The numerous explanations for why Earth's biodiversity is concentrated at low latitudes fail to explain variation in the strength and even direction of the gradient through deep time. Consequently, we do not know if today's gradient is representative of what might be expected on other planets or is merely an idiosyncrasy of Earth's history. We propose a hierarchy of factors driving the latitudinal distribution of diversity: (i) over geologically long time spans, diversity is largely predicted by climate; (ii) when climatic gradients are shallow, diversity tracks habitat area; and (iii) historical contingencies linked to niche conservatism have geologically short-term, transient influence at most. Thus, latitudinal diversity gradients, although variable in strength and direction, are largely predictable on our planet and possibly others.
Climate change is causing species ranges to shift, expand, and contract, with divergent and underappreciated consequences for local and global biodiversity. Widespread range shifts should increase local diversity in most areas but reduce it in the tropical lowlands. Widespread expansions should maintain diversity at low latitudes while increasing diversity elsewhere, leading to stable global biodiversity. Expansions and shifts are both common responses to climate change now and in the deep past. To understand how changing ranges will reshape Earth's biodiversity, we argue for three research directions: (i) leverage paleontological data to reveal long-term biodiversity responses, (ii) better monitor low-elevation and latitude limits to distinguish shifts from expansions, and (iii) incorporate dispersal barriers that can turn would-be shifts into contractions and extinctions.