Demographic Outcomes and Ecosystem Implications of Giant Tortoise Reintroduction to Española Island, Galapagos

Restoration of extirpated species via captive breeding has typically relied on population viability as the primary criterion for evaluating success. This criterion is inadequate when species reintroduction is undertaken to restore ecological functions and interactions. Herein we report on the demographic and ecological outcomes of a five-decade-long population restoration program for a critically endangered species of “ecosystem engineer”: the endemic Española giant Galapagos tortoise (Chelonoidis hoodensis). Our analysis of complementary datasets on tortoise demography and movement, tortoise-plant interactions and Española Island’s vegetation history indicated that the repatriated tortoise population is secure from a strictly demographic perspective: about half of tortoises released on the island since 1975 were still alive in 2007, in situ reproduction is now significant, and future extinction risk is low with or without continued repatriation. Declining survival rates, somatic growth rates, and body condition of repatriates suggests, however, that resources for continued population growth are increasingly limited. Soil stable carbon isotope analyses indicated a pronounced shift toward woody plants in the recent history of the island’s plant community, likely a legacy of changes in competitive relations between woody and herbaceous plants induced by now-eradicated feral goats and prolonged absence of tortoises. Woody plants are of concern because they block tortoise movement and hinder recruitment of cactus–a critical resource for tortoises. Tortoises restrict themselves to remnant cactus patches and areas of low woody plant density in the center of the island despite an apparent capacity to colonize a far greater range, likely because of a lack of cactus elsewhere on the island. We conclude that ecosystem-level criteria for success of species reintroduction efforts take much longer to achieve than population-level criteria; moreover, reinstatement of endangered species as fully functioning ecosystem engineers may often require large-scale habitat restoration efforts in concert with population restoration.

通过圈养繁殖恢复局域灭绝物种，通常以种群生存力（population viability）作为评估项目成功的核心标准。但若开展物种再引入的目标是恢复生态功能与种间互作，则该标准便不再适用。本文报道了一项针对极危‘生态系统工程师（ecosystem engineer）’物种——特有种埃斯帕诺拉加拉帕戈斯象龟（Chelonoidis hoodensis）——的长达五十年的种群恢复项目的种群统计与生态结果。我们结合象龟种群统计与移动轨迹、象龟-植物互作以及埃斯帕诺拉岛植被历史的多组互补数据集开展分析，结果显示：从严格的种群统计视角来看，放归回归的象龟种群已趋于稳定——1975年以来该岛放归的象龟中约有半数在2007年仍存活，野外繁殖现已达到显著水平，无论是否持续开展放归行动，未来灭绝风险均较低。不过，放归个体的存活率、躯体生长率与身体状况均呈下降趋势，这表明支撑种群持续增长的资源正日益匮乏。土壤稳定碳同位素（soil stable carbon isotope）分析显示，该岛植物群落近期的组成已显著偏向木本植物，这一变化大概率源于已被根除的野化山羊以及象龟长期缺失所引发的木本与草本植物竞争关系改变，进而留下的生态遗留效应。木本植物的扩张令人担忧，因为它们会阻碍象龟移动，并阻碍仙人掌——象龟的关键食物资源——的自然更新。尽管该岛大部分区域理论上具备象龟定居的条件，但目前象龟仅集中分布于岛内中部残存的仙人掌斑块以及木本植物密度较低的区域，这很可能是因为岛内其他区域缺乏仙人掌资源。我们的研究表明，评估物种再引入项目成功与否的生态系统层面标准，所需的实现时长远长于种群层面标准；此外，将濒危物种恢复为具备完整功能的生态系统工程师，往往需要将大规模栖息地恢复工作与种群恢复措施相结合。