Figure 1.

Schematic of mechanisms and direction of causation underpinning the three major hypotheses for macroecological correlations among range size, niche breadth and latitude. Bell curves represent locally evolved niches (i.e. individual or population level), and triangles represent range expansions. Ecological gradients are indicated via shading. The relevant scale of niche breadth expansion varies among hypotheses: in I, this must occur at the individual level (it must be heritable to spread); in II, niche breadth accrues at the species level; and in III, the population level is most relevant. The role of latitude also varies depending on the hypothesis: in I, latitude reflects environmental gradients that select locally for generalism versus specialism; in II, latitudinal patterns may emerge only indirectly, insofar as the grain of spatial environmental variation or species age varies with latitude; in III, latitude reflects net poleward range expansion of many species throughout the Holocene and Anthropocene. (Online version in colour.)