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Effects of deer herbivory on floristic heterogeneity in central Illinois
Brent E. Wachholder and Scott J. Meiners
Department of Biological Sciences, Eastern Illinois University
Introduction
White-tailed deer (Odocoilius virginiana Zimmerman)
population densities in the eastern U.S. are at historically high levels. Intense winter browsing by deer can slow growth and replacement
of canopy trees. Heavy grazing threatens rare herbs, especially early spring
ephemerals. Intense herbivory can produce alternate state communities and
alter succession.
Goals
Determine if deer herbivory in Illinois is reducing floristic
heterogeneity (species turnover) and species richness. Identify species threatened by herbivory. Predict future changes in Illinois forest communities.
Methods
Conducted a two part vegetation survey inside established deer
exclosures and adjacent control plots of identical size at Middle Fork and Salt
Fork County Forest Preserves, Champaign County. A four-way nested presence/absence sampling regime was used to
generate species-area curves for control and exclosure plots. Cover of all species was recorded for five subplots of one square
meter within both exclosures and controls. Floristic heterogeneity was compared using coefficients of
species accumulation (log-log regression slopes), which represent the number of
new species added per unit area.
Results
Total richness was higher in exclosure plots. Coefficients of accumulation for species-area relationships
were significantly higher within exclosures than controls at Middle Fork,
but not at Salt Fork. Woody cover was lower in controls at both sites, but the
differences was not significant. Enchanter’s nightshade (Circaea lutetiana) and oak (Quercus
spp.) seedlings were observed within both exclosures, but were not found
in either control plot.
| Species-area curves and
log-log species-area curves for deer exclosures and control plots at
Middle Fork and Salt Fork County Forest Preserves. |
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Middle Fork |
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Salt Fork |
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| Species richness
and coefficients of species accumulation for Middle Fork and Salt
Fork Preserves. |
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a. Coefficients of
accumulation are significantly different (t-test, P < 0.05). |
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Schematic of nested sampling regime
for a 128 m2 plot. Shaded areas represent cover sampling
subplots. The 32 m2 sampling regime is repeated once for
each corner, and species lists are added to determine richness of
larger plots. Dashed lines represent exclosure fencing. |
Examples of plant species
which may be adversely affected by deer herbivory include Enchanter’s
nightshade (Circaea
lutetiana) and Oak (Quercus
spp.) seedling. These species were present in both exclosures and
absent from both controls. Twayblade orchid (Liparis
lilifolia), an uncommon species was observed in Salt Fork exclosure
only.
Conclusions
Lower richness in both control plots and reduced species accumulation in Middle
Fork control suggest deer herbivory at its current level can reduce floristic
heterogeneity by eliminating rare or vulnerable species. No
significant change in coefficients of species accumulation was noted at Salt
Fork, suggesting effects of herbivory are dependent on site characteristics.
Lower woody cover in control plots implies herbivory is affecting survival of
woody plants.
Future studies will compare multiple exclosure sites and include an index of
deer herbivory to further clarify roles of site characteristics and changes in
intensity.
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