Applicability of the point-frame method as a food habit analysis method for omnivorous mammals: a case study on medium-sized carnivores
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Most studies on the food habits of mid-sized carnivora mammals in Japan, which are often omnivorous, have adopted the frequency method. The frequency method, however, cannot assess dietary compositions but only occurrences. In contrast, the point-frame method can assess not only food compositions but also occurrences. Therefore, in order to test the applicability of this method, we tried an analysis of the summer and winter stomach contents of raccoon dogs (Nyctereutes procyonoides) and masked palm civets (Paguma larvata). As many as 98 categories and 102 categories were found at 200 and 300 counts, respectively, for all the samples. However, only 6–10 categories were recovered at 200 counts for each sample. As counts increased, the food categories increased but very few categories were recovered after 200 counts. The number of samples which recovered 90–110% of the maximum category number at 300 counts was 31 (58.5%) out of 53 samples at 150 counts, and 52 (98.1%) at 200 counts. The necessary counts to reach the range of 90–110% of the occupancy at 300 counts ranged from 150 to 170, except for samples of masked palm civets during the summer, when 221 counts were needed. Sixteen and 28 (44.4%) samples among 36 samples (77.8%) entered this range at 150 and 200 counts, respectively. From these results, we judged that 200 counts are necessary and sufficient to recover the food categories as well as food compositions. The point-frame method took 25.9 minutes (36.7%) in average to analyze one sample whereas the weight method took 70.6 minutes. The interrelations between percentage frequency occurrence and percentage occupancy suggests a synthetic understanding of food availability and food intake by the animals. The correspondence between the point-frame method and frequency evaluation was weak, because samples included foods of "small amount but high frequency," and those of "large amount but low frequency". We showed by comparison of the point-frame method and weight evaluation that three-dimensional food categories like meat and large seeds were underestimated, while those like leaves and flowers were overestimated by the point-frame method. Important details of the point-frame method for the food habit analysis of omnivorous mammals were summarized.
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