Adaptation to a limiting environment: the phosphorus content of terrestrial cave arthropods

Abstract

AbstractStoichiometric imbalances (mismatches between elemental ratios of consumers and their food) are expected to be especially important in detritus‐based systems because poor resource quality may impose severe growth constraints. Such imbalances have been highlighted in producer‐based food webs and detritus‐based aquatic systems, but similar investigations of detritus‐based terrestrial ecosystems are absent. Cave animals are dependent on detrital subsidies from the surface and display adaptations to caves (e.g., decreased growth and metabolic rates). Here we examined how nutrient quality may constrain consumer strategies in caves. We found that the quality of cave resources was comparable to resources on the surface, but there was some evidence that cave animals may face nutritional constraints for at least a part of the year. Such constraints may be especially important for millipedes, whose C:P was particularly low (i.e., nutrient‐demanding) relative to cave detritus. Based on the growth rate hypothesis, we predicted that cave‐adapted species would have lower %P, lower %RNA, and a lower RNA/DNA ratio relative to surface‐dwelling counterparts; however, the differences we discovered between congeneric millipedes may not necessarily be due to P scarcity. Consistent with stoichiometric theory, we found significant negative %P allometry across phylogenetic groupings of 17 cave arthropods. We did not see this allometric relationship in millipedes, perhaps because of the lowered P content of the cave‐obligate species. Our results highlight the potential for stoichiometric challenges of caves to influence the adaptations of terrestrial cave animals. This novel explanation for cave adaptation may yield insights into cave biodiversity and biogeography.