Colonsay \ Rhum lona. « Barra
South Uist lura\ Shetland Orkney lura\ Shetland Orkney
Stronsay log S = 2.180 + 0.175 log A (S = 151.5 A0175) r2 = 0.39
Area in km2 (log)
• That the distance from the mainland is a very important determinant of the number of species on islands.
In addition, they assumed that the number of species on islands depends on the diversity of habitat and that other islands between the island and the mainland may play an important role as stepping stones for island hopping. It was also assumed that islands possess a limited capacity for taking in new species and that, therefore, an equilibrium between colonisation and extinction is established. It should be noted that some islands may be "oceanic" islands (e.g. formed by volcanic eruptions) and so never had direct contact to the mainland, or "continental" islands, where contact existed and where, therefore, a proportion of established species originated from that period of contact.
In summary, these observations and considerations lead to formulation of a general theory attempting to explain the different distributions of organisms. The most important parameters for such an explanation were, as previously, the size of the area and the distance to the mainland. Three statements were made:
• increased size of islands means increased number of species;
• increased distance from the mainland (supplier of propagules) means decreased number of species;
• with a constant number of species a continuous shift of species results because of colonisation and extinction.
The course of colonisation and extinction is shown schematically in Fig. 4.2.11 A where the intersection of both curves, number of species and turnover, shows the steady state (balanced state). The rate of establishment is dependent on the distance from the mainland. The larger the distance, the more difficult is the establishment. The quality of the "source" as well as the type and number of propagules that might settle
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