Professor McGill University Montreal, Quebec, Canada
Abstract: Animal dispersive movements play a key role in maintaining population dynamics, and density has been suggested to impact these movements either by driving or inhibiting them. Although the relationship between conspecific density and dispersal has been explored in various study systems, inconsistent results limit the formation of reliable general trends in terms of both the magnitude and the direction of density dependence. Here, we review empirical studies of animal dispersal in relation to conspecific density to examine the evidence for density-dependent dispersal among taxa and test whether reports of its occurrence are taxon-specific and/or biased by the methodology employed. We conducted a thorough review of the literature (2000 – 2021) and found k = 81 empirical studies of amphibians, birds, fishes, invertebrates, mammals, or reptiles that had tested for a correlation between conspecific density and animal dispersal. We extracted categorical variables for taxonomic group, sex, age, migratory behavior, study design, density and dispersal metrics, as well as temporal and spatial scale, to test each of their correlation with the effect of density on dispersal (Pearson’s r) using linear regressions and multilevel mixed-effect modelling. We tested for publication bias using funnel plots and trim-and-fill analysis to deduce asymmetry and examined heterogeneity among studies both across and within categories to ascertain the reliability of the results. Although there was no predominant trend for density effects on dispersal across the six taxonomic groups, results within groups were more harmonious for certain methodology categories. The direction and magnitude of the interaction between density and dispersal in empirical studies appears to be method-dependent, as the scales at which density and dispersal are measured significantly affect analytical results. In addition, the effect size varies depending on whether the animal undertakes migratory movements as a part of its life history or not. Negative results were also significantly under-reported, as we found proof of publication bias. Evidence that conspecific density is an effective driver of animal dispersal is equivocal, but empirical methods and animal migratory behavior can significantly impact study findings across taxa.
