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).