合歡山地區台灣高山田鼠與高山草原植物的交互作用：從優勢草本植物的營養與豐富度來探討

Abstract

動植物之間交互作用的機制是生態學及演化生物學裡令人著迷的重要議題。尤其是草食動物對植物的擇食現象，深深地影響各生態系的植被多樣性與組成。在全球暖化的陰影之下，了解高山生態系動植物之間交互作用的生態機制尤迫切需要。高山草原植物由於生長速度緩慢，草食動物的擇食對其影響甚鉅，過去的研究顯示小型草食獸的選擇性取食受到植物豐富度及可食度的影響，可食度又受到植物本身營養、次級代謝物及有毒物質含量的影響。在台灣，為冰河孓遺生物的特有種台灣高山田鼠(Microtus kikuchii)是合歡山地區玉山箭竹草原的優勢小型草食獸。為了瞭解台灣高山田鼠對當地高山草原植物的擇食狀況及其生態機制，我測試以下假說:(1) 台灣高山田鼠具有擇食行為(而非機會主義者)：對田鼠而言，不同植物具有不同可食度；(2)化學因子如營養、次級代謝物及有毒物質含量可以解釋可食度的差異：較可食的植物具有較高蛋白質、較低纖維素及多酚類；(3)田鼠的擇食符合兩階段理論：影響可食與否與可食程度者為不同決定因子；(4)田鼠喜食優勢植種：可食度越高的植物也是越優勢的植種；(5)田鼠的分佈受到食物資源的影響：田鼠的分佈與高可食植物的分佈有正相關，與低或不可食植物有負相關。我調查了合歡山區玉山箭竹草原裡的各種動植物(動物限定於小型哺乳動物)分佈及其豐富度，以餵食實驗測量優勢植物的可食度，並以化學分析測量優勢植物各化學因子。結果發現不同植物對台灣高山田鼠具有不同可食度，植物皆未發現有毒且不符合二階段理論，而可食度越高的植物為當地越優勢的植種，其半纖維素含量較高。而食物資源的分佈對台灣高山田鼠的分佈似乎也有影響。此擇食結果顯示台灣高山田鼠在合歡山地區玉山箭竹草原可迅速找到足夠食物並快速獲取熱量，並對當地植被可能具有抑制優勢種生長的效果。

Ecological mechanisms explaining how animals and plants interact are fascinating topics in ecology and evolutionary biology. Particularly, selective foraging by herbivores could alter plant species composition and diversity in various ecosystems. Previous studies have suggested that selective foraging by small mammalian herbivores is affected by the differential abundance and palatability of plants. The palatability is in turn influenced by chemical characteristics, like nutrition, secondary compound, and toxin contents, in plants. Under the growing threats of global warming, it is urgent to understand the ecological mechanisms in alpine meadow. In alpine meadow ecosystem vegetation is easily modified by the selective herbivory because of the low growth rates of plants. The endemic Taiwan vole (Microtus kikuchii), a glacial relic species, is the dominant small mammal herbivore in the Yushan cane (Yushania niitakayamensis) grassland in Taiwan. To understand the selective foraging and the involving mechanism, I proposed to test the following hypotheses: (1) Taiwan voles forage selectively (not an opportunist): different plants have different palatability to voles; (2) chemical characteristics, like nutrition, secondary compound, and toxin contents, in plants could account for the differential palatability: palatable plants have higher protein, lower fiber and lower protein-binding phenolics than less palatable ones; (3) food selection by Taiwan voles is a two-step process: the factors accounting for unpalatability and the degrees of palatability are different; (4) Taiwan voles prefer dominant plants: palatable plants have greater cover area than less palatable ones; (5) the distribution of Taiwan voles is influenced by the distribution of food resource: the distribution of voles is positively correlated with that of highly palatable plant species, negatively with less palatable ones. I surveyed the population of small mammals and the plant community in the Yushan cane (Yushania niitakayamensis) grassland in the Ho-huan Mountains to determine the distribution and abundance of different species. I assessed the palatability and chemical characteristics content of dominant plants by feeding trials and chemical analysis in the laboratory. The results showed that different plants had different palatability to voles, and the two-step forage selection process was not supported because no toxin in plants was found. Taiwan voles consumed greater amount of plants with greater cover areas and more hemicellulose contents. Such traits allowed adult voles find food and gain energy quickly. The selective foraging may prevent dominant plant species from over-dominating. The distribution of Taiwan voles also seemed to be affected by the distribution of food.