森林破碎化如何影響尼泊爾埋葬蟲（Nicrophorus nepalensis）與衿覆葬甲（N. schawalleri）的海拔分布界線

Abstract

森林破碎化是導致生物多樣性喪失的重要因素，森林破碎化除了直接改變棲地的物理條件，導致生物賴以為生的棲地喪失，也有可能改變物種的族群結構，並影響個體間的互動，抑或是種間關係，導致物種族群下降。然而森林破碎化如何透過這些機制影響物種的分布，是近年來愈來愈受重視的問題。我們透過研究森林破碎化如何影響中國四川夾金山區的尼泊爾埋葬蟲與衿覆葬甲的海拔分布界線，來探討這個重要的問題。我們沿著海拔及森林覆蓋度兩個環境梯度進行的族群密度調查與繁殖實驗，結果發現在兩種埋葬蟲海拔分布的交界帶上，隨著森林覆蓋度的降低，尼泊爾埋葬蟲的族群密度並不受影響，但衿覆葬甲的族群密度卻減少；尼泊爾埋葬蟲的繁殖成功率上升，但衿覆葬甲卻下降。在交界帶兩種埋葬蟲的族群密度透過影響在屍體上的相對群體大小而影響繁殖成功率，屍體上群體數量相對較大的物種，則其繁殖成功率愈高。我們的研究結果顯示，森林破碎化透過這個密度依存的機制，導致衿覆葬甲無法與尼泊爾埋葬蟲競爭，因此導致海拔分布界線上移。因此，在預測人為環境變動造成的物種分布改變時，不但需要考慮種間關係，也需要考慮族群密度變化所導致的種間競爭結果的改變。

Forest fragmentation is an important driver of current biodiversity loss. Forest fragmentation not only alters the physical conditions experienced by organisms, but also changes the population structure of species, as well as intra- and interspecific interactions, and ventually leads to population declines. However, how forest fragmentation affects species distribution through above mechanisms is an issue that has received increasing attention in recent years. Here, we address this question by examining how forest fragmentation affects the elevational range boundary between two burying beetle species, Nicrophorus nepalensis and N. schawalleri, in Mt. Jiajin, Sichuan, China. We conducted population density surveys and field breeding experiments along two environmental gradients: elevation and forest cover. We found, in the contact zone of the two species, decreases in forest cover reduced the population density of N. schawalleri, but not that of N. nepalensis. Decreases in forest cover also reduced breeding success of N. schawalleri, but increased that of N. nepalensis. Population densities of the two species in the contact zone jointly influenced their reproductive success by affecting their relative group sizes on the carcasses, with the species with larger group size on the carcasses having higher reproductive success. Forest fragmentation weakened the competitive ability of N. schawalleri against N. nepalensis through such a density-regulated mechanism, and in turn caused an upward shift in elevational range boundary between the two species. Therefore, we suggest the importance in considering density-regulated competitive advantages that alter the outcomes of interspecific competition when predicting changes in species distribution due to anthropogenic environmental changes.