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.

物种多样性纬度梯度的首要解释，必然落脚于物种为何无法在不同气候区之间拓展其分布范围。基于生态位保守性（niche conservatism）的物种梯度理论假定，整个演化支（clade）都会被限制在特定的气候区中，因为它们共有的性状限制了对其他气候区的适应能力。我们以喜马拉雅山脉从东南向西北鸟类物种丰富度下降一倍（即仅为东南侧的一半）这一现象为研究对象，对上述理论展开了检验。西北侧物种数量更少这一现象，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鸟类物种则呈现出相反的梯度格局。东南部中海拔区域（1000-2000米）的森林鸟类物种数量异常丰富，该区域拥有西北部不存在的温暖湿润气候，且这类物种中有很大比例无法将分布范围拓展至西北侧。尽管如此，仍有许多物种的种群或近缘类群沿海拔梯度或区域梯度跨越了不同的气候区，这表明基于气候的生态位保守性本身并不会对分布范围边界形成强烈限制。我们提出，由气候与竞争共同介导的资源分布格局，是决定物种北向分布边界的关键因素；同时研究表明，森林资源的一项衡量指标——叶片密度（foliage density）在西北部更低。