東亞遷徙線森林繁殖候鳥的族群變遷

Abstract

本篇博士論文聚焦於東亞遷徙線（East Asian Flyway, EAF）中的森林繁殖候鳥保育。EAF在全球的遷徙線之中被認為是鳥種多樣性最高同時也是受脅物種最多的候鳥遷徙線。同時，EAF橫跨不同緯度均以森林為主要生態系的特性，亦為其與其他遷徙線的關鍵生態特徵差異，並顯示了森林候鳥保育在這條遷徙線上的重要性。本研究主要探討兩個面向：在臺灣擁有已知全球最大的繁殖族群的全球受脅物種八色鳥（Pitta nympha），以及EAF森林繁殖候鳥的族群狀態及所遭受的威脅。主要研究目的是掌握EAF森林繁殖候鳥的族群趨勢、季節性的森林棲地偏好，以及造成族群減少的主要威脅因素。研究的核心是評估長期以來在EAF的假設與推論：究竟森林砍伐是不是造成EAF森林繁殖候鳥族群下降的主要原因，且主要威脅是來自繁殖地還是非繁殖地內。 本研究結合了在地尺度的族群數量調查、地區尺度的族群趨勢文獻回顧，以及遷徙線尺度的物種與棲地關係分析。研究的範疇包括了迄今為止最全面的東亞遷徙路徑上的族群趨勢整理。此外，本研究並建立一套棲地偏好的半定量的作法，描述不同的森林繁殖候鳥物種在繁殖季和非繁殖季的棲地偏好差異。本研究同時利用全球尺度的土地覆蓋資料，評估各鳥種在不同季節的分布範圍內的森林覆蓋變化，以及該變化對這些森林繁殖候鳥族群趨勢的影響。研究方法的目的為讓我們得以評估遷徙陸鳥在東亞遷徙路徑上的全年週期內，在繁殖季與非繁殖季兩個不同生活史階段面對森林砍伐的回應。 研究結果對於EAF上森林繁殖候鳥受森林變化影響的現況提供整體的評估與未來保育方向的指引。儘管名列國際自然保護聯盟（IUCN）紅皮書物種的八色鳥的族群趨勢令人擔憂，並且幾乎所有具有可分析的族群趨勢資料的森林繁殖候鳥物種的分布範圍內都有森林覆蓋減少的現象，但整體而言族群趨勢是增加和減少的物種比例大致相等。這顯示整體的森林繁殖候鳥群聚，即使在EAF熱帶地區高強度的森林砍伐下，目前並沒有經歷全面的大幅減少。此外本研究並發現，造成族群下降的主要驅動因素不能簡單歸因於繁殖地或非繁殖地內的森林砍伐。相對的，不同物種之間以及同物種在季節之間的森林棲地偏好的差異，則是解釋不同物種與不同地區間森林繁殖候鳥的族群趨勢差異的關鍵物種特性。 本研究結果對EAF上的遷徙性森林候鳥的保育策略具有關鍵意義。研究結果強調了在繁殖地對森林進行保育和復育，可能是減緩EAF森林繁殖候鳥族群趨勢減少的有效措施。而且即使在研究與保育的作為無法擴展至遭受高度森林砍伐的非繁殖地的情況下，這個策略很可能仍然至關重要。此外，本研究的發現有助於增進我們對EAF中鳥類遷徙生態學的認識，尤其支持了鳥類遷徙生態理論中的夏季限制假說。支持這個假說表示，未來的EAF森林繁殖候鳥的研究可再加強著重於與繁殖季節相關的族群結構變數，例如繁殖成功率。本研究更具體指出了EAF在洲大陸的區域的鳥類族群趨勢數據的缺乏。此外，候鳥的全年週期研究，特別是那些考察遷徙連結度的研究，對於理解遷徙線上不同國家與區域的保育作為的有效性將頗為關鍵。 本博士論文最後提出了一個未來跨區域合作和提高EAF森林繁殖候鳥的監測和保育研究效率的策略。考慮到本研究僅僅觸及影響EAF森林繁殖候鳥的族群動態的多種因素的一小部分，尚未探討的狩獵和氣候變遷等其他因素的研究都需要更多物種與更多區域的族群趨勢資料才能進行。本研究因此提出建立一套以EAF遷徙鳥類動態為目的的資訊基礎設施。該資訊基礎措施這應著重於收集、整合和以簡易視覺化的應用模式，並以追蹤研究和被動聲學監測技術的資料整合與運用為優先目標。同時。這樣的資訊基礎設施的開放共享作法對於解決當前EAF各區域與各鳥種研究的局限性以及為跨遷徙路徑的長期族群監測建立可靠的資料基礎至關重要。本博士論文作者相信這種方法為一保育森林繁殖候鳥所需的必要工具，更將是減緩EAF生物多樣性損失的重要一步。

