Population Dynamics of Harvest Mice (Reithrodontomys Fulvescens and R. Montanus) across a Nitrogen-Amended Old Field

By Clark, Jay E.; Hellgren, Eric C. et al. | The American Midland Naturalist, July 2005 | Go to article overview

Population Dynamics of Harvest Mice (Reithrodontomys Fulvescens and R. Montanus) across a Nitrogen-Amended Old Field


Clark, Jay E., Hellgren, Eric C., Jorgensen, Eric E., Leslie, David M., Jr., The American Midland Naturalist


ABSTRACT.-

We conducted a mark-recapture experiment to examine population dynamics of the fulvous harvest mouse (Reithrodontomys fulvescens) and plains harvest mouse (R. montanus) in response to low-level nitrogen amendments (16.4 kg N/ha/y) in an old-field grassland. The experimental design consisted of 16, 0.16-ha plots with four replicates of each treatment combination (fenced, nitrogen amendment; unfenced, nitrogen amendment; fenced, control; unfenced, control). We predicted that densities, survival, and transition probabilities would be greater for both species on nitrogen-amended plots because of greater aboveground biomass (i.e., enhanced concealment from predators). We observed no distinct patterns in survival or transition probabilities of R. fulvescens or R. montanus with regard to treatments. Although population densities of R. fulvescens did not exhibit any distinct patterns with regard to treatments, densities of R. montanus tended to be highest on nitrogen plots, but lowest on nitrogen-fenced plots during winter 1999-2000. As low-level nitrogen amendments continue to be applied, we predict survival and densities of R. montanus and R. fulvescens on control plots, especially fenced plots with no nitrogen amendment, will eventually exceed those on nitrogen-amended plots as a result of higher plant species diversity, food availability and better quality cover.

INTRODUCTION

Future atmospheric deposition of nitrogen from anthropogenic activities is expected to increase as the global human population and reliance upon fossil fuels increases (Galloway et al., 1994; U.S. Environmental Protection Agency, 1995). Thus, the amount of unretained nitrogen cycling through ecosystems is likely to increase and reasonably can be expected to cause future environmental problems. These biogeochemical alterations of the nitrogen cycle can dramatically change dominance hierarchies among species and decrease overall biodiversity of ecosystems by altering species composition, species diversity and structure of foodwebs (Vitousek et al., 1997a, b).

Nitrogen enrichment causes qualitative and quantitative changes in grassland vegetation. These changes result in an increase in biomass and decrease in plant species diversity (Grant et al., 1977; Tilman, 1987; Carson and Barrett, 1988; Hall et al., 1991; Tilman, 1996; Wilson and Tilman, 2002; Clark et al., 2003). For example, after 9 y of annual nutrient enrichment, Carson and Barrett (1988) reported 3 contrasting types of old-field communities with regard to structure and composition and a decrease in species diversity on nutrient-rich plots. Furthermore, long-term nitrogen amendments in artificially constructed plant communities dominated by C^sub 4^ prairie grasses resulted in nitrogen-mediated shifts to Cj nonnative grasses (Wedin and Tilman, 1996).

Population dynamics of some small-mammal species appear to be regulated by plantcommunity structure, vegetative cover, plant diversity and food quality (Hall et al., 1991). Thus, plant community changes associated with nitrogen amendments likely would impact dynamics and structure of small-mammal populations. Hall et al. (1991) reported changes in meadow vole (Microtus pennsylvanicus) populations resulting from changes in plant community structure, cover and diversity after supplementation of nitrogen. Although aboveground biomass was higher on nitrogen-amended plots, non-amended plots had higher plant diversity and quality of cover resulting in positive population growth rates and higher population densities, rates of recruitment and survivorship for voles (Hall et al., 1991). Similarly, Anderson and Barrett (1982) reported increased aboveground biomass and decreased plant species diversity on nutrient-amended sites resulting in decreased densities and survivorship of meadow voles.

Populations of fulvous harvest mice (Reithrodontomys fulvescens) and plains harvest mice (R. montanus) are sympatric and common in old-field habitats of central Oklahoma (Goertz, 1963). …

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