DISCUSSION
Our isotopic approach revealed the
spatial distribution pattern of crop-foraging female sika deer. Female
deer in proximity to agricultural crop fields during the winter and
spring were more likely to be crop-foraging individuals (Fig. 2).
Possible reasons why crop-foraging deer distributed in proximity to
agricultural crop fields as follows. Although seasonal migration is
well-known in cervids inhabiting middle- to high-latitude regions
(Mysterud, 1999; Igota et al., 2004; Monteith et al., 2011; Takii et
al., 2012a, b), sedentary behavior has also been observed in accordance
with food availability (Kufeld et al., 1989; Igota et al., 2004). In
general, the availability of both crops and natural food resources is
low during the winter and early spring. However, crops do not completely
disappear. In fact, deer aggregate in sown grasslands to consume pasture
grasses under snow during the snowy season (Hata et al., 2019), and many
leftover vegetables remain in agricultural crop fields for long periods
(Hata personal observation). Therefore, deer that are more likely to
consume crops might tend to distribute in proximity to agricultural crop
fields for crop consumption, even during the winter and spring. In fact,
it was reported that two female sika deer tracked with Global
Positioning System (GPS) inhabited areas near agricultural crop fields
throughout the year (Ishizuka et al., 2007).
The likelihood of crop consumption by females decreased by half as the
distance to agricultural crop fields increased to 10 km and fell to
essentially zero at a distance of approximately 40 km (Fig. 3).
Accordingly, crop-foraging female deer are more likely to distribute
within this range. The width of the range may depend on behavioral
variation among deer, such as migration. To elucidate the determinants
of behavioral differences among crop-foraging individuals, further
research is needed with consideration of additional factors, such as the
response to hunting risk (Kilgo et al., 1998; Kamei et al., 2010; Lone
et al., 2015; Little et al., 2016), genetic inheritance (Mueller et al.,
2013), and social learning (Hopkins, 2013). Nevertheless, our findings
can be useful to allocate management efforts such as culling at the
appropriate spatial scale to mitigate conflicts that arise from crop
consumption by deer.
In contrast to our results for
female deer, there was no significant trend in the spatial distribution
pattern of crop-foraging male deer (Fig. 2). As with many mammals (Pusey
1987), male deer show a greater tendency to disperse from their natal
area than female. In fact, dispersal movement has been observed in
juvenile male sika deer (Yamazaki & Furubayashi, 1995; Takii et al.,
2012b). Takii et al. (2012b) reported that some males dispersed at 1-2
years old with distances of 3.0 to 40.3 km. In our data, eight males
were 1-2 years old (Table S1). Even if they inhabited nearby
agricultural crop fields with their mothers and consumed agricultural
crops when they were young, they might disperse distant other habitats.
The differences between sexes in movement behaviors might affect spatial
distribution of crop-foraging deer. The small sample size might also
have resulted in the large variation in males.
Our results for the spatial distribution patterns of crop-foraging
female deer provide a basis for management at an appropriate spatial
scale to mitigate conflicts. Because crop-foraging female deer were more
likely to distribute within 10 km of agricultural crop fields, the
establishment of management areas, including zonation, based on this
result might be effective. Intensive culling in this zone would allow
for effective removal of crop-foraging female deer. Culling
crop-foraging female deer is expected to suppress deer population growth
because crop consumption induces precocious maturity and may promote
population growth (Hata et al., 2021). Moreover, intensive culling using
zonation can prompt deer to flee from nearby agricultural crop fields
because deer avoid hunting activity by altering their behavior and
habitat utilization (Kilgo et al., 1998; Kamei et al., 2010; Lone et
al., 2015; Little et al., 2016). In the future, it is necessary to
evaluate the effectiveness of intensive culling in the management areas
to decrease crop-foraging deer and subsequent crop damage and ecological
impacts.