Patterns of biodiversity

Biodiversity is not spread evenly over the earth. Two clear patterns stand out.

From the equator to the poles, the number of species generally falls. Near the equator, in tropical countries such as Colombia, there are many times more bird species than in New York, and far more than in Greenland. India, mostly in the tropics, has more than 1,200 bird species. The tropical Amazon rain forest in South America has the greatest biodiversity on earth.

Why are the tropics so rich? Ecologists suggest three reasons:

• Time: the tropics have been undisturbed for millions of years, while colder regions were repeatedly covered by ice in glaciations. So species there have had a long, steady time to evolve and diversify.
• Stable climate: tropical environments change little through the year, unlike temperate ones. This constancy lets species specialise into narrow roles (niches), so more species can live side by side.
• More energy: the tropics receive more solar energy, which means higher productivity, which can support more species.

Area. The German naturalist Alexander von Humboldt noticed that within a region, species richness rises with the area explored, though only up to a limit. On a log scale, the relationship is a straight line:

log S = log C + Z log A

where S is the number of species, A the area, C the Y-intercept, and Z the slope of the line (the regression coefficient). Within a region, Z is usually 0.1 to 0.2, whatever the region or group of organisms. But for very large areas such as whole continents, the slope is much steeper, 0.6 to 1.2.

Why does biodiversity matter for ecosystems? David Tilman's long-term experiments on outdoor plots found that plots with more species showed less year-to-year variation in total biomass, and that more diversity also meant higher productivity.

Paul Ehrlich explained the risk with the 'rivet popper' hypothesis. Picture an aeroplane (the ecosystem) held together by thousands of rivets (the species). If passengers keep pulling out rivets to take home, at first nothing happens. But as more are lost, the plane is weakened, and losing a rivet from a key part, such as a wing (a key species that drives a major ecosystem function), can be a disaster.