Experiments determining if habitat mosaics include the refugia from succession theorized to promote species coexistence

Abstract

Refugia within successional mosaics where localized conditions inhibit successional replacement may support large abundances of early colonizing species and their coexistence with strongly competitive late colonizers. Numerous habitats have been hypothesized as refugia from succession with important landscape-scale consequences from export of propagules, but their commonness among ecological systems is unknown because tests to demonstrate their existence have not been formulated and applied. In this study on an intertidal model system, an early successional tubeworm was highly abundant in a hypothesized refuge habitat-type where late successional algae could not establish. In adjacent non-refuge habitat, a change in species dominance involving tubeworms shifting to algae occurred from early to late succession following experimentally induced disturbance. No such change occurred in refuges where early successional tubeworm populations steadily increased throughout succession. Tubeworm recruitment was reduced in the presence of late successional algae, likely from competition in the non-refuge. The presence of habitats providing refugia from succession may have important consequences, e.g. promoting low but consistent levels of local-scale coexistence of early and late successional taxa observed here even without disturbance. Experimental tests such as these to identify refugia from succession will be useful to apply to larger-scale land/seascapes if, as in this study, the scale of experimentation is optimized for the species and processes of interest. If the inferences from these results are extrapolated to larger-scale systems, they may inform our understanding of spread of early successional species such as weeds with large impacts that are potentially influenced by this landscape feature.