Linking alternative food sources to winter habitat selection of herbivores in overbrowsed landscapes

Abstract

During winter, ungulates in boreal forests must cope with high energetic costs related to locomotion in deep snow and reduced forage abundance and quality. At high density, ungulates face additional constraints, because heavy browsing reduces availability of woody browse, the main source of forage during winter. Under these severe conditions, large herbivores might forage on alternative food sources likely independent of browsing pressure, such as litterfall or windblown trees. We investigated the influence of alternative food sources on winter habitat selection, by studying female white-tailed deer (Odocoileus virginianus) living in 2 landscapes with contrasted browse abundance, recently logged and regenerated landscapes, in a population at high density and on a large island free of predators. We fitted 21 female whitetailed deer with Global Positioning System (GPS) collars and delineated winter home ranges and core areas. We measured snow conditions in different habitat categories and sampled vegetation in the core areas and in the rest of the home ranges to determine how forage abundance, protective cover, and snow conditions influenced habitat selection within the home range. In both landscapes, deer were less likely to use open habitat categories as snow accumulated on the ground. At a finer scale, deer inhabiting the regenerated landscape intensively used areas where balsam fir cover was intermediate with greater balsam fir browse density than in the rest of the home range. In the recently logged landscape, deer were more likely to be found near edges between clear-cuts and balsam fir stands and in areas where windblown balsam fir trees were present; the latter being the most influential variable. Although balsam fir browse was sparse and mainly out of reach in this landscape, deer increased the use of areas where it was present. Our results offer novel insights into the resource selection processes of northern ungulates, as we showed that access to winter forage, such as woody browse and alternative food sources, depends on climatic conditions and stochastic events, such as abundant compacted snow or windthrows. To compensate for these scarce and unpredictable food supplies, deer selected habitat categories, but mostly areas within those habitat categories, where the likelihood of finding browse, litterfall, and windblown trees was greatest.

The effect of snow on space use of an alpine ungulate: recently fallen snow tells more than cumulative snow depth

“…For example it is known to decrease movements (von Hardenberg et al 2000;Fortin and Andruskiw 2003;Ratikainen et al 2007), activity levels (Beier and McCullough 1990), range size (Lovari et al 2006;Rivrud et al 2010;van Beest et al 2011), and resource accessibility (Mysterud et al 1997;Massé and Côté 2012;Robinson and Merrill 2012). In addition to the increasing energy costs of locomotion (Parker et al 1984;Dailey and Hobbs 1989), snow increases predation risk (Peterson and Allen 1974;Hebblewhite 2005;Robinson and Merrill 2012) and stress levels (Moen 1976) while modifying habitat use or selection (Poole and Mowat 2005;Bruggeman et al 2008).…”

Spatial correlations between browsing on balsam fir by white‐tailed deer and the nutritional value of neighboring winter forage

“…Browsing on fir could be more frequent near landscape features used by deer for purposes other than forage selection, such as forested patches used for protective cover against thermal stress and precipitation (Massé & Côté, ; Mysterud & Østbye, ), and thus influence the results of our previous analyses. We identified three landscape features that could influence deer distribution: (1) the proximity of residual forest patches, used as protective cover by deer (Massé & Côté, ); (2) proximity to forest roads not plowed in winter, which are avoided by deer because high snow accumulation increases locomotion costs (Parker, Robbins, & Hanley, ); and (3) proximity to fence lines, because of funneling of deer along fences, as suggested by observations of deer trails in enclosed areas. We calculated the distance of each plot to the nearest element of each feature (ArcMAP 10, ESRI ).…”

“…Approximately 200 white-tailed deer were introduced to the island at the end of the 19th century and thrived without natural predators. The population seems to be regulated by the availability of winter forage (Massé & Côté, 2012), and local deer density can exceed 20 deer/km 2 (Rochette & Gingras, 2007). Heavy deer browsing has modified the ecosystem, from the original forests dominated by balsam fir (Grondin, Berger, Landry, & Leboeuf, 2007) to white spruce-dominated (Picea glauca) stands (Potvin, Beaupré, & Laprise, 2003;Tremblay, Huot, & Potvin, 2006) and open parklands (Barrette, Bélanger, De Grandpré, & Ruel, 2014).…”

“…Browsing on fir could be more frequent near landscape features used by deer for purposes other than forage selection, such as forested patches used for protective cover against thermal stress and precipitation (Massé & Côté, 2012;Mysterud & Østbye, 1999), and thus influence the results of our previous analyses. We identified three landscape features that could influence deer distribution:…”

“…Low nutritional quality of neighboring plants should also be correlated with either high (associational susceptibility) or low (associational defense) browsing on fir. We investigated these relationships on a predator‐free island, where forage selection should mainly be driven by forage characteristics, deer nutritional state, abiotic environmental factors, and locomotion costs (Giroux, Dussault, Tremblay, & Côté, ; Massé & Côté, , ).…”