Rapid acquisition of an ecological inheritance: Niche constructed shells in hermit crabs

Background/Question/Methods

Through a process of niche construction, animals can shape their surrounding ecology in ways that produce new forms of ecological inheritance for future generations. However, few studies have investigated how niche construction can influence the speed with critical resources are absorbed into populations. Here I examine an ecological inheritance represented by gastropod shells, which are essential to the survival and reproductive success of hermit crabs. Through a process of niche construction certain species of terrestrial hermit crab (Coenobita compressus) architecturally remodel gastropod shells, creating an altered form of ecological inheritance that persists across generations. In field experiments in Osa Peninsula, Costa Rica I tested whether niche constructed shells (i.e., those architecturally remodeled by hermit crabs in the past) were incorporated into the population faster than were fresh shells (i.e., those acquired directly from gastropods, without any remodeling by hermit crabs). I further tested the extent to which these two alternative forms of ecological inheritance could catalyze a cascade of resource exchange within the population. Each experiment involved introducing empty shells of both types (niche constructed and fresh) across a range of sizes, and using high-definition video to record the ensuring behavioral dynamics and resource exchange among the crabs.

Results/Conclusions

Both niche constructed shells and fresh shells attracted large aggregations of crabs in periods of less than 1 h, with over 20 individuals accumulating in 0.3 x 0.3 m quadrats. Niche constructed shells were significantly more likely than fresh shells to be entered by the crabs, and were consistently occupied in less than 20 min. Once a crab entered a niche constructed shell it left behind its current shell, which was then entered by another crab, resulting in a cascading vacancy chain of shells. These vacancy chains resulted in up to seven times as many individuals in the population receiving replacement shells through the introduction of just a single niche constructed shell. The final shell leftover at the end of these vacancy chains was typically half the diameter of the original introduced shell and represented close to the smallest shell size in the population. Broadly, these results suggest that altered ecological inheritances, created through niche construction, have the potential to substantially alter the speed and dynamics with which critical resources are incorporated by future generations of the same species.