Mark D. Bertness

Positive interactions in communities

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.

Inclusion of facilitation into ecological theory

Investigations of the role of competition, predation and abiotic stress in shaping natural communities were a staple for previous generations of ecologists and are still popular themes. However, more recent experimental research has uncovered the largely unanticipated, yet striking influence of facilitation (i.e. positive species interactions) on the organization of terrestrial and aquatic communities. Modern ecological concepts and theories were well established a decade before the current renaissance of interest in facilitation began, and thus do not consider the importance of a wide variety of facilitative interactions. It is time to bring ecological theory up to date by including facilitation. This process will not be painless because it will fundamentally change many basic predictions and will challenge some of our most cherished paradigms. But, ultimately, revising ecological theory will lead to a more accurate and inclusive understanding of natural communities.

THE ROLE OF POSITIVE INTERACTIONS IN COMMUNITIES: LESSONS FROM INTERTIDAL HABITATS

Positive interactions that result from neighbors buffering one another from stressful conditions are predictably important community forces in physically stressful habitats. Here, we examine the generality of this hypothesis in marine intertidal communities. Intertidal communities have historically played a large role in the development of community ecology since they occur across pronounced physical gradients and are easily manipulated. Positive interactions, however, have not been emphasized in studies of intertidal communities. We first review studies of intertidal marsh plant communities that suggest that positive interactions play a dominant role in the structure and dynamics of these common assemblages. We then present the results of an experimental manipulation on New England rocky shores that suggests that group benefits are as important in maintaining the upper intertidal limits of dominant spaceholders on rocky shores as the negative forces of competition and predation are in maintaining lower d...

Physical Stress and Positive Associations Among Marsh Plants

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.

Zonation of Spartina Patens and Spartina Alterniflora in New England Salt Marsh

In New England salt marshes the boundary between frequently flooded low marsh habitats and less frequently flooded high marsh habitats is characterized by striking plant zonation. Spartina alterniflora monocultures dominate low marsh habitats while the seaward border of high marsh habitats is generally dominated by Spartina patens. In this paper I examine the role of interspecific competition in maintaining this zonation pattern. Spartina patens turfs and tillers transplanted into the low marsh were severely stunted with or without S. alterniflora neighbors, and low marsh bare patches bordering S. patens monocultures were not significantly colonized by S. patens in three growing seasons. The limited ability of S. patens to oxygenate its rhizosphere in anoxic soils appears to limit S. patens to high marsh habitats. In contrast, S. alterniflora transplants grew vigorously in the high and low marsh when buffered from neighbors, but were excluded from the high marsh in 2-3 yr when S. patens was present. S. alterniflora also rapidly invaded the high marsh in the absence of S. patens. These results support the hypothesis that S. alterniflora is restricted to low marsh habitats by competitive displacement. S. alterniflora thrives in anoxic low marsh habitats due to its ability to oxygenate its roots and rhizosphere. Rhizosphere oxidation by S. alterniflora, however, is not evident in seedlings and small colonizing patches and both seedlings and small colonizing patches of S. alterniflora are stunted in anoxic low marsh substrates. This suggests that the success of S. alterniflora in anoxic habitats is size dependent and may be driven by group benefits of rhizosphere oxidation. These results suggest that the maintenance of intertidal zonation in rocky beach and marsh plant communities is very similar. In both assemblages, competitive dominants monopolize physically benign habitats and displace competitive subordinates to physically stressful habitats.

Interspecific Interactions among High Marsh Perennials in a New England Salt Marsh

High marsh habitats in New England exhibit conspicuous zonation of vas- cular plants. Spartina patens and Juncus gerardi dominate the seaward and terrestrial borders of the high marsh, respectively, whereas Distichlis spicata is common in disturbed habitats. In this paper I examine the role of interspecific interactions among these marsh perennials in maintaining marsh plant zonation. Removal and transplant experiments were performed to examine species interactions in dense stands. In both types of experiments J. gerardi competitively dominated S. patens and D. spicata, while S. patens competitively dominated D. spicata. Each of these perennials showed maximum growth when transplanted into the J. gerardi zone without neighbors. J. gerardi appears to dominate the terrestrial border of the marsh by competitively excluding S. patens to lower marsh levels, while D. spicata is competitively restricted to disturbed habitats by both J. gerardi and S. patens. Species interactions during the colonization of bare patches were strikingly different than those found in dense vegetation. Colonization of bare patches by both S. patens and D. spicata facilitated the colonization of J. gerardi, the competitive dominant in dense vegetation. Bare patches in the high marsh are typically hypersaline due to increased surface evaporation in the absence of plant cover. Patch colonization by competitive subordinates, which are relatively salt tolerant, appears to reduce substrate salinity by passively shading the substrate and, as a by-product, facilitate the invasion of a superior competitor. While interspecific competition in dense vegetation dictates the zonation of New En- gland perennials in the high marsh, secondary succession in this assemblage may commonly be driven by facilitations. This scenario may not be uncommon among assemblages in harsh physical environments where plants or sessile animals ameliorate their physical environment.

Global shifts towards positive species interactions with increasing environmental stress

The study of positive species interactions is a rapidly evolving field in ecology. Despite decades of research, controversy has emerged as to whether positive and negative interactions predictably shift with increasing environmental stress as hypothesised by the stress-gradient hypothesis (SGH). Here, we provide a synthesis of 727 tests of the SGH in plant communities across the globe to examine its generality across a variety of ecological factors. Our results show that plant interactions change with stress through an outright shift to facilitation (survival) or a reduction in competition (growth and reproduction). In a limited number of cases, plant interactions do not respond to stress, but they never shift towards competition with stress. These findings are consistent across stress types, plant growth forms, life histories, origins (invasive vs. native), climates, ecosystems and methodologies, though the magnitude of the shifts towards facilitation with stress is dependent on these factors. We suggest that future studies should employ standardised definitions and protocols to test the SGH, take a multi-factorial approach that considers variables such as plant traits in addition to stress, and apply the SGH to better understand how species and communities will respond to environmental change.

