Why are there so many birds?

The main question behind my PhD research is why, and how, are there so many species in the Amazon rainforest. There really is a ridiculous number of species. Hundreds of bird and tree species can be found at single sites, dwarfing the numbers found in whole countries in Europe for example. The Amazon is not alone in being highly biodiverse, many tropical regions harbour vast numbers, but the Amazon and Andean lowlands have the greatest number found anywhere on earth.

It is mind boggling to try and grasp the magnitude of the diversity: whereas you’d be lucky to see one or two dozen bird species in your garden at home, in the rainforest hundreds of birds live in the same patch of forest. Many have had their minds boggled trying to work out why it is that diversity increases as you travel from the poles to the equator. And more still have concentrated on tropical rainforests, enthralled by the conundrum of so many species evolving in the first place, and then being able to coexist.

The particular hypothesis I am interested in is one that has roots in the earliest explorations of Amazonia. Alfred Russel Wallace, now celebrated for his insights into the theory of natural selection, set out in 1848, at 25 years old, to explore the Amazon rainforest. He noticed that some species were found on one side of a river, but never the other. The rivers themselves seemed to be the boundary, the limit of a species’ range. Since then numerous biologists have explored the extent to which rivers may limit species ranges, and more crucially, the extent to which they actually drive speciation (the evolution of two new species from one) or simply limit the distribution of species that have evolved elsewhere.

Chestnut-tailed antbird.
Photo credit: Will Minehart

My PhD tried to unravel a tiny fraction of the story, by looking at how populations of different species are structured across rivers. I was in Peru to get blood samples from my study species, so I could work out from genetic analyses the extent to which populations on opposite riverbanks were connected with each other. If rivers are barriers between populations, meaning that birds on one side very rarely mate with birds on the other, then with a great deal of time, these populations may evolve to become distinct species. The rest of my time was spent behind a desk back home, looking at hundreds of bird range distribution maps, and how they related to the many rivers that run through the Amazon basin.

Birds may seem a daft choice for investigating the ‘river barrier hypothesis’, as they have wings. But many tropical birds are surprisingly bad at flying across open spaces such as rivers. My study species the antbirds prefer to stay in the dark dense understorey, and therefore are some of the most likely to be affected by river barriers.

Although my research questions focus on the evolution of Amazonian diversity, they are part of a much broader set of questions trying to get at the root of how and why species evolve into new species. These PhD students studying coral ecosystems are dealing with the same evolutionary questions, and do a great job of explaining their research. They also convey a little of the frustration in carrying out field and lab work:

This is quite unrelated except as another brilliant example of creative science communication, prizewinning no less, by my talented colleagues at the EGI. Original music by Stuart Noah and choreography by Cedric Tan, with a cast of PhD students…

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