Köchy, M. (1991)
In the district of Dalby, 12 km south of Uppsala, Sweden, I explored the species-area relationship for vascular plants on field islands (vegetation stands on mounds or small hills of moraine or rock forming 'islands' in arable land). The area of the field islands varied between 10Êm2 and 1 ha.
I applied an approach that was suggested by Kelly et al. (1989). The islands were divided into a margin, a forest, and a meadow habitat. I laid out 16 m2 quadrats on the islands' meadows and counted the species. According to the equilibrium theory by MacArthur & Wilson (1963), the number of species in the quadrats should depend on the size of the field island because it is more probable on large islands that species that go extinct within the quadrat are replaced by other species living nearby but outside the quadrat. Two other hypotheses, the random placement and the habitat-diversity hypothesis infer that the number of species in quadrats is independent of island size.
The best two-parameter model for the number of species on the whole field island regressed on total island area was the logarithmic equation S = g + k lg(A). The z-value for the power equation model (S = c Az) differed remarkably between the transformed (lg(S) = lg(c) + z lg(A) and the untransformed equation (0.35 and 0.27 respectively). This difference can however be explained mathematically. The mean of both z-values, 0.31, coincides with the value range (0.20<z<0.35) predicted by the equilibrium theory, but is also expected by other hypotheses. One prediction by the equilibrium theory, the difference in steepness between mainland and island curve slopes was met, but may be due to a different relation between habitat diversity and area.
No relationship between species number in quadrats and field island area was found. This indicates that extinction rate is not lower for small field islands.than for larger ones The distance to the mainland was not correlated to species richness. Thus two of the basic assumptions of the equilibrium theory are violated. This means that other mechanisms than those proposed by the equilibrium theory determine the number of species.
It was possible to show that number of habitats and three habitat variables (disturbance by human activity, tree cover on the meadow part, and portion of boulder) depend on area: Small field islands consist of a single boulder with a thin soil cover, therefore the number of species is small. Big field islands are found where the amount of rock keeps the farmer from ploughing, but the soil cover allows a richer vegetation and even trees or small woods may grow on it.