Northeastern Naturalist 262 J.L.B. Pierce, et al. 22001155 NORTHEASTERN NATURALIST 2V2(o2l). :2226,2 N–2o7. 22 Scale-dependent Effects of Coyote-predation Risk on Patterns of White-tailed Deer Browsing Along Linear Forest Edges Joshua L.B. Pierce1, Sabrina A. Dalinsky1, Andre-Anne Chenaille1, Lewis M. Lolya1, Jennifer L. Maguder1, Chloe Mattilio1, Grace V. Mayhew1, Erin Regan1, and David A. Patrick2,* Abstract - Browsing by Odocoileus virginianus (White-tailed Deer) plays an important role in shaping forest structure in the northeastern US. Predation can influence patterns of browse through decreasing White-tailed Deer abundance and influencing patterns of habitat selection. Canis latrans (Coyote) is the dominant predator of White-tailed Deer in our focal region in northern New York. Optimal foraging theory suggests that White-tailed Deer browse in locations that maximize energy intake while minimizing predation risk. Linear forest edges provide abundant woody browse for White-tailed Deer, but are also used as travel-ways by Coyotes in our focal region. These locations therefore provide a venue for testing the extent to which predation risk influences patterns of White-tailed Deer browsing. We quantified White-tailed Deer browse use for edge and interior sites and the density of Coyote scats along seventeen 500-m transects to evaluate the scale at which White-tailed Deer herbivory was affected by Coyote predation risk. We did not observe a change in White-tailed Deer browsing at the landscape scale in response to variation in the risk of Coyote predation, or a mitigating role of escape cover. However, we did detect a tendency for White-tailed Deer to shift browsing from the forest edge to interior along transects with more Coyotes. Our study indicates that Coyotes in the focal area were likely influencing patterns of browsing primarily through behaviorally mediated indirect effects. Introduction Browsing of woody plants by ungulates, particularly Odocoileus virginianus Zimmermann (White-tailed Deer) is a dominant force shaping forest structure and function in the northeastern US (Bressette et al. 2012, Rooney and Waller 2003, Woods et al. 2012). For example, intense browsing by White-tailed Deer can lead to decreases in the abundance (Woods et al. 2012) and regeneration of preferred woody plant species (Waller and Alverson 1997) while benefitting species the animals avoid (Horsley et al. 2003). Patterns of browsing are driven by the abundance and behavior of White-tailed Deer (Alverson et al. 1988, Horsley et al. 2003) and habitat structure, particularly the availability of preferred species of woody and herbaceous browse (Kittredege and Ashton 1995). Understanding factors that influence abundance and/or behavior of White-tailed Deer is therefore important in predicting changes in forest structure. 1School of Natural Resources Management and Ecology, Paul Smith’s College, Paul Smiths, NY, 12970. 2Center for Adirondack Biodiversity, Paul Smith’s College, Paul Smiths, NY, 12970. *Corresponding author - david.patrick@tnc.org. Manuscript Editor:John Litvaitis Northeastern Naturalist Vol. 22, No. 2 J.L.B. Pierce, et al. 2015 263 Predation is one of the principal forces driving patterns of ungulate browsing (Beschta and Ripple 2009). For example, extirpation of top predators such as Canis lupus L. (Gray Wolf), has been linked to over-abundance of and over-browsing by ungulate species including White-tailed Deer, Cervus elaphus canadensis Erxleben (Elk), and Alces alces L. (Moose) (Beschta and Ripple 2009). In addition to limiting ungulate abundance, top predators also may influence patterns of behavior including where and when ungulates choose to browse (Halofsky and Ripple 2008, Ripple and Beschta 2004). The interrelationship between predators and prey and the subsequent effects on prey-habitat selection and patterns of feeding are well described by the concept of optimal foraging theory (MacArthur and Pianka 1966, Pyke 1984, Pyke et al. 1977). This general theory has more recently been adapted to specifically focus on predator–prey relationships between top carnivores and ungulate species, particularly through the use of resource-selection functions (Hebblewhite et al. 2005). Application of this theory demonstrates that predation can influence patterns of ungulate browsing at multiple spatial scales depending on the effects of the predator on prey abundance and/or prey behavior, and the structure of the environment (Altendorf et al. 2001, Frair et al. 2005, Halofsky and Ripple 2008, Jones and Hudson 2002). Canis latrans Say (Coyote) is the dominant predator of White-tailed Deer throughout much of the northeastern US and southeastern Canada (Messier et al. 1986). This species is a relatively recent