Examining Interspecific density-dependent dispersal in forest small mammals

The effects of conspecific densities on dispersal have been well documented. However, while positive and negative density-dependent dispersal based on conspecific densities are often shown to be the result of intraspecific competition or facilitation respectively, the effects of heterospecific densities on dispersal are examined far less frequently. This despite the potential for the analogous processes of interspecific competition and heterospecific attraction to influence dispersal. Here we use 51 years of live-trapping data on deer mouse (Peromsycus maniculatus), eastern chipmunk (Tamias striatus), red-backed vole (Myodes gapperi), and jumping mice (Napaeozapus insignis and Zapus hudsonius) to examine the effects of both conspecific and heterospecific densities on dispersal frequency. In terms of conspecific densities, jumping mice were more likely to disperse from areas of low conspecific densities, while red-backed voles and chipmunks did not respond to conspecific densities in their dispersal frequencies. When considering interspecific density effects, while there were no statistically clear effects of density on dispersal frequency, the effects of chipmunk and jumping mice densities on dispersal by red-backed vole were greater than the effects of conspecific densities, with voles more likely to disperse from areas of high chipmunk densities and low jumping mice densities. Likewise, the effect of chipmunk densities on dispersal by jumping mice was greater than the conspecific density effect. Conspecific densities clearly can affect dispersal by two of these four species, but the effects of heterospecific densities on dispersal frequency are less clear. Based on effect sizes it appears that there is potential for heterospecific effects on dispersal by some species in the community, but future experimental work could elucidate the strength and causes of these relationships.

Altitudinal patterns in breeding bird species richness and density in relation to climate, habitat heterogeneity, and migration influence in a temperate montane forest (South Korea)

Altitudinal patterns in the population ecology of mountain bird species are useful for predicting species occurrence and behavior. Numerous hypotheses about the complex interactions among environmental factors have been proposed; however, these still remain controversial. This study aimed to identify the altitudinal patterns in breeding bird species richness or density and to test the hypotheses that climate, habitat heterogeneity (horizontal and vertical), and heterospecific attraction in a temperate forest, South Korea. We conducted a field survey of 142 plots at altitudes between 200 and 1,400 m a.s.l in the breeding season. A total of 2,771 individuals from 53 breeding bird species were recorded. Altitudinal patterns of species richness and density showed a hump-shaped pattern, indicating that the highest richness and density could be observed at moderate altitudes. Models constructed with 13 combinations of six variables demonstrated that species richness was positively correlated with vertical and horizontal habitat heterogeneity. Density was positively correlated with vertical, but not horizontal habitat heterogeneity, and negatively correlated with migratory bird ratio. No significant relationships were found between spring temperature and species richness or density. Therefore, the observed patterns in species richness support the hypothesis that habitat heterogeneity, rather than climate, is the main driver of species richness. Also, neither habitat heterogeneity nor climate hypotheses fully explains the observed patterns in density. However, vertical habitat heterogeneity does likely help explain observed patterns in density. The heterospecific attraction hypothesis did not apply to the distribution of birds along the altitudinal gradient. Appropriate management of vertical habitat heterogeneity, such as vegetation cover, should be maintained for the conservation of bird diversity in this area.