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2007 SOUTHEASTERN NATURALIST 6(3):535–550
Effects of Ligustrum sinense Lour. (Chinese Privet) on
Abundance and Diversity of Songbirds and Native Plants
in a Southeastern Nature Preserve
Joshua Wilcox1 and Christopher W. Beck1,*
Abstract - Invasive plant species can have substantial negative impacts on native
flora and fauna. We investigated the effects of the invasive shrub Ligustrum sinense
(Chinese privet) on the abundance and diversity of songbirds in a southeastern forest
during summer, fall, and winter. We sampled 15-m x 15-m plots assigned to one of
three privet density categories (low, n = 5; medium, n = 4; high, n = 5). In addition,
we sampled all flora in each plot. Bird abundance and species richness varied only
during the winter, both increasing in high privet density. In general, the behaviors
and types of birds did not differ among privet-density categories. In contrast, abundance
and richness of native plants were reduced in high privet-density plots. Our
results suggest that removal of privet would improve native plant communities, while
having no substantial impact on songbird populations.
Nonindigenous species invasions are a major environmental threat both
to native habitats and the species that live in them. 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 (Simberloff et al. 1997). These
types of invasives tend to dominate their new environment, pushing out the
native species. They have been known to cause mass extinctions and habitat
destruction, as well as to cost billions of dollars annually in control measures
(Simberloff et al. 1997).
The effects of invasive plant species may extend beyond their effects on
native plants. Several recent studies have shown that specific bird species
are negatively impacted by invasive plants. For example, infestation of
grasslands by Euphorbia esula L. (leafy spurge) led to a reduction in the
density of Ammodramus savannarum Gmelin (Grasshopper Sparrows) and
Passerculus sandwichensis Gmelin (Savannah Sparrows)(Scheiman et al.
2003). In addition, Calcarius ornatus Townsend (Chestnut-collared Longspurs)
have reduced reproductive success in monocultures of the invasive
Agropyron cristatum (L.) Gaertn. (crested wheatgrass) as compared to in
native prairie (Lloyd and Martin 2005). Entire bird assemblages can be
negatively impacted by invasive plant species as well. Invasion of wetlands
by Lythrum salicaria L. (purple loosestrife) has resulted in a decline in
1Department of Biology, Emory University, 1510 Clifton Road, Atlanta, GA 30322.
*Corresponding author - email@example.com.
536 Southeastern Naturalist Vol. 6, No. 3
wetland bird species (Blossey et al. 2001). Similarly, Australian wetlands
invaded by Urochloa mutica (Forssk.) T.Q. Nguyen (para grass) have lower
numbers of birds than habitats without para grass (Ferdinands et al. 2005).
Although invasive plants often have negative effects on bird populations,
in some cases, invasive plants have negligible or positive effects on birds.
For instance, neither Dolichonyx oryzivorus L. (Bobolink) nor Sturnella
neglecta Audubon (Western Meadowlark) is influenced by the density of
leafy spurge in grasslands (Scheiman et al. 2003). In addition, a study of the
effect of invasive plant species on the native bird community in the Muddy
River drainage of the Mohave Desert found no effect on species richness of
birds, as long as the structural diversity in the plant community remained
unchanged (Fleishman et al. 2003). Furthermore, Blossey et al. (2001) found
that generalist bird species readily use purple loosestrife, although it leads to
the exclusion of wetland bird species.
Four species of privet have been classified as invasive: Ligustrum
vulgare L. (common privet), native to Europe; and Chinese privet,
Ligustrum japonicum Thunb. (Japanese privet), and Ligustrum lucidum
Ait. f. (glossy privet), native to east Asia. Chinese privet poses the biggest
threat to the native habitats of Georgia (Ward 2002). It is the most invasive
of the four species and the most suited to the Georgia climate. Introduced
in the mid-1850s, Chinese privet has naturalized itself in 19 states east of
the Rockies, ranging from Massachusetts to Texas, and as far south as
Florida (Ward 2002).