This PhD dissertation centers on the conservation of migratory forest birds in the East Asian Flyway (EAF), a flyway distinguished by its high species diversity and high number of threatened species. Uniquely characterized by forests as the predominant ecosystem across various latitudes, the EAF also presents a vital area for avian conservation research. The study focuses on two key aspects: the globally threatened Fairy Pitta, which has its largest known breeding population in Taiwan, and the broader migratory forest bird community within the flyway. The aim is to bridge gaps in our understanding of population trends, habitat requirements across seasons, and drivers of trends of these birds. A central aspect of the research is evaluating the long-standing hypothesis that deforestation, whether in breeding or non-breeding ranges, is a primary driver of population decline in EAF''s migratory forest breeding birds. In this dissertation, a multifaceted approach was employed, integrating local-scale field observations, regional-scale literature reviews, and flyway-scale species-habitat relationship analyses. The research involved compiling the most comprehensive review to date of population trends across the EAF. Additionally, this study developed a semi-quantitative dataset scoring habitat preferences for both seasons of migratory landbirds, including breeding and non-breeding season. Utilizing remote sensing global land cover data, I assessed forest cover changes and analyzed their impact on these migratory forest birds’ population trends. This integrated methodology provided a holistic evaluation of deforestation effects on the various life stages of migratory landbirds within the EAF over their annual cycle. The key findings of this research provide valuable insights into the potential effect of habitat loss on migratory landbirds in the EAF. Despite the concerning population decline of the IUCN red-listed Fairy Pitta and widespread deforestation across the distribution ranges of nearly all assessed migratory forest bird species, the proportion of species with increasing and decreasing populations is roughly equal. This suggests that the overall community of migratory forest birds may not be undergoing a widespread decline. Moreover, the study indicates that the primary drivers of population decline cannot be solely attributed to deforestation in either the breeding or non-breeding ranges. Instead, the impact of habitat change appears more pronounced when considering the variation in forest habitat preference among different species and across seasons. These findings crucially update the population status of these migratory species in the EAF and, for the first time, investigate their drivers of decline across breeding regions while considering the influence of non-breeding ranges. The implications of these research findings are pivotal for conservation strategies. They underscore the importance of preserving and restoring forest habitats in breeding regions as a key measure to protect migratory bird species. This approach remains crucial even in scenarios where conservation efforts cannot extend to the non-breeding ranges that are experiencing high rates of deforestation. Furthermore, these findings contribute to the understanding of bird migration ecology in the EAF by supporting the summer-limit hypothesis for migratory species. This hypothesis suggests the importance of focusing future research on breeding season-related demographic variables, such as productivity. The research also identifies several critical information gaps. Notably, there is a significant lack of data on population trends in the continental regions of the EAF. Additionally, full annual cycle studies, especially those examining migration connectivity, are essential for comprehending how conservation efforts in various regions will contribute to the population dynamics of the EAF migratory forest birds. The dissertation concludes by outlining a strategy for future cross-regional collaboration and enhancing the efficiency of monitoring and research projects within the EAF. Recognizing that the current study only begins to uncover the complex drivers affecting the population dynamics of migratory landbirds in the EAF, and that significant factors such as hunting and climate change require further exploration, I advocate for the development of open-shared information infrastructures. These should focus on collating, integrating, and visualizing data from tracking and passive acoustic monitoring techniques. Implementing such a system is crucial not just for addressing the limitations of current research but also for establishing a robust foundation for long-term population monitoring across the flyway. Such data will also provide the ground stone of enriching the demographic data available for more comprehensive migration ecological studies. Ultimately, this approach is aimed at strengthening the necessary tools for the effective conservation of migratory landbirds, marking a vital step towards mitigating biodiversity loss in the EAF.