Speakers at a 2016 colloquium on “Interactions of white-tailed deer and invasive plants in forests of eastern North America” explored these topics, and nine of their presentations are published in a Special Issue of AoB PLANTS. The papers describe interactive effects based on long-term studies of deer and invasive plants, explain the mechanisms underlying those effects, and introduce new research approaches.
Who is helping whom?
Does the arrival of deer help plants invade? By taking the tastiest plants from the understorey, do deer open up ground for colonisation?
Morrison looks into Effects of white-tailed deer and invasive plants on the herb layer of suburban forests. Cleared or predators, forests near humans provide a safe habitat for deer. Given the greater pressure on the herb layer from the deer, does this allow the annual grass Microstegium vimineum to invade? Morrison looked at the effect of deer fencing and Microstegium vimineum invasion to see what happened. Only when there was a combination of deer access and M. vimineum was there a strongly negative effect on woody native percent cover.
Heberling and colleagues looked at the Effects of deer on the photosynthetic performance of invasive and native forest herbs. The examined how deer affected photosynthesis in Alliaria petiolata (garlic mustard) and two palatable native herbaceous perennials, Maianthemum racemosum and Trillium grandiflorum. In the summer, herbivory led to smaller leaves with higher photosynthesis in Alliaria and Maianthemum racemosum, but not in Trillium, whose maximum photosynthetic rate dropped in unfenced areas. This shows that overabundant deer can cause physiological changes that benefit plant invasion.
But is it all about deer helping plants? Could the plants also be helping deer? Martinod and Gorchov say that the way White-tailed deer browse on an invasive shrub with extended leaf phenology meets assumptions of an apparent competition hypothesis. The idea is that invasive plant species with extended leaf phenology (ELP) elevate generalist herbivore populations and, when they no longer have ELP, this raise population then increases herbivory of native plants. Martinod and Gorchov tested this by estimating the proportion of the white-tailed deer diet comprised of Lonicera maackii, an invasive shrub with ELP, quantifying the seasonal pattern of deer browse on this invasive shrub, and comparing its nutritional quality to leafless woody stems. They found deer browsed L. maackii each month, but consumption was high in early spring and late summer. That meant that L. maackii was providing food to raise the deer population. As a result, the native species were hit comparatively harder in the other periods.
Are deer and invasive species a team?
Owings also look at a combination of L. maackii and deer. They examine Individual and interactive effects of white-tailed deer and an exotic shrub on artificial and natural regeneration in mixed hardwood forests.
The used exclosures and shrub removal at five sites, to study the effects of white-tailed deer and L. maackii both on underplanted seedlings of Castanea dentata and Quercus rubra and on the composition, species richness and diversity of naturally regenerated native tree seedlings. While they found effects, they did not observe interaction.
Bourg and colleagues studied Interactive effects of deer exclusion and exotic plant removal on deciduous forest understory communities. They predicted only deer exclusion and exotic plant removal in tandem would increase native plant species metrics anddeer exclusion alone would decrease exotic plant abundance over time. Deer exclusion caused significant increases in abundance and richness of native woody species >30 cm in height. Abundance changes in two focal members of the native sapling community showed that oaks (Quercus spp.) increased only with combined exotic removal and deer exclusion, while shade-tolerant maples (Acer spp.) showed no changes. They also found significant declines in invasive Japanese stiltgrass (Microstegium vimineum) abundance in deer-excluded plots, which would seem to agree to Morrison’s study.
Averill and colleagues conducted A regional assessment of white-tailed deer effects on plant invasion . They tested the effects of deer on the abundance and diversity of introduced and native herbaceous and woody plants across 23 white-tailed deer research sites distributed across the east-central and north-eastern USA and representing a wide range of deer densities and invasive plant abundance and identity. As deer density increased, the proportion of introduced species richness, cover and stem density all increased. Because the absolute abundance of introduced plants was unaffected by deer, the increase in the proportion of introduced plant abundance is likely an indirect effect of deer reducing native cover. Effectively they were nibbling a niche for invasive plants to fill.
How do you get the data?
The above studies might show there’s good reason to expect deer to change the local flora, but how do you measure the change over time? Blossey and colleagues use An indicator approach to capture impacts of white-tailed deer and other ungulates in the presence of multiple associated stressors. They planted cohorts in 2010 and 2011 into deer accessible and fenced 30 × 30 m plots at 12 forests in New York State. They used this plots as sentinels to watch what was going on. They found year and site-specific effects with high deer herbivory of unprotected individuals (70–90 % of oaks browsed by deer versus none in fenced areas) far exceeding the importance of rodent attacks.
Erickson and colleagues have tried Reconstructing a herbivore’s diet using a novel rbcL DNA mini-barcode for plants. They surveyed the diet of deer by examining what came out and comparing the results with comprehensive plant species inventory and corresponding reference collection of plant barcode and chloroplast sequences. In many cases they were able to assign taxonomy at the species level, which provides for the first time—sufficient taxonomic resolution to quantify the relative frequency at which native and exotic plant species are being consumed by white-tailed deer.
Nuzzo and colleagues assessed interactive effects of deer, earthworms, and invasive plants using 30 × 30 m paired fenced and open plots in 12 different forests from 2009 to 2012. They recorded not only plant community responses within 1 m2 quadrats, but also responses of select individual species. Deer-favoured forbs (Eurybia divaricata, Maianthemum racemosum, Polygonatum pubescens and Trillium recurvatum) grew taller and flowering probability increased in the absence of deer. Despite native forb cover increasing in response to deer reduction, the anticipated response of understory vegetation failed to materialize at the community level. They concluded Assessing plant community composition fails to capture impacts of white-tailed deer on native and invasive plant species.
The contributions in this Special Issue offer further confirmation for an already-strong scientific consensus on deer impacts, along with increasing evidence for facilitation of invasive plants by deer, via herbivory or non-consumptive mechanisms. The success of white-tailed deer in North America is a textbook example of wildlife management causing unintended consequences, and insights derived from this special issue may be applicable elsewhere.
AoB PLANTS is an Open Access journal, so you can access all the papers for free.