Justin P. Wright

Effects of biodiversity on the functioning of trophic groups and ecosystems

Over the past decade, accelerating rates of species extinction have prompted an increasing number of studies to reduce species diversity experimentally and examine how this alters the efficiency by which communities capture resources and convert those into biomass. So far, the generality of patterns and processes observed in individual studies have been the subjects of considerable debate. Here we present a formal meta-analysis of studies that have experimentally manipulated species diversity to examine how it affects the functioning of numerous trophic groups in multiple types of ecosystem. We show that the average effect of decreasing species richness is to decrease the abundance or biomass of the focal trophic group, leading to less complete depletion of resources used by that group. At the same time, analyses reveal that the standing stock of, and resource depletion by, the most species-rich polyculture tends to be no different from that of the single most productive species used in an experiment. Of the known mechanisms that might explain these trends, results are most consistent with what is called the ‘sampling effect’, which occurs when diverse communities are more likely to contain and become dominated by the most productive species. Whether this mechanism is widespread in natural communities is currently controversial. Patterns we report are remarkably consistent for four different trophic groups (producers, herbivores, detritivores and predators) and two major ecosystem types (aquatic and terrestrial). Collectively, our analyses suggest that the average species loss does indeed affect the functioning of a wide variety of organisms and ecosystems, but the magnitude of these effects is ultimately determined by the identity of species that are going extinct.

Impacts of plant diversity on biomass production increase through time because of species complementarity

Accelerating rates of species extinction have prompted a growing number of researchers to manipulate the richness of various groups of organisms and examine how this aspect of diversity impacts ecological processes that control the functioning of ecosystems. We summarize the results of 44 experiments that have manipulated the richness of plants to examine how plant diversity affects the production of biomass. We show that mixtures of species produce an average of 1.7 times more biomass than species monocultures and are more productive than the average monoculture in 79% of all experiments. However, in only 12% of all experiments do diverse polycultures achieve greater biomass than their single most productive species. Previously, a positive net effect of diversity that is no greater than the most productive species has been interpreted as evidence for selection effects, which occur when diversity maximizes the chance that highly productive species will be included in and ultimately dominate the biomass of polycultures. Contrary to this, we show that although productive species do indeed contribute to diversity effects, these contributions are equaled or exceeded by species complementarity, where biomass is augmented by biological processes that involve multiple species. Importantly, both the net effect of diversity and the probability of polycultures being more productive than their most productive species increases through time, because the magnitude of complementarity increases as experiments are run longer. Our results suggest that experiments to date have, if anything, underestimated the impacts of species extinction on the productivity of ecosystems.

The Concept of Organisms as Ecosystem Engineers Ten Years On: Progress, Limitations, and Challenges

Abstract The modification of the physical environment by organisms is a critical interaction in most ecosystems. The concept of ecosystem engineering acknowledges this fact and allows ecologists to develop the conceptual tools for uncovering general patterns and building broadly applicable models. Although the concept has occasioned some controversy during its development, it is quickly gaining acceptance among ecologists. We outline the nature of some of these controversies and describe some of the major insights gained by viewing ecological systems through the lens of ecosystem engineering. We close by discussing areas of research where we believe the concept of organisms as ecosystem engineers will be most likely to lead to significant insights into the structure and function of ecological systems.

An ecosystem engineer, the beaver, increases species richness at the landscape scale

Abstract. Ecosystem engineering – the physical modification of habitats by organisms – has been proposed as an important mechanism for maintaining high species richness at the landscape scale by increasing habitat heterogeneity. Dams built by beaver (Castor canadensis) dramatically alter riparian landscapes throughout much of North America. In the central Adirondacks, New York, USA, ecosystem engineering by beaver leads to the formation of extensive wetland habitat capable of supporting herbaceous plant species not found elsewhere in the riparian zone. We show that by increasing habitat heterogeneity, beaver increase the number of species of herbaceous plants in the riparian zone by over 33% at a scale that encompasses both beaver-modified patches and patches with no history of beaver occupation. We suggest that ecosystem engineers will increase species richness at the landscape scale whenever there are species present in a landscape that are restricted to engineered habitats during at least some stages of their life cycle.

