Deer Facilitate Invasive Plant Success in a Pennsylvania Forest Understory

Knight, Tiffany M.; Dunn, Jessica L.; Smith, Lisa; Davis, JoAnn; Kalisz, Susan

doi:10.3375/043.029.0202

Cited by 167 publications

(153 citation statements)

References 44 publications

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“…Because Alliaria cannot flourish in this forest without deer, our results suggest that previous hypotheses for Alliaria's invasion success (48) may be context dependent. In contrast to studies that demonstrate how native herbivores can increase biotic resistance by consuming invaders (18), our findings show that deer, a native generalist ungulate, find the invasive Alliaria completely inedible, which undoubtedly bolstered its fitness where deer had access (39,41). Thus, our data lend support to the hypothesis that a lack of palatability may be a general trait of highly successful invaders (16,53), particularly in communities with overabundant ungulates (54, 55).…”

Section: Discussioncontrasting

confidence: 49%

“…In addition to suppressing native species as shown here, a companion experiment at our study site (39) demonstrated that deer negatively affect the abiotic context, which could favor short-lived species like Alliaria (48). In that study, the area with deer present had significantly more bare ground relative to the area where deer were excluded (39). These openings are likely the combined results of soil disturbances by deer and leaf removal from palatable species in the understory by deer browse, which could increase light levels at the soil surface.…”

Section: Discussionmentioning

confidence: 96%

“…Our experiment was conducted in a beech-maple forest in southwestern Pennsylvania ( (39) used an indirect metric of plant performance and found that relative percent cover of Alliaria was lower and that there was significantly less bare ground where deer were excluded relative to sites where deer were present (39). However, in that study Alliaria nevertheless remained abundant (the second most abundant species) even where deer were excluded.…”

Section: Significancementioning

confidence: 90%

“…Relative to historical records, deer densities are currently 4-10 times higher than pre-European settlement densities across North America (47). Overabundant native deer in forests exert the same kinds of pressures as other ungulates (native and nonnative, wild and domesticated) globally, including perturbation of understory communities (22,27,39), exotic seed dispersal (27), and alteration of abiotic conditions (21,39). Likewise, Alliaria ranks among the most problematic forest invaders in North America (48).…”

Section: Significancementioning

confidence: 99%

“…However, in that study Alliaria nevertheless remained abundant (the second most abundant species) even where deer were excluded. That study (39), which used relative percent cover as a response metric, left several questions unanswered, including the following: Was Alliaria's relative decline due to the native species increasing in cover with no actual change in cover of the invader? Did the tenacious invader's population growth rate actually decline?…”

Section: Significancementioning

confidence: 99%

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

Kalisz

Spigler

Horvitz

2014

Proc. Natl. Acad. Sci. U.S.A.

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128

157

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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...

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