Master thesis at the Institute of Ecology and Evolutionary Biology, National Taiwan University (March 2023)
Author: Wu Yi-Mei (吳奕媺)
Advisor: David Zelený
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.
Keywords: agriculture, alien plant species, bioclimatic origin, human activity, introduced range, land-use, native range, roadside, species richness-elevation pattern, species temperature preference, Survey of Invasive Alien Plants in Taiwan, variation partitioning
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。