Why are there more species in the tropics?

Richness increases towards the equator for trees, birds, mammals and molluscs, on land and in the sea. A meta-analysis of nearly 600 published gradients found the decline holds across organisms, habitats and regions, more strongly at regional scales than at local ones. It reaches back before the dinosaurs, though perhaps not continuously: a 2014 review of the fossil record finds a tropical peak only in parts of the Palaeozoic and in the past 30 million years, with flatter gradients or temperate peaks in warmer times.

Exceptions exist, and they are informative. In the sea the gradient often looks bimodal, with a dip in richness at the equator and peaks to either side whose positions vary by group. How much of the dip is real is argued: a 2018 analysis traces most of it to how thinly tropical waters have been sampled. Fossil plankton, which preserve exceptionally well, show the dip opening as the ocean warmed after the last ice age, and the survey that described the bimodality suggests the equator may already be too hot for some species.

A 2007 review sorted the proposals into three kinds: ecological limits on how many species a place can hold, differences in the rate at which lineages diversify, and differences in how long the tropics have been there to diversify in. Counted region by region, movement between regions is a fourth term, and leaving it out is how the same data has been read two ways.

The review found accumulating evidence for higher diversification in the tropics, from phylogenies and from fossils, and attributed it in part to faster tropical speciation. How extinction rates vary with latitude, it said, was little known. It also counted the greater age and former extent of tropical environments as part of the answer, an idea that had had far less attention than ecological ones.

A 2006 study took the question to the marine fossil record, tracking genera and subgenera of bivalves through the past 11 million years and counting origination, extinction and immigration separately.

It found that taxa preferentially originate in the tropics and expand towards the poles without losing their tropical presence, a pattern the authors called out of the tropics.

That makes the tropics a cradle and a museum at once, against a dichotomy that had stood since 1974. A study of one genus of tropical American leaf beetles reached the same answer the same year, and the 2007 review took the pair as where the evidence stood.

If tropical richness reflects a slow accumulation over tens of millions of years, it cannot be rebuilt on any human timescale once lost.

If it reflects faster speciation, the loss is no less severe, because even fast speciation runs on a geological clock. The two stories still differ in what conservation is protecting: an archive in one, a source of new species in the other. The bivalve study put it flatly: a tropical diversity crisis would have evolutionary effects at every latitude.

Different groups may reach the same pattern by different routes, in which case looking for a single cause is a mistake in the framing of the question rather than a failure to find the answer.

A 2019 review says plainly that no consensus has been reached and that the observed patterns are often consistent with several explanations at once. That is less satisfying and better supported.