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Structural Interdependence: An Ecological Consequence of Morphological Responses to Crowding in Marsh Plants

C. D. G. Harley and M. D. Bertness
Functional Ecology
Vol. 10, No. 5 (Oct., 1996), pp. 654-661
DOI: 10.2307/2390176
Stable URL: http://www.jstor.org/stable/2390176
Page Count: 8
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Structural Interdependence: An Ecological Consequence of Morphological Responses to Crowding in Marsh Plants
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Abstract

1. Crowding is a common ecological phenomenon that has been widely studied from the perspective of competition for limiting resources. The extent to which crowded plants and sessile animals are positively affected by their neighbours, however, has received much less attention. Here, we use four salt-marsh plant species to examine the hypothesis that crowding leads to morphologically modified individuals that are dependent on their neighbours for structural support. 2. In the spring, crowded and isolated treatments were created for the four species (Juncus gerardi, Spartina alterniflora, Iva frutescens and Salicornia europaea). By the end of the growing season, crowded plants (a) were generally taller, thinner and produced less above-ground biomass, (b) were more susceptible to breaking and (c) grew closer to their theoretical maximum heights. 3. To explicitly test the hypothesis that crowding leads to plants that are dependent on their neighbours for support, we thinned crowded plant stands at the end of the growing season and compared the survivorship of these secondarily isolated plants with that of permanently isolated plants. For all four species, a significant number of previously crowded plants fell over, whereas plants grown in isolation remained upright. 4. Our results show that crowded plants often develop morphologies that cause them to be structurally dependent on their neighbours for support. Thus, the continuing presence of neighbours provides a positive benefit that may mitigate the negative effects of crowding. Our findings contribute to a growing body of evidence that positive interactions among crowded plants and sessile animals are a more pervasive feature of natural assemblages than is generally acknowledged.

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