The biogeographical effect on the dispersal-range size relationship
We also found that studies performed exclusively in temperate or subtropical regions showed more positive associations between dispersal-related traits and range size than studies performed exclusively in the tropics or that included multiple latitudes (Fig. 3). This suggests that (current) species range sizes in tropical organisms may be more independent from dispersal than in subtropical and temperate regions, and supports the hypothesis (Table 1) that species in tropical regions may have had more opportunity to attain and persist in their maximum range sizes compared to temperate species, because range sizes have not been altered as much by past environmental change (e.g., during the Quaternary period, such as lack of glaciations) (Svenning & Skov 2004) and time for range expansion. Indeed, if species in temperate regions are still spreading from their last glacial maximum refugia, their ranges might be smaller than their potential ranges (Svenning et al. 2008), and dispersal-related traits might be more directly affecting range sizes compared to those in tropical regions.
Alternatively, differences in evolutionary rates between temperate and tropical regions may explain the observed discrepancy in the dispersal-range size relationship. It has been argued that higher temperatures in the tropics accelerate evolutionary and ecological change (e.g., shorter generations times, fast mutation and selection rates), which may lead to increased speciation rates (Brown 2014). Higher speciation rates would negatively affect species range sizes, as new species rapidly arise, generally attaining a small range, and then give birth to new species again, thereby making them ‘ephemeral’ (Lester et al. 2007, Sheth et al. 2020). Therefore, in tropical regions there might be more species with small ranges independent of their dispersal abilities. This may be exacerbated by more narrow, specialized niches of tropical species that hinder the increase of range size and thus foster isolation, reproductive isolation and speciation (Janzen 1967). One way to circumvent this effect of evolution, is to consider species age (or evolutionary rates) directly when assessing the dispersal-range size relationship. Even though species age did not come out as a significant contributor in our analysis, we cannot discard its importance, because only 15 (1 in plants, 14 in animals) out of the 475 relationships considered species age.
Our results also suggested that studying multiple latitudes simultaneously prevents to find positive dispersal-range size relationships. This might be particularly relevant when combining latitudes for which the dispersal-range size relationship is different, like tropical vs. temperate regions together. Although a possible solution to circumvent this problem would be to correct for latitude, our results showed that correcting for latitude does not increase the chance of finding a positive dispersal-range size relationship. Nevertheless, only 30% of the relationships included a sort of latitude correction by either explicitly correcting for latitude (17 relationships) or restricting the study to a single latitudinal zone (107 relationships).