This study considered the influence of exotic shrubs on birds in urban patches of Yellow Box-
Blakely's Red Gum (E. melliodora-E. blakelyi) woodlands in the Australian Capital Territory,
Canberra. The aim of this research was to identify native and exotic birds that have their
abundance influenced by exotic shrubs. The purpose of this work was to provide more
information to managers of this woodland about the potential impacts of weed control on birds
living in woodland reserves adjacent to suburban areas.
Birds were sampled between 1996 and 1998 using the twenty minute area-search method to
derive estimates of bird abundance. Around 680 twenty minute area-searches were completed,
with 665 of these undertaken at 12 two hectare plots with varying levels of exotic shrub cover
over a period of ten months. The abundance of birds for each of the ten months sampled were
compared by pooling sites into four classes of exotic shrub cover. Classes of exotic shrub cover
were nil, light, moderate and dense. Seasonal inferences were drawn from non-parametric
analysis of variance. Non-parametric measures of association were used to test for correlation
between the mean abundance of bird species at different woodland sites and percentage foliage
estimates of exotic shrub cover. Percentage foliage estimates of native shrub cover were included
in tests for correlation between bird abundance and exotic shrub cover by applying partial
measures of association. To support statistical information, observations of birds in exotic shrub
cover were also recorded. In one woodland site birds were sampled before and after the removal
of exotic shrub cover. No statistical tests were applied to these samples because of a lack of
replication; however, descriptive graphs of the abundance of selected birds following weed
control are presented. To investigate the effect that exotic shrub invasion may have on the
composition of bird assemblages in woodland cluster analysis and ordination of the 12 sites using
the mean abundance of the 75 species recorded between July 1997 and June 1998 were also
undertaken.
The presence of exotic shrub cover in E. melliodora-E. blakelyi woodland in Canberra was found
to have differential effects on bird abundance. Wrens, finches, thornbills, whistlers and pigeons
were more abundant in woodland sites where exotic shrubs were present when compared to sites
with no or little exotic shrub cover. Fruit-eating birds, such as Silvereyes (Zosterops lateralis),
Pied Currawong (Strepera graculina) and Crimson Rosella (Platycercus elegans), were more
abundant in woodland with exotic shrubs in winter and autumn when these shrubs provided food
in the form of berries. The abundance of fruit-eating birds, and wrens and finches was reduced in
a single woodland site following the removal of most of the exotic shrub cover. The Common
Blackbird (Turdus merula) was the only exotic bird which showed a strong association with
exotic shrubs in woodland, while the Laughing Kookaburra (Dacelo novaeguineae) was
negatively correlated with exotic shrub cover, possibly because prey is harder to detect and
capture in woodland with a shrubby understorey.
Cluster analysis and ordination of the 12 woodland sites did not group sites into the four
experimental classes used to undertake analysis of variance. Multivariate analysis, however, did
reveal that seasonal peaks in the abundance of fruit eating birds affected the composition of bird
assemblages by increasing the mean abundance of these birds in densely invaded sites. Similarly,
structural differences in the understorey resulted in some birds being more abundant in woodland
sites invaded by exotic shrubs when compared to sites lacking a shrubby understorey. The
distance between some sites confirmed this stark difference in bird life when plotted in three
dimensions.
The results of this study suggest that exotic shrubs add food and structural complexity to
woodland habitat. Benefits of structural complexity for small native birds in woodland include
nest sites and protection from predators. These benefits may operate at certain thresholds of
invasion, as the woodland site with the densest level of exotic shrub invasion showed a slight
decrease in the number of wrens and finches. Adverse impacts from exotic shrub invasion may
include reduced open ground in which to forage and loss of floristic diversity in the understorey.
In effect, exotic shrubs add and remove resources in woodland habitat, benefiting some bird
species and limiting others.
These findings suggest that the removal of exotic shrub cover in woodland located in urban
landscapes simplifies the structural complexity of the understorey, reducing the quality of habitat
for some birds. Thus, adverse impacts on biodiversity arising from the invasion of exotic shrubs
in woodland need to be considered against the important role that a diverse bird population has in
maintaining ecosystem function.
| Identifer | oai:union.ndltd.org:ADTP/218918 |
| Date | January 2000 |
| Creators | Evans, Darren J., n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright Darren J. Evans |
Page generated in 0.0016 seconds