Abstract
Overabundant generalist herbivores can facilitate non-native plant invasions, presumably through direct and indirect modifications to the environment that affect plant performance. However, ecophysiological mechanisms behind ungulate-mediated plant invasions have not been well-studied. At a long-term Odocoileus virginianus (white-tailed deer) exclusion site in a temperate deciduous forest, we quantified deer-mediated ecophysiological impacts on an invasive biennial Alliaria petiolata (garlic mustard) and two palatable native herbaceous perennials, Maianthemum racemosum and Trillium grandiflorum. In mid-summer, we found that leaf-level light availability was higher in unfenced areas compared with areas fenced to exclude deer. Alliaria in unfenced areas exhibited 50 % higher mean maximum photosynthetic rates compared with fenced areas. Further, specific leaf area decreased by 48 % on average in unfenced areas, suggesting leaf structural responses to higher light levels. Similarly, Maianthemum had 42 % higher mean photosynthetic rates and 33 % decreased mean specific leaf area in unfenced areas, but these functional advantages were likely countered by high rates of deer herbivory. By contrast, Trillium exhibited significantly lower (26 %) maximum photosynthetic rates in unfenced areas, but SLA did not differ. Deer-mediated differences in light saturated photosynthetic rates for all three species were only significant during months with overstory tree canopy cover, when light availability in the herb layer was significantly lower in fenced areas. Alliaria's enhanced photosynthetic rates implicate overabundant deer, a situation that is nearly ubiquitous across its invaded range. Collectively, our results provide empirical evidence that generalist herbivores can alter non-native plant physiology to facilitate invasion.
Highlights
The globalization of human activities has led to widespread movement of plants around the world (van Kleunen et al 2015) and species invasions are widely appreciated as a major source of environmental change (Vitousek et al 1997)
In a temperate deciduous forest in North America, we investigated the effects of overabundant deer on the ecophysiological performance of invasive Alliaria petiolata (Brassicaceae, garlic mustard; hereafter Alliaria) and two commonly co-occurring native species preferred by deer as forage, Trillium grandiflorum and Maianthemum racemosum
To compare the effects of deer on Alliaria to those of resident species, we studied two native perennial herbs that commonly co-occur with Alliaria at the site, M. racemosum (Asparagaceae, false Solomon’s seal) and T. grandiflorum (Melanthiaceae, white/large-flowered trillium)
Summary
The globalization of human activities has led to widespread movement of plants around the world (van Kleunen et al 2015) and species invasions are widely appreciated as a major source of environmental change (Vitousek et al 1997). Deer can have direct consumptive effects through selective foraging (e.g. Wiegman and Waller 2006; Mason et al 2010; Averill et al 2016), indirect effects on biotic and abiotic properties (Rooney and Waller 2003; Heckel et al 2010; Kardol et al 2014; Heckel and Kalisz 2017) and can increase exotic seed dispersal (Myers et al 2004) Together, these effects have resulted in significant and negative community-level impacts (see Habeck and Schultz 2015 for a meta-analysis). Traditionally studied as separate phenomena, a growing literature has linked deer overabundance to increases in fitness and abundance of invasive plant species (Vavra et al 2007; Eschtruth and Battles 2009; Knight et al 2009; Christopher et al 2014; Kalisz et al 2014; Davalos et al 2015a; Shen et al 2016)
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