Coexistence of Habitat Specialists Under Environmental Change: Investigating Dietary Overlap in Two Brachyuran Species at Peritidal Stromatolite Ecosystems

Abstract

The extant stromatolite pools in Port Elizabeth, South Africa, provide a unique ecotone habitat at the interface of fresh groundwater seepage and the marine intertidal zone. Within this transition environment, we investigated the coexistence of two brachyuran species with different habitat preferences, i.e. marine and freshwater by quantifying niche overlap. The aim was to determine whether shifting environmental pressures or variability within the stromatolite pools (i.e. regular state shifts between marine and freshwater conditions) would invoke a dietary response or competitive interaction between these two species that are usually separated by their habitat tolerances. It was hypothesised that there would be little overlap between the two species but that this overlap would be greater in winter compared to summer due to reduced ectothermic activity. Stable isotope niche analysis revealed no dietary overlap between the two species at any site or in either season (summer or winter). Furthermore, isotope signatures suggest that both species feed on resources from their respective microhabitats. Despite the usual tendency of ecotone or edge populations to adopt a generalist diet, both species were able to remain habitat specialists, likely due to their highly mobile nature and access to suitable microhabitats within this dynamic freshwater/marine ecotone. This study is important from an environmental change perspective as it supports the hypothesis that populations occupying transitional environments such as these might be more tolerant of habitat shifts than their counterparts dwelling in more stable localities, and thus these populations are more likely to coexist.