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Presence and Absence of Habitat Shift in Some Widespread Lizard Species

Thomas W. Schoener
Ecological Monographs
Vol. 45, No. 3 (Summer, 1975), pp. 233-258
Published by: Wiley
DOI: 10.2307/1942423
Stable URL: http://www.jstor.org/stable/1942423
Page Count: 26
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Presence and Absence of Habitat Shift in Some Widespread Lizard Species
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Abstract

This study analyzes the circumstances under which certain lizards shift and fail to shift their habitats. At each of 20 localities, I measured the structural habitats utilized by all the diurnal arboreal lizard species as well as the availability of those habitats. I selected localities so as to include for four widespread species (Anolis grahami, A. sagrei, A. carolinensis, A. distichus) nearly all of the species-combinations in which they occur. Data were fitted to equations that (1) adjust for locality-specific differences in vegetation, and (2) estimate the direction and intensity of apparent interaction between sympatric forms. Shift was valuated both for species and separately for age and sex classes within species. Female-sized individuals shift more frequently than do adult @M @M. Linear equations that evaluate sympatric forms one at a time showed the strongest apparent competitors for a widespread from to be (1) adult @M @M rather than female-sized individuals, especially when adult @M @M represent the widespread species; (2) species of similar climatic habitat; (3) classes of similar size (especially against female-sized individuals of widespread species; and (4) classes of large size (especially against adult @M @M). The most abundant classes are the strongest apparent competitors for A. distichus but not for the other widespread species. These results are unchanged or strengthened when different habitat categories or nonlinear equations are used. Combining all sympatric forms into locality-specific linear equations supports Results 1 and 2 but is inconclusive for Results 3 and 4. The parameter proportional to the size of a refugium from interference is estimated for some cases to be significantly greater for female-size individuals than for adult @M @M. In general, results imply that animals similar to widespread forms in some niche dimension other than structural habitat are those most likely to cause shift in structural habitat. In addition, they suggest existence of a competition function with respect to size: in such a function intensity of competition is uniquely determined by the direction and amount of size difference, regardless of the competitors' absolute sizes. Competition intensity appears to (1) decrease overall with increasing difference in size, (2) be greater for a given size difference if the competitor is larger than if it is smaller, and (3) decrease at nonconstant rates, such that near complete size similarity there is a more rapid decline in intensity for smaller than larger competitors. Morphological differences between populations and short-term field observations suggest that both evolutionary and behavioral mechanisms regulate habitat shift.

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