DOI: 10.5061/dryad.bk272
title: Data from: In a long-term experimental demography study, excluding ungulates reversed invader’s explosive population growth rate and restored natives

description: 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.
description: Annual matrices for
 Alliaria petiolata and Trillium erectum and spatial
 weightingData 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