11:00 AM - 1:00 PM
[HTT18-P07] An isoscape analysis to test whether fecal δ15N of deer in agricultural landscapes indicate distribution of the crop-damaging individuals
Keywords:nitrogen isoscape, deer preferred plant, ecosystem management
A rapid increase in the number of sika deer (Cervus nippon) in the last decades aroused needs for quantitative estimation of crop herbivory by them to elucidate factors affecting damage-level they cause to agriculture. Analysis of ingredients in deer feces using observation under microscopes or molecular techniques is useful to reveal whether they consumed crops. Nevertheless, these techniques in common tend to underestimate crop herbivory, because crops are usually digested more easily than wild plants and less likely to remain in the feces. Measurement of fecal nitrogen stable isotope ratio (fecal δ15N) can be a complement to the molecular technique, considering that application of fertilizer leads to distinctively higher δ15N values of crops than the adjacent wild plants. In our previous study using captive deer individuals, we estimated the fecal δ15N values are 1.4 to 3.4‰ higher than those of deer diet.
Based on the test using captive individuals, the uncertainties of the fecal δ15N values associated with the variations in isotopic fractionation were far less than the difference of δ15N values between fertilized crops and wild plants. In this study, we first examined the relationship between spatial distribution of the fecal δ15N and surrounding landscapes based on a hypothesis that sika deer consumes higher proportions of crops in the habitats surrounded by crop field, and which lead to the higher fecal δ15N values. We also collected crops (rice) and wild plant (dwarf bamboo) to evaluate spatial patterns of δ15N of deer diet in the agricultural landscape for the estimation of importance of crops as diet.
Deer feces, crops, and wild plant were collected in 80 forested sites in the northern part of Osaka prefecture in winter seasons since 2014. The feces sampled in 2014, 2015, 2018, and 2019 were ground and homogenized for each of the fecal mass for δ15N measurements. In order to compare δ15N values of crops and wild plants in a same environmental condition, pairwise sampling of rice and dwarf bamboo was conducted in 2018, 2019, 2020, and 2021: rice sprouts were collected from a rice field and at the same time dwarf bamboo were collected in adjacence to each rice field. 3 to 5 leaves were ground and homogenized for measurements of δ15N. Based on GPS telemetry of sika deer in the region (Ishizuka et al., 2007), we assumed that short-time moving range of sika deer is around 500 m in radius and the accessibility of crops for the deer is reflected by surrounding land use within the distance from sampling points. Land use surrounding sampling points were categorized into four components; forest, crop field, herbal vegetation, residential area and area of each component were calculated. Land use data are obtained from MoE National Surveys on the Natural Environment and area (m2) of the surrounding sampling points. Spatial Autoregressive (SAR) analyses were conducted to analyze the effects of surrounding areas of these landscape components on the fecal δ15N values and concentrations of nitrogen in the feces. Spatial patterns of δ15N of crops and wild plants were visualized by interpolation and estimated δ15N values diet available in the fecal sampling sites.
The surrounding area of crop field was selected as a single variable to explain the fecal δ15N values, and the positive correlation between the fecal δ15N values and crop field area indicated the availability of crop to deer was a factor of the increase in the fecal δ15N values. Based on the result, this study supported our hypothesis that fecal δ15N values are an effective indicator of crop consumption by sika deer. In our presentation, we will further discuss about estimates about proportional contribution of crop in the diet of sika deer.
Based on the test using captive individuals, the uncertainties of the fecal δ15N values associated with the variations in isotopic fractionation were far less than the difference of δ15N values between fertilized crops and wild plants. In this study, we first examined the relationship between spatial distribution of the fecal δ15N and surrounding landscapes based on a hypothesis that sika deer consumes higher proportions of crops in the habitats surrounded by crop field, and which lead to the higher fecal δ15N values. We also collected crops (rice) and wild plant (dwarf bamboo) to evaluate spatial patterns of δ15N of deer diet in the agricultural landscape for the estimation of importance of crops as diet.
Deer feces, crops, and wild plant were collected in 80 forested sites in the northern part of Osaka prefecture in winter seasons since 2014. The feces sampled in 2014, 2015, 2018, and 2019 were ground and homogenized for each of the fecal mass for δ15N measurements. In order to compare δ15N values of crops and wild plants in a same environmental condition, pairwise sampling of rice and dwarf bamboo was conducted in 2018, 2019, 2020, and 2021: rice sprouts were collected from a rice field and at the same time dwarf bamboo were collected in adjacence to each rice field. 3 to 5 leaves were ground and homogenized for measurements of δ15N. Based on GPS telemetry of sika deer in the region (Ishizuka et al., 2007), we assumed that short-time moving range of sika deer is around 500 m in radius and the accessibility of crops for the deer is reflected by surrounding land use within the distance from sampling points. Land use surrounding sampling points were categorized into four components; forest, crop field, herbal vegetation, residential area and area of each component were calculated. Land use data are obtained from MoE National Surveys on the Natural Environment and area (m2) of the surrounding sampling points. Spatial Autoregressive (SAR) analyses were conducted to analyze the effects of surrounding areas of these landscape components on the fecal δ15N values and concentrations of nitrogen in the feces. Spatial patterns of δ15N of crops and wild plants were visualized by interpolation and estimated δ15N values diet available in the fecal sampling sites.
The surrounding area of crop field was selected as a single variable to explain the fecal δ15N values, and the positive correlation between the fecal δ15N values and crop field area indicated the availability of crop to deer was a factor of the increase in the fecal δ15N values. Based on the result, this study supported our hypothesis that fecal δ15N values are an effective indicator of crop consumption by sika deer. In our presentation, we will further discuss about estimates about proportional contribution of crop in the diet of sika deer.