不同層面的鳥類功能多樣性與環境梯度的關係

Abstract

功能多樣性描述群聚中性狀的多樣性，能夠詮釋一個地區生態系統功能。功能多樣性包括至少四個層面：功能相異度、功能分散度、功能豐富度、功能均勻度。目前已知功能多樣性在溫帶地區會受氣候和植物生產力的影響，但副熱帶地區仍缺乏相關研究，且較少有研究同時探討不同層面的功能多樣性。我們透過檢驗兩個假說來探討副熱帶地區環境梯度與不同層面功能多樣性的關係。第一，環境過濾假說：較高的溫度、較豐沛的降雨量、較低的降雨量變異係數（對功能性狀較不嚴苛的過濾條件）會導致較高的功能多樣性；第二，資源可得性假說：較高的植物生產力（較多的資源支持）會導致較高的功能多樣性。我們根據台灣繁殖鳥類大調查和五種重要的鳥類功能性狀，計算四個層面的功能多樣性指標。接著使用線性混合效應模型探討環境梯度（溫度、降雨量和植物生產力）與功能多樣性的關係。分析結果基本上支持兩個假說：較不嚴苛的環境條件和較多的資源支持增加了功能相異度、功能分散度與功能豐富度。本研究顯示台灣的氣候和植物生產力會影響功能多樣性，與溫帶地區的研究結果一致。然而在台灣，功能豐富度和溫度之間呈現二次曲線的關係，不同於溫帶地區的線性關係，代表相較於溫帶地區，氣候暖化更可能會降低副熱帶地區的功能豐富度。本研究也顯示不同層面的功能多樣性隨環境梯度變化的異同，並強調探討溫帶和副熱帶地區功能多樣性差異的重要性。

Functional diversity (FD), the diversity of functionally disparate traits in a community, critically influences ecosystem functioning. FD is a multifaceted concept that includes several key aspects: functional dissimilarity, functional dispersion, functional richness, and functional evenness. While FD was reportedly affected by climate and vegetation productivity in temperate regions, the effects remain to be investigated in the subtropics and different aspects of FD are rarely studied within one community. To help fill the knowledge gaps, we examined how environmental factors (i.e., climate and vegetation productivity) affect different aspects of functional diversity in the subtropics by testing the two hypotheses: 1) Environmental filtering: higher temperature, higher precipitation, and lower CV (coefficient of variation) of precipitation (less harsh environments for functional traits) will lead to higher FD, and 2) Resource availability: higher productivity (a higher resource supplement) will lead to higher FD. Specifically, we calculated the four FD aspects of bird communities in subtropical Taiwan based on the Taiwan Breeding Bird Survey (BBS Taiwan) data (2009 – 2015) and five important bird functional traits. We then investigated the FD pattern along environmental gradients (temperature, precipitation, and vegetation productivity), using linear mixed-effects models. The results generally supported both hypotheses: less harsh environments and higher amount of resources generally led to higher functional diversity (higher functional dissimilarity, dispersion and richness). Although these patterns agreed with those in temperate regions, the subtropical communities in this study showed a quadratic functional richness – temperature relationship, different from the linear functional richness – temperature relationship in temperate regions. This suggests that warming may reduce functional richness more likely in the subtropics than temperate regions. The results also showed that the four functional diversity indices may express different patterns along environmental gradients, suggesting various underlying mechanisms and highlighting a need to consider different indices in functional diversity studies. Based on the results, we urge a more holistic investigation to understand the similarity and difference in functional diversity between temperate and subtropical regions.