PLANT-SPECIES RICHNESS - THE EFFECT OF ISLAND SIZE AND HABITAT DIVERSITY

1 The objective of this study was to explore the interrelationship between island area, number of species, and habitat diversity. 2 A survey of dicotyledonous plant species was carried out on 45 uninhabited, unimproved, small islands off Shetland Mainland, plus two similar mainland headlands treated as islands. In addition, species were counted within 50-cm square quadrats randomly placed on island vegetation. The largest island surveyed was 100 ha; 81 plant species in all were found. 3 A total of 14 physical (abiotic) habitat types were classified. The number of habitats on each island was counted, and the habitat types characteristic of each plant species were recorded. Island areas were determined from Ordnance Survey maps. 4 There are close-fitting positive correlations between species number, island area, and the number of habitat types on an island. 5 Data on species number within quadrats of standard area reveal an increase in small-scale species richness on islands of increasing size - evidence for an effect of island area alone on species total. 6 Habitat types containing fresh water were largely absent from islands of less than one hectare in size. Species primarily associated with fresh-water habitats were generally also missing from these smaller islands - evidence for an effect of addition of habitat types on species total. 7 Path analysis confirms that island area contributes to species number both directly and indirectly, through habitat diversity, and that while the direct effects of area and habitats on species are roughly equal in magnitude, the total effect of area is nearly twice that of habitats. 8 Presence and absence of particular habitat types may be a function of island size.