To go or not to go: variable density-dependent dispersal in small mammals

Abstract

Population density has been widely understood to be a key influencer of dispersal behavior; however, the generality of density-dependent (DD) dispersal in vertebrates is unclear. We conducted a review of the available empirical data on small mammal DD dispersal, distinguishing between the three dispersal stages: emigration, immigration, and transience (dispersal distance). We focused on small mammals because they are a well-studied, functionally similar group of vertebrates, with a distinct ecological importance. We also examined the effect of season, body mass, study length, and study type on the strength and direction of DD dispersal. The majority of emigration and dispersal distance studies reported negative density dependence, while immigration was mostly independent of density. No correlative patterns were detected; however, interpretation of the available data was hindered by inconsistencies in experimental and analytical approach across studies. Our results suggest that the three phases of the dispersal process may be influenced differently by density and highlight the importance of distinguishing between emigration, immigration, and transience when considering the effects of density dependence. As well, our study identifies several limitations with the current available data which limit the ability to compare DD dispersal behavior across systems, and calls for future investigations that consider all three phases of dispersal in the same system.

Study methodology impacts density-dependent dispersal observations: a systematic review

“…Comparable to a recent review conducted on densitydependent dispersal in small mammals [85], we find that the reported observations on the effect of density on dispersal are limited and do not allow for the comparison or generalization of dispersal behavior across systems, even within taxonomic group. Another recent review reported that during 2009-2018, most studies on animal movement were in relation to external factors, but of those, only 38% were on movement in relation with other animals, conspecific or heterospecific [50].…”

“…However, there is potential for disparities among the results of different studies to be induced by taxonomic and/or methodological differences. Therefore, and unlike existing reviews on density-dependent dispersal [ 11 , 41 , 61 , 85 ], we particularly examined the extent to which several categories of study methodology, within and across taxonomic groups, impacted empirical results for the effect of density on dispersal. We explicitly considered heterogeneity and potential reporting of analytical biases, and tested whether reports of density-dependent dispersal were related to taxonomic group, sex, age, migratory behavior, study design, dispersal metric, density metric and variable type, and scales of space and time.…”