Causes of the Species-Area Relation: A Study of Islands in Lake Manapouri, New Zealand

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Abstract
The species-area relation consistently found among islands has given rise to the Random Placement hypothesis, the Habitat Diversity hypothesis and the Equilibrium hypothesis. In an attempt to test the Equilibrium hypothesis, twenty-three islands in Lake Manapouri, New Zealand were sampled for richness of vascular plant species. Sampling was restricted to two vegetation types (beech forest and manuka scrub) and was with a fixed-size quadrat (100 m2). Fixed-quadrat size sampling should eliminate the effect of island area on observed species richness if either the Random Placement or the Habitat Diversity hypothesis is correct, but should leave an effect of island area if the Equilibrium hypothesis is correct. In fact, the percentage of variation in species richness explained by island area was reduced from the 92% found in an earlier study by whole-island sampling to 17% and 10%, respectively, for the two vegetation types - both representing non-significant upward trends. However, the overall manuka scrub relation showed a significant quadratic relation, and examination of species-area relations within subgroups of quadrats selected for greater habitat uniformity showed one significant linear regression. Possible explanations, not normally considered in species-area studies, are the Incidence Function hypothesis, the Small Island Effect hypothesis, and the Small Island Habitat hypothesis - that there are inevitable habitat differences between small and large islands.