addition to the region, having expanded its range likely in response to the extirpation of the Canis lupus lycaon Schreber (Eastern Timber Wolf). By the late 1950s, Coyotes had arrived in our focal region in the Northern Adirondacks, NY, in appreciable numbers (Fener et al. 2005, Gompper 2002b). Although the ecology of Coyotes in the Northeast seems to be variable, there is strong evidence in our focal region indicating that Coyotes have adopted a functional role similar to the extirpated Eastern Timber Wolf, including group hunting and depredating adult White-tailed Deer, particularly during winter months (Gompper 2002a, Patterson and Messier 2001, Warsen 2012). Coyotes can potentially influence patterns of White-tailed Deer browsing at a number of scales including reducing their regional and local densities through predation, and influencing patterns of habitat use. Shifts in behavior are likely to occur in areas where there is an abundance of high-quality woody browse coupled with frequent occurrence of Coyotes. Linear forest edges represent areas where these conditions are present. Edges typically provide readily accessible woody browse and hence are heavily used by ungulates (Alverson et al. 1988, Jones and Hudson 2002, Williamson and Hirth 1985). However, linear edges are also easily accessible to and offer abundant food for canid predators (Heske et al. 1999). In our study region, White-tailed Deer spend the winter months in forested deer-yards, but use edges during the remainder of the year. While most Coyote depredation of adult White-tailed Deer is likely to occur in winter and hence not when deer are on edges, we observed White-tailed Deer bones in fresh Coyote scat in the fall; thus, it is likely that White-tailed Deer are at risk of predation by Coyotes throughout the year. The extent to which predator and Northeastern Naturalist 264 J.L.B. Pierce, et al. 2015 Vol. 22, No. 2 prey use of edges (and hence encounter frequency) translates into predation risk (either actual or perceived) may also be driven by fine-scale variation in habitat because habitat characteristics can influence the probability that an encounter will result in a successful kill (Hebblewhite et al. 2005). For example, denser cover may serve as either escape cover for ungulates such as White-tailed Deer or concealment cover for predators (Creel et al. 2005, Husseman et al. 2003). Forest roads created during timber harvests (and analogous disused railway lines) represent one of the most prevalent types of linear edges in the Adirondack Park. High-use roadways are often avoided by wild canids (Mladenhoff et al. 1995, Roy and Dorrance 1985), but low-use travel ways may be preferred corridors for movement (Whittington et al. 2005, but see Kolbe et al. 2010). The use of low-intensity roads and rail lines as travel ways is supported by observational evidence (tracks and scat) indicating linear movement of Coyotes along these pathways in the focal region throughout the year (D.A. Patrick, unpubl. data). The goal of our study was to evaluate the scale at which White-tailed Deer herbivory was affected by how frequently Coyotes used linear forest edges, and to use this information to understand the likely effects of predators in driving forest structure through influencing patterns of browsing on woody plants. To address this goal, we tested 3 hypotheses: (1) higher use of edges by Coyotes reduces the amount of browsing of woody plants at the landscape scale by reducing the abundance of White-tailed Deer via either predation or stimulating deer to locate home ranges in different areas, (2) higher use of edges by Coyotes causes White-tailed Deer to shift foraging into the forest interior but not to relocate home ranges, and (3) fine-scale habitat heterogeneity (escape cover) along forest edges mitigates some of the effects of Coyote use on White-tailed Deer browsing. Methods Study area We conducted our study from 2 October to 8 November 2013 in second-growth forest in the vicinity of Paul Smith’s College in the northern Adirondack Park, NY (44°26'N, 74°15'W). Focal stands consisted of mixed deciduous-coniferous forest with dominant overstory species including Acer saccharum Marsh. (Sugar Maple), A. rubrum L. (Red Maple), Betula alleghaniensis Britton (Yellow Birch), Fagus grandifolia Ehrh. (American Beech), Pinus strobus L. (Eastern White Pine), Tsuga canadensis (L.) Carrière (Eastern Hemlock), Picea rubens Sarg. (Red Spruce), and Abies balsamea (L.) Mill. (Balsam Fir). Dominant woody understory species included Viburnum lantanoides Michx. (Hobblebush) and Vaccinium spp. (blueberries). Forest harvesting in the focal area consisted of partial harvesting, typically for production of woody biofuels. Field sampling We