Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA.
Am Nat. 2011 Oct;178 Suppl 1:S97-108. doi: 10.1086/661926. Epub 2011 Aug 23.
The primary 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 midelevations (1,000-2,000 m) 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 that 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.
物种多样性的纬度梯度的主要解释必须在于为什么物种无法跨越不同气候区系扩展其分布范围。基于生态位保守主义的物种梯度理论假设整个进化枝都局限于特定的气候区系，因为它们所共有的特征限制了对替代区系的适应。我们在对喜马拉雅山脉从东南到西北的鸟类物种丰富度的两倍下降的分析中评估了这些理论。西北物种较少完全是由于森林物种数量急剧减少所致；占据更开阔栖息地的物种则呈现出相反的梯度。在东南地区（海拔 1000-2000 米），森林物种的数量异常高，那里气候温暖湿润，而西北则没有这种气候，这些物种中有很大一部分无法将其分布范围扩展到西北。尽管如此，许多物种确实有种群或近亲，它们在沿海拔梯度和/或区域梯度的不同气候区系之间跨越，这意味着基于气候的生态位保守主义本身并没有强烈限制范围极限。我们认为，气候和竞争介导的资源分布对于确定北方的范围极限很重要，并表明森林资源的一个衡量标准（叶密度）在西北较低。
