小族群台北赤蛙(Rana taipehehsis)的族群動態與年齡結構之探討

Abstract

台北赤蛙（Rana taipehensis van Denburgh, 1909）是一種中小型的蛙類，在台灣的分佈地點目前僅剩下三芝、楊梅、官田及內埔，且族群量都很少。本研究於2003 ~ 2004年，針對三芝、楊梅和內埔地區的族群進行野外調查及年齡結構的分析。 由標記-再捕捉的資料得知：三芝和楊梅地區的台北赤蛙生殖族群性比都偏離1：1，且三芝族群的性比（16.13：1）約是楊梅（6.97：1）的2倍。三芝地區的族群在2003及2004年的推估數量分別為：347、203隻；楊梅地區則為：544、226隻。楊梅地區台北赤蛙在兩次調查間的存活率，依雄、雌蛙不同分別隨著時間而變動；當次調查的遭遇率則依雄、雌蛙不同分別隨著累積降雨量而變動。兩者均是雄蛙比雌蛙高。 藉由趾骨鑑齡得知：台北赤蛙不論雄雌最長壽命均是4歲，但雌蛙（2.58 ±0.09歲）顯著較雄蛙（2.30 ± 0.03歲）大。推測雄蛙在1歲時即可達性成熟並加入生殖族群，雌蛙在2歲時才達性成熟。分別建立2003、2004年的年齡結構後，可推算出跨年的存活率。三芝地區2 --> 3 歲台北赤蛙的存活率：雄蛙 — 0.33；雌蛙 — 0.22。3 -->4歲的存活率：雄蛙 — 0.02；雌蛙 — 0。楊梅地區2 --> 3歲台北赤蛙的存活率：雄蛙 — 0.23；雌蛙 — 0.68。3 --> 4歲的存活率：雄蛙 — 0.06；雌蛙 — 0.12。經Multiple Linear Regression分析的結果，台北赤蛙的吻肛長因年齡、性別、年份及地點而有顯著的不同。 利用VORTEX模擬台北赤蛙的滅絕機率：當雌蛙每年的平均子代數為34隻幼蛙時，楊梅地區的族群到未來第100年時預測的滅絕機率為0；而三芝地區的族群約在未來第26年就完全滅絕了。如將三芝地區的雌蛙數量比例提高約1倍，就可使族群在未來第100年時仍有64%的存活機率，可見適當的性比對台北赤蛙族群的存續是非常重要的。而當雌蛙每年的平均子代數大幅下降時，對族群的存續也有很大的影響。Abstract Taipei grass frog (Rana taipehensis van Denburgh, 1909) is a small to median size frog. In Taiwan, the distribution of Taipei grass frog is fragmented in Sanchih, Yangmei, Kuantien, and Neipu. The population size of each study site is very small. From 2003 to 2004, we investigated the population dynamics of Taipei grass frog in Sanchih and Yangmei. After obtaining frog’s age with skeletochronology, we constructed the population age structures of each population. The sex ratio of reproductive populations in Sanchih and Yangmei both are skewed to males. The ratio for male to female frogs from Sanchih (16.13：1) is almost as twice as the one in Yangmei (6.97：1). Estimated population size with Schnabel method in Sanchih, we got 347 frogs in 2003, and 203 in 2004. Yangmei population was 544 in 2003 and 226 in 2004. In Yangmei, the survival probability changed with time and males have higher survival probability. The encounter probability changed with cumulative precipitation before each surveys and males also have higher encounter probability. Both in males and females, the oldest frog is 4 years old with skeletochronology. The age of female frog (2.58 ± 0.09LAGs) is significantly older than male (2.30 ± 0.03LAGs). Based on the age structure, males reached to sexual mature and join to the breeding population within 1 year while females in 2 years. The survival rates between age 2 to 3 in Sanchih are: male - 0.33, female - 0.22. The survival rates between age 3 to 4 are: male - 0.02, female - 0. In Yangmei, survival rates are male - 0.23, female - 0.68 between age 2 - 3 and male -0.06, female - 0.12 between age 3 - 4. With Multiple Linear Regression analysis, the result indicated that SVL is significantly different by age, sex, year, and place. With VORTEX, we simulated the extinction probability of Yangmei population in the 100th year starting from now is 0, when the offspring of a female reproduced is 34.

While in Sanchih will extinct in the 26th year. By doubling the female percentage, the population in Sanchih will have 64% survival probability in the 100th year starting from now. This indicates that sex ratio plays a major role in the persistence of Taipei grass frog. Mean offspring of a female reproduced may also have serious effect, when it dropped sharply.