Evolution of tropical biodiversity hotspots: Look to harsh species-poor areas

As they began to comprehend the depths of diversity across the planet, they noticed an interesting pattern. The number of species increases drastically from the poles to the equator. This phenomenon, known as the latitudinal gradient of species diversity, has helped define the tropics as home to most of the world's biodiversity. Scientists estimate that tropical forests contain more than half the species on earth, from plants and insects to birds, amphibians, and mammals.

These biologically rich areas are known as biodiversity hotspots. To qualify as a hotspot, a region must have at least 1,500 vascular plants species occurring nowhere else and have 30 percent or less of its original natural vegetation. In other words, it must be both irreplaceable and threatened.

While scientists agree that most biological diversity originated in the tropics, the jury is still out on how tropical species diversity formed and how it is maintained. A new study published in Science addresses these long-standing questions.

In "The evolution of tropical biodiversity hotspots," researchers argue that tropical species form faster in harsh species-poor areas but accumulate in climatically moderate areas to form hotspots of species diversity. Drawing on decades of expeditions and research in the tropics and the scientists' own knowledge and sampling of tropical bird diversity, the research team assembled a large and complete phylogenomic dataset for a detailed investigation of tropical diversification.

"This is our magnum opus," said Elizabeth Derryberry, associate professor in the University of Tennessee, Knoxville's Department of Ecology and Evolutionary Biology (EEB) and a senior author of the study. "This research is the product of a decades-long international collaboration to produce a completely sampled evolutionary history of a massive tropical radiation -- the 1,306 species of suboscine passerine birds."

Roughly one in three Neotropical bird species is a suboscine, making it the predominant avian group in the Neotropic terrestrial habitat -- which ranges from the Andes snow line to the Amazon lowlands -- and a perfect group to shed light on the origins of tropical biodiversity.

"The tropics are a natural laboratory for speciation research," said Michael Harvey, a recent EEB postdoctoral student and lead author of the study. "Many high-profile studies over the years sought answers to fundamental questions concerning species formation and maintenance." These earlier projects, he added, sampled only a minority of the existing species within the group being studied. In addition, said Derryberry, data analysis limitations in nearly all of the previous studies left them open to estimation errors.

For this study, Derryberry, Harvey, EEB Professor Brian O'Meara, and fellow researchers used a time-calibrated phylogenomic tree to provide information needed for estimating the dynamics of suboscine diversification across time, lineages, and geography. They also used the tree to test links between the dynamics and potential drivers of tropical diversity.

"We took no shortcuts in this study," Derryberry said. "We leveraged this unparalleled sampling of tropical diversity to illustrate the tempo and geography of evolution in the tropics. It is the first study to demonstrate conclusively that tropical biodiversity hotspots are linked to climates that are both moderate and stable."

The team discovered that species-rich regions in the tropics contain diversity accumulated during a protracted evolutionary period. A key result of their study is that the best predictor of elevated speciation rates in North and South American suboscines is low species diversity. In other words, new species form at higher rates in areas containing relatively few species.

"The qualities that nurture diversity, lower extinction, and promote the gradual accumulation of species are, paradoxically, not the ones that support biodiversity hotspots," Harvey said. "The hotspots are seeded by species born outside the hotspot in areas characterized by more extreme and less climatically stable climates."

The team discovered that, overall, extreme environments limit species diversity but increase opportunities for populations to evolve to become distinct species. Moderate climates, on the other hand, limit speciation but provide more opportunities for species diversity to accumulate.

"Our study will pave the way for future investigations of evolution in the world's diversity hotspots," Derryberry said. "This paper marks not only a change in our understanding of evolution in the tropics, but also in acknowledgement and valuation of the diversity of culture, expertise, and perspective in the field of ornithology."

International collaboration for the study included researchers from Colombia, Brazil, Uruguay, and Venezuela as well as ornithologists from groups underrepresented in the sciences, include Latino and women researchers.