STRUCTURAL INTERDEPENDENCE - AN ECOLOGICAL CONSEQUENCE OF MORPHOLOGICAL RESPONSES TO CROWDING IN MARSH PLANTS

1. Crowding is a common ecological phenomenon that has been widely stu died from the perspective of competition for limiting resources. The e xtent to which crowded plants and sessile animals are positively affec ted by their neighbours, however, has received much less attention. He re, we use four salt-marsh plant species to examine the hypothesis tha t crowding leads to morphologically modified individuals that are depe ndent on their neighbours for structural support. 2. In the spring, cr owded and isolated treatments were created for the four species (Juncu s gerardi, Spartina alterniflora, Iva frutescens and Salicornia europa ea). By the end of the growing season, crowded plants (a) were general ly taller, thinner and produced less above-ground biomass, (b) were mo re susceptible to breaking and (c) grew closer to their theoretical ma ximum heights. 3. To explicitly test the hypothesis that crowding lead s to plants that are dependent on their neighbours for support, we thi nned crowded plant stands at the end of the growing season and compare d the survivorship of these secondarily isolated plants with that of p ermanently isolated plants. For all four species, a significant number of previously crowded plants fell over, whereas plants grown in isola tion remained upright. 4. Our results show that crowded plants often d evelop morphologies that cause them to be structurally dependent on th eir neighbours for support. Thus, the continuing presence of neighbour s provides a positive benefit that may mitigate the negative effects o f 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 acknow ledged.