Patterns of Biodiversity

Beyond simply cataloguing how much biodiversity exists, ecologists have identified several broad, recurring patterns in how biodiversity is distributed across the planet -- patterns consistent enough, and observed across so many different taxonomic groups and geographic regions, that they are treated as general ecological principles rather than coincidences.

The species-area relationship is one such pattern: for a very wide range of taxonomic groups, the number of species found in a given area increases as the area surveyed increases, but not in a simple straight-line way -- the relationship instead turns out to be a fairly precise curve when plotted on ordinary axes, which becomes a straight line once both the area and the number of species are plotted on logarithmic scales. This logarithmic relationship is written as log S = log C + Z log A, where S is the number of species recorded, A is the area surveyed, C is a constant that depends on the taxonomic group and region being studied (essentially the line's starting height), and Z is the slope of the resulting straight line, which measures how quickly species number increases as area increases. Remarkably, when the areas being compared are relatively small and lie within a single, more or less continuous biogeographical region, the value of Z comes out close to the same narrow range -- roughly 0.1 to 0.2 -- regardless of which taxonomic group (plants, birds, insects, and so on) or which region of the world is being studied, suggesting this is a genuinely general biological pattern rather than a quirk of any one dataset. When the regions being compared are themselves very large, however -- for example comparing species richness across whole continents rather than smaller plots within a single region -- the slope Z becomes markedly steeper, typically in the range of about 0.6 to 1.2, reflecting the much greater ecological and evolutionary separation between species assemblages on different continents than between neighbouring plots in the same region. …