Can Rapoport’s Rule Be Rescued? Modeling Causes of the Latitudinal Gradient in Species Richness

Abstract

The latitudinal gradient in species richness, wherein species richness peaks near the equator and declines toward the poles, is a widely recognized phenomenon that holds true for many taxa in all habitat types. Understanding the causative mechanism or mechanisms that generate the latitudinal gradient in species richness (LGSR) has been a major challenge, and the gradient remains unexplained. A different latitudinal trend (named “Rapoport’s rule”), in which the mean size of species geographical ranges tends to decline toward the equator, has been hypothesized by G. C. Stevens to play a key role in generating the LGSR when coupled with a version of the “rescue effect,” in which local populations toward the fringes of geographical ranges are sustained by immigration. The Stevens hypothesis is now commonly cited as a potential explanation for the LGSR and has provoked numerous empirical studies in macroecology and biogeography. However, important aspects of the hypothesis are not obvious in Stevens’s verbal model and may go unrecognized, despite their major implications for empirical work related to large‐scale ecological and evolutionary processes. Here we present mathematical simulation models that test the logical structure of the Stevens hypothesis, examine effects on global patterns of species richness produced by the mechanisms (Rapoport’s rule and the rescue effect) explicitly identified by Stevens, and investigate the additional effect of competition. We find that Rapoport’s rule on its own generates an LGSR opposite that of the real world, with species richness peaking at the poles rather than at the equator. The same qualitative result (a “reverse” LGSR) appears when rescue‐effect regions, as described by Stevens, are added to the model. Building upon Stevens’s verbal model, we then develop an explicit version of competition and show that competition alone tends to equalize species richness across all latitudes. However, when both Rapoport’s rule and competition are included in the model, we find that a qualitatively correct LGSR is produced. Unlike previous hypotheses regarding the LGSR, this version of the model does not rely on a latitudinal gradient in the intensity of competition to produce an LGSR. However, detection of this LGSR depends on the spatial scale at which species richness is sampled, with the LGSR appearing only with regional, not local, sampling. In contrast, when competition is explicitly added to the model with both Rapoport’s rule and the rescue effect, an LGSR that is qualitatively consistent with that of the real world does appear in both local and regional samples. This expanded version of the Stevens hypothesis potentially could explain the real‐world LGSR, but all three elements (Rapoport’s rule, the rescue effect, and competition) are crucial and must operate sufficiently strongly and in specific ways. The LGSR becomes apparent in the model only when parameter values for Rapoport’s rule and the rescue effect are large, possibly unrealistically so, and when all points on Earth are ...

Rapoport's Rule Revisited: Geographical Distributions of Human Languages

“…Our results provide empirical evidence in support of the theoretical models of Taylor and Gaines [40] and have clear implications for understanding latitudinal patterns in language diversity. Unlike most species ranges for the taxa considered to date, human language ranges do not tend to overlap.…”

“…In the database used in this study, two or more languages co-exist on only 5% of the land area that support humans. As was the case in the simulations of Taylor and Gaines [40], this constraint on language range overlap helps explain the strong correlation between language range size and language richness. Therefore, we suggest that two key factors lead to the latitudinal gradient in language diversity: Rapoport's rule and the strong group boundary formation between socio-linguistic groups that prevents overlap.…”

“…In fact, in some cases the models produced higher richness at the poles. However, when Taylor and Gaines [40] included a measure of competition in their model the result matched the observed greater species richness in the tropics. Because competition limited the overlap of species ranges in the model, the authors attributed the geographic patterns produced to ‘species packing’, in which more species of smaller ranges can be packed into a similar area.…”

“…Gaston and Chown [39] pointed out a critical limitation by noting that key climatic variables, such as temperature, tend to level off across the tropics, while species richness in the region still varies considerably with latitude. Taylor and Gaines [40] simulated species ranges and found that Rapoport's rule alone or in combination with demographic sink perimeters did not produce the expected latitudinal gradient in species richness. In fact, in some cases the models produced higher richness at the poles.…”

Phytogeography and Floristics of Pinyon-Juniper Woodlands in Northern Arizona

“…Rapoport's latitudinal rule contends that as latitude increases so do the latitudinal ranges of species (Taylor and Gaines 1999). In other words, equatorial species are typically more latitudinally restricted, while temperate and circumpolar species are more wide ranging.…”