Why some taxa are more species-rich towards higher latitudes

How niche conservatism in colonizing and sedentary species shape a latitudinal gradient.

Niche conservatism has often been used as an elegant explanation for why there are more species in the tropics—i.e. most taxa originated in the tropics, had more time to diversify therein, and tended to retain ancestral climatic affinities making range shifts out of the tropics less likely. However, researchers have rarely referred to niche conservatism to explain why some taxa exhibit inverse patterns of increasing diversity towards the poles. We wondered whether niche conservatism would also make a valid explanation for taxa exhibiting inverse richness gradients. In other words, would clades originated under extra-tropical climates still be more diverse outside the tropics?

Image: Illustration of examples of clades that participated in the Biotic Exchanges through Bering in either direction, from the Palaearctic to the Nearctic and from the Nearctic to the Palaearctic. The species belonging to these clades are classified as ‘colonizers’ and assumed to be adapted to cooler temperatures to some extent.

Read the Full (open) Access EDITORS’ CHOICE article on which this post is based:
Morales‐Castilla, I, Davies, TJ, Rodríguez, MÁ. (2019) Historical contingency, niche conservatism and the tendency for some taxa to be more diverse towards the poles. Journal of Biogeography 47:783– 794.

While it is hard, if at all possible, to pinpoint the geographic origin of current clades, palaeontological records provide highly valuable information. For example, the fossil record contains information about past distributions and historical dispersal events. We thought of the mammalian biotic exchanges through the Bering Strait as a natural experiment allowing classification of species into colonizers—i.e. current species descending from ancestors that crossed Bering—and sedentaries—i.e. species whose ancestors did not cross Bering. Assuming that colonizers would have been adapted to cooler climates, we tested if, despite the long timespan since these dispersal events took place, colonizers would still distribute preferably over the extra-tropics.

The answer is yes, while sedentaries follow the classical Latitudinal Diversity Gradient (LDG) –i.e. they are more diverse toward lower latitudes– and show negative temperature-richness relationships, colonizers exhibit the reverse latitudinal trend increasing in diversity with cold. What was surprising to us was that, being classified as a colonizer according to the (admittedly sparse) fossil record would better explain climate-richness relationships than usual suspects in macroecology such as body size or range size. Altogether, our results step towards reconciling views explaining the LDG based alone in climatic determinism, evolutionary rates, or tropical conservatism. Cold adapted ancestors were able to cross Bering, but their descendants had a harder time moving towards tropical latitudes because extra-tropical niches are conserved too.

Our paper confirms that historical contingency, dispersal and niche conservatism, matter when it comes to understanding contemporary diversity gradients. This would have implications for species currently adapted to cold climates, in a context where regions with cooler climates are shrinking. More importantly, our findings lead us to ask new questions such as, are historical dispersal events as important for plant and animal taxa other than mammals? Are there physiological traits able to explain why some clades and not others participated in biotic exchanges? How much further can we incorporate palaeontological findings into the biogeographic agenda? How long back in time, can we track the imprints of history and evolution into the contemporary distribution of biodiversity? Would any of this information be useful to hindcasts or forecasts of species distributions? We are excited to tackle some of these questions and hope that fellow biogeographers will join us in this fascinating research venue.

Written by: Ignacio Morales-Castilla
Postdoctoral Fellow, Global Change Ecology and Evolution (GloCEE), Department of Life Sciences, University of Alcalá, Spain

Additional information: https://moralescastilla.github.io/

Differing diversity-climate relationships—i.e. species richness vs. temperature—between sedentary species, which conform to the LDG and show positive slopes and higher species richness towards the tropics (yellow boxplot and ribbon); and colonizers, which show a higher proportion of negative slopes and higher species richness across temperate and cooler climates.

Published by jbiogeography

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