Species Richness: Small Scale


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.

Keywords: biodiversity; community ecology; competition; diversity; scale; ecosystem function; species richness

Figure 1. Log‐log species–area curve of vascular plant richness from fine scales to global land area, starting from within the Carolinas, USA. The lowest six values are mean richness values from 1472 vegetation plots of 0.1 ha of the Carolina Vegetation Survey (CVS), with solid line as the mean species accumulation rate for these scales. Global richness is an estimate of 250 000 species. Reproduced with permission from Fridley et al. (2005). ©Ecological Society of America.
Figure 2. Species–area curves for four habitats, Las Cruces, Costa Rica, showing different rates of species accumulation along a transect in old‐growth forest, a forest gap, an open field and under a shade tree in the open field (data from Rebecca Brown). Area in this example refers to distance along the transect.
Figure 3. Species richness peaks at low to intermediate levels along a gradient of fertility or primary productivity in grasslands and open savannas, after which competition for light becomes important and the shorter species are lost, reducing richness. Grasslands with very high richness are chronically disturbed by some factor such as fire or grazing or mowing that reduces plant height and thus reduces competition for light, allowing richness to increase along a fertility gradient longer before the asymmetric competition for light becomes a factor. Reprinted from Peet et al. (2014) © Oxford University Press.
Figure 4. With increasing canopy cover from open grasslands through open woodlands to closed canopy temperate forests, light available to the herbaceous layer steadily declines. As the available light declines, it takes greater fertility for the herbaceous layer to become sufficiently robust for asymmetric light competition to become important, with the consequence that the peak in richness shifts progressively toward more fertile sites. Reprinted from Peet et al. (2014) © Oxford University Press.


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Brown, Rebecca L, Reilly, Lee Anne J, and Peet, Robert K(Feb 2016) Species Richness: Small Scale. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020488.pub2]