暖化下物種海拔分布變遷與物種內體型變化對尺蛾群聚體型結構的影響

Abstract

生態群聚中物種或個體的體型分布，又稱群聚體型結構(community body size structure)，是影響群聚食物網結構、動態、和穩定性的重要因子。群聚體型結構可能受到氣候暖化的影響，例如暖化可引起物種種內體型大小的變化(intraspecific body size change)以及物種分布範圍和物種組成的改變(range-shift-induced composition change)，進而改變原有的群聚體型結構。然而，我們對這些機制如何共同影響群聚體型結構所知甚少。本研究以馬來西亞婆羅洲神山(Mt. Kinabalu)的尺蛾(Geometrid moth)群聚作為研究系統(共396物種，海拔範圍1,450-3,675 m a.s.l.)，探究在四十二年(1965-2007)的區域性暖化下 (1) 尺蛾群聚體型結構(以物種間平均體型為指標)是否有改變、(2) 尺蛾物種的種內體型大小是否有變化、(3) 海拔分布範圍的變遷是否造成各海拔物種組成的改變、(4) 前兩者對於群聚體型結構是否造成影響，兩機制間孰強孰弱。結果顯示，在四十二年的暖化下，高海拔的尺蛾群聚體型結構有小型化的現象，造成此現象的原因並非是暖化改變了尺蛾的種內體型大小，而是暖化改變了物種的海拔分布範圍，進而改變了群聚的物種組成與體型結構。在高海拔區域，物種分布的變遷造成群聚平均體型下降，代表小物種/個體在群聚中所佔的比例顯著上升，這主要是由於往高海拔遷徙的物種(平均體型較小)在高海拔地點數量增加、以及往低海拔遷徙的物種(平均體型較大)在高海拔地點數量減少所造成。上述發現有助於進一步瞭解熱帶高山生物群聚在氣候變遷下的整體反應，同時也警示了氣候暖化對於生物體型的衝擊可能展現在不同的生態層級，即使暖化在物種層級的影響尚未顯著，仍可以在同樣的時間尺度下對群聚層級造成顯著的影響。

Community body size structure (e.g. the distribution or average of body size of different species) is fundamental in determining food web structure, dynamics and stability. This size structure across environmental gradients (e.g. altitude) can be affected by climate warming, because warming has reportedly led to changes in intraspecific body size or species range (affecting species composition), consequently shaping community body size structure. However, few studies examined both mechanisms together, and therefore little is known about whether and how intraspecific body size change and species range shift will collectively affect community body size structure under warming. We collected Geometrid moths in the year of 1965 and 2007 from Mt. Kinabalu (396 species from 1,450 to 3,675 m a.s.l.), which had experienced a 0.7oC warming during these 42 years. This study investigated 1) whether community body size structure (i.e. average interspecific body size) of Geometrid moths across altitude had shifted over the 42 years of warming, 2) whether intraspecific body size had changed under this warming, 3) whether species range shift had occurred, consequently changing species composition, and 4) what is the relative contribution of the two mechanisms above (intraspecific body size change and species range shift) to changes in community body size structure, if any. The results showed that species range shift, instead of intraspecific body size change, significantly contributed to the detected change in community body size structure. In specific, species range shift changed local species composition and reduced both average species body size and average individual body size at high altitude, based on either non-abundance-weighted or abundance-weighted analyses. This altitude-dependent pattern was driven mainly by an increase in the number and population size of upward-shifting species (relatively small body size) and a decrease in the population size of downward-shifting species (relatively large body size). This study is among the first ones to show that species range shift may precede intraspecific body size change in terms of shaping community body size structure under climate warming. In other words, this study provides a warning that we may not observe significant warming impact at lower ecological level (e.g. species-level body size), but warming may have caused significant impact at higher level (e.g. community level) during the same time period (e.g. 42 years in this study).