Rapoport’s rule
Rapoport’s rule states that the latitudinal ranges of plants and animals are greater at higher compared to lower latitudes (Stevens 1989). Stevens (1989) originally proposed a mechanism underlying this rule that relates to the climatic conditions under which different organisms have evolved. Specifically, species at higher latitudes must be able to handle greater temporal variability in climate compared to species at lower latitudes. As a result, species that have evolved at higher latitudes should be able to occupy larger latitudinal extents than species at lower latitudes. The rule was later extended to elevational gradients with the expectation that at higher elevations, climate is more variable and thus high elevation species should have broader elevational ranges (Stevens 1992). Other possible mechanisms for the rule have since been proposed, such as decreasing land area available to species at lower latitudes, reduced competition at higher latitudes, or extinction of species with narrow ranges at high latitudes due to glaciation (Gaston et al. 1998).
Studies examining Rapoport’s rule in plants and animals suggest that the rule holds for certain taxa, in certain locations, and that multiple mechanisms are likely contributing to the observed pattern (Rohde 1996; Gaston & Chown 1999). The pattern tends to be weaker in marine compared to terrestrial systems and in the Southern compared to Northern Hemisphere (Ruggiero & Werenkraut 2007). The rule has received such mixed support that some researchers have questioned whether it is “time for an epitaph” for Rapoport’s rule (Gaston et al. 1998). Despite this, ecologists are still intrigued by this ecogeographical rule and continue to examine whether it holds in a host of taxa, including microorganisms.
Thus far, support for Rapoport’s rule across latitude and elevation in microorganisms is equivocal at best. In terrestrial systems, the latitudinal ranges of fungal taxa (Tedersoo et al. 2014; Coxet al. 2016) and ciliates appear to increase toward the poles (W. Foissner, pers. comm. to Azovsky and Mazei (2013)). By contrast, Learet al. (2017) observed smaller range sizes of bacterial taxa at higher latitudes in 204 streams along a 1000 km latitudinal gradient. In marine systems, a global study of bacteria found strong support for Rapoport’s rule, with bacteria in the tropics having smaller ranges than bacteria in temperate regions (Amend et al. 2013). The relationship was so strong that the most abundant bacteria in the tropics were completely absent from higher latitudes, but even narrowly distributed bacteria in temperate regions were found at lower latitudes (Amend et al. 2013). A second study of marine bacteria also found that bacteria showed narrower ranges at lower compared to higher latitudes, but this was only true for bacteria in the phylaBacteroidetes and Cyanobacteria and the classes α-, β-, and γ- of the phylum Proteobacteria (Sul et al. 2013). In contrast, bacteria in the phyla Firmicutes , Chlamydiae ,Chloroflexi , Planctomycetes and those in the class Epsilonproteobacteria did not follow Rapoport’s rule (Sul et al.2013). Researchers have found a reverse Rapoport’s rule in marine benthic ciliates. One study found that ciliates had narrower ranges at higher latitudes (Azovsky & Mazei 2013). Support is also mixed for an elevational Rapoport’s rule. In a study across fungal phyla (Basidiomycota , Ascomycota , Zygomycota ,Chytridomycota and Glomeromycota ), fungi at higher elevations had greater distributional ranges (Ogwu et al. 2019). In contrast, bacteria and diatoms in streams in Asia and Europe did not follow an elevational Rapoport’s rule (Teittinen et al. 2016; Wang & Soininen 2017).
Researchers have identified several possible mechanisms leading to the distributions of microorganisms across latitudinal and elevational gradients. Range sizes of some taxa have been associated with variation in temperature and precipitation (Lear et al. 2017), suggesting that environmental selection determines ranges. However, evidence from other studies suggests that dispersal limitation may be the primary driver of range sizes in microorganisms at larger spatial scales, with environmental selection, competition, and niche differentiation becoming important factors at smaller spatial scales (Mittelbach & Schemske 2015). More manipulative experiments are needed to uncover the mechanisms driving range sizes of microorganisms to provide greater understanding of the importance of various processes at different spatial scales.