Search

Landscape fragmentation and exotic species invasions are two modern-day forces that have strong and largely irreversible effects on native diversity worldwide. The spatial arrangement of habitat fragments is critical in affecting movement of individuals through a landscape, but little is known about how invasive species respond to landscape configuration relative to native species. This information is crucial for managing the global threat of invasive species spread. Using network analysis and partial Mantel tests to control for covarying environmental conditions, we show that forest plant communities in a fragmented landscape have spatial structure that is best captured by a network representation of landscape connectivity. This spatial structure is less pronounced in invasive species and exotic species dispersed by animals. Our research suggests that invasive species can spread more easily in fragmented landscapes than native species, which may make communities more homogeneous over time. Key word: beta diversity; graph theory; habitat fragmentation: invasion: seed dispersal; spatial pattern.

Rapid changes in biodiversity are occurring globally, yet the ecological impacts of diversity loss are poorly understood. Here we use data from marine invertebrate communities to parameterize models that predict how extinctions will affect sediment bioturbation, a process vital to the persistence of aquatic communities. We show that species extinction is generally expected to reduce bioturbation, but the magnitude of reduction depends on how the functional traits of individual species covary with their risk of extinction. As a result, the particular cause of extinction and the order in which species are lost ultimately govern the ecosystem-level consequences of biodiversity loss., Marine coastal ecosystems are among the most productive and diverse communities on Earth (1) and are of global importance to climate, nutrient budgets, and primary productivity (2). Yet, the contributions [...]

Rapid environmental changes have fostered debates and motivated research on how to effectively preserve or restore ecosystem processes. One such debate deals with the effects of biodiversity, and the loss thereof, on ecosystem processes. Recent studies demonstrate that resource-use complementarity, now known as the 'niche-differentiation effect,' and the presence of a competitive species with strong effects on ecosystem processes, now known as the 'sampling effect,' can explain why productivity and nutrient retention are sometimes enhanced with increasing species richness. In a well-replicated outdoor mesocosm experiment, we tested these and other alternative mechanisms that could explain the effects of submersed aquatic plant (macrophyte) diversity on wetland ecosystem processes. Algal biomass increased and phosphorus loss decreased as species richness increased. This result can best be explained by an indirect sampling effect caused by one of the weakest competitors, which appeared to facilitate algal growth and thereby filtering of particles, and thus phosphorus, from the water column. The dominant competitor also appeared to decrease phosphorus loss through direct effects on phosphorus availability in the soil and water. Thus, the effects by one of the weakest and the most dominant competitors combine to produce a diversity effect on phosphorus loss. Macrophyte biomass was not enhanced, but converged toward the intermediate biomass of the most competitive species. Such an 'inverse sampling effect' may be produced when the most competitive species is not the most productive species owing to species-specific feedbacks and adaptations to the wetland environment. In summary, we reject the niche-differentiation effect as the dominant mechanism in our macrophyte communities and expand on the role of sampling effects in explaining the relationship between plant communities and ecosystem processes. In particular, indirect and inverse sampling effects combine to drive the relationship between species richness and wetland ecosystem processes. Thus, we demonstrate that plant diversity may affect wetland ecosystem processes when inferior competitors drive system productivity and nutrient retention. To ensure coexistence of such species with superior competitors, wetland systems may need to be maintained in a nonequilibrium state, such as with hydrologic disturbances, which would maintain both higher diversity and enhance ecosystem functioning. Key words: algal colonization; competitive ability; diversity; ecosystem functioning; indirect sampling effect; inverse sampling effect; nutrient retention; productivity; sampling effect; submersed aquatic macrophytes; wetland.

WE ARE PLEASED WITH M. SOLAN ET AL.'S findings that 'species extinction is generally expected to reduce the depth of bioturbated sediments' ('Extinction and ecosystem function in the marine benthos,' [...]

Abstract: Key differences exist between tidal fresh- and saltwater marshes, such as the relative importance of mineral versus organic sedimentation and plant species diversity, that likely result in different drivers of sedimentation. In tidal freshwater marshes, we hypothesize that vegetation composition, along with physical marsh features (i.e., elevation and tidal channels), play a critical role in sedimentation. This hypothesis is evaluated in Dyke Marsh Preserve (Potomac River, VA, USA) by examining sediment character (grain size, organic content) and deposition rates across the marsh in spring and summer 2010 and 2011. 7Be is especially well suited to capture seasonal sedimentation patterns owing to its short half-life (53.3 d) and ability to assess both sediment deposition and erosion. However, its use in marshes can be challenging, especially due the presence of vegetation. In this study, 7Be-derived sedimentation rates are compared with sediment deposition observed on ceramic tiles to assess its utility in tidal freshwater marshes, and biophysical influences on sediment deposition are examined through statistical models. 7Be- and tile-derived sedimentation rates show similar spatial and temporal patterns, with highest rates occurring at sites closer to tidal channels, highlighting the importance of sediment availability. In addition, complex feedbacks between sedimentation and the plant community are discussed. [Copyright &y& Elsevier]