Size-dependent interference competition between two sea star species demographically affected by wasting disease

ABSTRACT: In systems where species interaction strengths change with body size, perturbations which result in size-selective mortality can potentially alter interactions by affecting not only population density, but also size structure. This study examined the size-dependency of interference competition between the sea stars Pisaster ochraceus and Evasterias troschelii using field surveys and behavioral experiments to better understand the indirect demographic effects of sea star wasting disease (SSWD) on competitive interactions. In Puget Sound, Washington, where these species utilize similar prey resources and intertidal habitat, SSWD has reduced population densities and body sizes similarly for both species. Interference behaviors were not apparent among stars <4 cm arm length, but among larger individuals, Pisaster was dominant. In the field, small stars were found in aggregations of conspecifics and heterospecifics more often than were large stars. In behavioral trials, small stars responded neutrally towards conspecifics and heterospecifics, but large Evasterias responded submissively towards large Pisaster. Additional observations suggested that aggressive use of pedicellariae by Pisaster was the mechanism for these avoidance behaviors. Although demonstration of limiting resources is required to conclude that competition occurs in the field, our results suggest that SSWD has decreased the intensity of potential competitive interactions between Pisaster and Evasterias by reducing not only population density, but also body size. Prior to SSWD and if/when Pisaster recovers, interference competition may allow Pisaster to dominate in preferred habitats. However, if Pisaster recovers more slowly than Evasterias, Evasterias may move into areas formerly dominated by Pisaster, and potentially fill Pisaster’s ecological role.