Resource movements across landscapes play vital roles in many food webs and ecosystems. Extensive work in aquatic food webs has revealed that nutrient and energy subsidies from external environments can profoundly impact the structure and function of recipient ecosystems. However, understanding subsidies’ effects on trophic structure remains challenging due to the many pathways linking most aquatic and terrestrial ecosystems, the heterogeneity of autochthonous primary production, and the complexity of recipient communities. Aquatic caves provide compelling models for subsidy research due to their prevalent resource limitation and depleted taxon richness. Most aquatic cave communities are thought to rely entirely on intermittent subsidies of photosynthetic material from the surface, but some ecosystems are also supported by in situ chemosynthetic bacteria, which may contrast in temporal availability and potentially support increased diversity. Though long dismissed as rare, patterns in groundwater chemistry along many tropical limestone coasts suggest that chemosynthetically-supported caves may be far more common than previously thought. In order to test the importance of chemosynthetic and photosynthetic resources on community structure, we surveyed aquatic caves in Mexico and compared trophic structure and species richness between communities that depend entirely on intermittent subsidies and communities suspected to rely in part on chemosynthesis.
Results/Conclusions
We conducted visual surveys to assess macro crustacean community composition and species richness, and used stable isotopes (δ13C and δ15N) to compare food web complexity, specialization, and basal resource use between caves. Of the six caves examined, consumers in four communities had isotopic compositions consistent with a chemosynthetic energy source. Communities supported in part by chemosynthesis were more speciose than subsidy-dependent communities, but differences are largely due to the presence of more salt-tolerant taxa such as Xilbansus tulumensis and Bahalana mayana that were absent from subsidy-dependent communities due to patterns in groundwater chemistry. Our results reveal that utilization of photosynthetic and chemosynthetic material differs by taxon and functional feeding group, with filter-feeders exhibiting high trophic flexibility compared to detritivores. Broadly, this work advances our understanding of how resource dynamics influence community structure.