using gut content analysis of 891 tadpoles and insects to identify 1793 linkages from four food webs: pools and riffles with and without tadpoles. For each food web, fourteen structural properties were estimated and the effect size was measured between pre- and post-decline food webs. Simulations using pre-decline food webs that reflected the loss of tadpoles and cascading extirpations of macroinvertebrates, were then compared with the observed post-extirpation food web to quantify the robustness of food web structure with respect to these perturbations.
Amphibian extirpations resulted in the loss of 52% of macroinvertebrate taxa, but connectance was reduced by an average of only 3%. The mean difference in the values of the fourteen food web properties from pre- to post-decline was <10%. Simulated loss of tadpoles and macroinvertebrates overestimated the effects of species extirpations on food web properties by an average of 8%. This difference between predicted and observed effects of species loss was explained by 46% of the linkages post-decline being new trophic linkages, reorganizing the food web and mitigating the effects of species loss while retaining the structural properties of the original food web. Persistence of food web structure after species loss suggests that food webs are shaped by forces independent of the individual trophic linkages of which they are composed.