The role of species interactions within plant communities is emerging as a key factor in species invasions. Recently, generalist ungulate herbivores were hypothesized to provide biotic enhancement of exotic plants’ success rather than resistance to invaders because ungulates alter key biotic and abiotic factors within plant communities that could facilitate exotic plant invaders. Additionally, if an exotic species is less palatable to generalist ungulate herbivores than resident native plants, then selective foraging on native species has been proposed as a multiplier of invasion success. I tested the role of deer in Alliairia petiolata’s (GM) invasion success of forests in seven paired deer exclusion (DEER-)/deer access(DEER) plots. Within each plot, I censused three complete GM cohorts (2003-04, 2004-05, 2005-06) to obtain demographic transition probabilities among the seed, rosette and adult stage classes for each cohort and plot. Demographic projection analyses produced λ, the finite rate increase, for each matrix. Finally, to assess deer effects in the plots, permanently-tagged focal native palatable species (~2,000 plants/yr) were censused annually and the proportion of reproductive individuals in all plots and the % deer browsed in DEER plots (2003-2009) were calculated.
On average, deer browsed 26% of the available focal palatable species annually (range 19-53%), and never browsed GM resulting in a constant low proportion fruiting (~2%) in (DEER) plots. Conversely, native palatable stems increased by 30% in (DEER-) plots (8782003 vs.11452009 individuals) accompanied by a significant increase in the proportion fruiting focal species (2% in 2003→ 26% in 2009) in (DEER-) plots. In contrast, deer positively affect GM’s population λ and density. At the start of the experiment, GM 's λ in the two plot types did not differ (DEER plots mean λ=1.15) vs. (DEER- plots mean λ=1.35). Over the next two cohorts in (DEER-) plots, GM's λ declined steeply (mean λ = 0.75), with a concomitant drop in population density. In contrast, there was no change in GM’s λ or density in the DEER plots. These results demonstrate that deer sustain high population growth rates of GM, likely mediated through indirect interaction with native species. Both the invader’s population growth rate and density decline significantly where deer are excluded, directly linking this ungulate with this invaders success. My results implicate deer in the invasion success of GM in forests and the facilitative role of ungulates in unpalatable invaders’ success through alteration in the competitive interactions with native species.