Strong effects of weak interactions in ecological communities

The loss or removal of individual species can cause dramatic changes in communities(1-5). Experiments indicate that in many communities only a few species will have such strong effects, whereas most will have weak effects owing to small per capita effects and/or low abundance(3,6-15,16). But extinction of these 'weak' interactors could significantly alter natural communities because they play important stabilizing or 'noise-dampening' roles(14,15,17-23). I demonstrate here that some 'weak' interactors may also be important by magnifying spatiotemporal variation in community structure. An analysis of published interaction strength data shows that the greatest variation in species effect occurred for the weakest interactions. A field experiment corroborates this and shows how indirect interactions can generate an inverse relationship between the mean and variance of a consumer's impact on its prey. When a species' effects are highly variable in sign and magnitude, they may average to seem weak over broad scales but be strong in local contexts. Thus, what is frequently considered to be 'noise' in species interaction data may be a critical part of the signal.