INTRODUCTION
With the expansion of human activities, reports of anthropogenic food consumption by wildlife are increasing worldwide (Oro et al., 2013). Access to anthropogenic foods by wildlife can increase human–wildlife conflict, resulting in economic losses (e.g., predation on agricultural crops and livestock) as well as increased risk of vehicle collisions and infectious disease transmission among wildlife, humans, and livestock (Cote et al., 2004; Becker et al., 2015; Honda et al., 2018; Johnson et al., 2020). Additionally, anthropogenic food consumption can affect wildlife population dynamics and local ecosystems via alterations in behavioral traits and physiological conditions (Prange et al., 2004; Oro et al., 2013; Gaynor et al., 2018; Tucker et al., 2018; Petroelje et al., 2019; Hernando et al., 2020). To mitigate these conflicts, it is important to develop management strategies for individuals that consume anthropogenic foods.
For managing anthropogenic food-foraging wildlife, understanding their spatial distribution pattern is important to determine the efficient spatial allocation of management efforts. In particular large terrestrial mammals, even they consumed anthropogenic foods in artificial landscapes such as agricultural crop fields, they move across distant other habitats (Takada et al., 2002; Hata et al., 2017). Therefore, understanding the spatial distribution patterns of anthropogenic food-foraging individuals is necessary to prioritize areas where limited management effort should be allocated to mitigate conflicts. Previous studies have investigated the spatial distribution of large terrestrial mammals by direct and indirect observations, such as radiotelemetry and camera trap surveys (Sanderson, 1966; Rubenstein & Hobson, 2004). However, these techniques have some limitations, including substantial effort and costs, and inability to obtain information about the diet and geographical location of animals simultaneously (Rubenstein & Hobson, 2004; Hobson, 2005). Stable isotope analysis is an alternative tool that overcomes these weaknesses of conventional approaches (Crawford et al., 2008; Hobson et al., 2010). The stable isotope ratios in animal tissues are related to those in the diet (DeNiro & Epstein, 1978, 1981); when the isotope values differ between anthropogenic and natural food resources, animal tissues reflect the foraging history of anthropogenic foods (Mizukami et al., 2005; Ditmer et al., 2016; Hata et al., 2017, 2021; Demeny et al., 2019). However, only few studies have described the spatial distribution patterns of anthropogenic food-foraging animals (Walter, 2014; Hata et al., 2017).
Deer are typical large terrestrial mammals that move across multiple landscapes. In middle- to high-latitude regions, deer often migrate seasonally in accordance with snow depth and food availability (Kufeld et al., 1989; Ball et al., 2001; Sabine et al., 2002; Igota et al., 2004). The consumption of anthropogenic foods, including agricultural crops, by deer has been documented and causes serious economic losses in many countries (Fagerstone & Clay, 1997; Putman & Moore, 1998; McCullough et al., 2009). Moreover, crop consumption has the potential to induce deer population growth (Iijima et al., 2013; Hata et al., 2021), which can increase agricultural crop damage and induce ecosystem changes; the increment of browsing pressure on the forest understory, the inhibition of woodland regeneration, and the promotion of fluctuations in the population and community structure of various taxa, from insects to mammals (Cote et al., 2004). To mitigate conflicts that arise from crop consumption by deer, it is necessary to understand spatial distribution patterns of crop-foraging deer to manage them at an appropriate spatial scale.
In Japan, the consumption of crops, such as vegetables and pasture grasses, by sika deer (Cervus nippon ) is well-documented and causes serious economic losses (Ministry of Agriculture, Forestry and Fisheries, 2018). To mitigate conflicts, damage prevention management such as fencing and culling are conducted. Although many deer are killed every year (e.g., about 600,000 individuals were killed in Japan in 2019, in which about 23% were by hunting and 77% by culling) (Ministry of the Environment, 2020), agricultural damage by deer still amounts to over 50 million dollars every year (Ministry of Agriculture, Forestry and Fisheries, 2018). More efficient and effective management strategies are needed to mitigate conflicts arising from crop consumption by deer. Sika deer inhabit various landscapes from plains to high-altitude areas (Takatsuki, 2009) and migrate seasonally, as do most ungulates at middle and high latitudes (Igota et al., 2004; Takii et al., 2012a, b). Therefore, agricultural crop-foraging deer may not consistently inhabit near agricultural crop fields throughout all seasons and may move long distances during the winter and spring, when crop production decreases. Because the food resources for deer are limited during the winter and spring (Yokoyama et al., 2000; Seto et al., 2015), deer culling is suitable during these seasons when the bait-trap success improves, and shooting also can be easy with better visibility without leaves. Therefore, clarifying the spatial distribution pattern of crop-foraging deer during these seasons will facilitate the efficient spatial allocation of management efforts to mitigate conflicts.
In this study, we examined the spatial distribution pattern of crop-foraging sika deer during the winter and spring in central Japan. We investigated crop consumption by performing nitrogen stable isotope analyses of bone collagen samples. The nitrogen stable isotope ratios (δ15N) of bone collagen were expected to reflect the foraging history and crop consumption by individual deer (Hata et al., 2021). We examined whether deer likely to consume crops live closer to agricultural crop fields, even during the winter and spring.