Plant species invasions alter the composition and structure of invaded communities

Background/Question/Methods

A major tenant of invasive species ecology suggests that invaders cause a significantly negative impact on affected communities. Researchers have quantified these community changes in many ways, yet consensus has remained elusive. We provide a robust data set across a broad landscape to address the question, “How do invasive plant species affect the communities they invade?”

We located forested patches dominated by one of two invasive shrubs, Ligustrum sinense or Elaeagnus umbellata,identifying all vascular plant species within the plot at six different spatial grains and measuring all stems that reached breast height.  We also collected soil samples and assessed several abiotic components of the location. We then identified nearby communities that were similar to invaded communities in geography, community type and land-use history, with the only detectable difference being fewer invader stems. These communities served as controls for our invaded communities, isolating the impacts of invasion in a paired-plot design.

We assessed site-level species richness using the log-log species area relationship and community composition changes between the paired control-invaded plots using NMDS. Finally, we examined changes in various groups within the sampled communities (vines, saplings, etc.) to determine how community structure responded to invader dominance.

Results/Conclusions

Contrary to previous findings, invaded communities demonstrated an increase in small-scale richness measures. Both focal invaders correlated with increased richness at scales up to 1 m², indicating that exotic species may facilitiate establishment by successful propagules, including other exotic species. There was no significant change in large-scale richness with either invader, though Ligustrum sinense demonstrated a qualitative decrease relative to the control at all scales above 1 m².  There was no general relationship of large-scale richness between invaded and control communities.

Species composition changed noticeably within invaded communities, reflecting an increased presence of exotic species, with a general decrease in less common species in invaded communities. These invaded communities may occur on propagule-rich sites, thereby explaining the increased richness of all species and of exotic species within those invaded sites. Finally, invaded communities exhibited drastic changes in forest structure, namely a decreased abundance of tree saplings and increase in vine stems. These structural changes, paired with the observed compositional shifts, indicate that future forests may differ significantly from their current state, particularly where invasive shrubs can become dominant.  If left unmanaged, invasive species may reduce tree regeneration, opening up the canopy and leading to altered succession favoring exotics and common species.