赤背條鼠與臺灣森鼠間溫度棲位區隔與海拔分佈之研究

Abstract

環境溫度是影響物種分佈範圍的重要限制因子之一。臺灣小型哺乳動物在地理分佈上有明顯的海拔區隔。以姬鼠屬(Apodemus)的兩個物種為例，赤背條鼠(Apodemus agrarius)主要分佈在低海拔，臺灣森鼠(A. semotus)主要分佈在中高海拔。由於環境溫度與海拔梯度關係緊密，生物的海拔分佈可能是反映其對溫度的容忍度。且根據前人研究，兩物種具有溫度生理上的差異；赤背條鼠較不耐低溫，而臺灣森鼠在高溫時的散熱能力較差。因此本研究以赤背條鼠及臺灣森鼠為材料，探討溫度是否為造成近緣小型哺乳動物物種海拔分佈區隔的原因。本研究利用操控溫度試驗觀察兩物種在低溫(15oC-21oC)、中溫(21oC-27oC)及高溫(27oC-33oC)下的體重變化，建立兩物種的基礎溫度棲位，並建立兩物種的潛在分佈；同時，以生態棲位模擬（Ecological niche modeling, ENM）預測赤背條鼠與臺灣森鼠的潛在分佈，擷取潛在分佈範圍內的氣溫數值以建立兩物種之實際溫度棲位，將結果相互比較，以檢視兩物種的海拔分佈是否受其自身基礎溫度棲位所限制。溫度試驗的結果顯示赤背條鼠於低溫環境下體重下降，而臺灣森鼠在三個溫度處理下體重變化並無差異，以溫度試驗結果建立的潛在分佈預測圖，赤背條鼠的潛在分佈侷限於台灣低海拔地區，與生態棲位模擬所建立的潛在分佈範圍大致相符；臺灣森鼠以溫度試驗結果所建立的潛在分佈包含中低海拔地區，與生態棲位模擬所預測於中高海拔範圍的結果並不相符。赤背條鼠的基礎溫度棲位與實際溫度棲位大致相符，顯示赤背條鼠的分佈主要受其自身溫度棲位所限制，而臺灣森鼠則不相符，顯示其分佈可能是受到溫度以外的其他因子進一步限制。

Ambient temperature is one of the key mechanisms determining species range limits. Small mammals in Taiwan are known for their distinct elevational distribution. For example, of the two Apodemus species, A. agrarius distributes at lower elevation and A. semotus distributes at higher elevation. Given the close relationship between temperature and elevation, species’ elevational distribution may reflect their thermal niche. Previous studies showed that the two Apodemus species are different in their thermal physiology in that A. agrarius has lower tolerance for low temperature whereas A. semotus has more difficulties to dissipate their body heat in high temperature. In this study, I used the pair of A. agrarius and A. semotus as a model system to investigate if thermal niche influences elevational distribution of closely related small mammal species. I used laboratory experiments to establish their thermal niche by subjecting them to low (15oC-21oC), intermediate (21oC-27oC) or high (27oC-33oC) temperature environment, and monitoring their body condition. I predicted their geographic distribution based on the fundamental thermal niche established in the laboratory. In addition, I used ecological niche modeling to predict habitat suitability of these two species, which was then used to extract temperature data that represent their realized thermal niche. The laboratory experiment showed that A. agrarius lost weight under low temperature treatment whereas A. semotus were not influenced by temperature treatments. For A. agrarius, the laboratory experiment and ecological niche model showed consistent patterns in predicted distribution and thermal niche, suggesting this species’ geographic distribution is likely limited by its own thermal niche. In contrast, according to the fundamental thermal niche from the current laboratory experiment as well as a previous experiment, A. semotus should be widely distributed across the entire Taiwan, which was inconsistent with the predicted distribution from the ecological niche model. For A. agrarius, the thermal niches established by extracting temperature data of the predicted distribution were similar whether the prediction distribution was based on the laboratory experiment or ecological niche model. In contrast, the thermal niches based on the laboratory experiment and ecological niche model were dissimilar for A. semotus. Therefore, this study suggests that the geographic distribution of A. agrarius is primarily restricted by its fundamental thermal niche, but that of A. semotus may be further restricted by factors other than temperature.