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Small Islands and the Equilibrium Theory of Insular Biogeography

Donald R. Whitehead and Claris E. Jones
Evolution
Vol. 23, No. 1 (Mar., 1969), pp. 171-179
DOI: 10.2307/2406492
Stable URL: http://www.jstor.org/stable/2406492
Page Count: 9
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Small Islands and the Equilibrium Theory of Insular Biogeography
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Abstract

Anomalous area-diversity patterns may well appear on very small islands. For example, for certain island systems one can demonstrate essentially no increase in species number with island size until a certain critical area is attained; after this the slope of the area-diversity curve increases sharply. As suggested by MacArthur and Wilson (1967), this might be due to area-independent extinction rates on the smaller, unstable islands. This explanation has been advanced for the peculiar area-diversity pattern observed for the Florida Sand Keys and for Kapingamarangi atoll. Although their explanation may well apply to the Sand Keys, our analysis of Niering's (1963) data on the flora of Kapingamarangi suggests another interpretation. Our studies indicate that: (1) the area-diversity curve does not "inflect" as sharply as the published illustration indicates; (2) the relatively rapid rise in species number on larger islands is at least in part an artifact resulting from the inclusion of recent introductions in the data; (3) the "corrected" area-diversity curve (excluding recent introductions) does have a slightly greater slope for islands larger than 3.5 acres, but it is not nearly as striking; (4) when one plots the number of strand and salt-tolerant species against island area, there is no inflection and only a slight increase with size; (5) non-strand species are extremely rare on the smaller islets and increase appreciably on islands larger than 3.5 acres. It is thus evident that the major "inflection" of the area diversity curve is a result of the pattern of appearance of non-strand or salt-intolerant species; species that presumably require relatively fresh water. Data on the occurence of fresh water on the islets indicates that the smallest dimension must exceed 350 ft before an island can maintain an internal lens of fresh water. Given the fact that few islets are isodimensional, 3.5 acres may well be the minimal size required to sustain a reservoir of water. All of this indicates that the inflection relates not to area-independent extinction rates on theoretically unstable small islets, but instead to the number of strand species available in the source region and barriers to establishment of non-strand species posed by small, homogeneous islets. Establishment will be limited to salt tolerant, drift-dispersed species. Since the ecologically equivalent portions of the source region contains a limited number of such species, species number can not climb appreciably until the islands are large enough and diverse enough to permit the establishment of non-strand species. The relationships to be expected in any system of small islands are indicated in Figure 7. Here one can ascertain the correlations between island size and (1) total number of species, (2) "strand" species, (3) "non-strand" species, (4) introduced ("weedy") species, and (5) indigenous species. Strand species increase only slowly with area. Non-strand species are essentially absent from the smaller islets, but increase rapidly with area as soon as islets are large enough to maintain a freshwater reservoir. Introduced species begin to appear on islands large enough and diverse enough to be useful to man (for agriculture and habitation). A consideration of the factors influencing immigration rate suggests that island size may be extremely important. Larger islands should have a higher immigration rate because: (1) they present a larger target, hence will capture more propagules per unit time; (2) smaller islands possess less ecological diversity, hence establishment will be limited mainly to drift-dispersed, salt-tolerant species. The required adjustment of the slope of the immigration curve would result in larger values of dS/dA for both near and far islands, but will not affect the generalization that species number will increase with area more rapidly on far islands than on near. Data on dispersal mechanisms suggest that distance will operate most strongly against wind-dispersed species, while driftdispersed species should be least affected. Thus for islands far removed from the source region, the frequency of arrival of wind-dispersed propagules should be low, that for bird-transported taxa somewhat higher, and that for drift-dispersed species highest.

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