Shared strategy or unique niches: Using functional traits of invasive shrubs of Michigan to test competing hypotheses of invasion

Background/Question/Methods

Invasive species are a substantial threat to native communities, threatening to alter habitat structure, displace native species, and even alter ecosystem function.  A number of hypotheses of invasion have been proposed, but substantial work remains to determine their generality and fit to specific systems.  Some try to define invasive species by a suite of shared trait; others argue that increased intraspecific variability allows local adaptation. Alternatively, invasive species may displace local species through competition for the same niche space.  Within Southeast Michigan, several common shrub species are invasive in the understory of existing forests.  Prior work comparing traits of an invasive with those of native species suggested that the study species may be “cheating” on the growth-survival tradeoff, allowing it to persist in the understory when its traits suggest it should not.  Here, we ask if the suite of invasive shrubs all share these similar traits, and a similar advantage in the understory, or if each species is exploiting a unique pathway into the understory by comparing their traits to each other and those of the native community. We utilize a standard suite of plant functional traits to address this question.

Results/Conclusions

In the forest understory of our research site in Lenawee County, Michigan, we found substantial populations of Elaeagnus umbellata, Rosa multiflora, and Lonicera maackii, along with more scattered individuals of Berberis thunbergii and Euonymos alatus.  For specific leaf area, three invasive species showed the highest SLA, while the other two placed in the bottom 7 among all species. Similar results were found for leaf dry matter content (at the low end of the spectrum) and leaf nitrogen content (high content in the invasives), but not for leaf laminar area.  For the first three traits, the five invasive species were statistically indistinguishable from one another, leading us to conclude that invasive shrubs in this Michigan forest share similar ecological strategies for invasion. Their position at the extreme (light-demanding) end of trait values suggests these species are living outside of the expected habitat for their traits. Finally, trait variation (as measured by standard deviation) does not differ among the invasive species or between invasive and native species.