Trillium grandiflorum height is an indicator of white-tailed deer density at local and regional scales

Abstract

High densities of white-tailed deer ( Odocoileus virginianus) in the U.S.A. and Canada are reducing or preventing forest regeneration, and as a result, deer function as a keystone species in some sites. Management decisions about deer require reliable population density data, which are challenging to obtain at both regional and local scales. We tested the broad-scale applicability of the indicator species approach in which forage plant height was used to estimate deer density. The maximum heights of marked and unmarked plants of the widely distributed, spring-flowering polycarpic herb, white trillium ( Trillium grandiflorum) were measured across southern Ontario, Canada, over a 15-year period. A significant negative relationship was found at 10 sites between maximum plant height and estimates of deer population densities, which were derived from counts of live or culled animals and varied from 7 to 40 deers km −2. Maximum plant height could be reliably measured within a 4-week period. The underlying mechanism driving the negative relationship between plant height and deer density was attributed to deer preferentially selecting taller plants that grew less than ungrazed plants in the subsequent year. In 16 additional sites with locally high deer populations, the mean maximum height of T. grandiflorum appeared to be a more reliable indicator of deer density than estimates based on hunter returns across the broader regional scale of the Wildlife Management Unit. The ability to assess local scale white-tailed deer densities based on measuring heights of an easily identifiable, widespread plant, provides local residents and landowners with a tool for estimating the potential impacts of deer browsing and grazing in local woodlots and forests, improving local knowledge about herbivory pressure.