Colonization, establishment, and impacts of three notorious invasive species over five decades in southern Wisconsin broadleaf forests

Background/Question/Methods Although invasive plants clearly accompany declines in floristic quality, we often lack empirical evidence that the establishment of invasives causes declines in native species richness.  We compare data collected from 128 S Wisconsin forest stands in the 1950s to samples from the 2000s to assess shifts in the distribution and abundance of Alliaria petiolata, Rhamnus cathartica, and Lonicera spp. and their impacts. Using a before-after-control-impact (BACI) framework, we identify how these invasions affect native species richness at these sites. 

Results/Conclusions All three species were absent from all study sites 50 years ago, yet at least one is now present in 77% of the stands and all three are present in 12% of the stands. Alliaria petiolata occupied 45% of the sites re-surveyed, Rhamnus cathartica, 44%, and Lonicera spp., 38%. Native species richness has declined on average by 30.6% at these sites, but declines are similar at invaded and uninvaded sites suggesting that they are unlinked to exotic species invasions.  This absence of effect may reflect the fact that external factors are driving both invasions and declines in native species richness.  Alternatively, site richness may be an insensitive response variable, time lags may occur between invasions and their impacts, or invader impacts may occur at a scale smaller than the whole site.  We therefore tested for associations within 1 m² quadrats between invaders and native species that have increased or decreased over the past 50 years.  Initial results reveal no clear evidence of (dis)association, again suggesting that these invaders are not the primary drivers of native species declines. Local declines in species richness often reflect succession as mesic hardwoods replace oaks, contributing to losses of shade-intolerant woodland herbs.  Invasive plants appear thus far to be ‘passengers’ rather than drivers of local species losses.