棲地改變與艾里效應對尼泊爾埋葬蟲海拔分布範圍之影響

Abstract

人類活動造成的棲地改變除了影響環境的非生物因子，也可能改變生物之間的互動，進而影響物種分佈或存續。在探討限制物種分佈的因子時，較多研究支持非生物因子，支持生物因子則較少，主要機制為種間競爭，種內的互動則鮮少被提及。種內互動可能的機制是艾里效應，其描述當族群密度下降時，族群內個體的適存度跟著下降的現象，理論上會因棲地改變影響族群密度而發生。然而鮮有研究討論棲地改變與物種分佈範圍的關係時，同時考慮上述三種因子。尼泊爾埋葬蟲（Nicrophorus nepalensis）在台灣的中部橫貫公路廣泛地分佈，而在中橫西部有大規模的開墾，東部則無，剛好適合我們測試棲地改變如何透過生物與非生物因子造成此分佈改變。我們提出了（1）微氣候條件假說：棲地改變影響微氣候因子而限制了物種的生理；（2）種間競爭假說：棲地改變提高競爭者競爭能力或增加競爭者數量而增加了種間競爭壓力；（3）艾里效應假說：棲地改變致使族群密度下降，影響物種在環境壓力下的表現，進而改變物種分佈。我們在兩地沿著海拔進行尼泊爾埋葬蟲密度調查與繁殖實驗，以繁殖成功定義分佈範圍。發現相同微氣候條件下，東部繁殖範圍較廣，同時東部種間競爭壓力較大，故拒絕了前兩項假說。我們發現東部族群密度較高，族群密度與繁殖群大小呈正相關，而在海拔較低處群大小又與成功率呈正相關。在兩區域沿海拔操作繁殖個體數以模擬高與低密度條件則發現，在海拔較低處，皆是放入6隻個體共同繁殖時較僅2隻時繁殖成功率高，且兩樣區沿著海拔操作密度的趨勢相同，證明造成兩區域埋葬蟲分佈與繁殖差異的關鍵因子在於族群密度，證實艾里效應假說。我們的研究顯示，棲地改變對埋葬蟲的分佈造成影響，並非直接由非生物因子或種間競爭造成，而是透過降低族群密度，使埋葬蟲溫度高競爭壓力較強的海拔無法對抗其競爭對手，限縮了埋葬蟲可繁殖的海拔下界。我們的結果闡明了種內生物因子也是棲地改變影響族群分佈不可或缺的機制，因此保育研究應當更全面地探討生物之間以及生物與環境的關係，方能有效避免生物多樣性持續快速消失。

Habitat alteration impacts the distribution of species and population dynamics through changing biotic or abiotic factors. Most studies have focused on the changes in abiotic factors, such as temperature, when considering the effect of habitat alteration on species distribution. The role of biotic factors, such as interspecific competition, remains largely unexplored. Furthermore, there are even fewer studies investigating the synergistic effects among abiotic, inter- and intraspecific factors on the distribution of species. Here, we used the burying beetle, Nicrophorus nepalensis, a widely distributed cooperatively breeding species, to study the ecological impact of habitat alteration on the distribution of species. Our study was conducted along two elevational gradients in the central cross-island highway of Taiwan, where the habitats on the west side are extensively altered but nature forests on the east side remain intact. Specifically, we test three alternative, but non-mutually exclusive, hypotheses: (1) “microclimate condition hypothesis”, which means that microclimate alteration in the habitat limits the reproduction of N. nepalensis; (2) “inter-specific competition hypothesis”, which argues that habitat alteration enhances the competitive ability of interspecific competitor, flies; and (3) the “Allee effect hypothesis”, which states that habitat alteration decreases the population density of N. nepalensis, which, in turn, impacts its competitive ability against the flies. Our results showed that, by comparing the microclimate data and group size of flies in natural forests along the two elevational gradients, we rejected the first two hypotheses because the microclimate conditions of the two elevational gradients were similar and the numbers of the flies on the east side were even larger than it was on the west side. Interestingly, we found that the group size was positively correlated with the population density of N. nepalensis, and the probability of successful breeding was positively correlated with the group size of N. nepalensis at the lower elevation. Also, we experimentally manipulated the population density by placing high density (three males and three females) and low density ( one male and one female) in the breeding boxes in the field at two gradients. We found that the probabilities of successful breeding were the same between the two gradients, where the high-density treatment had a higher successful rate than the low-density treatment, small group, supporting the Allee effect hypothesis. In conclusion, our experiment shows that habitat alteration negatively impacts the fitness of burying beetles and, thus, limits their elevational distribution through lowering their population density, which leads to Allee effect and weakens their interspecific competitive ability against flies. Our study, therefore, demonstrates that understanding the synergistic effect of biotic and abiotic factors is crucial for determining the ecological impacts of habitat alteration and conservation.