Population growth rate of a common understory herb decreases non-linearly across a gradient of deer herbivory

Abstract

(247 words).Overabundant white-tailed deer (Odocoileus virginianus) are a significant management problem in North America that exert unprecedented herbivory pressure on native understory forest communities. Conserving understory plant populations requires quantifying a sustainable level of deer herbivory. To date, most population projection models consider only deer presence and absence. To estimate population growth rate along a gradient of herbivory, we focused on Trillium grandiflorum because it is a common understory species and a bellwether of deer effects and forest decline. We used matrix population models, and employed both prospective and retrospective analyses using a regression life table response experiment (LTRE).Deer affect size, stage and population dynamics of T. grandiflorum. Because deer target flowering and large non-flowering stages of T. grandiflorum, these individuals do not produce seed in the year they are browsed and are more likely to regress in stage and size in the following growing season relative to non-browsed plants. Importantly, sustained high browse levels result in populations dominated by small, non-flowering individuals. Our LTRE revealed a significant negative and decelerating relationship between herbivory and λ. This non-linearity occurs at the highest herbivory levels because highly browsed populations become dominated by stages that deer do not consume and are thus buffered from rapid decline. However, population extinction is expected when herbivory is greater than the pivotal value of ~15%. Our study demonstrates that levels of deer herbivory commonly experienced by forest understory perennials are sufficient to cause the loss of T. grandiflorum and likely other co-occurring palatable species.

White‐tailed deer (Odocoileus virginianus) positively affect the growth of mature northern red oak (Quercus rubra) trees

“…Those studies mainly show that herbivory by deer and other large mammalian herbivores have strong negative effects on plant growth, reproduction, survival, and terrestrial carbon storage (Crête et al 2001, Russell et al 2001, Côté et al 2004, Knight et al 2009, MacDougall et al 2010. Herbivory on younger life cycle stage trees will eventually influence the structure of the future mature forest (Husheer et al 2006, Long et al 2007 Effects of deer on the growth of mature canopy trees have rarely been reported.…”

“…Belowground competition can be intense (Casper andJackson 1997, Bloor et al 2008) and has been demonstrated for both saplings and adult trees in other contexts (Bloor et al 2008, Montgomery et al 2010. Because understory vegetation represents an important fraction of forest biomass (Knight et al 2009, Heckel et al 2010 and is a substantial contributor to ecosystem transpiration (Yepez et al 2003), competition for water and nutrients may be important in structuring forest communities and could be strongly influenced by deer herbivory. Increased resource competition following reductions in deer population is suggested by a study involving muntjac deer (Muntiacus reevesi ) in a copice woodland in England where growth, survival, and recruitment of Corylus avellana and Crataegus laevigata all significantly decreased following a reduction in the deer population ).…”

“…As deer populations have grown, they have increasingly impacted the biomass and species composition of understory woody plant communities in forest ecosystems. Many studies have shown that in forest ecosystems, deer have contributed to the decreased abundance and diversity of palatable herbs and tree seedlings (Gill and Beardall 2001, Kuiters and Slim 2002, McGraw and Furedi 2005, Knight et al 2009, Heckel et al 2010. Naturally, changes in tree seedling community can alter the future structure of adult tree communities.…”

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

“…In a nonexperimental study, deer browse levels within a population were negatively correlated with population growth rate for another species in the genus, Trillium grandiflorum (51). Accordingly, Trillium represents a model for understanding the impact of deer on native species, and the loss of such browse-sensitive species can be a metric of decline in forest integrity (52). We predicted that, if ungulates disrupt the native community and enhance exotic invasion success, then in plots experimentally protected from deer: (i) native species would have higher reproductive success, (ii) Trillium fitness would increase and its density would increase, (iii) Alliaria fitness would decrease and its density would decline.…”

Population status of the tree Sacoglottis holdridgei (Humiriaceae) at Isla del Coco National Park, Costa Rica

“…The negative effects of animals in the forest are fully demonstrated. For example, deers affect regeneration by foraging limiting its development, affecting the composition, structure and successional forest development (Green et al, 1997;Gill, & Beardall, 2001;O'Dowd et al, 2003;Rooney, & Waller, 2003;Knight, Caswell, & Kalisz, 2009;Takatsuki, & Ito, 2009;Koda, & Fujita, 2011;Shelton, Henning, Schultz, & Clay, 2014). Also, it has been demonstrated that a preference for palatability of certain plant species exists (Takatsuki, & Ito, 2009;Koda, & Fujita, 2011).…”