屏東縣綠鬣蜥(Iguana iguana)主要入侵範圍族群監測:密度與體型變化

綠鬣蜥原產於中南美洲，曾透過寵物貿易、非法交易等途徑大量進入臺灣寵物市場；然而市場規模有限與飼養成本隨體型增長大幅提升，導致低價購入及滯銷個體被棄養的機率大幅增加；且入侵初期並未及時評估入侵風險、儘早研擬並執行移除防制策略，現已於臺灣西南部平原及部分零星地點建立野化族群，其族群現況與族群監測應立即著手進行，藉此規劃調整移除策略與評估控制成效。本研究於屏東縣隘寮溪排水系統及其周圍水道、萬丹大排及牛稠溪水道主要族群分布範圍進行監測調查，利用point transect方法各選定59及52處樣點，以Distance Sampling估算族群密度並分析變動趨勢，並評估作為監測野化族群方法之可行性；同時利用2014年至2022年所捕獲綠鬣蜥平均吻肛長變化，評估族群數量增長變化趨勢。竹田及萬丹樣區捕獲綠鬣蜥平均吻肛長變化並不一致，竹田樣區公母蜥平均體型變化於2019年開始有明顯下降，之後便未出現明顯差異；而萬丹公母蜥平均體型變化於2018年後有明顯下降，但在2021年開始上升後隔年再度下降。野化綠鬣蜥體型組成變化是評估移除成效指標之一；民眾或政府委託的廠商選擇性捕捉較容易捕獲之幼蜥、亞成蜥，控制族群增長成效有限，優先移除繁殖潛力高之成蜥對抑制野化族群數量增長有幫助。密度監測結果顯示將距離資料右截斷(right truncation) 5%離群值及樣點的選擇儘量包括不同棲地類型，能有效提高偵測率(detection probability)，密度估算的結果也能更精確。於隘寮溪排水樣區2021年3月到11月的密度估算值在20.7-32.4隻/公頃間變動，於2021年12月密度快速下降，達最低值7.3隻/公頃，然而12月過後密度快速回升，在2022年3月出現最高值46.7隻/公頃，整體密度變動趨勢不減反增的；於萬丹大排及牛稠溪樣區密度則呈雙高峰，分別於2021年1-2月及2022年4月(25.8隻/公頃及28.6隻/公頃)，其餘月份密度在9.7-19.2隻/公頃間變動，整體密度雖有緩降的趨勢但還是有突然回升的情形。本研究結果顯示密度並未出現明顯下降，於隘寮溪排水樣區甚至出現持續上升。在利用經濟誘因鼓勵以捕獲數量換取更多獎勵或利益的控制策略，未能配合其他控制方法，對於野外綠鬣蜥族群數量的控制未必有效，在經濟誘因不足時移除成效受到限制，族群密度甚至會在短時間內回升；利用有限資源與人力、配合族群監測結果調整移除策略，應是主管機關需考量的控制機制。

關鍵字

綠鬣蜥；入侵物種； Distance Sampling

並列摘要

Green iguanas (Iguana iguana), native to Central and South America, have been massively imported to Taiwan for the demand of pet market through pet trade and illegal trafficking. However, the pet market size was small and cost of owning pet iguanas increased significantly with the growth of body size, resulting in iguanas being abandoned more frequently. Moreover, the risks of iguana invasion has not been seriously and we have not responsed promptly in the early stage of invasion, feral green iguana population have established in southwestern Taiwan successfully. An effective monitoring program should be conducted for the evaluation of removal plan effectiveness and feral population status. In this study, I have conducted a monitoring survey for the density of feral green iguanas at Ailiao Stream drainage system, Wandan drainage system and Niuchou Stream, the mainly invasive areas in Pingtung, using point transect method and Distance Sampling. Additionally, the changes of body size composition (snout-vent length, SVL) in removed green iguanas from 2014 to 2022 were evaluated for the effectiveness of eradication programs. The trends in average SVL of removed iguanas in Jutain and Wandan were inconsistent. In Jutain, the average SVL of females and males decreased significantly in 2019, but then kept relatively stable in the following years. In Wandan, the average SVL of females and males decreased in 2018, but rebounded significantly in 2021 and the dropped again in 2022. The consistently decrease in the body size of removed iguanas will be expected if the eradication programs are really effectively. Quantity-based bounty or eradication program may encourage locals or contracted hunters to removed small-sized juveniles and subadults to pursue financial rewards effectively, the highly size-biased results might reduce the effectiveness in the management of the growing iguana population, as the larger adults have the higher reproductive potential then smaller ones. In my results of density estimation showed that distance data with 5% right truncation and survey areas included more diverse habitat types would lead to higher detection probability and more precise density estimates than the data without truncation and survey areas with simple habitats types. From March to November 2021, the estimates of density in Ailiao Stream drainage ranged from 20.7 to 32.4 individuals per hectare, then dropped rapidly in December 2021, to the lowest density of 7.3 individuals per hectare, but soon rebounded rapidly in March 2022, reaching the highest density of 46.7 individuals per hectare. Overall, the trends in density estimates remained increasing. In Wandan drainage system and Niuchou Stream, the density estimates showed a bi-modal pattern, dropped in January-February 2021 and April 2022 respectively (25.8 and 28.6 individuals per hectare). In the other months, the density ranged from 9.7 to 19.2 individuals per hectare. Although the overall trends in density estimates decreased slowly but fluctuated and rebounded seasonally. In my results of feral green iguana monitoring by using Distance Sampling, the density of iguanas in the main invaded areas in Pingtung has not decreased significantly as anticipated after conducted extensively control program; the estimates increased in the Ailiao Stream drainage. According the results of density estimates and body size composition of removed animals, the effectiveness of current control programs were uncertain. With the limit of available resources, it is strongly suggested that the responsible agencies have to adjust the eradication and management strategies flexibly based on the results of reliable monitoring program.

並列關鍵字

green iguana ； invasive species ； Distance Sampling

參考文獻

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