Estuarine and coastal transformation is as old as civilization yet has dramatically accelerated over the past 150 to 300 years. Reconstructed time lines, causes, and consequences of change in 12 once diverse and productive estuaries and coastal seas worldwide show similar patterns: Human impacts have depleted >90% of formerly important species, destroyed >65% of seagrass and wetland habitat, degraded water quality, and accelerated species invasions. Twentieth-century conservation efforts achieved partial recovery of upper trophic levels but have so far failed to restore former ecosystem structure and function. Our results provide detailed historical baselines and quantitative targets for ecosystem-based management and marine conservation.

/pdf/depletion-degradation-and-recovery-potential-of-estuaries-1hinljyave.pdf

Depletion, Degradation, and Recovery Potential of Estuaries and Coastal Seas

Summary 1 Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized It is timely to consider the progress made by research in this field 2 We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant‐plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory We then consider future directions for facilitation research 3 With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies 4 There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity 5 Certain ecological modelling approaches (eg individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes 6 Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms

/pdf/facilitation-in-plant-communities-the-past-the-present-and-572accn7oh.pdf

Facilitation in plant communities: the past, the present, and the future

Biotic resistance describes the ability of resident species in a community to reduce the success of exotic invasions. Although resistance is a well-accepted phenomenon, less clear are the processes that contribute most to it, and whether those processes are strong enough to completely repel invaders. Current perceptions of strong, competition-driven biotic resistance stem from classic ecological theory, Elton’s formulation of ecological resistance, and the general acceptance of the enemies-release hypothesis. We conducted a meta-analysis of the plant invasions literature to quantify the contribution of resident competitors, diversity, herbivores and soil fungal communities to biotic resistance. Results indicated large negative effects of all factors except fungal communities on invader establishment and performance. Contrary to predictions derived from the natural enemies hypothesis, resident herbivores reduced invasion success as effectively as resident competitors. Although biotic resistance significantly reduced the establishment of individual invaders, we found little evidence that species interactions completely repelled invasions. We conclude that ecological interactions rarely enable communities to resist invasion, but instead constrain the abundance of invasive species once they have successfully established.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-0248.2004.00657.x

A meta‐analysis of biotic resistance to exotic plant invasions

Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density- and risk-driven ecological processes, and not demographic persistence alone.

/pdf/predicting-ecological-consequences-of-marine-top-predator-4r2j56kh15.pdf

Predicting ecological consequences of marine top predator declines

Summary 1. The stress-gradient hypothesis (SGH) predicts that the frequency of facilitative and competitive interactions will vary inversely across abiotic stress gradients, with facilitation being more common in conditions of high abiotic stress relative to more benign abiotic conditions. With notable exceptions, most tests of the SGH have studied the interaction between a single pair or a few pairs of species, and thus have evaluated shifts in the magnitude and direction of pair-wise interactions along stress gradients, rather than shifts in the general frequency of interactions. 2. The SGH has been supported by numerous studies in many ecosystems, has provided a crucial foundation for studying the interplay between facilitation and competition in plant communities, and has a high heuristic value. However, recent empirical research indicates that factors like the variation among species and the nature of the stress gradient studied add complexity not considered in the SGH, creating an opportunity to extend the SGH’s general conceptual framework. 3. We suggest that one approach for extending the SGH framework is to differentiate between the original idea of how ‘common’ interactions might be along stress gradients and the ubiquitous empirical approach of studying shifts in the strength of pair-wise interactions. Furthermore, by explicitly considering the life history of the interacting species (relative tolerance to stress vs. competitive ability) and the characteristics of the stress factor (resource vs. non-resource) we may be able to greatly refine specific predictions relevant to the SGH. 4. We propose that the general pattern predicted by the SGH would hold more frequently for some combinations of life histories and stress factor, particularly when the benefactor and beneficiary species are mostly competitive and stress-tolerant, respectively. However, we also predict that other combinations are likely to yield different results. For example, the effect of neighbours can be negative at both ends of the stress gradient when both interacting species have similar ‘competitive’ or ‘stress-tolerant’ life histories and the abiotic stress gradient is driven by a resource (e.g. water). 5. Synthesis. The extension of the SGH presented here provides specific and testable hypotheses to foster research and helps to reconcile potential discrepancies among previous studies. It represents an important step in incorporating the complexity and species-specificity of potential outcomes into models and theories addressing how plant‐plant interactions change along stress gradients.

