Nonlinear thermal gradients shape broad-scale patterns in geographic range size and can reverse Rapoport’s rule

Aim: Species living at latitudes that have greater annual temperature variations are expected to achieve broader geographic ranges than species living at latitudes that have smaller annual temperature variations, generating a positive relationship between range size and latitude (Rapoport's rule). However, this prediction fails to take into account the greater latitudinal extent of tropical temperatures relative to those at higher latitudes. Here we model the contributions of the broader latitudinal extent of equal-temperature habitats at low latitudes and the greater annual temperature variation at high latitudes to the range size–latitude relationship, and test whether the latitudinal variation in geographic range size in marine bivalves can be explained by models that account for both annual temperature variation and the steepness of latitudinal thermal gradients. Location: Western Pacific, eastern Pacific and western Atlantic. Methods: We use a null model where geographic ranges are placed on the ocean surface independently of thermal gradients, and a range-expansion model where the minimum and maximum temperatures encompassed by the geographic range of a species (macroecological thermal ranges) are positively related to annual temperature minima and maxima at the location where the species originated. We compare results with a database containing 40,820 occurrences of 4760 marine bivalve species. Result: Models incorporating temperature-limited range expansion along realistic thermal gradients predict an inverse relationship between range size and latitude, in opposition to Rapoport's rule. The distribution patterns of marine bivalves match this prediction. Main conclusions: The poleward trend in latitudinal range size is determined by the nonlinearity of the latitudinal gradient of temperature minima and maxima and less by the latitudinal gradient of the local seasonal range in temperatures. Although tropical species do have narrower macroecological thermal ranges than high-latitude species, the nearly constant temperatures over wide areas of the tropics allow tropical species to achieve broad latitudinal ranges.

研究目的：栖息于年温度变幅更大纬度带的物种，其地理分布范围预计较年温度变幅更小纬度带的物种更为广阔，进而形成分布范围大小与纬度间的正相关关系，即拉波波特法则(Rapoport's rule)。然而该预测未纳入考量：热带温度相较于高纬度温度，拥有更大的纬度跨度。本研究通过建模，分析低纬度等温栖息地的更大纬度跨度，以及高纬度地区更大的年温度变幅，对分布范围大小-纬度关系的贡献；同时检验海洋双壳类动物的地理分布范围大小的纬度变化，能否通过同时纳入年温度变幅与纬度热梯度陡度的模型得到合理解释。
研究区域：西太平洋、东太平洋与西大西洋。
研究方法：本研究采用两类模型：其一为零模型，即地理分布范围随机放置于海洋表面，与热梯度无关；其二为分布扩张模型，即物种地理分布范围所涵盖的最低与最高温度（宏观生态热范围），与物种起源地的年温度最小值、最大值呈正相关关系。我们将模型结果与包含4760种海洋双壳类动物的40820条物种出现记录数据库进行对比分析。
研究结果：纳入沿真实热梯度的温度限制型分布扩张的模型，预测得到分布范围大小与纬度间呈负相关关系，与拉波波特法则相悖。海洋双壳类动物的分布格局与该预测结果相符。
主要结论：分布范围大小沿纬度向极地的变化趋势，由温度最小值与最大值的纬度梯度非线性性所决定，而非受局地温度季节变幅的纬度梯度影响。尽管热带物种的宏观生态热范围确实窄于高纬度物种，但热带区域广阔范围内近乎恒定的温度，使得热带物种能够拥有更广的纬度分布范围。