In natural areas, Chinese privet may be able to outcompete the native
understory species of a habitat through characteristics common of many
species of privet, such as high fruit production, rapid seed dispersal, high
germination rates, clonal reproduction, ability to capture sunlight, and
shade tolerance (Debussche and Isenmann 1994, Lavergne et al. 1999,
Matlack 2002, Morris et al. 2002). Preferring moist, well-drained soils
with large amounts of sunlight, Chinese privet is commonly found in
lowland floodplains, but has been known to populate widely varied habitats
and can tolerate drought-like conditions and low soil fertility. It will
often form large monospecific stands, completely dominating the area
(Langland and Craddock Burks 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 (Louisiana
Invasive Plants Database 2001).
Although Chinese privet is known to be invasive, its effects on native
species of flora and fauna are not well-known, with a few exceptions. For
example, Merriam and Feil (2002) noted a decrease in the abundance of
herbaceous plants, but not small trees, in a mixed-hardwood forest that had
been invaded by privet. In contrast, Stromayer et al. (1998) found that
Odocoileus virginianus Boddaert (white-tailed deer) depend on privet for
2007 J. Wilcox and C.W. Beck 537
browse. Data on the effects of privet on native flora and fauna will allow us
to determine the importance of privet-control efforts in natural areas.
The main objective of our study was to examine how the density of privet
affects the abundance and species richness of songbirds. Therefore, we
recorded the number and species of songbirds that occurred in plots that
differed in privet density. We sampled in the summer, fall, and winter to
determine whether the effect of privet on abundance and diversity of birds
was consistent across seasons for two reasons. First, different assemblages
of birds can be found in the nature preserve during each season. Second, the
phenology of privet might influence which birds are attracted to plots with
high densities of privet. In Georgia, privet is evergreen, flowers between
May and June, and ripens fruit between September and November (Radford
et al. 1968). In addition to sampling songbirds, we also determined the
abundance and diversity of native flora in each plot, because the effects of
privet on songbirds might be mediated through its impact on native understory
Anecdotal observations of birders suggest that songbirds are often found
in privet, because its dense cover provides nesting habitat in the summer and
protection for fall migrants. Miller and Miller (2005) suggest that privet has
a moderate value as cover for birds. In addition, in the winter, privet berries
might be an important food source for some species of songbirds, although
privet is considered to have a low value as a food source (Miller and Miller
2005). Therefore, we predicted that we would see a greater number of birds
in high privet-density plots. However, areas with high privet density were
predicted to exhibit a general decline in native plant biodiversity, as was
found by Merriam and Fiel (2002) in a similar forest.
Clyde Shepherd Nature Preserve is a 28-acre property located in Decatur,
GA, inside metropolitan Atlanta. The Preserve contains a 4-acre beaver
pond, wetlands, secondary growth pine groves, and wooded uplands. Much
of the land is a floodplain for South Peachtree Creek, which borders the
Preserve. Walking paths run through portions of the Preserve for public use.
Various plant and animal species can be found in the park including beavers,
raccoons, foxes, turtles, fish, snakes, frogs, rabbits, and a large variety of
tree and plant species. To date, 168 bird species have been observed in the
park, which has been recognized by the Atlanta Audubon Society as a
The Preserve is a prime example of secondary growth in a southeastern
piedmont floodplain forest. Much of the floodplain area is surrounded by
dense stands of privet that dominate the understory. The Preserve contains
three distinct forest areas. The floodplain area is generally at a low elevation
in the Preserve. These areas are poorly drained, often with standing
538 Southeastern Naturalist Vol. 6, No. 3
water several cm above the soil. They are dominated by Liriodendron
tulipifera L. (tulip poplars), Acer negundo L. (boxelder maples), and Acer
rubrum L. (red maples). The pine groves appeared physically similar to the
floodplain areas, but drier and dominated by young Pinus taeda L.
(loblolly pine). The uplands forest areas are at a higher elevation within the
Preserve, are drier than both the floodplain forests and the pine groves, and
are dominated by Fagus grandifolia Ehrh. (American beech) and various
Quercus (oak) species.
