臺灣黑熊族群存續力分析

外文標題: 
Population viability analysis of Formosan black bears (Ursus thibetanus formosanus)
校院系所: 
屏東科技大學野生動物保育研究所
指導教授: 
黃美秀
出版年份: 
2013年
主題類別: 
摘要: 

物種滅絕為當今保育的重要課題,野外族群除了面臨人為威脅之外,同時面臨其他的潛在風險,包含族群統計隨機性、環境隨機性、自然災難又或是遺傳隨機性等。對於受威脅的族群來說,探究物種或族群的滅絕風險,並釐清關鍵因素,有助於適時地提出有效的經營管理方案,以提升族群未來的存續力。其中族群存續力分析(population viability analysis, PVA)為當今最有力且普遍應用的保育工具之一。
欲了解族群的存續力,則必須探討導致目標物種之所以造成族群衰退的原因。全球八種熊類,普遍受到的威脅即人類的過度利用,以及棲息地破壞(Garshelis, 2009)。臺灣黑熊雖為法定的瀕臨絕種野生動物,但同樣存在上述兩種的風險,而人為狩獵活動是目前威脅臺灣黑熊生存的重要因素(Hwang, 2003)。本研究旨在利用VORTEX模型,模擬不同的狩獵程度下,臺灣黑熊之族群存續力的表現,並評估關鍵的狩獵閾值,以及藉由敏感性分析探討影響結果之關鍵參數。有鑑於臺灣黑熊實地資料的缺乏,特別是現今全島族群量與承載量,藉由回顧相似物種的數據,予以評估、選擇出最適合應用的參數,以能呈現最貼近事實的結果。
藉由美國東南部美洲黑熊的族群密度推算現今族群量可能為100到700隻,承載量有5,000到10,000隻之多。從預測結果顯示,當沒有任何狩獵的威脅壓力時,無論初始族群量為多少隻,承載量為7,000隻的狀況下,族群數量皆有顯著的成長,於未來100年可達2,000隻以上。然而,面對實際的狩獵壓力時,即便在初始族群量是相對較大的數量下(600~700),於狩獵相當低的程度,就可導致族群成長趨勢的下降,並隨著狩獵程度的增加,更加劇族群趨向滅絕的命運。當以增加死亡率來模擬時,死亡率增加3%族群就開始呈現負成長,這表示對狩獵程度的容忍力僅能接受2%,換算成初期狩獵量,相等於有2~14隻,隨著初始族群量的增加可接受較多的個體死亡。然就收穫量的模擬之下,又更清楚呈現當初始族群量為100~300隻時,完全不能忍受每年5隻的收穫量,當初始族群量為400~500隻僅接受每年5隻的收穫量,而初始族群量為600~700隻可容許每年10隻的收穫量。藉由敏感性分析的檢測,確定雌性(亞成體與成體)死亡率,以及狩獵是最為影響臺灣黑熊未來的族群存續力,於未來應努力進行此兩方面的研究。
本研究所模擬的結果顯示,狩獵對臺灣黑熊族群未來存續有關鍵性的影響。然而,這些結果仍有可能是處於高估的狀態,因為本研究並未模擬棲息地破碎化、族群隔離效應等威脅的作用,所設定的假設條件與模型結構也是相對較為保守。面對現今全島族群量還未明朗之時,為確保族群的永續力,本研究認為實際上可能還無法容許少數個體的損失。因此,特別呼籲應加強非法狩獵取締等相關工作,同時也須多元發展其他的保育措施,方以達成終極臺灣黑熊族群永續的目標。

外文摘要: 

Extinction is an important issue in conservation. Species survival is threatened by human, as well as catastrophes, demographic, environmental, genetic stochastic events. Population viability analysis(PVA)provides a quantitative means for predicting the probability of extinction and for prioritizing conservation needs. This is one of the most powerful and pervasive tools in conservation biology.
To understand population viability, it is essential to know what has caused the population to decline. For bear populations two principal factors are direct human exploitation and habitat destruction. In Taiwan, poaching is one of the primary threats to Formosan black bears(Ursus thibetanus formosanus). In this study, a population viability analysis was used to evaluate the impact of hunting on Formosan black bear, and to estimate critical hunting levels that could be used as quasi-extinction thresholds for future population modeling. Furthermore, sensitivity analysis was conducted to know which model parameters are key factors that strongly determine population dynamics.
We applied the VORTEX(Version 9.99b) program where each scenario was simulated 1,000 times over 100 years. Assuming an initial population size of 100~700 and a carrying capacity of 7,000 bears, the results suggested that when there was no hunting, the population size continued to increase rapidly, with the final population size growing to more than 2,000 individuals after 100 years. But when simulating hunting by increasing the mortality rates to 3%, all initial population would be in decline. However, when we simulate the impact of hunting with harvest(kill/year), the probability of extinction within the next 100 years was over 10% for harvesting 15 bears every year. And results suggest that the female mortality rate and hunting rate have the most significant impact on population viability. To conduct a more extensive PVA, it requires a systematic monitoring program and more focused research.
In summary, hunting is a key factor that strongly determines the population dynamics of bears, regardless of the fact that other potential risks have not been taken into account. Therefore, proactive conservation actions to reduce illegal hunting will be critical for ensuring long-term bear population viability.