PS 18-251 - Allee effect strength varies across Allee mechanisms, but not taxonomic groups

Abstract: The Allee effect is a density-dependent phenomenon in which individual fitness increases as population density increases. Over the past few decades, a growing number of studies have identified Allee effects in populations using experimental approaches and statistical modelling techniques. These studies have investigated multiple Allee mechanisms (e.g., mate-finding, predation, resource limitation), across a range of systems and taxa (e.g., plants, vertebrates, invertebrates). This meta-analysis aims to synthesize experimental demonstrations of the Allee effect, with the goal of determining whether the “strength” of the Allee effect varies with Allee mechanism across taxonomic groups. A total of 2305 studies were screened, and 62 of these studies met the meta-analysis inclusion criteria by reporting an experimental manipulation of population density. Within these 62 studies, 9 different Allee mechanisms were identified across 6 broad taxa. Standardized effect sizes (Fisher’s Z) were calculated for the studies to show the direction and strength of the density dependence, with positive values indicating positive density dependence (i.e., the Allee effect), and negative values indicating negative density dependence. When grouped by Allee mechanism and taxa, the strength of the Allee effect differed across mechanisms (z-test = 0.12, p = 0.044, k = 155), whereas taxonomic group was less useful at explaining variation in Allee effect strength (z-test = 0.148, p = 0.383, k = 155). Of the 9 Allee mechanisms identified, competition resulted in the strongest Allee effects, followed by fear, pollen limitation and mate limitation. This suggests that Allee effects may be more dependent on mechanism than taxa, and that Allee effects may function similarly within different taxonomic groups. However, as the majority of experimental Allee effect studies included in this meta-analysis focused on plants and invertebrates, more research is needed on Allee effects in other taxonomic groups to confirm this conclusion. This meta-analysis provides the first quantitative synthesis of Allee effect research in ecology, offering novel insight into how the Allee effect can manifest in populations.