A major goal in ecology is to understand mechanisms that increase invasion success of exotic species. A recent hypothesis implicates altered species interactions resulting from ungulate herbivore overabundance as a key cause of exotic plant domination. To test this hypothesis, we maintained an experimental demography deer exclusion study for 6 y in a forest where the native ungulate Odocoileus virginianus (white-tailed deer) is overabundant and Alliaria petiolata (garlic mustard) is aggressively invading. Because population growth is multiplicative across time, we introduce new metrics to correctly integrate experimental effects across treatment years, the cumulative population growth rate, λc, and its geometric mean, λper-year, the time-averaged annual population growth rate. We determined λc and λper-year of the invader and of a common native, Trillium erectum. Our results conclusively demonstrate that deer are required for the success of Alliaria; its projected population trajectory shifted from explosive growth in the presence of deer (λper-year = 1.33) to decline toward extinction where deer are excluded (λper-year = 0.88). In contrast, Trillium’s λper-year was suppressed in the presence of deer relative to deer exclusion (λper-year = 1.04 vs. 1.20, respectively). Retrospective sensitivity analyses revealed that the largest negative effect of deer exclusion on Alliaria came from rosette transitions, whereas the largest positive effect on Trillium came from reproductive transitions. Deer exclusion lowered Alliaria density while increasing Trillium density. Our results provide previously unidentified, definitive experimental support that interactions with overabundant ungulates enhance demographic success of invaders and depress natives’ success, with broad implications for biodiversity and ecosystem function worldwide.
Annual matrices for Alliaria petiolata and Trillium erectum and spatial weighting
Data for the demographic matrices were collected on individually tagged plants growing in paired plots located in Trillium Trail. Paired plot locations were chosen in Spring 2002 spanning the range of habitats in this forest where our focal species were found to co-occur. In Fall 2002, we established paired plots (n = 6 pairs of 14 x 14-m plots). One plot per pair was randomly assigned to a fenced treatment that excluded deer, eliminating only deer while allowing all other animals free access. Fenced plots were enclosed with 3-m-high, 15 x 15-cm steel mesh. Fences were maintained continuously, creating two treatments: deer access and deer exclusion. Each plot contained 36, 4-m-square subplots, with footpaths every 4 m to ensure minimal disturbance by data collectors.
For Alliaria, which was abundant in all plots, we created separate annual matrices for each plot, six matrices for fenced plots (not accessible to deer) and six matrices for unfenced plots (accessible to deer), for each of the four transition years. Within the range of areas where our focal species were present in Trillium Trail, we chose plots to span the gradient in topography from level to sloped, locating matched pairs along this gradient. We then determined the proportion of the total area of Trillium Trail that was similar to each matched pair. Thus, we were able to apply a weighted average to the data that was representative of the habitats where our focal species were found at the study site. Each plot pair (1–6) was weighted 10%, 10%, 20%, 20%, 20%, and 20%, respectively. To scale up to the level of the entire study site, we created a spatial average by weighting our single plot results accordingly. This resulted in a site-wide spatial average for each transition year and treatment, where the weighting for Alliaria was by abundance of the habitat at the site.
For Trillium, which varied in abundance among the pairs of plots (and was absent from the habitat represented by one of the pairs), we created a single matrix for each treatment and transition year, pooling data across all plots of a given treatment (fenced vs. unfenced, abbreviated NO_DEER and DEER) for each of the transition years (2003–2004, 2004–2005, 2005–2006, 2006–2007, abbreviated 2003, 2004, 2005, and 2006). This resulted in a site-wide spatial average, where the weighting for Trillium was by abundance of individuals in each plot.
Trillium erectum and Alliaria petiolata demographic matrices from Kalisz et al 2014.xlsx