/pdf/refining-the-stress-gradient-hypothesis-for-competition-and-384869d37d.pdf

Refining the stress-gradient hypothesis for competition and facilitation in plant communities

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.

https://www.pnas.org/content/pnas/99/3/1395.full.pdf

Anthropogenic modification of New England salt marsh landscapes

Although a great deal of research has focused on the effects of nutrient supply on plant competition, few studies have explored how these processes interact with non- resource factors to determine community-level patterns. This study examined how resource competition interacts with physical stress to structure salt-marsh plant communities across a natural gradient in tidal stress. First, nutrient additions at naturally occurring species borders at zonal and patch boundaries in two Rhode Island (USA) marshes revealed that competitive outcomes were typically reversed when nutrients were abundant. These results, which are consistent with earlier findings in a third southern New England marsh, suggest that a nutrient- dependent competitive hierarchy is a general characteristic of salt marshes in this region. To test whether these shifts in competitive outcomes occur only at naturally occurring species borders or can lead to more significant shifts in zonation patterns, lower marsh species were transplanted into the matrix of each zonal species at higher tidal elevations, and the outcomes of plant competition in fertilized plots and unfertilized plots were compared. Results of this experiment indicate that nutrient effects on the competitive relations of marsh plants were independent of where the interactions took place along the tidal gradient. The stress-tolerant species were consistently the best competitors in fertilized treatments, showing that an increase in nutrient availability can lead to drastic shifts in the distributions of plants across marshes. Finally, a third experiment examined the interaction between nutrient supply and the above- ground and belowground components of plant competition using a reciprocal transplant design coupled with nutrient-addition and neighbor-removal treatments. Results suggest that com- petition is primarily belowground under ambient marsh conditions but is aboveground at high nutrient levels. Thus the mechanism underlying the nutrient-dependent competitive hierarchy may be driven by a trade-off between belowground and aboveground competitive abilities, although the potential interaction between above- and belowground effects was not examined. Together, the results of these experiments suggest that nutrient supplies may significantly affect the competitive dynamics between salt-marsh perennials and their resultant zonation across an environmental gradient in tidal stress. The result that stress tolerators can be dominant competitors is not predicted by any current model of plant competition and must be considered in future empirical and theoretical studies.

/pdf/competition-and-salt-marsh-plant-zonation-stress-tolerators-1r9qqdwrco.pdf

Competition and salt-marsh plant zonation: stress tolerators may be dominant competitors

The influence of predation on rocky intertidal community structure has long emphasized the importance of indirect interactions. Most efforts in this area have focused on the density-mediated, or lethal effects, of predators on prey density. Recently, there has been growing interest in trait-mediated indirect interactions (TMIIs): the presence of a predator in the environment influences the interaction between two other species (prey and their resource) by altering a trait of the prey species. For example, waterborne cues released by predators can cause changes in prey species behavior, such as feeding rates, thereby altering the impact of the prey species on their resources. Thus, TMIIs represent the nonlethal effects of predators that contrast with the more traditional emphasis on lethal indirect effects. Marine ecologists are just beginning to explore the role of TMIIs in their systems.