We chose fourteen 15-m x 15-m plots, each with a 30-m buffer from any
other plot. The plot size was chosen due to the small size of the nature preserve
and the lack of large plots with consistent densities of privet. Plots were
selected based on observed privet density in three categories: high, medium,
and low privet density. Five plots with the maximum density of privet
available were selected for the “high” category (90.6 ± 4.6 % cover); five plots
containing very little (i.e., privet seedlings) to no privet were selected for the
“low” category (0.4 ± 0.6 % cover); and four plots of intermediate privet
density, ranging between maximum privet density and minimum privet density,
were chosen to represent “medium” privet density (13.6 ± 1.5 % cover).
The actual density of privet differed significantly across the three categories
(Kruskal-Wallis: H = 11.7, P = 0.003) and between each category (Mann
Whitney U tests: P < 0.015 for all pairwise comparisons).
For the summer analysis, birds in each plot were surveyed once a week
over the months of June–August 2003 using a 20-minute stationary survey
point count, following Blondel et al. (1981). Sorace et al. (2000) note that
10-minute counts are sufficient to detect approximately 80% of birds, but
longer counts ensure that rare species are counted. Longer counts may
increase the probability of counting the same individual more than once.
However, our plots were sufficiently small that individual birds could be
tracked within a plot, thus avoiding overcounting. We conducted sampling
using sight and sound between one-half and three and one-half hours after
sunrise. Plots sampled were rotated randomly for days of the week and time
slots in each day. Each plot was surveyed a total of 10 times. No surveys
were conducted within 30-minutes of any rainfall or during other abnormally
adverse weather conditions. Since understory birds are the only birds that
would be affected by the shrubby nature of privet, only birds occurring
within 15 m of the ground were counted. In addition, only birds detected
within the plot boundaries were counted.
We collected data during each survey including number, species, and
behavior of the birds observed (e.g., Anderson 1981, Dawson 1981). We
2007 J. Wilcox and C.W. Beck 539
recorded the behaviors of birds to determine how birds were using the plots.
The following categories of bird behavior were utilized (following Ekert
1999): fly through (a bird that flies through the plot and performs no other
behavior before leaving the plot or the termination of the observation period),
perch (a bird remaining stationary on a perch for a time), call (a bird
making a vocal call while in the plot), sing (a bird making a vocal song while
in the plot), forage (a bird foraging for or eating any type of food while in the
plot), and nest (a bird constructing a nest or visiting an existing nest inside
Fall and winter sampling were conducted in essentially the same manner,
with some slight exceptions. Two plots from each privet-density category
were chosen randomly for each sample day to ensure that single days were
not skewing the data for a particular privet density. So that all six plots could
be surveyed in a single day in order to detect transient migrants, the observation
length was reduced to 15 min, which is still sufficiently long to observe
greater than 90% of birds (Sorace et al. 2000). The order in which plots were
sampled in a given day was randomized. These new methods were employed
for the last four days of the fall sampling (five total samples between
September 15 and September 27, 2003, based on the arrival of fall migrants)
and all of the winter sampling (seven samples between December 27, 2003
and January 13, 2004).
All plants occurring inside each plot were surveyed between August 4
and August 17, 2003. We recorded species identity, number, condition, and
dbh (for trees over 2 m tall). We determined cover of privet and other
comparable understory trees and shrubs using the crown-diameter method
(Mueller-Dombois and Ellenberg 1974). Because of the tendency of Chinese
privet to grow in large monospecific stands with numerous individual
trunks and overlapping branches, the crown-diameter method was modified
to assess the combined canopy size of all privet individuals occurring
in the plots.
In all three seasons, bird abundance and species-richness data were
not normally distributed and could not be transformed to normality. As a
result, we used nonparametric statistics for all analyses. For all three
seasons, we examined the effect of privet density on bird abundance and
species richness in two ways. First, we compared bird abundance and species
richness across plots in different privet-density categories using
Kruskal-Wallis tests. If there was a significant effect of privet-density
category, we carried out pairwise comparisons of privet-density categories
with Mann-Whitney U tests with = 0.05/3 = 0.017 to control the
experiment-wise error rate. Second, we used Spearman rank correlations
to examine the relationship between actual privet density and bird
540 Southeastern Naturalist Vol. 6, No. 3
abundance or species richness. During the summer, each plot was
surveyed every week. Therefore, we could examine changes in bird abundance
and species richness across the season. Individual plots were
ranked based on bird abundance or species richness across all weeks, with
plots with the lowest abundance or richness having the lowest rank. Then,
the effect of plot, time, and the interaction between the two on ranks were
determined using a repeated-measures analysis of variance (Conover
1999). In addition to effects on bird abundance and species richness, we
examined the effect of privet density on behavior (e.g., foraging, singing)
by comparing the frequency of behaviors in the three privet-density categories
with a chi-square test. Finally, we categorized birds into groups
based on primary and secondary diet (e.g., frugivores, insectivores),
foraging behavior (e.g, ground gleaners, foliage gleaners), and primary
nesting habitat (e.g., ground nesters, shrub nesters) based on Ehrlich et al.
