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An Unpredictable Indirect Effect of Algal Consumption by Gulls on Crows

Masakazu Hori and Takashi Noda
Ecology
Vol. 82, No. 11 (Nov., 2001), pp. 3251-3256
Published by: Wiley
DOI: 10.2307/2679848
Stable URL: http://www.jstor.org/stable/2679848
Page Count: 6
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An Unpredictable Indirect Effect of Algal Consumption by Gulls on Crows
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Abstract

Indirect interactions among species may be important for understanding community dynamics because species generally interact with more species indirectly than directly. Although various kinds of indirect effects have been documented, estimates of indirect effects are based on estimates of the direct interactions occurring on a short time scale. In the present study, we report that predicted indirect effects may not represent the actual indirect effects in a rocky intertidal habitat. Gulls feed on green foliose algae during winter, and these foliose algae inhibit crows from foraging on chitons. This is because algal mats cover the chitons during spring. By connecting these direct interactions, gulls are expected to indirectly increase crow foraging. However, gulls indirectly decreased crows foraging because gull feeding increased the number of foliose algal mats during spring. This result indicates that the consequence of a direct effect may occasionally change temporally from negative to positive before an indirect effect appears. Therefore, indirect effects should not be estimated merely by connecting direct effects on a short-term basis.

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