We examined whether risk cues released by a ubiquitous crab predator (Carcinus maenas) influence the abundance of two dominant species in the rocky intertidal zone (barnacles [Semibalanus balanoides] and fucoid algae [Ascophyllum nodosum]) by altering the behavior of two of its snail prey (Nucella lapillus and Littorina littorea). We found that the presence of green crab risk cues can have strong cascading indirect effects on the abundance of barnacles and fucoid algae. N. lapillus exposed to risk cues consumed up to 29% fewer barnacles compared to conspecifics feeding in the absence of risk cues, whereas L. littorea exposed to risk cues consumed 459% fewer fucoids compared to conspecifics feeding in the absence of risk cues. These cascading interactions appear to reflect suppression of snail feeding by predator risk cues. In both food chains, snails exhibited more refuge-seeking behavior and grew less in the presence of risk cues. Our experiments suggest that TMIIs may have an important and underappreciated influence on species interactions that shape community dynamics on rocky intertidal shores.

Corresponding Editor: P. T. Raimondi

/pdf/trait-mediated-effects-in-rocky-intertidal-food-chains-qbdwc9pelo.pdf

Trait‐mediated effects in rocky intertidal food chains: predator risk cues alter prey feeding rates

The recent expansion of the reed Phragmites australis in western Atlantic salt marshes has become a conservation concern. Historically, Phragmites was restricted to the terrestrial border of marshes, but now it aggressively invades lower elevations. To explore factors influencing this expansion, we examined (1) the effects of physical factors and competitive interactions on the performance of Phragmites and (2) the role of clonal integration on the ability of Phragmites to invade low marsh habitats. We transplanted Phragmites into vegetation along an elevation gradient, with and with- out neighboring plants. Phragmites died when transplanted to the lowest marsh zone but survived and expanded in higher marsh zones. This suggests that the low oxygen availability characteristic of waterlogged soils limits Phragmites growth in the low marsh. Neighboring vegetation reduced Phragmites growth in all zones, and severing Phragmites rhizomes invading the low marsh reduced the survivorship and photosynthetic rate of ramets. These results suggest that Phragmites may invade low marsh habitats by initially es- tablishing itself in the high marsh and then expanding into lower, less favorable habitats using clonal integration. This has important implications for understanding the changing dynamics of New England salt marsh plant communities and developing management strat- egies for the control of Phragmites.

https://www.jstor.org/stable/pdfplus/2641020.pdf

Clonal integration and the expansion of phragmites australis

Summary


1
Northern New England marshes are characterized by mid-elevation areas of sparse vegetation, but high species diversity, which are not found in the well-studied marshes further south. These forb pannes occur in habitats that are more waterlogged and anoxic than adjacent clonal turf-dominated areas.

2
When the clonal turfs were transplanted into forb pannes, they showed reduced growth with and without neighbours present, suggesting that they are physically limited from invading the forb pannes. Conversely, when forb panne species were transplanted into surrounding turf areas, they flourished when neighbours were removed, but were suppressed when neighbours were present, suggesting that interspecific plant competition restricts the forbs to panne habitats.

3
Recovery of disturbance-generated bare patches was extremely slow, taking twice as long as in southern New England marshes. Secondary succession in northern New England marshes was driven by competitive interactions rather than by facilitative processes typical in southern New England marshes.

4
Differences in the structure and dynamics between northern and southern New England marsh systems may be due to differences in climate and human impacts. Waterlogged pannes are probably more common in northern marshes because they are generally larger and less ditched and drained. Reduced solar radiation and summer heating in northern New England reduces the potential for high soil salinities seen in southern marshes. This lack of the need for amelioration by neighbours probably explains the absence of facilitation.

/pdf/structure-and-organization-of-a-northern-new-england-salt-32hpz496cn.pdf

Patrick J. Ewanchuk

Papers

Anthropogenic modification of New England salt marsh landscapes

Competition and salt-marsh plant zonation: stress tolerators may be dominant competitors

Trait‐mediated effects in rocky intertidal food chains: predator risk cues alter prey feeding rates

Clonal integration and the expansion of phragmites australis

Structure and organization of a northern New England salt marsh plant community