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[[Image:SAR(Species-area curve).gif|thumb|The species-area relationship for a contiguous habitat]]

In [[ecology]], a '''species-area curve''' is a relationship between the area of a [[habitat]], or of part of a habitat, and the number of [[species]] found within that area.  Larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to follow systematic mathematical relationships.<ref name=Preston>Preston, F.W.  1962.  The canonical distribution of commonness and rarity: Part I.  Ecology 43:185-215 and 410-432.</ref>  The species-area relationship is usually constructed for a single type of organism, such as all [[vascular plant]]s or all species of a specific [[trophic level]] within a particular site.  It is rarely, if ever, constructed for all types of organisms if simply because of the prodigious data requirements.  It is related to, but not identical with, the [[species discovery curve]].

Ecologists have proposed a wide range of factors determining the slope and elevation of the species-area relationship.<ref name="Rosenzweig, M.L 1995">Rosenzweig, M.L.  1995.  Species Diversity in Space and Time.  Cambridge University Press, Cambridge.</ref>  These factors include the relative balance between immigration and extinction,<ref name="Wilson 1967">MacArthur and Wilson. 1967. ''The Theory of Island Biogeography.'' Princeton University Press: Princeton, NJ.</ref> rate and magnitude of disturbance on small vs. large areas,<ref name="Wilson 1967"/> predator-prey dynamics,<ref>Brose, U., A. Ostling, K. Harrison, and N.D. Martinez.  2004.  Unified spatial scaling of species and their trophic interactions.  Nature 428:167-171.</ref> and clustering of individuals of the same species as a result of dispersal limitation or habitat heterogeneity.<ref>Green, J.L. and A. Ostling.  2003.  Endemics-area relationships: The influence of species dominance and spatial aggregation.  Ecology 84:3090-3097.</ref>  The species-area relationship has been reputed to follow from the [[second law of thermodynamics|2nd law of thermodynamics]].<ref>{{cite journal|title=Roots of diversity relations | author= Würtz, P. and Annila, A.| journal = J. Biophys| year=2008| doi=10.1155/2008/654672 |url=http://www.hindawi.com/journals/jbp/2008/654672.html|volume=2008|pages=1–8}}</ref>  In contrast to these "mechanistic" explanations, others assert the need to test whether the pattern is simply the result of a random sampling process.<ref>Connor, E.F. and E.D. McCoy.  1979.  The statistics and biology of the species-area relationship.  American Naturalist 113:791-833.</ref>

Authors have classified the species-area relationship according to the type of habitats being sampled and the census design used. [[Frank W. Preston]], an early investigator of the theory of the species-area relationship, divided it into two types: samples (a census of a contiguous habitat that grows in census area, also called "mainland" species-area relationships), and isolates (a census of discontiguous habitats, such as islands, also called "island" species-area relationships).<ref name="Preston"/>  [[Michael Rosenzweig]] also notes that species-area relationships for very large areas—those collecting different biogeographic provinces or continents—behave differently from species-area relationships from islands or smaller contiguous areas.<ref name="Rosenzweig, M.L 1995"/>  It has been presumed that "island"-like species-area relationships have higher slopes (in [[log-log space]]) than "mainland" relationships,<ref name="Rosenzweig, M.L 1995"/> but a recent [[metaanalysis]] of almost 700 species-area relationships found the former had lower slopes than the latter.<ref>[http://www3.interscience.wiley.com/journal/118634038/abstract Drakare S, Lennon J.L., Hillebrand H., 2006 ''The imprint of the geographical, evolutionary and ecological context on species-area relationships'' Ecology Letters 9 (2), 215–227]</ref>

Regardless of census design and habitat type, species-area relationships are often fit with a simple function.  Frank Preston advocated the power function based on his investigation of the lognormal [[species-abundance distribution]].<ref name=Preston />  If S is the number of species, A is the habitat area, and z is the slope of the species area relationship in log-log space, then the power function species-area relationship goes as:

<math>S = cA^z</math>

Here c is a constant which depends on the unit used for area measurement, and equals the number of species that would exist if the habitat area was confined to one square unit. The graph looks like a straight line on [[Log-log graph|log-log axes]].  In contrast, [[Henry Gleason]] championed the semilog model:

<math>S = c + z log(A) = log(cA^z),</math>

which looks like a straight line on [[Semilog graph|semilog axes]], where area is logged and the number of species is arithmetic.  In either case, the species-area relationship is almost always decelerating (has a negative second derivative) when plotted arithmetically.<ref>Arrhenius, O. 1921. "[http://www.jstor.org/pss/2255763 Species and Area]" J. Ecol. 9: 95-99</ref>

Species-area relationships are often graphed for islands (or habitats that are otherwise isolated from one another, such as woodlots in an agricultural landscape) of different sizes.<ref name="Wilson 1967"/>  Although larger islands tend to have more species, it is possible that a smaller island will have more than a larger one.  In contrast, species-area relationships for contiguous habitats will always rise as areas increases, provided that the sample plots are nested within one another.

The species-area relationship for mainland areas (contiguous habitats) will differ according to the census design used to construct it.<ref>Scheiner, S.M.  2003.  Six types of species-area curves.  Global Ecology and Biogeography 12:441-447.
</ref>  A common method is to use quadrats of successively larger size, so that the area enclosed by each one includes the area enclosed by the smaller one (i.e. areas are nested).

In the first part of the 20th century plant ecologists often used the species-area curve to estimate the minimum size of a quadrat necessary to adequately characterize a community.  This is done by plotting the curve (usually on arithmetic axes, not log-log or semilog axes), and estimating the area after which using larger quadrats results in the addition of only a few more species.  This is called the '''minimal area'''.  A quadrat that encloses the minimal area is called a '''relevé''', and using species-area curves in this way is called the relevé method.  It was largely developed by the [[Switzerland|Swiss]] ecologist [[Josias Braun-Blanquet]].<ref name=Barbour>Barbour, M. G., Burk, J. H., & Pitts, W. D. (1980).  ''Terrestrial plant ecology''. Menlo Park CA: Benjamin/Cummings.  Pp. 158-160.</ref>

Estimation of the minimal area from the curve is necessarily subjective, so some authors prefer to define minimal area as the area enclosing at least 95 percent (or some other large proportion) of the total species found.  The problem with this is that the species area curve does not usually approach an [[asymptote]], so it is not obvious what should be taken as the total.<ref name=Barbour />  In fact, the number of species always increases with area up to the point where the area of the entire world has been accumulated.<ref>Williamson, M., K.J. Gaston, and W.M. Lonsdale.  2001.  The species-area relationship does not have any asymptote!  Journal of Biogeography 28:827-830.</ref>

==See also==
*[[Scaling pattern of occupancy]]
*[[Species richness]]
*[[Storage effect]]
*[[Unified neutral theory of biodiversity]] (UNTB)

==References==
{{reflist|2}}

==External links==
*  [http://math.hws.edu/~mitchell/SpeciesArea/speciesAreaText.html The Species-Area Relation]

{{modelling ecosystems|expanded=other}}

[[Category:Population ecology]]
[[Category:Biodiversity]]