The altitudinal distribution of species richness is affected by climate and habitat changes, and the balance between these drivers will produce an idiosyncratic pattern in each mountain range. However, the potential effect of habitat change has rarely been included in studies designed to detect the effect of climate change on mountain biodiversity. This paper explores the changes in the altitudinal distribution of forest bird richness in the Guadarrama Mountains (600–2400 m a.s.l., Central Spain) over the last several decades. These mountains are affected by global warming and increased tree density resulting from rural abandonment. Nothing is known, however, about the way these changes have affected the altitudinal distribution of bird richness. Bird counts carried out in 1976–1980 were repeated in 2014–2015 along an altitudinal succession of forest belts. The results show that the relationship of bird richness to elevation shifted from a hump-shaped model typical of dry mountains to a monotonic negative model characteristic of humid mountains. These trends diverged from predictions on the effects of climate warming and were related to a loss of bird richness in endemic Scots pinewoods in the upper parts of the mountains. Variations in the composition of bird assemblages suggested that these pinewoods have experienced a process of tree densification and understory loss. As a result, the reshuffling of the altitudinal distribution of bird richness produced by habitat changes has eclipsed any effects of climate warming. These results emphasize the importance of considering habitat and climate interactions when exploring the altitudinal shift of species richness in the context of global change.

中文翻译：

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地中海山脉鸟类丰富度的长期高度变化：栖息地的变化解释了这种趋势

物种丰富度的高度分布受气候和栖息地变化的影响，这些驱动力之间的平衡将在每个山脉中产生特有的格局。但是，旨在检测气候变化对山区生物多样性影响的研究很少包括栖息地变化的潜在影响。本文探讨了过去数十年来瓜达拉马山脉（西班牙中部600–2400 m asl）的森林鸟类丰富度的高度分布变化。这些山脉受到全球变暖和农村遗弃导致树木密度增加的影响。但是，关于这些变化影响鸟类丰富度的垂直分布的方式一无所知。1976-1980年进行的鸟类计数在2014-2015年沿森林带的高度演替重复进行。结果表明，鸟类丰富度与海拔的关系从旱山典型的驼峰状模型转变为湿山特征的单调负模型。这些趋势与对气候变暖的影响的预测背道而驰，并且与山区上部的苏格兰特产松林中鸟类的丰富度丧失有关。鸟类组合物组成的变化表明，这些松木经历了树木致密化和林下损失的过程。结果，栖息地变化所产生的鸟类丰富度的高度分布的重新改组掩盖了气候变暖的任何影响。