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Introduction
Over 2000 exotic plant species are believed to be growing in “natural” communities in the United States (Hobbs and Humphries 1995). The relationship between diversity and invasion is a controversial topic in the current literature. Charles Elton has influenced, to a great degree, the contemporary study of ecosystem stability and highlighted the potential for invasion. In his book, The Ecology of Invasions by Animals and Plants (1958), he stated that “the balance of relatively simple communities of plants and animals is more easily upset than that of richer ones”. This sweeping generalization has sparked great debate on the actual evidence for this hypothesis, be it observational, mathematically derived, or purely speculative. Different investigative methods have provided conflicting, yet equally plausible conclusions. Levine (2000) observes that the experimental studies tend to ignore the various factors aside from diversity and thus skew the relationships, if any. On the other hand, field studies (see Levine 2000) have shown that species-rich communities are more prone to invasion than those of low diversity.
The reasons for the positive relationship between native species diversity and invasion have been proposed to be caused by the limitation of resources, which acts similarly on native and exotic species (Wiser et al. 1998). Conversely, Lotka-Volterra models have shown that the probability of invader success decreased with the size and structure of the community (see Stohlgren et al. 1999). Others have found that lower biodiversity increased invasibility by reducing competition from the plant community and thus allowing exotics to establish (Naeem et al. 2000). McCann (2000) stated that a decrease in diversity will result in greater instability of a community and therefore, a greater susceptibility to exotic species invasions.
The difficulty in predicting invasibility based upon diversity lies in the fact that variation in diversity covaries with disturbance, resource availability, competition, etc. These factors are known to influence invasibility itself (Levine and D’Antonio 1999). In terms of interspecific competition, Law and Morton (1996) have argued that communities of species that evolved in regions of greater diversity have been tested by a greater variety of competitors, and are less likely to be invaded by species from less diverse communities. Sax and Brown (2000) propose that these invasions occur, not because of any disturbance, but because native species do not necessarily possess the optimal traits for their habitat.
There is also the problem of causal relationships. Although several studies have shown that invaded sites are less diverse than their equivalent pristine sites, this may be due to the effect of the invader on the native diversity rather than the reverse (Meekins and McCarthy 1999, Levine and D’Antonio 1999). For example, Webb et al. (2000) found that, in New Jersey, understory richness was significantly lower beneath Norway Maple (Acer platanoides) than under the native species, Sugar Maple (Acer
saccharum).
The success of the invasion depends not only on the diversity of the community being invaded, but also on several abiotic characteristics, as well as the characteristics of the invading plant (Hobbs and Humphries1995). Most of the exotic species introduced to North America have been intended for the landscape trade (Reichard and Hamilton 1997). Because these plants are imported mainly on their appearance, and not on their fitness in the host environment, the percentage of successful introductions is low and the number of invasions even lower.
However, invasions by some species have been extremely successful. In New York state, the average richness of exotic plant species in the flora is 36% (Yost et al. 1991). In the London, Ontario area, this value is 15% (Bowles et al. 1994). Species that have been successful invaders in the eastern deciduous forest include Lonicera japonica, L. maackia, Berberis thunbergii, Rhamnus frangula, Euonymus alatus, and Alliaria petiolata, but there are many more (Webb et al. 2000).
One of the primary mechanisms that allows for the initial establishment of invasive plants into communities is some type of disturbance (Burke and Grime 1996, Prieur-Richard and Lavorel 2000). Pyle (1995) described disturbance as an “event that can change community and ecosystem structure and composition by changing the physical environment and/or resource availability”. An established cover of indigenous plants is one of the main factors reducing the probability of successful invasion (Burke and Grime 1996), indicating that disturbance opens up previously occupied niches for invaders (Wiser et al. 1998, Prieur-Richard and Lavorel 2000).
The concerted effect of several disturbance types often leads to the greatest rate of invasion (Prieur-Richard and Lavorel 2000, Anderson 1999). However, whether the community richness and composition is affected differentially by the disturbance versus the actual competitive effects of the invader, and to what extent, has not been compared.
Studies have shown that exotic species frequency increases with increased disturbance and that fragmentation of forests increased the number of introduced plant species (Pyle 1995). The “edge effect” also results in an altered community structure (Anderson 1999).
In yet another study, the type of invader that is successful in a given site has been found to be dependent on the native plant community type, not on the magnitude and /or type of disturbance (Larson et al. 2001).
Garlic Mustard (Alliaria petiolata (Bieb.) Cavara & Grande) is a dominant invasive species in northeastern deciduous woodlands (Meekins and McCarthy 1999). Garlic Mustard is a herbaceous perennial of the Brassicaceae, native to Europe (Cavers et al. 1979). It was first recorded in North America in 1868 in Long Island, New York (Cavers et al. 1979). By 1990, it was commonly found in three provinces (Anderson et al. 1996). Disturbed communities have been found to be particularly vulnerable to Garlic Mustard invasion (Byers and Quinn 1998, Anderson et al. 1996, Nuzzo 1991). Ten years after initial establishment, Garlic Mustard may significantly decrease diversity and cover of native species in some regions (Nuzzo 1991). It is often found exploiting localized disturbances, such as trails, and exposed soil at the base of large trees (Nuzzo 1991, Anderson et al. 1996, Byers and Quinn 1998). An interesting possibility is the dispersal by raptors that feed upon granivores that, in turn, have foraged upon exotic plant species (Higgins and Richardson 1999). This may explain the commonly observed presence of Garlic Mustard at the base of large trees used by hawks for perching (Moc personal observation).
Removal experiments have shown that the diversity of herbaceous plants increased after Garlic Mustard was removed. This was under dense cover of Garlic Mustard (McCarthy 1997, Byers and Quinn 1998). It is for this reason that there is some concern that A. petiolata may compete with native spring ephemerals and be a threat to already rare species (Byers and Quinn 1998, Anderson et al. 1996). This may be due to an innately superior competitive ability of Garlic Mustard and its early seasonal growth (Meekins and McCarthy 1999).
Biomass allocation patterns in Garlic Mustard are not affected by density, indicating a low level of intraspecific competition (Meekins and McCarthy 2000). Stohlgren et al. (1999) propose that, because of this variation in invader characteristics, focus should be upon species-specific responses to disturbance and habitat variation rather than on general, all-encompassing theories such as those originated by Elton. For example, Garlic Mustard appears to be a particular threat in Red Oak (Quercus rubra) stands, where it suppresses the seedling biomass of Q. rubra (Meekins and McCarthy 1999).
The objectives of this study were to determine whether native species richness in vascular plants was decreased by the presence or abundance of exotic vascular plant species in woodlands dominated by Sugar Maple (Acer
saccharum).
Focus was on sites in relatively disturbed Sugar Maple woodlots in and surrounding urban environments in London, Ontario. Since Garlic Mustard was by a significant degree the most dominant exotic plant in the ground layer of these communities, it was used as an indicator species of invasion.
Examining the degree to which disturbance, whether anthropogenic, or natural, had affected native and alien species richness and abundance, was also a goal.
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