established 17 forest-edge transects, each 500 m in length and separated by a minimum Euclidean distance of 500 m along gated logging roads and disused railroad lines running into forest stands. No active use of roadways by vehicles Northeastern Naturalist Vol. 22, No. 2 J.L.B. Pierce, et al. 2015 265 occurred during the study period. We set the transects at a minimum of 50 m from the nearest paved highway. We only sampled transects with limited herbaceous vegetation and leaf litter to ensure detection of Coyote scats. We identified Coyote scats based on size, shape, and the presence of Whitetailed Deer hair and larger bone fragments (Elbroch 2003). To determine the abundance of Coyote scats, 2 technicians walked the transect once, recorded each scat, marked its position, and tallied all scats regardless of their level of decomposition. During preliminary studies, we also walked an additional 500 m beyond the end of the transect to ensure that Coyote scats were not spatially clumped at a scale that would influence our results; they were not. We assessed browsing of woody plants by sampling one time within 5 paired edge and interior plots (1 m x 2 m, from ground level to 2 m high). Paired plots were located at 100-m intervals along the transects on alternating sides. We placed forestedge plots with the outside edge of the plot (closest to the forest edge) established on the woody stem nearest to the road- or railway. We placed the forest interior plots 50-m into the forest from the edge plot perpendicular to the linear forest edge. We chose 50 m as the distance from edge for our interior plots based on preliminary observations indicating that this distance provided complete visual obscurity at the majority of our sites. To enumerate browsing within each plot, we counted the total number of terminal buds on twigs >2.5 cm in length for each woody plant species present and the number browsed. We considered twigs browsed if the tip of the apical meristem was missing. We only counted twigs if they occurred in the 3-dimensional sampling area. We then calculated the proportion of available twigs of preferred woody plant species browsed within each plot. We used data provided by the New York State Department of Environmental Conservation (NYSDEC) to identify preferred browse species (NYSDEC 2014), and defined preferred browse as all woody species not considered starvation food for White-tailed Deer. We quantified escape cover at each browse plot using a 20 cm x 100 cm visual obscurity board (divided into 80 alternating black and white squares) positioned horizontally at 1 m from the ground surface and placed in the center of the plot. We took 4 measurements of obscurity at each plot. For the edge plots, we took 2 measurements along the forest edge (1 in each direction down the roadway) and 2 perpendicular to the forest edge. In the interior plots, we took measurements of obscurity in the 4 cardinal directions. For each measurement, an observer stood 15-m from the board and counted the number of squares that were not obscured in any way; we then converted these data into the mean percent obscurity per plot. Statistical analyses We initially compared the number of preferred woody browse species in edge versus interior plots using a 2-tailed t-test to assess if differences in food availability could potentially mask any effects of differences in perceived predation risk. To assess if higher use of edges by Coyotes reduced the amount of browsing of woody plants at the landscape scale, we employed a linear regression with the mean proportion of preferred woody species browsed per transect (i.e., averaged Northeastern Naturalist 266 J.L.B. Pierce, et al. 2015 Vol. 22, No. 2 across edge and interior plots) as the dependent variable and the number of Coyote scats as our independent variable. To test if higher use of edges by Coyotes corresponded with more White-tailed Deer browsing in the forest interior, we first quantified the difference in the proportion of preferred species browsed between the edge and interior at each pair of plots. This metric provided a measure of the relative difference in edge and interior browsing with a value of ~0 indicating no difference between the edge and interior, positive numbers indicating more browsing in the interior, and negative values indicating more browsing on the edge. We then used linear regression to compare the mean difference in edge/interior browsing per transect to the number of Coyote scats. To assess if escape cover mitigated some of the effects of Coyote use on White-tailed Deer browsing, we compared the mean preferred species browsed per transect with the mean percent escape cover. All statistical analyses were conducted using Program R, version 3.0.2; we set