Incorporating positive interactions in aquatic restoration and conservation

© The Ecological Society of America www.frontiersinecology.org P interactions are traditionally defined as interactions in which one species benefits from the presence of another species, without harm (and, potentially, with benefit) to the latter. Common examples include mutualisms (both species benefit), commensalisms (one species benefits with no measurable effect on another species), and facilitation (one species makes conditions more favorable for another species). These interactions have long been recognized as important structuring forces in natural communities (eg Clements 1916; Gleason 1927), yet only recently have they been formally included in ecological theory (Bertness and Callaway 1994; Bruno and Bertness 2001; Bruno et al. 2003). Experimental work over the past 15 years in rocky intertidal, salt marsh, and montane plant communities has revealed that positive relationships among species can be as important as, or even more important than competition and predation in regulating ecosystem structure and function (Bertness and Leonard 1997; Callaway and Walker 1997; Bruno et al. 2003; Leslie 2005), yet negative interactions (eg competition, predation) have tended to dominate ecological research over much of the past century (Bertness and Callaway 1994; Bruno and Bertness 2001; Bruno et al. 2005). This focus has, in turn, strongly influenced contemporary approaches to restoration and conservation. Here, we propose a broader scope of positive interactions that acknowledges (1) how individuals within a population can benefit one another (intraspecific interactions) and (2) that positive interactions can be conducted across very large scales, even if individuals are never in contact (Table 1). Minimum population sizes that avoid Allee effects (positive relationship between population density and the reproduction and survival of individuals) and positive density dependence (eg spawning aggregations) are clear examples of the former, while cross-ecosystem linkages and subsidies that derive from species in one ecosystem and benefit other species in distant ecosystems illustrate the latter. These examples meet the traditional definition of positive interactions, in that one individual or species benefits without harm to the other, but expand the nature and scale of our traditional understanding of positive interactions. We review how restoration and conservation have been conducted in the past, reflect on how terrestrial restoration efforts have benefited from consideration of positive interactions, and assess how aquatic management may also benefit from the explicit inclusion of positive interREVIEWS REVIEWS REVIEWS