利用生態棲位模擬探討鄰域鳥種間之棲位分化

Abstract

物種競爭是決定物種分布範圍、群聚結構以及生態種化的關鍵因素之一。而競爭排除被認為是棲位相近的二物種，在地理上鄰域分布的主要原因。然而，要在自然的情況下證明競爭排除則相當困難。本研究以分布於亞洲的二近緣鳥種：棕頭鴉雀(Paradoxornis webbianus)以及灰喉鴉雀(Paradoxornis alphonsianus)，利用生態棲位模擬來預測其潛在分布。由現今的潛在分布顯示二物種在中國四川具有一潛在共域區，而二物種在此區域內的實際分布則為非隨機分布(宗頭鴉雀分布於東北方，而灰喉鴉雀位於西南)。棲位相同檢測的結果顯示二物種在此潛在共域區內佔有不同的氣候棲位。此外，判別分析的結果顯示，溫度變異最可能是導致二物種分化的棲位面向：棕頭鴉雀可能比灰喉鴉雀更能忍受氣溫的變動。模擬二物種在末次冰河最盛期(約二萬一千年)的潛在分布顯示，二物種自從二萬五千年至三萬年分化以來，皆維持一至的共域區。因此，二物種在現今潛在共域區內的空間劃分很可能是導因於競爭排除而不是近期的次級接觸。本研究的結果支持物種間的競爭能夠維持二近緣物種的鄰域分布。因此，由競爭以及棲位分化所導致的生態種化，很可能是造成近緣物種間(如棕頭鴉雀及灰喉鴉雀)快速分化的重要因素。

Competition is one of the key mechanisms determining species range limits, community structures, and ecological speciation. At geographic scale, competitive exclusion is often proposed to be a cause of parapatric distribution between species that have similar niche requirements. However, it is exceedingly difficult to demonstrate competitive exclusion in natural settings. In this study, I used ecological niche modeling to predict potential distributions of two closely-related avian species in Asia, Paradoxornis webbianus, and P. alphonsianus. The current-day distributions of the two species indicate that they share an area of potential sympatric zone in Southwestern China, within which both species exhibit non-random spatial distributions (P. webbianus in the northeastern region, and P. alphonsianus in the southwestern region). The niche identity test shows that the two species occupy different niches within the sympatric zone. Furthermore, the discriminant analysis points to temperature variability as the most likely niche dimensions along which P. webbianus and P. alphonsianus differentiate. Paradoxornis webbianus appears to be more tolerant of temperature fluctuations than P. alphonsianus. The potential distributions of the two species during the last glacial maximum (21,000 years ago) suggest that they have maintained a similar area of potential sympatry since their divergence approximately 30,000 to 25,000 years ago. Therefore, their spatial segregation within the current-day sympatric zone is more likely a result of competition than secondary contact. In conclusion, this study provided strong support for the role of competition in maintaining parapatric distribution between two recently-diverged species. Ecological speciation through competition and niche partitioning, therefore, might play a key role in the rapid divergence among closely-related species such as P. webbianus and P. alphonsianus.