Spring- and groundwater-associated organisms often exhibit restricted geographic ranges, high levels of endemism, and sensitivity to changes in habitat conditions. It is also thought that subterranean habitats are relatively resource poor, leading to simple food webs populated by generalist species exhibiting little functional diversity. However, some subterranean species exist in transition zones between subterranean and surface habitats and frequently co-occur with surface-species, potentially competing for resources. The purpose of this study was to examine patterns in resource and habitat use in the invertebrate communities found at two spring complex ecosystems (Comal Springs and Finegan Springs) in Texas, USA. We assessed the resource use of surface and subterranean species, several of which are found in the transition zone between subsurface and surface habitats. Food web structure and basal food resources at each site were assessed via bulk stable isotopes of carbon (δ13C) and nitrogen (δ15N) in organisms and their potential food sources. We additionally assessed basal food resources supporting a subset of the community using a suite of compound-specific stable isotopes (i.e., amino acids). Finally, we additionally examined the degree of niche overlap between of several species that occupy surface - subsurface transition zones.
Results/Conclusions
Although both subterranean and surface species were found at spring opening interfaces in both Comal and Finegan Springs, isotopic mixing models indicated that there was a substantial amount of niche partitioning among species. In both spring systems, endemic spring-associated riffle beetle species (Heterelmis) derived >70% of their diet from terrestrial-derived organic matter, whereas closely-related but more cosmopolitan Elmid species derived a majority of their diets from periphyton. Interestingly, in the Comal Spring system, organisms which were clearly adapted for subterranean existence (i.e., blind, unpigmented) derived as much as 70% of their diet from terrestrially-derived organic matter. In addition, a subterranean adapted and predatory amphipod (Stygobromus pecki) was the apex predator in the Comal Springs invertebrate food web, consuming prey from both surface and subsurface environments. Results of this study indicate that invertebrate species found in subterranean – surface ecotones are finely partitioning food resources and exhibit narrow feeding niches. Indeed, the degree of feeding niche specialization among these interface species is likely related to energy availability in this low resource environment.