In a long-term experimental demography study, excluding ungulates reversed invader's explosive population growth rate and restored natives

Abstract

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 metrics that correctly integrate experimental effects across treatment years, the cumulative population growth rate, λ c , and its geometric mean, λ per-year , the time-averaged annual population growth rate. We determined λ c and λ per-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 (λ per-year = 1.33) to decline toward extinction where deer are excluded (λ per-year = 0.88). In contrast, Trillium's λ per-year was suppressed in the presence of deer relative to deer exclusion (λ per-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 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.life table response experiment | herbivory | biotic resistance | temperate deciduous forest conservation | forest understory herbs S teadily increasing pressure by invasive plant species on native biodiversity (1) disrupts both community and ecosystem function (2) and results in staggering economic costs worldwide (3, 4). A major goal in ecology is to understand how changes over time in species interactions affect invasion success of exotic species (5-8). According to ecological theory, the ability of the resident community to limit the success of invading exotics [biotic resistance (9, 10)] will depend upon ecological context that includes the suite of local interactors (11-15). The abundance of herbivores and their local impacts (11,14,16) can play a prominent role in how fast plant populations grow or shrink and how much the relative abundance of plant species changes over time (5, 15), including changes associated with plant invasions (11,(16)(17)(18)(19). Recently, increased browsing pressure by overabundant ungulate herbivores on native plant communities has been proposed as a fundamental cause of a shift from native to exotic plant domination in forests and rangelands worldwide (11,16,20). Wild and domes...

Climate change both facilitates and inhibits invasive plant ranges in New England

“…S19), providing many opportunities for other invaders to fill vacated niches or A. petiolata to exploit fluctuating resources should local adaptation arise or refugia persist. A. petiolata also modifies the soil where it has colonized; hence, understory communities seem to be at risk even after its disappearance (17,26,27).…”

Herbaceous invaders in temperate forests: a systematic review of their ecology and proposed mechanisms of invasion

“…Rooney et al 2004;Wiegmann and Waller 2006;Rogers et al 2008;Winter et al 2009;Kalisz et al 2014). Even intact temperate forest understories are not immune to invasion (reviewed in Martin et al 2009).…”

Independent Effects of Invasive Shrubs and Deer Herbivory on Plant Community Dynamics

“…Deer herbivory can accelerate the proliferation of invasive species, especially in conjunction with canopy disturbances that increase light to understory plants [17]. Non-native species can benefit in areas of high deer abundances because of increased available growing space [18], seed transport [19], and unpalatability of some invasive species [20]. While unpalatable invasive species flourish where there is heavy deer browse [17], especially invasive grasses [10,14], growth of palatable invasive species can be severely depressed by herbivory [21,22].…”