(1988), because certain types of birds, such as shrub nesters in the
summer or frugivores in the winter, might be more likely to use high
privet-density areas. We considered both primary and secondary diet to
account for changes in the diet of species across seasons. We compared
the frequency of the different foraging or habitat groups in the three
privet-density categories with chi-square tests.
To determine the effects of privet density on native plants, we divided
plant species into trees (over 2 m tall), shrubs, and ground cover. Small trees
(< 2 m tall) were classified as shrubs and grouped together as a single type.
Although this reduced species-richness values, it should not bias comparisons
among privet-density categories. For each group, we compared the
number of individuals and the number of species or types across plots in
different privet-density categories using Kruskal-Wallis tests. If there was a
significant effect of privet-density category, we carried out pair-wise comparisons
of privet-density categories with Mann-Whitney U tests with =
0.5/3 = 0.017 to control the experiment-wise error rate. We also calculated
Spearman rank correlations between actual privet density and number of
individuals or number of species for each group.
Cardinalis cardinalis L. (Northern Cardinals), Thryothorus
ludovicianus Latham (Carolina Wrens), Cyanocitta cristata L. (Blue Jays),
and Dumetella carolinensis L. (Gray Catbirds) were the most abundant
species of songbirds. However, the number of individuals and species of
birds varied across season and privet-density category (Table 1). Yet, during
the summer, privet density did not significantly affect the number of birds
(Kruskal-Wallis H = 0.39, df = 2, P = 0.83; Fig. 1a) and the number of
species of birds (Kruskal-Wallis H = 2.15, df = 2, P = 0.34; Fig. 1b) recorded
2007 J. Wilcox and C.W. Beck 541
Table 1. Identified bird species by season and privet density.
Summer Fall Winter
Species Low Medium High Low Medium High Low Medium High
American Crow (Corvus brachyrhynchos Brehm) 6 2
American Robin (Turdus migratorius L.) 1 31 37
Blue Jay (Cyanocitta cristata L.) 12 6 8 3 1 3 2 2
Blue-Gray Gnatcatcher (Polioptila caerulea L.) 1
Brown-headed Cowbird (Molothrus ater Boddaert) 2 1
Brown Thrasher (Toxostoma rufum L.) 2 1 1
Carolina Chickadee (Poecile carolinensis Audubon) 1 3 2 1 1
Carolina Wren (Thryothorus ludovicianus Latham) 15 10 39 4 2 3 1 4 11
Cedar Waxwing (Bombycilla cedrorum Vieillot) 1 9 2 4 6
Eastern Towhee (Pipilo erythrophthalmus L.) 3 3 10 1 3 1
Gray Catbird (Dumetella carolinensis L.) 16 1 5 3 1 2 2
Great Crested Flycatcher (Myiarchus crinitus L.) 1 1
Hairy Woodpecker (Picoides villosus L.) 1
House Wren (Troglodytes aedon Vieillot) 1
Mourning Dove (Zenaida macroura L.) 1 3
Northern Cardinal (Cardinalis cardinalis L.) 11 42 42 2 9 8 3 8 8
Red-headed Woodpecker (Melanerpes erythrocephalus L.) 2 2 2 1 1
Tufted Titmouse (Baeolophus bicolor L.) 1 2 1
White-breasted Nuthatch (Sitta carolinensis Latham) 1
White-eyed Vireo (Vireo griseus Boddaert) 1
Wood Thrush (Hylocichla mustelina Gmelin) 8 1 7 3 1
542 Southeastern Naturalist Vol. 6, No. 3
on plots. In addition, neither was significantly correlated with proportion
of privet cover within plots (number of birds: r = -0.26, P = 0.37; number of
Figure 1. The effect of privet-density category on the abundance and species richness
of birds by season. Data are means ± 1 SE. In all cases except abundance during the
winter, abundance and species richness was not significantly affected by privetdensity
category. For abundance in the winter, bars with the same letter are not
significantly different based on a Mann-Whitney U test, after significance level was
adjusted for multiple comparisons.
