The increase in species number from poles to the equator is one of the most fundamental patterns in ecology. Although several hypotheses have been proposed, there is a lack of consensus on the mechanisms underlying this pattern. While most hypotheses provide plausible explanation for high tropical diversity of tropical clades, it is unclear if similar mechanisms drive the diversity of extra-tropical clades. Here, we investigated the environmental drivers influencing the diversity pattern of a Mediterranean plant genus Medicago and dissect their effects across continents and biomes. We compiled a comprehensive dataset on the distribution of all Medicago species and mapped their distribution at the spatial resolution of 100 × 100 km². We used generalized linear models to quantify the relative effects of environmental factors on the richness patterns of Medicago and its two life forms. Next, using geographically weighted regression, we explored the variation in richness–environment relationship across latitude. We found that Quaternary climate change and environmental energy were important explanatory variables, and their effects were consistent at global, continental and biome scales. However, we found contrasting effect of environmental energy in temperate and tropical regions, with species richness increasing with energy in the temperate and decreasing in the tropics. We also found strong affinity of Medicago species to its ancestral Mediterranean climatic niche. Our results suggest that the spatial variation in Medicago richness patterns may be strongly influenced by heat tolerance as opposed to the tropical clades that depend upon cold tolerance. This dependency on heat tolerance can be attributed to strong niche conservatism exhibited by Medicago species. Our findings provide new insights into the richness-energy hypothesis and suggest that high environmental energy may not necessarily yield high species diversity but may also lower species diversity, especially of extra-tropical clades towards the tropics.

中文翻译：

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利基保守主义限制了紫花苜蓿在热带地区的分布

从两极到赤道物种数量的增加是生态学中最基本的模式之一。尽管已经提出了几个假设，但对于这种模式背后的机制缺乏共识。虽然大多数假设为热带分支的高热带多样性提供了合理的解释，但尚不清楚类似的机制是否推动了热带分支的多样性。在这里，我们调查了影响地中海植物属紫花苜蓿多样性模式的环境驱动因素，并剖析了它们对大陆和生物群落的影响。我们编制了一个关于所有紫花苜蓿物种分布的综合数据集，并以 100 × 100 km ²的空间分辨率绘制了它们的分布图. 我们使用广义线性模型来量化环境因素对紫花苜蓿及其两种生命形式的丰富度模式的相对影响。接下来，使用地理加权回归，我们探索了跨纬度的丰富度-环境关系的变化。我们发现第四纪气候变化和环境能量是重要的解释变量，它们的影响在全球、大陆和生物群落尺度上是一致的。然而，我们发现温带和热带地区环境能量的对比效应，物种丰富度随着温带能量的增加而增加，而在热带则减少。我们还发现了紫花苜蓿的强亲和力物种到其祖先的地中海气候生态位。我们的研究结果表明，苜蓿丰富度模式的空间变化可能受到耐热性的强烈影响，而不是依赖于耐寒性的热带进化枝。这种对耐热性的依赖可归因于紫花苜蓿物种表现出的强生态位保守性。我们的研究结果为丰富能量假说提供了新的见解，并表明高环境能量可能不一定会产生高物种多样性，但也可能降低物种多样性，特别是热带外向热带的进化枝。