Facilitation in Mangrove Ecosystems

Ecologists have studied ‘mutual aid’ (or facilitation) since at least the time of Kropotkin (1842–1921). But the ‘watchword’ which he heard with such force has not always been heeded, remains poorly understood and is rather loosely defined. A seminal early use comes from Connell and Slatyer (1977), where building on the work of Clements (1916), they used ‘facilitation’ to describe a model of community succession in which pioneer species modify the habitat allowing the colonisation of later ones. While this successional implication remains common in the literature, the word is also applied more broadly to positive interactions between individuals and species. Some authors restrict the term to plant– plant interactions (e.g., Krebs, 2001), while others treat it as synonymous with ‘positive species relationships’ (He et al., 2013), although Munguia et al. (2009) argue that any relationship that does not cause evolutionary changes to both parties is not a true ‘interaction’. Others expand the term to describe any positive relationships between individuals. For example Schöb et al. (2014) state that facilitation describes ‘the positive effects of one organism on others’, while Singer (2016) defines it as ‘interactions between two organisms, or two species, that benefit at least one of them and harm neither’. The latter definition covers interactions at all levels of complexity, from individual behaviour through to ecosystems, and includes obligate symbiosis, commensalism and looser associations. The term therefore always involves the idea of benefit from association with another organism. This benefit is sometimes mutual, sometimes one-way and sometimes even negative for one partner, particularly over time (the facilitation model of succession allows for the competitive exclusion of the pioneer species by those that they facilitate). Given this broad use, any review of facilitation is in danger of losing focus or becoming overwhelmed. Here, we adopt another influential definition from Bertness and Calloway (1994), which includes interactions within and between species, but restricts the ambit of the term through a focus on stress: ‘the benefits to an organism by the minimisation by neighbouring organisms of biotic or physical stress’. Applications of this definition in the literature have