7842

• UCREFRP Catalog Number: 7842
• Author: Knopf, F. L.
• Title: Biological Diversity in Wildlife Management.
• USFW Year: 1992.
• USFW - Doc Type: 1992.
• Copyright Material: NO

highest densities on the small patches. Western harvest mice became rare after 1987
for reasons still unclear. One cause may be due to changes in the plant community.
Western harvest mice may prefer early successional grasses over later successional
forbs (Birkenholz 1967, Fitch et al. 1984, Johnson and Gaines 1988). The annual
and perennial grasses have been gradually replaced by perennial fortis since 1984
(Foster and Gaines 1991). Due to low numbers, western harvest mice are not con-
sidered here.
Significant differences in persistence rates between patch sizes indicate that pop-
ulation processes are affected by habitat fragmentation, but these effects vary among
species (Foster and Gaines 1991, Gaines et al. 1992). Although sample sizes of
cotton rats on medium and small patches are small, persistence rates of cotton rats
are highest on large patches and essentially zero on small patches. Persistence rates
of prairie voles are lowest on small patches, presumably because small patches have
less suitable habitat. That persistence rates of prairie voles are highest on large patches
and intermediate on medium patches leads us to suggest that vote densities should
be greatest on large patches, but this is not the case (Foster and Gaines 1991). The
lower density of voles inhabiting large patches rather than medium patches could be
due to negative competitive interactions with cotton rats (Gaines et al. 1992), although
the few voles establishing themselves on large patches are maintaining their territories.
Persistence rates of deer mice are either highest on smaller patches (Foster and
Gaines 1991) or equal across all habitat types (Gaines et al. 1992), depending on
the season. This contrast between deer mice and the two larger species may be
explained by the manner in which deer mice utilize the mowed "interstitial areas"
between habitat patches. Based on trapping data, deer mice appear to exploit the
interstitial areas between habitat patches, whereas the other species do not (Foster
and Gaines 1991). This ability to exploit resources in the most unsuitable habitats
may explain why deer mice can persist and maintain high densities on small patches.
Individuals residing in interstitial areas may move freely onto small patches. More-
over, the small patches and interstitial areas may serve as refuges from larger and
more aggressive prairie voles and cotton rats. Competition and competitive refuge
effects comefrom negative correlations in abundances between the deer mice and
the two larger species (Gaines et al. 1992). From 1984 to 1987, deer mouse densities
increased in the interstitial areas and declined on large and medium patches as prairie
vole densities increased (Foster and Gaines 1991). Deer mouse densities remained
lowest on large and medium patches from 1987 to 1991, while cotton rat densities
increased on large patches and prairie vole densities increased on medium patches
(Gaines et al. 1992).
Our system consists of three metapopulations, one for each species, made up of
different subpopulations based on patch size. Source habitat patches where individuals
persist the longest should contain a high number of dominant individuals who establish
territories and are reproductively active. Subordinate individuals born in these source
populations should disperse to less suitable sink habitats when carrying capacities in
the source habitats are exceeded. A source/sink structure appears to occur in cotton
rats, prairie voles and deer mice (Gaines et al. 1992). In our earlier studies (Foster
and Gaines 1991, Gaines et al. 1992), we made no attempt to determine the age
structure and reproductive activity of source and sink populations. However, this
information is useful for population viability analyses (Mace and Lande 1991).
Habitats in which individuals of a species persist the longest should have the
Population Processes ? 257