2007 J. Wilcox and C.W. Beck 543
species: r = -0.23, P = 0.43). When we considered abundance and species
richness among plot types across sample dates, we also found no significant
effects of privet-density category, sample date, or the interaction between
the two (Table 2).
Similar to the summer, we found no significant differences among
privet-density categories in the number of birds (Kruskal-Wallis H = 1.27, df
= 2, P = 0.53; Fig. 1c) and the number of species of birds (Kruskal-Wallis H
= 3.31, df = 2, P = 0.19; Fig. 1d) in the fall. Again, the proportion of privet
cover within plots was not significantly correlated with the number of birds
(r = 0.27, P = 0.34) or the number of species (r = 0.43, P = 0.12).
In contrast to both the summer and the fall, in the winter, significantly
fewer birds were found on plots in the low privet-density category as
compared to plots in the high privet-density category (Kruskal-Wallis H =
6.33, df = 2, P = 0.04; Fig. 1e). However, the number of birds did not differ
between plots in the medium privet-density category as compared to plots in
either of the other privet-density categories (Fig. 1e). Yet, the number of
birds was positively correlated with the proportion of privet cover in a plot
(r = 0.61, P = 0.02). The number of species also was positively correlated
with the proportion of privet cover in a plot (r = 0.59, P = 0.03). However,
the number of species did not differ among the privet-density categories
(Kruskal-Wallis H = 5.27, df = 2, P = 0.07; Fig 1f).
The types of behaviors observed varied significantly among birds in the
different privet-density categories during the summer (2 = 19.6, df = 10, P =
0.03), but not in the fall (2 = 14.1, df = 10, P = 0.17) or winter (2 = 7.2, df
= 8, P = 0.51) (Table 3). The significant effect of privet-density category on
Table 3. Frequency of different behaviors by season and privet-density category.
Summer Fall Winter
Behavior Low Medium High Low Medium High Low Medium High
Call50 50 56 75 7 14 13 9 12 22
Fly10 10 19 28 1 5 3 4 4 5
Forage 2 3 11 0 0 3 1 5 16
Nest 0 0 1 0 0 1 0 0 0
Perch 77 70 116 12 10 21 12 23 37
Sing 24 17 13 3 5 1 2 3 3
Table 2. Effect of privet-density category and sample date on abundance and species richness
Number of birds Species richness
Source df MS F P MS F P
Privet category 2 12,744.4 2.6 0.12 9967.7 2.8 0.10
Error 11 4831.4
Time 9 1537.5 1.5 0.16 1678.2 1.4 0.19
Time x Privet category 18 1313.4 1.3 0.23 972.3 0.82 0.68
Error (Time) 99 1035.8 1189.3
544 Southeastern Naturalist Vol. 6, No. 3
behavior seems to be due to birds singing more often in low privet-density
plots and less often in high privet-density plots (Table 3).
When birds were grouped by primary diet, the frequency of different diet
types varied among privet-density categories during the summer (2 = 29.8,
df = 6, P < 0.001) and the winter (2 = 12.6, df = 4, P = 0.01), but not during
the fall (2 = 5.65, df = 6, P = 0.06) (Table 4). In both the summer and winter,
a greater number of omnivores were found in the low privet-density plots
than expected (Table 4). In addition, in the summer, we observed a greater
number of birds that eat berries (primarily Bombycilla cedrorum Vieillot
[Cedar Waxwings]) in the medium privet-density plots (Tables 1 and 4).
The results for secondary diet were qualitatively similar to those for
primary diet, except that the frequency of different diet types varied among
privet-density categories in all seasons (P < 0.05 in all cases; Table 5). As
with the primary diet, a greater number of omnivores were found in the low
privet-density plots in the winter than expected (Table 5). However, in
contrast to the primary diet, bird species whose secondary diet is fruits were
Table 6. Frequency of birds grouped by foraging type for each season and privet-density
category. Groupings based on Ehrlich et al. (1988).
