Species richness, defined as the number of species per unit area, is the simplest measure of biodiversity. Small‐scale species richness generally refers to species richness at the scale of a single community, habitat or microhabitat. Understanding the factors that affect and are affected by small‐scale species richness is fundamental to understanding how ecological communities are assembled and function and how biodiversity is maintained. Several factors affect small‐scale species richness, including geographic factors such as the regional species pool, dispersal distance and ease of dispersal, biological factors such as competition, facilitation, and predation as well as environmental factors such as resource availability, environmental heterogeneity and disturbance frequency and intensity. The importance of these factors varies with scale of observation. Further, small‐scale richness can impact aspects of ecosystem function including productivity, stability, and invasibility. Key Concepts Small‐scale species richness is the number of species per unit area at the scale of a single community, habitat or microhabitat. Species richness is similar to alpha (α) diversity, or the number of species occurring at the local scale in a relatively homogeneous area. Many factors affect small‐scale species richness, including geographic (e.g. species pool, dispersal), biotic (e.g. competition, predation, facilitation) and abiotic (e.g. resource availability, environmental heterogeneity, disturbance frequency and intensity). The species pool is the set of species adapted to a site that are regionally available to colonise that site, and, in conjunction with dispersal processes, define the universe from which the ecological community is assembled. Immigration is affected by the distance between suitable habitat sites and the ability of propagules to become established at the local site, and can significantly increase species richness. Competition tends to decrease small‐scale richness, and its impact is shaped by resource availability, the specific resources being competed for and ecological disturbances that reduce competition. Interactions among predators and prey, or pathogens and parasites and their hosts, can maintain or alter small‐scale diversity, the impact being largely dependent on the level of dominance of the species directly impacted. While there is some evidence of a positive relationship between small‐scale richness and productivity, other factors such as the number and identity of functional groups may be the direct cause of the relationship. Small‐scale richness has complex effects on the stability and invasibility of local sites.