Caring for and Healing the Earth

Alien Plants


The effect of alien plant species on native plant richness and community composition in urban mid-age Sugar Maple (Acer saccharum) dominated forests in London, Ontario

Page 7 of 9


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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