Summer Fall Winter
Foraging type Low Medium High Low Medium High Low Medium High
Bark glean 2 2 4 0 0 1 0 0 0
Foliage glean 2 13 5 0 1 0 5 5 6
General glean 47 21 67 8 4 8 0 0 0
Ground glean 31 48 51 5 9 9 14 18 29
Hawk 1 0 1 0 0 0 1 0 0
Table 5. Frequency of birds grouped by secondary diet for each season and privet-density
category. Groupings based on Ehrlich et al. (1988).
Summer Fall Winter
Diet Low Medium High Low Medium High Low Medium High
Fruits 26 5 14 3 1 2 5 3 8
Omnivores 22 8 11 4 0 2 6 2 2
Seeds 19 50 59 2 11 11 3 10 8
Small vertebrates 15 10 39 4 2 3 1 4 11
Insects 1 11 5 0 0 0 0 4 6
Table 4. Frequency of birds grouped by primary diet for each season and privet-density
category. Groupings based on Ehrlich et al. (1988).
Summer Fall Winter
Diet Low Medium High Low Medium High Low Medium High
Berries 1 9 2 0 0 0 0 4 6
Insects 59 67 112 9 14 16 9 17 27
Omnivores 22 8 11 4 0 2 6 2 2
Seeds 1 0 3 0 0 0 0 0 0
2007 J. Wilcox and C.W. Beck 545
found in greater densities than expected in low privet-density plots during
the summer. In the fall, fewer seed-eaters were found in low privet-density
plots than expected.
The frequency of different foraging types also varied among the privetdensity
categories during the summer (2 = 30.2, df = 8, P < 0.001), but not
in the fall (2 = 6.25, df = 6, P = 0.40) or winter (2 = 7.2, df = 4, P = 0.13);
(Table 6). During the summer, a greater number of foliage gleaners and a
lower number of general gleaners than expected were observed in medium
privet-density plots (Table 6). As with diet type, the high number of foliage
gleaners in the medium privet-density plots was due to the number of Cedar
Waxwings that we observed in those plots (Table 1).
When we grouped birds by primary nest location, the frequency of birds
in each nest-location category did not differ among privet-density categories
in any season (summer: 2 = 8.57, df = 4, P = 0.07; fall: 2 = 5.6, df = 4, P =
0.23; winter: 2 = 3.5, df = 4, P = 0.47; Table 7).
The effects of privet density on the abundance and species richness of
native plants were similar for trees, shrubs, and ground covers (Table 8,
Fig. 2). The abundance and species richness of native plants were higher in
low and medium privet-density plots as compared to high privet-density
plots (Fig. 2). The one exception was the species richness of trees, which did
not differ significantly among privet-density categories (Table 8, Fig. 2).
When we examined the correlation between proportion of privet cover in a
plot and abundance and species richness of native plant species, we found
significant negative correlations for all comparisons (r < - 0.56, P < 0.35),
except for species richness of trees (r = -0.21, P = 0.48).
Table 8. Results of Kruskal-Wallis tests for the effect of privet-density category on native plant
abundance and species richness.
Variable Kruskal-Wallis H df P
Number of trees 9.05 2 0.01
Number of tree species 4.76 2 0.09
Number of shrubs 9.23 2 0.01
Number of shrub types 10.4 2 0.006
Number of ground-cover plants 6.11 2 0.05
Number of ground-cover species 7.08 2 0.03
Table 7. Frequency of birds grouped by primary nest location for each season and privet-density
category. Groupings based on Ehrlich et al. (1988).
Summer Fall Winter
Nest location Low Medium High Low Medium High Low Medium High
Ground 3 3 10 0 1 3 0 1 0
Shrub 29 44 48 6 10 10 5 8 8
Tree 51 84 70 7 3 5 10 14 27
546 Southeastern Naturalist Vol. 6, No. 3
The abundance and species richness of songbirds were not significantly
affected by the density of privet, contrary to what we had expected. The
Figure 2. The effect of privet-density category on the abundance and richness of native
plants by plant type. Data are means ± 1 SE. See Table 7 for results of statistical
analysis. Bars with the same letter are not significantly different based on a Mann-
Whitney U test, after significance level was adjusted for multiple comparisons.
