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

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Influence of Site Size

    Overall, vascular plant species richness varied among the sites. Larger sites had higher numbers of vascular plant species (Fig. 2, p=0.024, R2=0.49). Similarly, native species richness increased with increasing area of the site (Fig. 3, p=0.014, R2=0.55). Larger, and hence, richer sites also had higher richness of species encountered in quadrats (Fig. 4). Therefore, site richness was significantly (p<0.01) reflected in total richness from quadrats. The greater numbers of species in larger sites, were being effectively represented in the quadrats.


Figure 2. Effect of site area on total site plant species richness.


Figure 3. Effect of area on total site native plant species richness. NS indicates a non-significant relationship.


Figure 4. Relationship between total site species richness and total quadrat species richness.

Effect of Alien Species Richness on Native Species Richness
    A regression of alien plant richness on native richness was not significant (Fig. 5, p=0.34). However, since this was analyzed using total native richness and total alien richness, another test was done to see if the relationship was affected by differences in site area (Fig. 6). No relationship was found between the size of the site and the ratio of alien to native species (p=0.85, R2<0.00).

Figure 5. Total native species richness against total alien species richness at each site.


Figure 6. Effect of site area on mean alien : native species richness ratio in the quadrats. NS indicates a non-significant relationship.

    A one-way ANOVA of native richness in quadrats with or without alien species showed (Fig. 7) a small but highly significant difference, with more native species in quadrats with no aliens (p<0.001). Therefore, at the quadrat level, the presence of an alien species was associated with reduced native richness. However, when native species richness was tested against the number of alien species in quadrats containing alien species (Fig. 8), native species richness was found to increase significantly with increasing with alien richness. However, the test was highly unbalanced, with only 2 of 200 quadrats containing three alien species.

Figure 7. Mean native species richness in quadrats with alien plant species present or absent in the quadrat with 95% confidence intervals. Different letters indicate a significant difference in means.


Figure 8. Mean native species richness in the presence of varying numbers of alien plant species within the same quadrat with 95% confidence intervals. Different letters indicate a significant difference in means.


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