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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Relationships Between Native and Alien Cover Abundance

    Plant species abundances were compared using percent cover values, taken at the mid-points of the Domin-Cover-Abundance intervals. Alien plants often have high abundance, even though richness is relatively low. There was a significant (p<0.01), negative relationship between native cover and alien cover (Fig. 9). However, this was quite weak (R2=0.030). Total native cover decreased with increases in total alien cover.

    Garlic Mustard (Alliaria petiolata), was the dominant alien species and the main species of interest in this study. Garlic Mustard cover accounted for 92% (p<0.01) of the variation in total alien cover. Therefore, it is used, in this study, as an effective measure of alien cover. Garlic Mustard cover did not significantly affect native richness (Fig. 10).

    Total native cover was negatively related to Garlic Mustard cover (Fig. 11). Although this relationship was significant (p=0.012), the amount of variation explained was low (R2=0.032).


Figure 9. Relationship between total native cover and total alien cover within quadrats.


Figure 10. Relationship between Garlic Mustard cover and native richness within quadrats. NS indicates a non-significant relationship.


Figure 11.Relationship between Garlic Mustard cover and total native cover within quadrats.

Alien Richness and Native Species Community Composition

    Mean Conservatism Coefficient (MCC) per quadrat was tested against native richness per quadrat (Fig. 12). The relationship was positive and highly significant (p<0.001), but weak (R2 =0.17). Thus, sites with a greater number of native species tended to have a greater community importance of conservative plants.


  Figure 12. Relationship between quadrat Mean Conservatism coefficients and quadrat native species richness.

    MCC was tested in the presence and absence of alien species (Fig. 13). The difference was highly significant (p<0.0001) with MCC higher in the absence of aliens. MCC was also tested to determine if there were differences between quadrats with different numbers of alien species (Fig. 14). The difference was not significant (p>0.05). The general trend was similar to the relationship between mean native richness versus alien richness (Fig. 8).

Figure 13. Quadrat Mean Conservatism Coefficient in the presence or absence of alien species with 95% confidence intervals. Different letters indicate significantly different means. 


Figure 14. Quadrat Mean Conservatism Coefficient in the presence of varying numbers of alien species with 95% confidence intervals. Different letters indicate significantly different means.

    MCC was tested against garlic mustard cover (Fig. 15) to see if greater Garlic Mustard cover reduced the community importance of conservative plants. This relationship was indeed negative and significant (p<0.05). It was weak however, with R2=0.026.

Figure 15. Relationship between quadrat Garlic Mustard cover and quadrat Mean Conservatism Coefficients.

    The relationship between MCC and MWS was also tested (Fig. 16). This relationship was highly significant (p<0.001), but fairly weak (R2=0.103). It should be kept in mind that a lower MWS means that there is a greater representation of strong weeds due to the negative scale. Therefore, the more invasive weeds were associated with the less conservative native plant species.

Figure 16. Relationship between quadrat Mean Weediness Score and quadrat Mean Conservatism Coefficient.   


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