Anthropogenic modification of New England salt marsh landscapes

Salt marshes play a critical role in the ecology and geology of wave-protected shorelines in the Western Atlantic, but as many as 80% of the marshes that once occurred in New England have already been lost to human development. Here we present data that suggest that the remaining salt marshes in southern New England are being rapidly degraded by shoreline development and eutrophication. On the seaward border of these marshes, nitrogen eutrophication stimulated by local shoreline development is shifting the competitive balance among marsh plants by releasing plants from nutrient competition. This shift is leading to the displacement of natural high marsh plants by low marsh cordgrass. On the terrestrial border of these same marshes, shoreline development is also precipitating the invasion of the common reed, Phragmites, by means of nitrogen eutrophication caused by the removal of the woody vegetation buffer between terrestrial and salt marsh communities. As a consequence of these human impacts, traditional salt marsh plant communities and the plants and animals that are dependent on these habitats are being displaced by monocultures of weedy species.

TESTING THE RELATIVE CONTRIBUTION OF POSITIVE AND NEGATIVE INTERACTIONS IN ROCKY INTERTIDAL COMMUNITIES

In contrast to many other biotic forces, such as competition and predation, the role played by habitat modification by plants and sessile animals in natural communities has not been given the experimental attention it deserves. To test the hypothesis that habitat modification by seaweed canopies can have direct positive effects on rocky intertidal com- munities, we quantified habitat amelioration by Ascophyllum nodosum canopies and its consequences on understory organisms in the Gulf of Maine, USA. At the upper and lower elevational borders of the algal canopy, we examined the recruitment, growth, and survi- vorship of common benthic organisms in canopy removal, canopy control, and shaded canopy removal plots intended to mimic canopy habitat modification. The algal canopy greatly reduced potential physical stresses, particularly at high tidal heights. Maximum daily rock temperatures were 5?-10?C lower and evaporative water loss was an order of magnitude less under the canopy than in canopy removal plots. The response of understory organisms to canopy removal, however, was species specific and somewhat idiosyncratic. Nonetheless, in general, at the high intertidal border of the canopy the re- cruitment, growth, and survival of understory organisms were enhanced by the canopy, whereas at the low intertidal border canopy effects were negative or neutral. Nearly half of the interactions we studied were positive in the high zone. In contrast to positive canopy effects on understory organism recruitment and growth at high tidal heights, consumer pressure was severe under the canopy, particularly at low tidal heights. Green crab predation is likely responsible for limiting understory mussel densities, while grazing by the snail, Littorina littorea, keeps understory substrate clear of algal recruits. The amelioration of harsh physical conditions by algal canopies can have strong direct positive effects in high rocky intertidal communities by enhancing organism recruitment, growth, and survival. These canopy effects, however, may often be offset by increased consumer pressure at low tidal heights. These types of habitat modification effects are likely to be pervasive in many other terrestrial and marine communities exposed to harsh physical conditions.

Competition and facilitation in marsh plants.

Historically facilitative interactions were widely believed to be important structuring forces in nature (Clements et al. 1928; Allee et al. 1949; Odum 1969). This point of view, however, has received little recent support (see Connell and Slayter 1977 and Boucher et al. 1982 for reviews of the evidence). In contrast, contemporary ecologists have devoted tremendous effort o documenting the role of competition in nature. Studies of the role of competition in natural communities have found it to be generally stronger and more important in communities under benign than harsh physical conditions (see, e.g., Connell 1972; Keddy 1989). Many ecologists, however, have cautioned that the relationship between interspecific interactions and environmental stress has not been given adequate attention (Connell 1975; Fowler 1986; Goldberg 1990; Dunson and Travis 1991). Moreover, a number of ecologists have suggested that facilitative or positive interactions among species may be more characteristic of harsh physical environments where neighbors can potentially buffer one another from physical stress (Allee et al. 1949; Connell and Slayter 1977; Bertness 1989). The role of physical stress in mediating the relative importance of facilitative interactions in natural communities, however, has received little direct attention. Here we present the results of a field experiment hat examines the relationship between competitive and facilitative interactions in salt marsh habitats. Specifically, we test the hypothesis that facilitation is common in secondary succession under harsh physical conditions but that competition dominates under benign physical conditions. The seaward border of high marsh habitats in New England is dominated by the grass Spartina patens, whereas dense standsof the rush Juncus gerardi characterize the terrestrial border (Miller and Egler 1950; Nixon 1982). Competition determines this zonation. Juncus competitively excludes Spartina from the terrestrial border, while both Juncus and Spartina restrict a third perennial, Distichlis spicata, to low densities and disturbed habitats (Bertness 1991). Natural disturbance in these habitats frequently occurs when tidally transported plant debris smothers underlying plants (Reidenbaugh and Banta 1980; Bertness and Ellison 1987). Secondary succession in the resulting bare patches is very predictable. Bare patches are initially colonized by the succulent annual Salicornia europaea (fig. 1), which is competitively displaced by perennial turfs within 2-3 yr (Ellison 1987). Distichlis by vegetative growth rapidly invades disturbed patches, but within 3-4 yr it too is competitively displaced by the zonal dominants (Bertness and Ellison 1987). Because bare patches in marshes become hypersaline (30%o100%o) without plant