Probability of Persisting Re-introduced Population of Japanese Crested Ibis, Nipponia Nippon, for 50 Years on Sado Island, Niigata

27 Mar Probability of Persisting Re-introduced Population of Japanese Crested Ibis, Nipponia Nippon, for 50 Years on Sado Island, Niigata

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Abstract

トキの野生復帰個体群が50年間存続可能かどうかについて，中国の野生個体群から得られている個体群変数と佐渡島で利用可能な変数を用いて個体群存続可能性分析（PVA）を行なった．佐渡島においても，中国の野生個体群と同等の個体群変数が達成できれば個体群を維持し成長できるが，捕食圧等の繁殖を阻害する要因があると再導入個体群はいずれ絶滅する．また，野生個体群の巣立ち率が飼育下の孵化率と同等であれば，捕食がなくても個体群はいずれ絶滅する．そのため，再導入個体群を維持するには，個体群変数が中国の野生個体群と同等になるまで放鳥を継続する必要がある．PVA モデルを使って感度分析を行なった結果，環境変動や巣立ち率は再導入個体群の存続確率に影響を与え，生存率は個体群の成長率に大きな影響を与える．巣立ち率が閾値を超えている場合，生存率が向上することで，ある程度の捕食圧をカバーし個体群が存続できることがわかった．

English Abstract

The Japanese crested ibis (Nipponia nippon) used to be widespread in Japan, but the species was extinct in the wild in 1981. In order to re-establish a wild population, a reintroduction programme has been implemented on Sado Island since 2005. This programme aims to reintroduce 60 ibises to Japan by 2015. To date 10 ibises were hard-released in 2008, and 68 were soft-released between 2009 and 2011. In order to evaluate whether reintroduced population will persist for 50 years on Sado Island, population viability a lysis (PVA) was conducted by using population parameters of wild population in Chi and captive one in Japan. Re-introduced population will persist for 50 years if it has the same population parameters as Chinese ones exposed to less than 25% environmental fluctuation. After 2016, continuous releases of a minimum number of individuals will be necessary to maintain more than 60 ibises on Sado Island, if breeding success will not reach to Chinese level. To establish reintroduced population of the ibis, it is necessary to improve the breeding success and survival rate at first year.