Explaining global patterns of biodiversity is a fundamental goal in biology. Understanding how the tens of millions of species on earth have arranged themselves into populations, communities, and ecosystems, is critical for conserving them in the face of a rapidly growing human population and global climate change.
The latitudinal gradient in species diversity is perhaps the most famous such pattern, and it has confounded biologists for decades. Almost invariably across taxonomic groups, hemispheres and continents, as one moves from polar regions towards the equator, species diversity increases (see the figure for a depiction of global bird diversity). The concept of diversity here can be broken down into three parts: “alpha diversity” or the diversity of species in a single location; “beta diversity”, or the turnover of species observed when moving among locations; and “gamma diversity” or the diversity of species found in an entire region. The latitudinal diversity gradient holds true for all three elements.