探討臺灣外來植物豐富度沿海拔變化的模式：物種溫度偏好或人類活動強度的影響？

Abstract

外來種指的是由人類帶入原生地以外地區的物種。世界各地的研究大多指出外來植物物種豐富度沿海拔上升而下降，然而其背後機制尚未明瞭，因此我們提出兩個假說來解釋這個模式，其一為物種溫度偏好假說，外來種物種豐富度與其在引入地的外來種物種庫大小相關，而外來種物種在其原生地的溫度偏好決定引入地的物種庫大小。另一為人類活動假說，我們假設外來種物種豐富度與人類活動的強度成正比，因為人類活動產生的拓殖體壓力以及干擾有助於外來種成功入侵引入地。我們使用全國外來入侵植物調查的樣區資料檢視外來植物物種豐富度沿海拔變化模式，並量化比較兩個假說對這個模式的貢獻，為物種溫度偏好假說提供更多的證據支持。 我們以現有的系統性調查樣區資料，使用標準化樣格內努力量的物種豐富度繪製臺灣外來植物物種豐富度沿海拔變化模式。為確保不同海拔梯度有足夠的樣本數分布，我們分析的範圍限制在路邊和農業用地這兩種棲地。使用潛在物種庫和樣格周邊的土地利用比例分別代表物種偏好假說與人為活動假說，對標準化物種豐富度進行多重線性迴歸和變異分配，比較兩假說的貢獻。在物種層級的分析中，我們比較每種外來植物其在原生地和臺灣的偏好溫度，將兩地之溫度差與物種特質(例:生物氣候起源、經濟用途)做迴歸分析，尋找在原生地及引入地較大溫度差的物種擁有的特質。 結果發現臺灣的外來植物物種豐富度沿海拔是單調遞減的模式。相較於人類活動假說，物種溫度偏好假說對此模式解釋較多的變量。外來種在其原生地與臺灣的物種偏好溫度成高度相關(r = 0.67)，本研究的396種植物中，有257種在兩地的溫度偏好相符。來自於溫帶和非園藝用途的外來種在臺灣傾向生長較原生地溫暖的地方。外來種在臺灣的分布上，大多是由低海拔地區至某個海拔，而分布在海拔較高的大都是溫度上的廣適應物種，可以在高或低溫生活，並來自溫帶地區。這些證據顯示臺灣的外來種物種豐富度沿海拔模式受物種原生地的溫度偏好影響。外來種由低海拔向上擴散，並逐漸根據外來種原生地的溫度偏好一一被溫度梯度過濾，導致單調遞減的外來種物種豐富度沿海拔變化模式。

Alien are those species that were brought from their native ranges to the introduced ranges by humans. Studies around the world described decreasing species richness-elevation patterns of alien plants. However, the mechanisms behind these patterns remain unclear. Hence, we proposed two hypotheses to explain this pattern. The first is the species preference temperature hypothesis, stating that alien species richness results from the size of the species pool determined by temperature preferences of these species in their native range. The second is the human activity hypothesis, assuming that the intensity of human activities, serving as the source of propagule pressure and disturbance, is positively related to alien species richness. We used the survey data from the Survey of Invasive Alien Plants in Taiwan (led by Dr. Shan-Hua Wu) to describe the species richness-elevation pattern in Taiwan, to quantify the relative importance of these two hypotheses, and to further find evidence for both hypotheses. For describing the alien species richness-elevation pattern in Taiwan, we used existing systematical sampling survey data and standardized sampling effort in each 1 km × 1 km cell by plot-based rarefaction. We confined the analysis to two habitat types, roadside and agricultural land, which have enough sample size and are distributed widely along the elevation gradient. For quantifying the relative importance of these two hypotheses, we used potential species pool size and the proportion of land-use in the surrounding area of cells to represent species temperature preference hypothesis and human activity hypothesis, respectively. Multiple linear regression with variation partitioning was performed on standardized species richness with explanatory variables which represent two hypotheses. In species-level analysis, we compared the preference temperature of individual species between Taiwan and their native ranges. We regressed temperature deviation between Taiwan and their native ranges on several species attributes, such as bioclimatic origin and economic usage, to identify a possible reason for the larger temperature deviation of some species. The results showed that the alien species richness-elevation pattern is monotonically decreasing in Taiwan. The species temperature preference hypothesis explained more variation in alien species richness than the human activity hypothesis. Species-level analyses showed that species preference temperatures in the native range and Taiwan are well correlated (r = 0.67), and species preference temperatures in Taiwan match their native range in the case of 257 out of 396 species. However, temperate-origin and non-ornamental alien species in Taiwan tend to occur in habitats warmer than in their native ranges. The elevation distribution of alien species in Taiwan is from lowland to certain elevations. Many alien species occurring in higher elevations are temperature generalists, adapting to both lower and higher temperatures, and most of them are of temperate origin. We conclude that alien species mainly spread from the lowland and are gradually being filtered out along the elevation gradient based on their native temperature preference (bioclimatic origin), resulting in the decreasing alien species richness-elevation pattern in Taiwan.