2007 J. Wilcox and C.W. Beck 547
dense cover in high privet-density plots might have made it more difficult to
detect birds in those plots (Richards 1981), thus reducing our estimates of
abundance and species richness. However, given the small size of our plots,
it seems unlikely that detection differences biased our results. In contrast to
the general pattern of no effect of privet density on abundance and species
richness of songbirds, during the winter, we found a greater number of birds
on high privet-density plots, as compared to low privet-density plots
(Fig. 1e), and both abundance and species richness were positively correlated
with actual privet density. During the winter, birds might have been
found in high privet-density plots more often because they were foraging on
privet berries. However, birds were not any more likely to be observed
foraging in high privet-density plots than in low privet-density plots
(Table 3). In addition, birds found in high privet-density plots during the
winter were not predominantly frugivores (Table 4).
In addition to effects of privet density on abundance and species richness
of birds in the winter, the behaviors observed and the types of birds observed
varied among plot types, although most often during the summer rather than
the fall or winter (Tables 3–7). The effect of season is most likely due to the
low number of birds observed during the fall and winter. As a result,
detecting a significant effect of privet-density category on behavior or bird
type was more difficult in these seasons. However, the significant effects
found during the summer are instructive. Birds in low privet-density plots
were more likely to be singing than birds in other plots (Table 3). This
suggests that areas without dense underbrush might be used by birds for
attracting mates. In addition, although most species used all plot types,
certain species preferred particular plot types more than others (Table 1). In
particular, cedar waxwings frequented medium privet-density plots more
often that the other two plot types during the summer, which led to a
significant effect of privet density on foraging type and primary diet type
during the summer (Tables 4–5).
In contrast to the general lack of an effect of privet density on songbirds,
privet substantially impacted the abundance and diversity of most types of
flora (Fig. 2). However, since we only sampled plants in August, our results
do not account for winter annuals or spring ephemerals. Yet, for plants found
in the summer, we found significantly reduced densities of trees, shrubs, and
ground covers in high privet-density plots. In addition, richness of shrubs
and ground covers were reduced in high privet-density plots. These results
are in agreement with those of Merriam and Feil (2002), who also found a
decrease in both abundance and species richness of native plants under
privet. How Chinese privet leads to a reduction in abundance and richness of
native plants is unknown. However, glossy privet, a related species of privet,
has been shown to increase mortality of saplings of native trees in Argentina
(Lichstein et al. 2004). In some high and medium privet-density plots, two
other invasive plant species—Hedera helix L. (English ivy) and Pueraria
548 Southeastern Naturalist Vol. 6, No. 3
montana (Lour.) Merr. (kudzu)—were abundant. Although these species
may have contributed to the low abundance and richness of native plants in
these plots, we found no discernable differences in the native plant community
in plots with ivy and kudzu as compared to those without these species.
The co-occurrence of privet, English ivy, and kudzu does suggest that
certain ecological factors may make some forested areas more prone to
invasion by exotic species.
Taken together, the results of our bird and plant sampling suggest
several things about the control of privet in southeastern piedmont forests.
Songbirds are not relying on low privet-density plots for habitat at
our site. Therefore, removal of privet is not necessary to maintain bird
populations. However, birds are not using high privet-density plots to the
exclusion of other areas, even during the winter when privet berries might
represent an important food source. Thus, removal of privet should not
adversely affect bird populations. Yet, removal of privet would prevent
birds from being active dispersers of privet seeds, thereby slowing its
spread (Debussche and Isenmann 1994, Gosper et al. 2005). In addition,
native plants clearly would benefit from the removal of privet. In fact,
Merriam and Feil (2002) showed that the abundance and diversity of
native plant species increased after privet removal in a mixed-hardwood
forest in North Carolina. Thus, the ecological benefits of removing privet
seem to outweigh any costs.
We thank Andy Davis for comments during the design and implementation of
this study as well as comments on the manuscript, and Dave Butler for providing
access to the Clyde Shepherd Nature Preserve.
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