Showing papers by "Mark D. Bertness published in 1994"

Positive interactions in communities.

Abstract: Current concepts of the role of interspecific interactions in communities have been shaped by a profusion of experimental studies of interspecific competition over the past few decades. Evidence for the importance of positive interactions — facilitations — in community organization and dynamics has accrued to the point where it warrants formal inclusion into community ecology theory, as it has been in evolutionary biology.

Physical Stress and Positive Associations Among Marsh Plants

Abstract: The contribution of positive interactions such as facilitations and mutualisms to the structure and organization of natural communities has received little recent attention. Here we show that distribution patterns of New England salt-marsh plants are strongly influenced by facilitative associations among neighboring plants. Positive interactions among marsh plants appear to be the simple by-product of neighbors buffering one another from potentially limiting physical stresses and thus only occur in physically harsh habitats. Positive associations such as these are likely common but unappreciated forces in harsh environments that have been largely overlooked by contemporary ecologists because of their preoccupation with competitive phenomena.

Cooperative and Competitive Interactions in the Recruitment of Marsh Elders

Abstract: While most studies of plant recruitment focus on competitive interactions, recruitment can be influenced by both positive and negative interactions. We studied recruitment in the marsh elder, Ivafrutescens, to examine the relationship between positive and negative forces in recruitment. Marsh elder seedling establishment was competitively precluded in undisturbed habitats by the dense perennial turfs that dominate marsh habitats. Adult marsh elders, however, passively trapped tidally transported plant debris that kills underlying vegetation. As a result, adult Iva created bare space and positively influenced seedling recruitment by fa- cilitating the formation of seedling safe sites. Within bare patches, Iva seedling survivorship was high under adult conspecifics and at high seedling densities. In contrast, solitary Iva seedlings without adult neighbors suffered extremely high mortality. These positive associations among neighbors were experimentally shown to result from neighbor buffering of hypersaline soil conditions. Salinities in un- shaded bare patches were elevated due to increased exposure to radiation and surface evaporation. Both adult nurse plants and high seedling densities shaded the soil, ameliorated high soil salinities, and thus had positive effects on seedling performance. Under adult canopies or when soil salinities were experimentally reduced by shading or watering, pos- itive associations did not occur and all interactions among seedlings or between seedlings and adults were competitive. Our results indicate that marsh plant recruitment is dictated by a balance between positive and negative forces. Moreover, since positive interactions were only seen once densities and physical forces had been experimentally manipulated, our results warn that positive interactions may be important forces in assemblages even where they are not conspicuous. These findings contribute to growing evidence that positive interactions, particularly those mediated by neighbor amelioration of harsh physical conditions, are important community level processes in a wide range of plant and animal assemblages.

Patch Size Effects on Marsh Plant Secondary Succession Mechanisms

Abstract: eters yi, from which it follows that invertebrates would be more likely than vertebrate ectotherms, which would be more likely than endotherms, to be involved in food chain chaos; and (b) certain ranges of values of the mass ratios m,,/m,and mC /mR, which affect the growth of oscillation magnitude in ways that are simple but fairly lengthy to describe explicitly (but see Yodzis and Innes [1992] for a complete discussion). Chaos in food chains (and, surely, in other wholesystem models as well) ought to be quite common when the resource productivity is sufficiently high. The obvious question, how high is sufficiently high, is particularly difficult to answer in this context, for the appropriate measure of resource carrying capacity is relative to the half saturation density of consumers in the system. There are few data available on this point, but one would expect a great deal of variation among taxa. Functional responses under field conditions, while difficult to measure, are vital for theoretical understanding.