|
Effects of shrub cover on the abundance and distribution of small mammals.
Sara Conrad, NSF REU Program, St. Cloud State University.
Introduction
An exotic species is a species that is found outside of its
original home range. Many exotic plant species have been intentionally introduced by
humans. In the Midwest, several exotic shrubs were introduced for conservation
purposes, for erosion control or to provide food and cover for wildlife. Such
was the case for autumn olive (Elaeagnus umbellata) and multiflora rose (Rosa
multiflora). Both provide a source of food and shelter from predators and
weather for small mammals.
These exotic shrubs have an architecture that is different from that of native
shrubs. The exotics are multi-branched and radiate out from the ground being
wider than they are tall. In contrast, the native shrubs tend to grow taller
than wide, thus potentially providing less protective cover to small mammals.
Small mammals make decisions about where to forage based on the quality and
quantity of vegetative cover. Since many small mammals are important seed
predators, seed predation rates may be higher in areas that provide better
vegetative cover. If exotic shrubs provide better cover and small mammals spend
more time foraging under them, higher rates of seed predation could reduce the
survival of tree seeds and seedlings, and alter successional patterns in old
field communities.
Small mammals prefer different types of habitat. For example, deer mice (Peromyscus
maniculatus) tend to prefer more open fields whereas white-footed mice (Peromyscus
leucopus) are typically woodland inhabitants and prefer dense woody cover.
Further, the diversity of small mammals is not equal among the stages of old
field succession, being generally highest in habitats that have dense, tall, and
diverse vegetative cover. Population densities also are higher in areas with
more vegetative cover.
Objectives
1. Assess relative abundance of small mammals in fields invaded
by exotic shrubs vs. open old fields.
2. Contrast species composition and diversity in fields invaded by exotic
shrubs vs. open old
fields.
3. To compare vegetative cover at sites used by each small mammal species.
4. Evaluate whether exotic shrub cover is used more than native shrub
cover.
Methods
The five study sites were former agricultural fields surrounded
by upland forest located near Fox Ridge State Park in east-central Illinois
(Figure 1). Field size ranged from 0.2 ha to 2.9 ha; all were within
1 km of
each other. Three fields were heavily invaded by native and exotic shrubs
(Figure 2) and two fields were relatively open (Figure 3). A ten by ten
meter trapping grid was set up in each field. Sherman box traps (Figure 4)
were used to live trap animals using sunflower seeds as bait. Animals were
trapped in both open and invaded fields simultaneously. Captured animals
were identified to species, sexed, aged, given an identification number and
released at the capture site (Figures
6 and 7). Other visitors to
the trap sites included raccoons (Figure 9), opossum (Figure 10) and woodchuck
(Figure 11). Vegetation coverage was measured around a
one meter radius of
each trap station. Measurements included percent of woody cover, height of
herbaceous cover and whether the shrub cover was native or exotic. Exotic
shrubs were removed from randomly chosen trapping stations after the first
trapping period and the fields were re-trapped.
Results
Invaded fields had a greater abundance of mammals than open
fields (Figure 13).
Species composition differed between exotic invaded (Figure 14)and open
fields (Figure 15).
Chipmunks and short-tailed shrews predominated in invaded fields
whereas prairie voles and deer mice predominated in open fields.
 |
 |
 |
| Figure 13. |
Figure 14. |
Figure 15. |
Exotic shrub removal did not decrease capture rates (Figure 16).
Correlates of trap site and captures multiple regression model:
Capture rate = 0.5(% woody cover) + 0.5 (herb height), r = 0.326,
p-value < 0.001
Small mammals preferred sites with shrub cover over sites without
shrubs (Table 1).
Small mammals did not prefer exotic shrub cover over native shrub
cover (Table 2).
 |
 |
 |
| Figure 16 |
Table 1 |
Table 2. |
Conclusions
1. Total abundance of small mammals was greater in invaded fields
than in open fields.
2. Chipmunks and short-tailed shrews predominated in invaded fields, where as
prairie voles and deer mice predominated in open fields. Invaded fields had
greater species richness, but open fields had greater species evenness.
3. The best predictors of habitat use are percent woody cover and mean height of
the herbaceous cover.
4. Small mammals did not prefer exotic woody cover over native woody cover.
Acknowledgements
Funding for this study was provided by the
National Science Foundation’s
Research Experience for Undergraduates (REU) Program
and
Eastern Illinois University.
Thanks to
Dr. Tom Nelson, Nan Clarke and the rest of the REU
staff and students.
|
