| Discussion Influence of Site Size
The increase in species richness of vascular plant species
with increasing area of the site was expected, since the greater the available
space, the more niches are possible. This is a well studied phenomenon known as
the “area effect”, first described by Preston (1960). Similarly, the larger
sites had greater numbers of native species. They did not, however, have greater
numbers of alien species. Timmins and Williams (1991) found that area of sites
accounted for the positive correlation between native and alien plant diversity,
whereas MacDonald et al. (1989) found that site size was not correlated
with the number of exotic species. It appears that larger sites may promote more
diverse native species communities, while having no effect on the richness of
exotic species. Alien species richness appeared to be controlled by other
factors.
Interestingly, site richness also translated into quadrat
richness. Therefore, the larger, richer sites had their higher species richness
effectively represented within the quadrats.
Effect of Alien Species Richness on Native Species Richness
There was no relationship found between the total alien
site richness and the total native site richness. Area effects may have skewed
this relationship, so a more appropriate method of measure is the use of quadrat
data. Native richness in quadrats increased with increasing alien richness,
although fewer than 8% percent of quadrats had more than one alien species.
A commonly held hypothesis of invasibility, originated by
Elton (1958) and reiterated by Naeem et al. (2000) is that high native
richness reduces the invasibility of a site to some degree. Perhaps the invasion
of one strongly dominant alien species can reduce the native diversity of a
site, but a very diverse site can support relatively large numbers of alien
species with little change to the native species composition. Only one of my
sites (KOM2) had high native richness and high quadrat frequency of alien plant
species.
Overall, native species richness was significantly reduced
in the presence of alien species at the quadrat level, which agrees with Elton’s
hypothesis. This was due to the effect of the presence of alien species, not the
richness of alien species. The results strongly support the presence of an alien
species being associated with reduced native species richness within a quadrat.
Since there is usually one highly dominant invasive plant, it is a large
number of these individuals evenly distributed throughout a site that may be
linked to a decrease in native species diversity.
A distinction should be made between a site that is highly
invaded in the sense that it contains a high frequency of alien plants in
relation to individual native plants and a site that contains a large number of
alien species. There is great limitation in generalizing Elton’s hypothesis to
include the presence of a small population of alien species in the definition of
an invasion. In terms of a highly invasive species such as Garlic Mustard, it
appears that a low native richness enhances its ability to invade a site.
Although the my data are limited, it appears that a larger
number of alien species that are not particularly invasive can coexist with
native species in a highly diverse site. The distinction must be made between
the characteristics of exotic plant species, as some, such as Helleborine (Epipactus
helleborine), may have minimal effect on native diversity, while others,
such as Garlic Mustard have major effects in many locations, depending on their
frequency of distribution. In the former case, the data is in conflict with
Elton’s hypothesis.
Wiser et al. (1998) found that, in New Zealand, the
exotic species Hieracium lepidulum occurred predominantly in species-rich
sites. They proposed a general theory that species-rich sites favour invasion,
as invaders are limited by resources to the same degree as native species are
and thus perform better in sites rich in resources. Although this is in conflict
with my results, it indicates the role of the biology of the particular invader
in determining its effect on the native plant community. As Wiser et al.
(1998) state, the community structure of the specific ecosystem determines the
effect of the invader. Obviously, these cannot be separated and they both play
an important role in the effects of invasion.
Relationships Between Native and Alien Cover Abundance
Cover of native species showed no relationship with cover
of alien species. In this study, one alien species, Garlic Mustard, was the
dominant alien species in all sites. The very weak negative relationship between
total native species cover and Garlic Mustard cover and the high amount of
unexplained variation suggests that the amounts of cover of the two groups of
plants are not directly related, but controlled by factors other than direct
competition.
In a study performed by McCarthy (1997), experimental
removal of Garlic Mustard increased the richness and abundance of understory
species. McCarthy’s study dealt with very high Garlic Mustard cover
abundances, which may exaggerate the effect of Garlic Mustard cover on native
species cover. My study did not find a significant relationship between Garlic
Mustard cover and native species richness. A possible explanation for this is
that I did not limit my sample points to sites with very high Garlic Mustard
cover but sampled a wider range of sites with less specific limits. On a smaller
scale, I would agree with McCarthy that microsites densely populated by Garlic
Mustard have reduced cover of natives species, but these sites are limited to
locations with highly favourable growing conditions for the dominant invader.
These sites seem to have been characterized by frequent and rather intensive and
regular disturbance, as well as nutrient rich soils. McCarthy’s sites were in
a wooded floodplain, an ideal location for colonization by Garlic Mustard, which
would allow it to reach its maximum potential in reproduction and growth.
Alien Richness and Native Species Community Composition
The increase of native species richness in quadrats with
MCC per quadrat indicates that richer communities may have a greater abundance
of conservative species. However, the unexplained variation was quite high, so
this conclusion must be viewed with caution.
MCC was found to be higher in quadrats where no aliens
were present. It was not affected by the number of alien species, just whether
they were present or not. This suggests that once aliens are established, the
native community composition is altered, with a decrease in the number of
conservative plants. Perhaps an alteration of the native community composition
allows for the establishment of alien species.
The MCC was not significantly affected by the extent of
garlic mustard cover in quadrats where aliens were present. Therefore, MCC did
not increase where there was only a small abundance of Garlic Mustard. It
remained at a relatively fixed low value. This may indicate a threshold effect;
once the pristine native community is altered and exotic species are able to
establish, the composition of native species reaches minimum conservative
species content. This may explain the similarity between sites that have
significant populations of exotic plants.
It appears that the shift in community composition of
native plants to more generalist species coincides with the presence of alien
species. In quadrats where alien species were absent, native species richness
was not only higher but of a better floristic quality, with a higher proportion
of conservative species.
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