Variation in species-specific responses to habitat fragmentation and land cover structure in urban small mammal communities

Abstract

Urbanization is a key driver of habitat loss and fragmentation worldwide, yet many urban ecosystems contain vegetated habitat patches that support diverse wildlife communities. Managing urban systems to support robust wildlife communities requires us to understand the mechanisms that drive the response of species to the urban environment. Small mammals are key components of terrestrial ecosystems (e.g., seed predators, prey) and likely also carry out these roles in urban ecosystems; however, the effects of urbanization on small mammal communities are understudied. To identify how species-specific responses to urban environments shape community composition, we quantified both overall community and species-specific changes in small mammal abundance across an urbanization gradient in a Midwestern US metropolitan area. We combined small mammal trapping and land cover data to generate a hierarchical community abundance model. Species diversity increased with increasing proportional cover of human-modified land cover (i.e., impervious surfaces and turfgrass). This finding is driven by high species diversity on sites bordering streams in mowed parks and low diversity on sites with high tree canopy closure. Additionally, modeling results indicated that species responded differently to landscape attributes, leading to variation in small mammal community composition across the urbanization gradient: prairie-associated species tended to be more abundant in tall vegetation bordering mowed parks while habitat generalists tended to be more abundant on sites with greater canopy closure and shrub cover. Our results suggest studies that focus on community-level responses (e.g., species richness) to urbanization may miss important species-specific responses. It may be particularly important to assess both species-specific and community-level responses in cities at ecotones (e.g., between forest and grassland) where species with different habitat requirements may replace one another in different types of green spaces, thereby changing community composition without affecting species diversity or richness. Our findings also indicate that vegetated urban patches, especially patches with tall vegetation cover and low canopy cover, are important habitat for prairie-associated small mammal communities, providing conservation options in heavily altered landscapes.

Keywords: abundance; community; habitat fragmentation; rodent; shrew; species richness; urban.

Fig. 1.

The Iowa City metropolitan area of Iowa, United States, including the locations of the 45 survey plots where small mammal trapping took place from 2021 to 2022. Inset map shows the location of the study area in the broader context of Iowa and the United States.

Fig. 2.

Small mammal species diversity as a function of a principal component score (PC1) describing land cover characteristics (sites with more negative scores have higher cover of imperviousness and turfgrass and sites with more positive scores have higher canopy closure with greater shrub and herbaceous cover) in the Iowa City, Iowa metropolitan area, United States, 2021 to 2022. Dark gray dots are species diversity for individual sites, the black line is the model-estimated mean, and the light gray ribbon is the standard error around the model mean.

Fig. 3.

Persistence probability of the small mammal community on average and species-specific estimates in the Iowa City, Iowa metropolitan area, United States, 2021 to 2022. PC1 (A) is the principal components score of a site, ranging from high cover of impervious surfaces and turfgrass (negative values) to high cover of natural vegetation (positive values). The contagion index (B) of a site is a measure of habitat fragmentation with low values indicating low dispersion of different land cover types (i.e., high connectivity) and high values indicating high interspersion (i.e., low connectivity). Colored lines are species-specific responses; the black line is the community mean response. Gray ribbon is the 95% credible interval for the community mean. Species with statistically significant relationships have their lines labeled with an asterisk.

Fig. 4.

Population gain (recruitment) of small mammals during (A) spring, (B) summer, and (C) fall in the Iowa City, Iowa metropolitan region, United States, 2021 to 2022. The contagion index measures habitat fragmentation with low values indicating low dispersion of different land cover types (i.e., high connectivity) and high values indicating high interspersion (i.e., low connectivity). Colored lines are species-specific responses; the black line is the community mean response. Gray ribbon is the 95% credible interval for the community mean. Species with statistically significant relationships have their lines labeled with an asterisk.