了解影響物種繁殖表現的因子與其機制，一直是生態學上重要的議題。物種繁殖表現的時空變化是受到許多因子共同影響的結果，然而過去的研究多偏重於探討影響物種繁殖表現的空間差異，造成物種繁殖表現季節變化的機制則較少被研究。在本研究中，我們以尼泊爾埋葬蟲(Nicrophorus nepalensis)為研究對象，探討影響物種繁殖表現季節變化的因子與機制，並測試下列假說: 1) 生理限制假說：環境因子的季節變化影響物種生理反應，進而影響繁殖表現；2) 艾里效應假說：族群密度的季節變化造成繁殖表現的的正密度依賴性，導致繁殖表現的季節變化。本研究解果發現，在低海拔地區，由於夏季尼泊爾埋葬蟲的族群密度有些微的上升，使得埋葬蟲能形成大的合作群體，以合作生殖的方式抵抗不適宜的高溫環境，因而維持了稍低但卻穩定的繁殖成功率。相反的，中海拔地區，因族群密度隨季節的遞嬗而逐漸遞減，增加了埋葬蟲形成大合作體的困難，因而不利於抵抗外在惡劣的的環境，使得繁殖成功率逐季節遞減。然而，高海拔地區，由於低的族群密度導致埋葬蟲找到屍體的機率降低，以至於繁殖成功率也隨族群密度降低而遞減。我們的結果指出，尼泊爾埋葬蟲繁殖成功率的季節差異，是受到族群密度的季節變化的影響，符合艾里效應假說。我們的結果意味著，族群密度因極端氣候事件與人為的棲地破壞而降低，使得物種不利於抵抗外在的惡劣環境，而限制其分布之外，也可能導致能繁殖的季節縮減，加速物種的滅絕。

關鍵字：繁殖表現、季節變化、族群密度、艾里效應、生理限制
 

SUMMARY
Understanding factors affecting reproductive performances of species and the underlying mechanisms is an important goal for macroecological studies. However, the mechanism of how reproductive performances change across seasons remains poorly understood. Here, we used burying beetle (Nicrophorus nepalensis) to test two hypotheses: 1) physiological constraint hypothesis, which denotes that seasonal variation in reproductive performance is directly limited by environmental factors; 2) Allee effect hypothesis, which denotes that seasonal variation in reproductive performance is modulated by positive density-dependent population regulation. We found that, at low elevation, the slightly higher population density in summer allowed burying beetles to form larger cooperative groups to overcome stronger interspecific competition at warmer environments and maintain a relatively stable probability of breeding successfully across seasons. At intermediate elevation, probability of breeding successfully decreased with decreasing population density across season, since burying beetles failed to successfully cooperate at low population density. However, at high elevation, probability of breeding successfully also decreased with decreasing population density across season, since burying beetles failed to locate the carcasses at low population density. Our results show that seasonal variation in reproductive performance of burying beetles was strongly modulated by population density, as predicted by the Allee effect hypothesis. Our findings imply that decrease in population density will reduce the ability of species to cope with environmental stress. Our results also highlight the need to maintain population density in order to promote population sustainability under future climate and land-use changes.

Key words: reproductive performance, seasonality, population density, Allee effect, physiological constraint

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