The contribution of autochthonous resource to the diet of aquatic consumers is unrelated to its spatial distribution in tank bromeliads

Abstract

Autochthonous biomass is heterogeneously distributed within freshwater ecosystems, shaping community structure and ecosystem functioning, yet few studies have evaluated how the spatial heterogeneity in the availability of the autochthonous resource affects its contribution to the diets of aquatic consumers. To fill this gap, we conducted a field survey to test whether the spatial heterogeneity of the autochthonous resource within freshwater ecosystems formed in tank bromeliads determines its contribution to the diet of aquatic consumers. Tank bromeliads are Neotropical plants with densely arranged leaves in a rosette shape that accumulate rainwater and terrestrial plant material, providing suitable habitat for a diversity of aquatic invertebrates. We sampled 3 regions (north, central, and south) of the bromeliad ecosystem to provide a natural gradient in the availability of the autochthonous resource (i.e., algae biomass). To trace the diets of aquatic consumers, we used stable isotope analysis of hydrogen coupled with Bayesian stable isotope mixing models. The availability of the autochthonous resource varied among the tank-bromeliad regions with light availability, but it was not related to its relative contribution to the diets of aquatic consumers. Instead, aquatic consumers from all regions showed similarly high degrees of autochthony. The capacity of aquatic consumers to move within the plant did not affect the contribution of the autochthonous resource to their diets. We reason that the contribution of the autochthonous resource to the diets of consumers is not related to its spatial distribution because aquatic consumers preferentially feed on this resource, even when its stocks are limited. In addition, water flow among tanks could provide a subsidy of the autochthonous resource from more productive to less productive regions. We conclude that spatial heterogeneity in the availability of the autochthonous resource does not limit its contribution to the diet of aquatic consumers because of active foraging on the autochthonous resource and the flux of this resource among compartments in freshwater tank-bromeliad ecosystems.