Using Stable Isotopes to Analyze Food Partitioning of Two Small Rodent Communities in He-huan Mountains

共域的物種在食物資源的使用上可能會有不同的生態棲位。本研究使用穩定碳與氮同位素分析探討合歡山地區箭竹草原與冷杉林兩種不同棲地內的共域小型哺乳類（台灣高山田鼠Microtus kikuchii、台灣森鼠Apodemus semotus與高山白腹鼠Niviventer culturatus）在不同季節的食物資源分配。我們於合歡山冷杉林及箭竹草原設立樣區，進行生長季（春夏季）與非生長季（秋冬季）小型哺乳類毛髮的採樣，並採集可能的食物來源（植物葉片、真菌與無脊椎動物）。各樣本以同位素分析儀得到穩定碳與氮同位素值，並以SIAR（Stable isotope analysis in R）運算可能食物來源對小型哺乳類的食物貢獻比例，資料分析分為三個方向：棲地性、季節性及物種間差異，同時亦利用同位素值計算各物種的棲位寬度與變異程度。結果顯示，共域的物種在棲地間存在差異：兩個季節，台灣高山田鼠的草原個體均較森林個體取食較多植物；而台灣森鼠的森林個體不同於草原個體，兩個季節均以真菌為主要的取食對象。季節性的差異僅存在於森林的台灣高山田鼠與高山白腹鼠族群：在非生長季的時候會較生長季取食較多的無脊椎動物。在物種間的比較發現，台灣高山田鼠不論季節或棲地，均較其他鼠類取食較多的植物，相較森林的其他鼠類則取食較少的真菌；此外高山白腹鼠在非生長季的時候會較其他鼠類取食較多的動物性資源。運用同位素計算棲位寬度與變異程度，整體來說森鼠在兩個棲地擁有最廣的棲位寬度與最高的變異程度，但各物種在季節間存在著差異。結果支持三種共域的物種在食物資源的使用上有不同的生態棲位，而真菌可能在合歡山冷杉林內小型哺乳類的取食上扮演一個重要的角色。Coexisting species may occupy different ecological niche in food resource utilization. We used stable carbon and nitrogen isotope techniques to analyze the food partitioning of rodent communities ( Taiwan vole: Microtus kikuchii, Formosan mouse: Apodemus semotus, and White-bellied rat: Niviventer culturatus ) in the alpine meadow and fir forest in the He-huan mountains. Rodent furs and potential food sources ( plants, fungi, and invertebrates ) were sampled in growing and non-growing seasons during 2009-2010 in both habitats, followed by carbon and nitrogen isotopic analyses. SIAR ( Stable isotope analysis in R ) was used to calculate the percentage contribution of food sources to the consumers. Three aspects were examined: habitat, seasonal, and species differences, along with food niche width measures to reveal variations within the populations. Habitat difference focused on coexistent species: Taiwan voles had similar trends in both seasons, with meadow individuals consuming more plants than forest ones. Formosan mice in the forest differed from the meadow ones, with forest individuals using fungi as their main food sources. Seasonal difference in diets only existed in Taiwan voles and white-bellied rats in forest, with more invertebrates consumed in non-growing than growing seasons. For species comparison, Taiwan voles ate more plants than other species in both habitats and less fungi in the forest. Besides, white-bellied rats consumed more animal sources than others in non-growing seasons. Formosan mice had wider food niche width than other species in both habitats, but seasonal differences existed. Overall, the three species did partition their food resources. The results also revealed that fungi might play an important role in rodents’ diet in the forest in He-huan mountains