α ≤ 0.05 as the threshold for statistical significance. Results We found no significant difference in the amount of available preferred woody browse in edge versus interior plots (t32 = 0.776, P = 0.443). The frequency of use by Coyotes varied considerably among transects (mean ± SE = 3.2 ± 0.6, range = 0–9). Similarly, the proportion of preferred twigs browsed varied from low levels of browsing to transects where almost half of the available twigs were browsed (mean ± SE = 0.21 ± 0.03, range = 0.08–0.44). We found no significant relationship between the mean proportion of preferred species browsed at the transect level and the number of Coyote scats (F1,15 = 0.182, P = 0.675, r2 = 0.01; Fig. 1). However, we did find a marginally significant effect of Coyotes on the difference in the proportion of preferred species browsed between the edge and interior, with a trend towards a higher proportion of browsing in the interior as the frequency of Coyote scats increased (F1,15 = 4.452, P = 0.052, r2 = 0.23; slope = 0.036 ± 0.017, y-intercept = -0.165 ± 0.068; Fig. 2). The mean escape cover at the transect scale was not a significant driver of the proportion of preferred species browsed (F1,15 = 3.95, P = 0.065, r2 = 0.21), although we did observe a tendency towards a higher proportion of browse where there was less escape cover (Fig. 3). Discussion Our results support the findings of prior research in showing that the response of ungulates to predation pressure was scale-dependent (Altendorf et al. 2001, Anderson et al. 2005, Jones and Hudson 2002, Kittle et al. 2008). Specifically, we found that White-tailed Deer did not respond to predation risk (as inferred from the number of Coyote scats) at the transect scale, but shifted to foraging more in the forest interior. Similar risk-driven shifts in patterns of habitat selection have been demonstrated for Elk in the presence of wolves in the western US (Creel et al. 2005); however, we are unaware of any previous evidence of a similar relationship between Coyotes and White-tailed Deer in the northeast. The scale of the response Northeastern Naturalist Vol. 22, No. 2 J.L.B. Pierce, et al. 2015 267 Figure 1. The relationship between the number of Coyote scats and the proportion of preferred woody plant species browsed along 17 linear transects sampled in the northern Adirondacks, NY. Each transect consisted of 5 browse plots on the forest edge and 5 browse plots in the forest interior. Figure 2. The relationship between the number of Coyote scats and the mean difference between the proportion of preferred browsed tree species in the forest interior and forest edge along 17 linear forest-edge transects sampled in the northern Adirondacks, NY. Positive values represent more browse in the forest interior compared to the edge, and negative values the opposite trend. Northeastern Naturalist 268 J.L.B. Pierce, et al. 2015 Vol. 22, No. 2 we observed likely represented shifts in the feeding ecology of individual animals, i.e., within-population effects, and supported the findings of previous studies in showing that non-lethal effects of predators may be more prevalent than lethal effects at the population scale (Altendorf et al. 2001, Brown 1999, Brown and Kotler 2004, Creel et al. 2005, Schmitz et al. 1997). We did not find strong evidence that White-tailed Deer prefer to browse in areas with greater escape cover; this finding may represent a flaw in the logic of our study in that denser cover may serve as concealment for predators. However, we hypothesize that the trend we observed toward less browsing in areas with more cover is due to dense cover in our study area primarily representing Red Spruce and Balsam Fir, which are considered to be starvation foods for White-tailed Deer. Optimal foraging theory (MacArthur and Pianka 1966, Pyke 1984, Pyke et al. 1977) provides a strong theoretical framework for understanding the patterns we observed in our data. In general, organisms should choose to adopt a foraging strategy that minimizes costs, while maximizing energy intake (Brown and Kotler 2004, Gilliam and Fraser 1987). The costs associated with foraging in a patch include (1) the metabolic costs of foraging, (2) missed-opportunity costs of not being able to engage in other activities, and (3) predation costs (Brown 1988). Because we found no significant difference in the amount of available browse in edge versus interior plots, we are able to exclude this potentially confounding variable from our analyses and focus on differences in perceived predation risk. Individuals can demonstrate habitat selection, i.e., the manifestation of optimal foraging theory as it relates to the spatial distribution of organisms, at 2 scales: (1) the location of Figure 3. The