環境因子與競爭對台灣無尾目群集的影響

Abstract

群集組成受到決定性機制（如環境篩選與競爭）以及隨機性機制（如擴散與生態漂變）的共同影響。理解這些機制的相對貢獻，對於預測生物多樣性如何回應環境變遷至關重要。由於蛙類具有複雜的生活史，並依賴同時依賴水域與陸域環境，因此對環境變化特別敏感。儘管在熱帶與亞熱帶地區，群集組成機制受到越來越多的關注，但對於島嶼（如臺灣）中這些機制（如環境篩選及競爭）的運作方式及其在時間尺度下的變化，目前仍所知有限。本研究使用臺灣兩棲類資料庫中自2005年至2021年間，於3,534個樣區進行的標準化調查資料，探討臺灣蛙類群集組成的機制及其時空變化。我們運用典範對應分析（Canonical Correspondence Analysis, CCA）評估物種與環境因子的關係，並進行共現分析（co-occurrence analysis）以探討物種間的共域關係（如潛在的競爭）。分析分別針對整體期間（2005–2021）與兩個五年子時期（2005–2009與2017–2021）進行，以揭示時間變化趨勢。CCA結果顯示，溫度、濕度與微棲地結構是影響物種組成的主要環境因子，支持環境篩選為重要機制。然而，這些變數的解釋力隨時間而下降：2005–2009年的總解釋變異為36.93%，而2017–2021年為17.31%，整體期間僅為9.91%。這可能反映出環境擾動增加、棲地同質化，或近期資料中噪音增多等問題。部分物種在不同時期維持穩定的環境關聯，例如斯文豪氏赤蛙固定出現在溪流環境，而長腳赤蛙偏好低溫環境；另一些物種，如黑眶蟾蜍與莫氏樹蛙，則顯示其環境關聯性隨時間出現變化，可能與環境變遷有關。我們亦檢測到環境條件的時序變化：2005–2009年間樣區層級的平均溫度為24.66°C，至2017–2021年間下降了1.37°C（具統計顯著性），而相對濕度則顯著上升。儘管微棲地類別的變化主要反映分類系統的調整，這些氣候變化仍可能影響物種與環境之間的關係。共現分析結果顯示，正向物種共域關係（物種同時出現）在所有時期皆較為常見，佔整體與近期期間中物種配對的60–65%；而早期（2005–2009）則有較高比例的隨機關係（43%），顯示該時期群集結構較鬆散。拉都希氏赤蛙、腹斑蛙與布氏樹蛙等物種在各時期皆與多數其他物種呈現正向共現，而莫氏樹蛙與梭德氏赤蛙則常與其他物種呈現負向共現，可能反映出棲位差異或競爭排除。整體而言，我們的研究結果強調了：1) 環境篩選與正向的物種共域關係（而非許多先前研究所指出的競爭）所扮演的角色；2) 無尾目群集組成的時間變化； 3) 環境變化對物種與環境的關係及物種間共域關係的影響。本研究凸顯了長期生態資料對於偵測群集層級受環境改變的重要性，並透過強調保護棲地異質性與監測物種動態變化的必要性，為未來在氣候與土地利用持續變遷下的保育工作提供參考方向。

Community assembly is shaped by both deterministic processes, such as environmental filtering and competition, and stochastic forces like dispersal and ecological drift. Understanding the relative contributions of these processes is essential for predicting biodiversity responses to environmental change. Anurans are particularly sensitive to such changes due to their complex life cycles and dependence on both aquatic and terrestrial habitats. However, despite increasing recognition of community assembly mechanisms in tropical and subtropical ecosystems, relatively little is known about how these processes (e.g., environmental filtering and competition) operate in island systems such as Taiwan, and how they change over time. In this study, we investigated anuran community assembly mechanisms in Taiwan and its temporal change using a standardized dataset from the Taiwan Amphibian Database, covering 3,534 survey sites between 2005 and 2021. We applied Canonical Correspondence Analysis (CCA) to evaluate species–environment relationships and co-occurrence analysis to explore species–species co-occurrence associations (e.g., potential competition). Analyses were conducted for the entire study period (2005–2021) and two five-year sub-periods (2005–2009 and 2017–2021) to examine temporal changes. The CCA results indicated that temperature, humidity, and microhabitat structure were key environmental variables shaping species composition, supporting the role of environmental filtering. However, the explanatory power of environmental variables declined over time, with the 2005–2009 period explaining 36.93% of variation in species distribution, compared to 17.31% in 2017–2021 and only 9.91% in the full dataset. This decline may reflect increasing environmental disturbance, homogenization of habitat conditions, or noise in more recent data. Some species showed consistent environmental associations across time periods, such as Odorrana swinhoana with running water and Rana longicrus with low temperatures. Others, like Duttaphrynus melanostictus and Zhangixalus moltrechti, shifted their associations over time, possibly in response to changing environmental conditions. Temporal changes in environmental conditions were also detected. Between 2005–2009 and 2017–2021, site-level mean temperature decreased significantly by 1.37°C, while relative humidity increased slightly but significantly. While the change in microhabitat composition was largely attributed to classification system updates, these environmental changes likely contributed to shifts in species–environment relationships. Co-occurrence analysis revealed that positive associations between species co-occurrence were consistently more common than negative ones, accounting for 60–65% of species pairs in the full and recent periods. In contrast, the earlier period (2005–2009) had a higher proportion of random associations (43%), suggesting a less structured community. Species like H. latouchii, N. adenopleura, and P. braueri consistently exhibited strong positive co-occurrence patterns, while species such as Z. moltrechti and R. sauteri showed high levels of negative co-occurrence, potentially reflecting niche differentiation or competitive exclusion. Overall, our findings highlight 1) the role of environmental filtering and positive species co-occurrence associations—rather than competition, as suggested by many previous studies, 2) dynamic shifts in anuran community composition over time, and 3) the effect of environmental change on species–environment associations and co-occurrence relationships. The study underscores the importance of long-term ecological data for detecting community-level responses and informs future conservation efforts by emphasizing the need to preserve habitat heterogeneity and monitor shifting species dynamics under ongoing climate and land-use change.