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Invasive plant species are known to have substantial
effects on the native flora and fauna of the habitats they invade.
Chinese Privet (Ligustrum sinense) invades forested areas
creating dense mono-specific stands of very dense understory. We
predicted that the highest density and greatest species diversity
of birds would be found in areas with the highest densities of privet
expecting that the dense privet would provide desirable cover for
the birds. To determine the effects of Chinese Privet (Ligustrum
sinense) on bird species we performed an observational study
in the piedmont of Georgia. Plots were chosen based on privet density
and each plot was surveyed for birds seven times over a ten-week
period recording number species and behavior of the birds. On any
given survey day the highest density of birds and the largest species
variation could be found in the plots with high privet density as
compared with the plots of low privet density. When these numbers
were pooled across all seven samples we found no significant effect
of privet density on either bird density or species diversity. Therefore
privet density does seem to have an impact on the bird species of
a southeastern piedmont forest providing a more desirable habitat
on a day to day basis for these birds as compared to the native
vegetation. However our data suggests that removal of invasive privet
would not adversely affect bird populations over longer time scales.
Recently conservationists have begun to realize that
non-indigenous species invasions are a major environmental threat
both to native habitats and the species that live in them. This
threat was basically overlooked until 1958 when Charles Elton published
his book The Ecology of Invasions by Plants and Animals propounding
the risk that invasive species pose. Many species of exotic plants
and animals are able to outcompete native species for resources
through superior reproductive potential quick growth alleopathic
qualities and a number of other survival mechanisms. These types
of invasives tend to dominate their new environment pushing out
the native species. They have been known to cause mass extinctions
habitat destruction as well as costing billions of dollars annually
in control measures (Simberloff 1997). Ligustrum sinense is such
a species. Privet is a member of the family Oleaceae the olive family.
There are three species of privet that have been classified as invasive
common privet (Ligustrum vulgare) native to Europe Chinese
privet (Ligustrum sinense) and Japanese privet (Ligustrum
japonicum) (Ward 2002). Chinese privet poses the biggest threat
to the native habitats of Georgia being the most invasive of the
three species and the most suited to the Georgia climate. Introduced
in the mid 1850s Chinese privet has naturalized itself in nineteen
known states east of the Rockies ranging from Massachusetts to Texas
and as far south as Florida. It was brought to this continent as
a cheap fast growing alternative to common privet (Ligustrum
vulgare) a native of Europe used as a hedge for farming and
as an ornamental bush in horticulture. Common privet was more susceptible
to the native pests of the New World than its Asian counterpart
which proved to be a hardier species surviving the variety of adverse
weather conditions found in the Americas. Chinese privet is still
a popular landscaping species and is easily purchased at most nurseries
in the country making it particularly difficult to limit its spread
(Ward 2002). Chinese privet is able to outcompete the native understory
species of a habitat through its rapid seed dispersal and quick
growth to reproductive maturity. Producing an abundant amount of
berries that are dispersed by animals it is able to spread through
an area with surprising speed. It is also able to clonally reproduce
by root suckering making it especially difficult to completely remove.
Preferring moist well-drained soils with large amounts of sunlight
Ligustrum sinense is commonly found in lowland floodplains but has
been known to populate widely varied habitats and can tolerate drought-like
condition and low soil fertility. It will often form large mono-specific
stands completely dominating the area (Langland 1998). These stands
can have dramatic effects on the abiotic environment because the
privet plants are able to catch sediment in their root structures
during floods elevating the soil and making the habitat more suitable
for their reproduction (LIPD 2001). Removal of Chinese privet can
be a long and arduous process. Cutting of mature trees will actually
lead to an increased regrowth and uprooting is extremely difficult
because of their extensive root structure and ability to clonally
reproduce. Application of glyphosate herbicides has been found to
be the most successful but because of their non-discriminatory nature
they are also dangerous to most native species (Miller 1998). Stump
cutting accompanied with a small localized application of this herbicide
on the site of the cut is the most effective method of removal that
seems to work on both large and small specimens (LIPD 2001). Much
of the problem with privet is the lack of research on how it affects
native species of flora and fauna. There has been some research
done on deer grazing on privet leaves during the winter but no studies
to date have looked at the bird dispersal of privet berries or its
direct effects on resource competition with native understory and
groundcover plant species. In particular the use of privet by various
bird species has also been ignored leaving a big gap in the literature.
Plot Selection
- The study site selected was an urban nature preserve in Dekalb
County containing approximately 30 acres of secondary growth forest
(South Peachtree Creek Nature Preserve)
- 14 plots were selected based on overall privet density inside
the plot; 5 high density privet 5 low density privet and 4 medium
density privet
- Each plot was a 15m by 15m square
- Each plot was a minimum of 30 meters away from any other plot
Bird Sampling
- Birds were observed using a stationary survey point in the center
of the plot for a period of 20-minutes
- Species number of birds and behavior were recorded for each
plot
- All fourteen plots were surveyed once a week over a period
of three days
- All plots were surveyed during the hours of 7:00am and 11:00am
times for each plot were selected randomly each week
- No surveys were completed during rain or other abnormally adverse
weather conditions
A graph shows the average number of birds observed
in each type of plot on each survey day divided into low medium
and high-density privet plots. The data in the graph indicates that
on any given day the largest number of birds could be found in the
high-density privet plots. There is a significant difference in
the number of birds found in high-density privet plots as compared
to low-density privet plots (P = 0.004) but that there is no significant
difference between mid-density and either high or low-density pots.
The data in another graph indicates that on any given day the largest
number of species of birds could be found in the high-density privet
plots. There is a significant difference in the number of bird species
found in high-density privet plots as compared to low-density privet
plots (P = 0.003) but that there is no significant difference between
mid-density and either high or low density plots. The data in a
thrid graph indicates that the largest number of birds will be found
in high-density privet plots over a longer period of time but there
are no significant differences between any of the plot types. The
data in the last graph indicates that when looking at a longer period
of time each plot type has approximately the same number of bird
species with no significant differences between any of the plot
types.
On any given day, a larger number of birds and a larger variety
of bird species can be found in high-density privet areas as compared
to low-density privet areas. This data indicates a larger number
and variety of birds make the daily choice to enter areas of high
privet density. Over a longer period of time however the significance
of these difference disappear indicating that birds are not necessarily
making their habitat choice long term and any privet removal would
not adversely affect bird species on a long time span. The unsignificant
results may prove significant with further testing and more plots
giving a larger data bank. Due to time constraints this project
cannot be presented in its completed form. In the future ten sample
days for each plot will be recorded. Other future examinations will
include individual behavior of the birds individual preferences
of different species and the significance of location within the
park. In addition an in depth analysis of the effects privet has
on native flora will also be conducted.
This material is based upon work supported by the Howard Hughes
Medical Institute under Grant No. 52003727 and the support of the
South Peachtree Creek Nature Preserve. Thanks to Chris Beck and
Andrew Davis for their help thus far.
Invasive species are different types of plants or
animals that are not native to a certain habitat but are able to
thrive there. These species are a major threat to the other plants
and animals that normally live in the invaded habitat. Chinese Privet
a leafy bush is an example of such an invasive species. My research
looked at how different species of birds use privet for habitat
and nesting. To do this I observed the birds that came into areas
with large amounts of privet and the birds that came into areas
with very little privet.
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