Plant species dispersed by Galapagos tortoises surf the wave of habitat suitability under anthropogenic climate change

Native biodiversity on the Galapagos Archipelago is severely threatened by invasive alien species. On Santa Cruz Island, the abundance of introduced plant species is low in the arid lowlands of the Galapagos National Park, but increases with elevation into unprotected humid highlands. Two common alien plant species, guava (Psidium guajava) and passion fruit (Passiflora edulis) occur at higher elevations yet their seeds are dispersed into the lowlands by migrating Galapagos tortoises (Chelonoidis spp.). Tortoises transport large quantities of seeds over long distances into environments in which they have little or no chance of germination and survival under current climate conditions. However, climate change is projected to modify environmental conditions on Galapagos with unknown consequences for the distribution of native and introduced biodiversity. We quantified seed dispersal of guava and passion fruit in tortoise dung piles and the distribution of adult plants along two elevation gradients on Santa Cruz to assess current levels of ‘wasted’ seed dispersal. We computed species distribution models for both taxa under current and predicted future climate conditions. Assuming that tortoise migratory behaviour continues, current levels of “wasted” seed dispersal in lowlands were projected to decline dramatically in the future for guava but not for passion fruit. Tortoises will facilitate rapid range expansion for guava into lowland areas within the Galapagos National Park where this species is currently absent. Coupled with putative reduction in arid habitat for native species caused by climate change, tortoise driven guava invasion will pose a serious threat to local plant communities.

加拉帕戈斯群岛（Galapagos Archipelago）的本土生物多样性正遭受外来入侵物种的严重威胁。在圣克鲁兹岛（Santa Cruz Island），加拉帕戈斯国家公园（Galapagos National Park）的干旱低地中，外来植物的丰度较低，但随着海拔升高，在未受保护的湿润高地中其丰度逐渐升高。两种常见外来植物——番石榴（guava，Psidium guajava）和西番莲（passion fruit，Passiflora edulis）——分布于高海拔区域，但其种子会通过迁徙的加拉帕戈斯陆龟（Chelonoidis spp.）被扩散至低地。陆龟会将大量种子长距离运输至当前气候条件下几乎或完全无法萌发和存活的环境中。然而，预计气候变化将改变加拉帕戈斯群岛的环境状况，其对本土和外来生物多样性分布的影响尚不明确。我们对陆龟粪便团中的番石榴和西番莲种子扩散情况，以及圣克鲁兹岛两条海拔梯度上的成年植株分布进行了量化分析，以评估当前无效种子扩散的水平。我们针对这两个物种分别构建了当前及预测未来气候条件下的物种分布模型（Species Distribution Models）。假设陆龟的迁徙行为保持不变，预计未来低地的无效种子扩散水平将在番石榴种群中大幅下降，但西番莲种群不会出现这一变化。陆龟将推动番石榴快速向加拉帕戈斯国家公园内当前无该物种分布的低地区域扩张其分布范围。再加上气候变化可能导致本土物种的干旱生境缩减，陆龟介导的番石榴入侵将对当地植物群落构成严重威胁。