Abstract

Species diversity is a measure of the number of species, either with or without consideration of relative abundances. Statistical analysis estimates that there are approximately 8.7 million (±1.3 million SE) eukaryotic species on Earth. Several global patterns of species diversity have been described, including a peak in species diversity in tropical latitudes and fewer taxa as latitude decreases toward the poles. The mechanisms of species coexistence, and therefore maintenance of species diversity, at local scales is an active field of ecological theory. Models of coexistence include both equilibrium and non-equilibrium explanations. Human activities including land-use change, overexploitation, and, in the future, climate change have significantly influenced species numbers locally and globally by increasing the rates of extinction. Efforts to preserve species and limit rates of extinction have been justified both in terms of the benefits that species provide in economic and social terms (e.g., “ecosystem services”) and in ethical terms.