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

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    The three groups of sites that were revealed by the site PCA corresponded to sites that had high disturbance, sites with high species richness, and sites with high conservatism scores. This suggests that disturbance may be an important factor contributing to reduction in conservative species abundance and native species richness. The PCA done on quadrat data produced two groups: sites with no aliens, and sites with aliens. Quadrats that contained aliens tended to be found in sites with a high disturbance loading in the PCA by site. Therefore, sites with high disturbance tended to be more likely to contain aliens. Conversely, sites with no aliens tended to have higher species richness and higher MCC per quadrat. Alien presence is the dominant variable, rather than the number of alien species or their total cover.

    It seems, therefore, that sites that were disturbed had fewer conservative species, as well as fewer native species in general and had higher frequency of quadrats containing alien species. On the other hand, sites that were of high quality in respect to mean conservatism and greater numbers of native species were less disturbed and tended to have very few quadrats containing alien species.

    Several authors have indicated that disturbance favours invasion (Burke and Grime 1996, Prieur-Richard and Lavorel 2000, Wiser et al. 1998, Anderson 1999, Pyle 1995). Few, however, have dealt with the dynamics of conservative species in these situations. The reduction in conservative species frequency as a result of disturbance or as a result of alien species presence or both is an important mechanism in the overall process of invasion. Figure 32 shows a possible multiple regression model for the interaction of the frequency of alien species occurrence throughout a site and the effect of MCC per quadrat, which is negative, and disturbance index. This particular model explains 79% of the variation at P<0.005.

Figure 32. Relationship between MCC per quadrat combined with disturbance index and the frequency of occurrence of alien species in quadrats by site.

    There was significant difference in mean native quadrat richness between the sites. FAN2, the most disturbed site, had the fewest native species. Futhermore, FAN2 had the lowest MCC per quadrat of all the sites and the second highest mean Garlic Mustard cover by quadrat. Garlic Mustard cover differed significantly between the groups, with KOM1, FAN2, FAN1, and RES1, the sites that were heavily affected by disturbance, having the highest values for Garlic Mustard cover.

    Site KOM2 differed from the overall pattern in that it had high mean native richness but also had the most Garlic Mustard cover per quadrat and the highest alien richness per quadrat. This site had great disturbance and exhibited the same trends as other sites that had great disturbance, except that it had high native quadrat richness, yet low mean native species cover. Perhaps this site was in the initial stages of invasion, with native cover being reduced by disturbance, yet with some remnant of native species richness. The decrease in native species cover may allow an invader such as Garlic Mustard, which had high cover in this site, to establish itself.

    Another factor in KOM2's unusual position may be its close proximity to a large and relatively continuous tract of highly diverse forest, which may provide a rich supply of seed influx from a variety of nearby native species.

    Alien species cover increased and native species cover decreased with increased overall site disturbance. This trend was strong. Burke and Grime (1996) also found that the cover of introduced species responded very positively to disturbance. They stated that in the absence of disturbance, alien species typically had less than 25% of the total cover, but in highly disturbed plots this increased to 40%. Pyle (1995) also found that exotic species abundance increased in disturbed habitats. Since the presence of aliens cannot be a causal factor in disturbance, the disturbance is probably a determining factor in whether aliens are present or not. Whether disturbance is responsible for the decline in native richness or the presence of aliens or both is unclear. However, sites that had high native diversity tended to be less disturbed even when aliens were present. In addition, alien species cover was quite low in such sites.

Additional Factors

    The reduced alien-to-native ratio in the presence of increasing litter cover may be explained if lack of litter allows seeds of alien species greater access to germination sites in the mineral soil and greater success under these conditions. Litter cover is often reduced in disturbed sites. Conversely, the cover of native species increased with an increase in litter cover. Litter may help provide optimal conditions for pre-adapted native species, with established populations, to continue growth.

    Alien cover increased as tree basal area increased. This was particularly evident in sites with numerous large trees. Garlic Mustard, in particular, often formed dense colonies at the base of very large trees. This has been observed by Nuzzo (1991) as well. The implications of these results are particularly important in the conservation of ecologically significant, mature forest communities. When combined with disturbance, these appear to be under particular threat of Garlic Mustard invasion. If possible, efforts should be made to limit public access to sites with large trees to reduce the damage done to fragile native understory species. Regular monitoring of such sites and removal of vigorous invaders would reduce the ecological degradation of these ecosystems.

    Mean native species richness was lower in quadrats with aliens present. The abundance of highly invasive aliens is the most important variable I have tested in reducing native species richness. Disturbance appears to be an important factor in the spread of a particularly invasive species throughout a site. The type of exotic species appears to be very important in its effect on the native community. Exotic, yet weakly invasive species appear to be able to coexist within highly diverse native species communities in low numbers without altering the native composition or affecting the native species richness. Disturbance seemed to reduce the cover of native species, allowing Garlic Mustard to spread. The percent cover of the invader did not seem to have an effect on native species richness or cover unless the conditions for that invader are ideal and it heavily colonizes an area. The structure of a site is altered by high frequency of alien species and/or external disturbance. The occurrence of conservative plant species appeared to decline to a minimum, fixed value in the presence of a highly invasive plant species. This value did not change with the abundance of aliens.

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