Why are some places so species-rich?

Our newest paper is out in Science Advances - the culmination of 3 years of hard work on our biodiversity hotspots project.

The idea of biodiversity hotspots (you can see ours mapped below) came into the mainstream with Norman Myers’ now classic paper published in 2000. He reported that nearly half of all vascular plant species and a little more than one-third of all mammals, birds, reptiles, and amphibians are confined to 25 “regions” that comprise only 1.4% of the Earth’s land surface. Recent estimates have raised this number to about 20% of the Earth’s land surface holding more than 50% of all vertebrate species. While Myers’ paper has been tremendously influential for conservationists in guiding their interventions, much less thought has been given to how this remarkable pattern came to be in the first place.

Biodiversity hotspots for mammals estimated in 100 km by 100 km grid cells. Red cells are hotspots.

Biodiversity hotspots for mammals estimated in 100 km by 100 km grid cells. Red cells are hotspots.

In our paper, we have tried to explain why biodiversity hotspots have arisen by analysing data from over 5,000 mammal species and 11,000 bird species. We discovered that hotspots followed different routes to high diversity depending on their location. Hotspots in tropical regions were characterised by increased rates of species generation in the last 25 million years compared to their surroundings. By contrast, hotspots in temperate zones were shaped by higher rates of migration from their surroundings.

Environmental differences between hotspots and their surrounding areas, such as greater topographic and habitat complexity, could explain how their extraordinary diversity has been generated and maintained through time. This is because these environmental differences could promote both greater speciation and immigration within hotspots.

The results also matter for two reasons. First, they highlight that tropical hotspots are important “arenas” where evolution is playing out. We might therefore want to conserve these areas to ensure that diversity - and the evolutionary processes supporting it - are maintained, particularly in the face of global environmental change. Although humans are changing the planet much faster than most species can adapt, areas where evolution is happening quicker would arguably give species a better chance to respond and carry more genetic variation for doing so. Second, the results underscore the need to protect and enhance habitat connectivity in temperate hotspots, which export species to their surrounding areas.

To build on this work, we hope to shift systems to plants.  The much greater diversity of flowering plants (300,000+ species) is advantageous because it enables us to analyse much finer spatial patterns, which we can directly associate with environmental features to make broader generalisations about the processes that generate hotspots. Nonetheless, mammals and birds were a good start because almost all species have complete spatial distributions and information about their evolutionary relationships, which are needed to reconstruct rates of species generation.  Therefore, we used these two animal groups to develop a methodological approach for analysing hotspots that we now hope to transfer to plants.  Wish us luck!

Andrew Tanentzap