The outcomes of ecological interactions fall along a continuum from cooperative (mutually beneficial) to antagonistic (detrimental to one or both parties). This applies at both interspecific (e.g. plant-animal interactions) and intraspecific levels (e.g. male-female interactions). Phenotypic variation among individuals is increasingly being recognised as an important factor in ecological processes, and trait variation in either interacting party may determine the outcome of the interaction, such as whether a symbiont provides net costs or benefits to their host, or whether two conspecifics decide to cooperatively forage or to compete over food. This systematic review investigates the role of intraspecific trait variation (‘ITV’) in determining the cooperative-antagonistic outcome of ecological interactions. Based on a literature collection of 95 empirical and theoretical publications meeting our inclusion criteria, we give an overview of the various mechanisms that can lead to shifts between antagonism and cooperation within or between species. We describe two broad classes of interrelated mechanisms that may drive shifts in outcomes. First, trait frequency effects occur when processes influencing a population’s composition of traits linked to cooperation or antagonism (e.g. aggressive personality types, cheater/exploiter phenotypes etc.) lead to net shifts in interaction outcome. Second, systemic variance effects occur when changes in the amount of ITV in the population (as opposed to the mean phenotype) is the factor driving shifts. Both heritable genetic differences among individuals and phenotypic plasticity are important sources of phenotypic variation. The specific mix of heritable vs. plastic ITV may determine whether a change from cooperative to antagonistic, or vice versa, is likely to be short-term (i.e. context-dependent) or lead to more persistent shifts (e.g. mutualism breakdown). To guide future research on this topic we describe knowledge gaps and divergences between empirical and theoretical literature, further highlighting the value of applying research synthesis methods in ecology and evolution.
Shifts between cooperation and antagonism driven by individual variation: A systematic synthesis review
