The abundance of species within an ecosystem. Alpha diversity is the diversity of species within a particular area or ecosystem, expressed by its species richness (Moreno et al. (2006) INCI 31, 1). Beta diversity is the difference in diversity of species between two or more ecosystems, expressed as the total number of species unique to each of the ecosystems compared; see Vellend (2001) J. Veg. Sci. 12, 4 on beta diversity and species turnover.