Extirpations caused by plant invasions are explained by resource-use overlap between invasive and common, native species

Background/Question/Methods

While invasive plants cause drastic declines in native diversity at local scales, they cause few extinctions at broad spatial scales.  In a previous study, we found that the small number of extinctions at broad scales is, in part, due to a proportionately smaller effect of invaders on the abundance of rare compared to common species.  We examined the mechanisms causing larger declines in the abundance of common species using the mid-story forest invader, Lonicera maackii.  We first characterized the abiotic differences in uninvaded compared to L. maackii-invaded habitats, including differences in light, leaf litter, soil moisture, and soil nutrients.  We then conducted field and greenhouse experiments to test resource-use overlap between L. maackiiand rare and common, native species.

Results/Conclusions

Significant differences in uninvaded and L. maackii-invaded habitats were mainly driven by a decrease in light (i.e. photosynthetically active radiation), which resulted from dense L. maackii stands shading the forest understory.  We also found that the abundance of common species was best explained by changes in the light environment, while the abundance of rare species was best explained by soil variables, regardless of the presence or absence of L. maackii.  We supplemented these results with a controlled greenhouse experiment that mimicked uninvaded and L. maackii-invaded light environments.  Our greenhouse experiment revealed that a larger decline in native species’ relative growth rates due to light was strongly correlated with the effect of L. maackii on their abundance in the field.  L. maackii is a shade-intolerant invader that thrives in high-light conditions, and our results suggest that competitive dominance and niche overlap of an ubiquitous resource (in this case light) with common species best explains the effects of an invasive plant on native species’ extinctions.