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. E lucidating the causes and consequences of human modification of natural ecosystems is one of the most pressing ecological issues of our times (1). Although the direct impacts of habitat destruction are usually obvious, the indirect effects of many human activities, such as nitrogen loading, have been more difficult to detect, but are likely just as pervasive and critical to understand (2). In this paper, we show how a mechanistic understanding of the organization of salt marsh communities reveals that these ecologically and economically important habitats (3) are currently being dramatically degraded by localized shoreline development and human-induced nitrogen enrichment.Salt marshes have attracted the attention of scientists because their simple structure makes them amenable for studying the mechanisms that generate ecosystem and community patterns (4-6). New England salt marsh plant communities have a striking vertical zonation (Fig. 1) that has long interested researchers (7, 8). The cordgrass Spartina alterniflora dominates low marsh elevations that are flooded daily by tides as a dense, clonally propagated monoculture. On the seaward border of the high marsh, cordgrass is replaced by marsh hay, Spartina patens, whereas the terrestrial border of the high marsh is dominated by a monoculture of the black rush, Juncus gerardi. A band of the shrub Iva frutescens typically demarcates the terrestrial border of these marshes. A number of less-abundant halophytic forbs commonly coexist with the high-marsh dominants (9).This striking spatial segregation of plants in salt marshes is the product of both plant competition and the strong, physical gradients characteristic of these habitats. Physical stress on plants in salt marshes (e.g., water-logging and salinity) generally decreases with increasing tidal height (10, 11), and there is an inverse relationship between the ...

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

Classical views of trophic cascades emphasize the primacy of consumptive predator effects on prey populations to the transmission of indirect effects [density-mediated indirect interactions (DMIIs)]. However, trophic cascades can also emerge without changes in the density of interacting species because of non-consumptive predator effects on prey traits such as foraging behaviour [trait-mediated indirect interactions (TMIIs)]. Although ecologists appreciate this point, measurements of the relative importance of each indirect predator effect are rare. Experiments with a three-level, rocky shore food chain containing an invasive predatory crab (Carcinus maenas), an intermediate consumer (the snail, Nucella lapillus) and a basal resource (the barnacle, Semibalanus balanoides) revealed that the strength of TMIIs is comparable with, or exceeds, that of DMIIs. Moreover, the sign and strength of each indirect predator effect depends on whether it is measured in risky or refuge habitats. Because habitat shifts are often responsible for the emergence of TMIIs, attention to the sign and strength of these interactions in both habitats will improve our understanding of the link between individual behaviour and community dynamics.