The tropical alpine ecosystem in eastern Africa is highly fragmented among biological ‘sky islands’, where populations of frost-tolerant organisms are isolated from each other by a ‘sea’ of tropical lowlands. One-third of the species in the afroalpine flora are exclusively alpine, but the other species can to varying degrees extend into grasslands and open forests of lower vegetation belts. A long-debated question is whether colonization of the alpine zone of these mountains and subsequent intermountain gene flow entirely depend on long-distance dispersal across unsuitable habitats, or whether suitable habitats shifted far enough downslope under past colder climates to form bridges enabling gradual migration. Here we address this question using a classification tree model. We mapped the extent of the current alpine habitat and projected it to the last glacial maximum (LGM) climate to assess whether gradual migration was possible for exclusively alpine taxa during this glacial period, and thus potentially also during earlier Pleistocene glaciations. Next, we modelled landcover under current and LGM climates to assess whether grassland and open forests could have served as migration corridors for alpine taxa that today extend into lower vegetation belts. We estimated that the LGM treeline was about 1000 m lower and the alpine habitat was about eight times larger than that today. At the LGM, we found that most of the currently fragmented alpine habitat of the Ethiopian highlands was interconnected except across the Great Rift Valley, whereas the solitary mountains of East/Central Africa remained isolated for exclusively alpine species. However, for drought-tolerant alpine species that today extend below the treeline, gradual migration through habitat corridors may have been possible among mountains during the dry glacial periods, and possibly also under the current climate before agriculture transformed the low-lying landscapes.

中文翻译：

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东部非洲“天空诸岛”中高山植物的迁徙走廊：它们存在还是存在？

东部非洲的热带高山生态系统在生物“天空岛”中高度分散，那里的耐霜冻生物种群被热带低地的“海洋”相互隔离。在非洲高山植物群中，只有三分之一是高山植物，但是其他物种可以不同程度地扩展到草木和低植被带的开阔森林中。一个长期争论的问题是，这些山脉的高山地带的定居和随后的山间基因流动是否完全取决于在不合适的生境中的长距离扩散，还是合适的生境在过去的较冷气候下是否向下坡移动了足够远的距离，从而形成了能够逐步迁移的桥梁。在这里，我们使用分类树模型解决这个问题。我们绘制了当前高山栖息地的范围，并将其投影到最后的冰川最大气候（LGM），以评估在这个冰川时期是否可能仅对高山分类群进行逐步迁移，因此也可能在较早的更新世冰川时期进行。接下来，我们对当前和LGM气候下的土地覆盖物进行建模，以评估草地和开阔的森林是否可以作为今天延伸到较低植被带的高山生物群的迁移通道。我们估计，LGM树线大约比现在低1000 m，而高山栖息地大约是它的八倍。在LGM上，我们发现埃塞俄比亚高地目前大部分零散的高山栖息地都相互关联，除了穿越东非大裂谷，而东非/中非的孤山仍然只为高山物种而孤立。但是，对于今天延伸到林线以下的耐旱高山物种而言，在干旱冰川时期，甚至在当前气候下，在农业转变低洼地貌之前，可能有可能在山区之间逐渐通过栖息地走廊迁移。