Shifts between cooperation and antagonism driven by individual variation: A systematic synthesis review

The outcomes of interspecific and intraspecific ecological interactions can be considered to fall along continua from cooperative (mutually beneficial) to antagonistic (detrimental to one or both parties). Furthermore, the position of an interaction outcome along the continuum, for example whether a symbiont provides net costs or benefits to its host, or whether two conspecifics cooperatively forage or compete for food, is often not fixed but can change over time or across contexts. In this systematic review, we investigate the role of intraspecific trait variation ('ITV') in one or both interacting parties in determining the cooperative-antagonistic outcome of inter- and intraspecific ecological interactions. Based on a literature collection of 96 empirical and theoretical publications meeting our inclusion criteria, we give an overview of the types of interaction continua involved; traits related to outcome variance; and mechanisms as well as constraints on shifts in interactions outcomes. We propose that ITV can lead to shifts in interaction outcomes via two interrelated mechanisms. First, trait frequency effects occur when there are changes in a population's composition of traits linked to cooperation or antagonism (e.g. aggressive personality types, cheater phenotypes etc.), leading to net shifts in interaction outcomes. Second, systemic variance effects occur where the level of ITV in a trait in a population (as opposed to the mean value) is the factor that influences the cooperative-antagonistic outcome. Heritable trait differences and phenotypic plasticity are sources of phenotypic variation among individuals, and both the degree of heritability and plasticity of the trait involved may determine whether shifts between cooperation and antagonism are likely to be short-term (i.e. context-dependent) or lead to more persistent shifts (e.g. mutualism breakdown). To guide future research, we describe knowledge gaps and divergences between empirical and theoretical literature, highlighting the value of applying evidence synthesis methods in ecology and evolution.