Abstract
Management of ungulates is contested ground that lacks stakeholder agreement on desirable population sizes and management approaches. Unfortunately, we often miss information about extent of local impacts, for example on plant communities, to guide management decisions. Typical vegetation impact assessments like the woody browse index do not assess herbaceous plants, and differences in browse severity can be a function of deer density, deer legacy effects, localized deer feeding preferences and/or differences in plant community composition. Furthermore, in heavily affected areas, few remnant plants may remain for assessments. We used a sentinel approach to assess impact of white-tailed deer (Odocoileus virginianus), rodent attack, invasive earthworms and three invasive plants on survival and growth of 3-month-old red oak (Quercus rubra) individuals. We planted cohorts in 2010 and 2011 into deer accessible and fenced 30 × 30 m plots at 12 forests in New York State. We 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 importance of rodent attacks. Oaks planted at low earthworm density sites were at significantly higher risk of being browsed compared with oaks at high earthworm density sites, but there was no detectable negative effect of invasive plants. Surviving oaks grew (~2 cm per year) under forest canopy cover, but only when fenced. We consider planting of oak or other woody or herbaceous sentinels to assess deer browse pressure a promising method to provide quantifiable evidence for deer impacts and to gauge success of different management techniques. The strength of this approach is that typical problems associated with multiple stressor impacts can be avoided, areas devoid of forest floor vegetation but under heavy deer browse pressure can still be assessed and the method can be implemented by non-specialists. Implementation of regular assessments can guide ungulate management based on meaningful evidence.
Highlights
Many populations of mammals, including large predators and ungulates, are rapidly declining, often in response to shrinking habitats and excessive hunting pressure (Donlan et al 2006; Maxwell et al 2016)
We recorded rodent attack and other mortality factors on 51 oaks and deer browse on remaining oaks (157 out of 240 oaks planted in open plots) resulted in 83 % mean browse rate across sites
Rodent attack and other mortality factors reduced the number of oaks available for deer consumption: at site 12 rodents attacked half of planted oaks (N = 10) and at site 5 six oaks in the open plot died for unknown reasons within 73 days of planting
Summary
Many populations of mammals, including large predators and ungulates, are rapidly declining, often in response to shrinking habitats and excessive hunting pressure (Donlan et al 2006; Maxwell et al 2016). A few adaptable ungulate species thrive in human-dominated landscapes in Europe, Australia, Japan, New Zealand and North America (Forsyth et al 2010; Tanentzap et al 2012; Foster et al 2014; Howland et al 2014; Perea et al 2014; Iijima and Ueno 2016) These gains and losses of browsers and grazers, that function as important drivers of plant community composition, can have transformative consequences for ecosystems (Wardle and Bardgett 2004; Wardle et al 2011). The state wildlife agencies established to manage recovery of white-tailed deer were effective in rebuilding populations but philosophically and financially they continue to be poorly equipped to switch from conservation to management of damage and impact of abundant deer (Halls 1984; Brown et al 2000; Rooney 2001; Jacobson et al 2010)
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