New Museum-led research takes a closer look at why biodiversity on Earth is greatest in the tropics and gradually diminishes toward the poles. The study, published recently in the journal PLOS Biology, reveals that tropical species have more genetic variation than temperate species.
“One of the most ubiquitous patterns in nature is that as you get closer to the equator, you see higher diversity,” said Brian Smith, the lead author of the study and an assistant curator in the Museum’s Department of Ornithology. “It’s also one of the most controversial patterns. Scientists have been arguing over exactly why this phenomenon exists for a long time.”
The pattern, known as the latitudinal biodiversity gradient, is true not just for birds but across many groups of plants and animals, including mammals, insects, amphibians, and fish. Understanding how Earth’s millions of species have arranged themselves is critical for conserving them in the face of a rapidly growing human population and global climate change.
There are a number of hypotheses for why this pattern might exist. Could it be that animals in the tropics have been there longer than animals in more northern climates, with more time to diversify? Is it because tropical species are less likely to go extinct because their environment is much more stable than in temperate zones? Or is it a combination of several effects? Smith and his collaborators at Louisiana State University and the University of Michigan focused their investigation of these questions on birds, taking a finer-scale look than has been previously attempted by looking at diversity within species.
“The bulk of the research out there is focused on how diversity gradients form across deep evolutionary timeframes,” Smith said. “We looked at changes in diversity throughout the entire Western hemisphere but with a shallower timeframe, focusing on genetic patterns within the species themselves.”
The researchers collected genetic data for 210 bird species representing a broad range of locations in North America and South America to compare genetic variation within species in temperate zones with that of tropical birds. They found a clear pattern, a genetic-level parallel to the established species-level gradient that marks tropics as species-rich relative to cooler climates: On average, tropical bird species are more genetically diverse and genetic diversity persists longer than in temperate species.
The finding is consistent with explanations that point to older species and stable environmental conditions as key factors fueling greater biodiversity in the tropics.