Data from: Determinants of northerly range limits along the Himalayan bird diversity gradient

The first-order explanation for the latitudinal gradient in species diversity must lie in why species fail to expand ranges across different climatic regimes. Theories of species gradients based in niche conservatism assume that whole clades are confined to particular climatic regimes, because the traits they share limit adaptation to alternative regimes. We assess these theories in an analysis of the twofold decline in bird species richness along the Himalayas from the southeast to the northwest. The presence of fewer species in the northwest is entirely due to a steep decline in the number of forest species; species occupying more open habitats show a reversed gradient. Forest species numbers are exceptionally high at mid-elevations (1,000-2,000m) in the southeast, which experience a warm wet climate not present in the northwest, and a high proportion of these species fail to expand their range to the northwest. Despite this, many species do have populations or close relatives which straddle different climatic regimes, along altitudinal gradients and/or the regional gradient, implying that climate-based niche conservatism per se does not strongly constrain range limits. We argue that climate- and competition-mediated resource distributions are important in setting northerly range limits and show that one measure of forest resources (foliage density) is lower in the northwest.

对物种多样性纬度梯度的首要解释，必然聚焦于物种为何无法跨越不同气候域拓展分布范围这一核心问题。基于生态位保守性（niche conservatism）的物种梯度理论假定，整个演化支都会被局限于特定的气候域，因为它们共有的性状限制了其对其他气候域的适应能力。我们通过分析喜马拉雅山脉从东南向西北的鸟类物种丰富度下降一倍的现象，对上述理论展开评估。西北部的物种丰度更低，完全源于森林鸟类物种数的急剧下降；而栖息于更开阔生境的物种则呈现出相反的梯度格局。东南部海拔1000-2000米的中海拔区域气候温暖湿润，这一气候条件在西北部完全不存在，该区域的森林鸟类物种数量异常丰富，且其中极高比例的物种都未能将分布范围拓展至西北部。尽管如此，仍有许多物种的种群或近缘类群沿着海拔梯度和/或区域梯度跨越了不同的气候域，这意味着基于气候的生态位保守性本身并不会对分布范围上限形成强烈限制。我们认为，由气候和竞争共同介导的资源分布格局，是决定物种北部分布上限的关键因素；同时我们的研究还表明，森林资源的一项衡量指标——叶片密度（foliage density）——在西北部更低。