How three rocky shore snails coexist on a limited food resource |
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Authors: | Tom M Spight |
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Institution: | (1) Woodward-Clyde Consultants, 3 Embarcadero Center, Suite 700, 94111 San Francisco, California |
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Abstract: | Summary Competition theory poses a major problem when several species coexist on what appears to be one resource. ThreeThais species living on the Pacific Northwest Coast provide an example: at many sites, all three depend primarily on one barnacle
species. Growth rates of three species were measured for 3 years and these provide an indirect means to assess how these snails
use their common food resource. Major temporal differences were observed:T. lamellosa grew 0–1 mm/mo during the spring and 2–3 mm/mo during the summer, whileT. emarginata andT. canaliculata grew 2–3 mm/mo during the spring and 0–1 mm/mo during the summer. However, all species are opportunists when food is available,
and seasonal and interspecific differences disappeared when all three species were kept well fed together in the laboratory.
Therefore, temporal differences arise from spatial segregation rather than from intrinsic differences in activity, and must
arise because barnacle abundance patterns differe consistently from one area of the shore to another. Species-specific activity
patterns lead each species to a food intake that is independent of the food supply on the shore as a whole and is also independent
of food intake by the other two species. Where two snail species depend upon a single food species, their use of the food
supply appears to make it function as two different resources. This resource use is possible because prey quality is markedly
dependent on shore level. |
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