relationship between mean percent escape-cover and the mean proportion of preferred woody plant species browsed per transect along 17 linear forest-edge transects sampled in the northern Adirondacks, NY. Northeastern Naturalist Vol. 22, No. 2 J.L.B. Pierce, et al. 2015 269 the home range in relation to available habitat, and (2) the areas utilized within the home range (Anderson et al. 2005, Johnson 1980). Our study was conducted within a relatively small spatial extent within forest-stand types known to host high abundances of Coyotes in the focal region (Kays et al. 2008). These site characteristics may explain why we did not observe patterns of browsing indicative of White-tailed Deer relocating home ranges to areas with fewer Coyotes: dispersal to unfamiliar territory would likely have been energetically costly, and the probability of Whitetailed Deer finding an area with fewer Coyotes was low within our study region. Conversely, the concentration of Coyotes along linear forest/shrub edges—as observed in our focal region and in prior research (Knowlton et al. 1985)—represented a more predictable risk that was avoided by moving into the forest interior. Optimal foraging theory typically assumes that there is a trade-off between predation risk and the quality of browse, i.e., the best places to browse are likely to have the highest risk of predation (Brown and Kotler 2004). However, research has shown that this is not always the case (Pierce et al. 2004). Our study provided evidence that the most energetically efficient place for White-tailed Deer to browse woody vegetation was on the forest edge because these areas were favored when Coyote use of the edge was low. Our study was based on the premise that the density of scat was indicative of the intensity of use by Coyotes, and that this intensity of use translated into predation risk for White-tailed Deer. This assumption was supported by prior research in our study area that found that higher numbers of Coyote scat were associated with a larger number of individuals at the landscape scale (Kays et al. 2008). Our method for quantifying browsing inevitably included some twigs that were removed by abiotic processes or by other species of browsers. Although this likely led to an overestimation of the proportion of browsing, we have no reason to suspect that abiotic damage to twigs varied among transects. Additional species that may have consumed woody vegetation in the region include Lepus americanus Erxleben (Snowshoe Hare) and Moose. Snowshoe Hares in the northern Adirondacks tend to be concentrated in stands with dense cover (J. Favreau, School of Natural Resources and Management, Paul Smith's College, Paul Smiths, NY, unpubl. data) that are dissimilar to our focal stands; we observed very little sign of browsed twigs that were cut (hares) rather than torn (ungulates). We excluded the few twigs that were clearly browsed by Snowshoe Hares from our analyses. Moose occur at very low densities in our focal region (Hickey 2008, Sage et al. 2003), with the NYSDEC estimating a maximum of 1000 individuals in the 2,400,000-ha park (i.e., less than 0.0004/ha). In comparison, White-tailed Deer densities in the central Adirondacks range from 0.019/ha to 0.104/ha (Sage et al 2003). Browsing by White-tailed Deer is a dominant force that shapes forest structure and function in the northeastern US (Sage et al. 2003). Of particular concern to land managers is the potential for White-tailed Deer browsing to reduce the recruitment of commercially valuable tree species including birch species and Eastern Hemlock (Rooney and Waller 2003). Current approaches to mitigating the effects of deer browsing on forest composition tend to focus on directly reducing Northeastern Naturalist 270 J.L.B. Pierce, et al. 2015 Vol. 22, No. 2 the abundance of White-tailed Deer through hunting (Sage et al. 2003). However, the efficacy of hunting as a tool for widespread reductions in White-tailed Deer densities is uncertain (Brown et al. 2000). Natural processes such as predation may offer an alternative approach to hunting if they result in patterns of White-tailed Deer browsing and forest regeneration that meet forest-management objectives. While restoring top carnivores following their extirpation is controversial, understanding the consequences of natural colonization/recolonization events such as the movement of Coyotes into the northeastern US on patterns of browsing is of great importance. Our study, while limited in scope, indicated that Coyotes may be driving local browsing patterns primarily through behaviorally mediated indirect effects as has been demonstrated with other taxa (Creel et al. 2005, Schmitz et al. 1997). 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