Low Concentrations of Silver Nanoparticles in Biosolids Cause Adverse Ecosystem Responses under Realistic Field Scenario

A large fraction of engineered nanomaterials in consumer and commercial products will reach natural ecosystems. To date, research on the biological impacts of environmental nanomaterial exposures has largely focused on high-concentration exposures in mechanistic lab studies with single strains of model organisms. These results are difficult to extrapolate to ecosystems, where exposures will likely be at low-concentrations and which are inhabited by a diversity of organisms. Here we show adverse responses of plants and microorganisms in a replicated long-term terrestrial mesocosm field experiment following a single low dose of silver nanoparticles (0.14 mg Ag kg−1 soil) applied via a likely route of exposure, sewage biosolid application. While total aboveground plant biomass did not differ between treatments receiving biosolids, one plant species, Microstegium vimeneum, had 32 % less biomass in the Slurry+AgNP treatment relative to the Slurry only treatment. Microorganisms were also affected by AgNP treatment, which gave a significantly different community composition of bacteria in the Slurry+AgNPs as opposed to the Slurry treatment one day after addition as analyzed by T-RFLP analysis of 16S-rRNA genes. After eight days, N2O flux was 4.5 fold higher in the Slurry+AgNPs treatment than the Slurry treatment. After fifty days, community composition and N2O flux of the Slurry+AgNPs treatment converged with the Slurry. However, the soil microbial extracellular enzymes leucine amino peptidase and phosphatase had 52 and 27% lower activities, respectively, while microbial biomass was 35% lower than the Slurry. We also show that the magnitude of these responses was in all cases as large as or larger than the positive control, AgNO3, added at 4-fold the Ag concentration of the silver nanoparticles.

Effects of Silver Nanoparticle Exposure on Germination and Early Growth of Eleven Wetland Plants

The increasing commercial production of engineered nanoparticles (ENPs) has led to concerns over the potential adverse impacts of these ENPs on biota in natural environments. Silver nanoparticles (AgNPs) are one of the most widely used ENPs and are expected to enter natural ecosystems. Here we examined the effects of AgNPs on germination and growth of eleven species of common wetland plants. We examined plant responses to AgNP exposure in simple pure culture experiments (direct exposure) and for seeds planted in homogenized field soils in a greenhouse experiment (soil exposure). We compared the effects of two AgNPs–20-nm polyvinylpyrrolidine-coated silver nanoparticles (PVP-AgNPs) and 6-nm gum arabic coated silver nanoparticles (GA-AgNPs)–to the effects of AgNO3 exposure added at equivalent Ag concentrations (1, 10 or 40 mg Ag L−1). In the direct exposure experiments, PVP-AgNP had no effect on germination while 40 mg Ag L−1 GA-AgNP exposure significantly reduced the germination rate of three species and enhanced the germination rate of one species. In contrast, 40 mg Ag L−1 AgNO3 enhanced the germination rate of five species. In general root growth was much more affected by Ag exposure than was leaf growth. The magnitude of inhibition was always greater for GA-AgNPs than for AgNO3 and PVP-AgNPs. In the soil exposure experiment, germination effects were less pronounced. The plant growth response differed by taxa with Lolium multiflorum growing more rapidly under both AgNO3 and GA-AgNP exposures and all other taxa having significantly reduced growth under GA-AgNP exposure. AgNO3 did not reduce the growth of any species while PVP-AgNPs significantly inhibited the growth of only one species. Our findings suggest important new avenues of research for understanding the fate and transport of NPs in natural media, the interactions between NPs and plants, and indirect and direct effects of NPs in mixed plant communities.

PREDICTING EFFECTS OF ECOSYSTEM ENGINEERS ON PATCH-SCALE SPECIES RICHNESS FROM PRIMARY PRODUCTIVITY

Ecosystem engineering—the physical modification of habitats by organisms—can create patches with altered species richness relative to adjacent, unmodified patches. The effect of ecosystem engineering on patch-scale species richness is likely to be difficult to predict from the identity of the engineer, the resources altered as a result of engineering, or the identities of the affected species. Here we develop a simple conceptual model that predicts the effects of ecosystem engineers on species richness based on how the habitat modifications caused by engineers affect primary productivity, assuming a hump-shaped relationship between productivity and species richness. We review data from 35 studies that contained 60 comparisons of species richness on patches that had been modified by ecosystem engineers vs. unmodified patches. We found no general patterns in whether species richness at the patch scale was increased or decreased by ecosystem engineering. However, 14 of these studies also contained data on pr...

LOCAL VS. LANDSCAPE CONTROLS ON PLANT SPECIES RICHNESS IN BEAVER MEADOWS

There is considerable interest in determining whether the species richness of communities is determined by forces controlling dispersal into patches that operate at the landscape scale, or forces controlling persistence that act at the local scale. Understanding the relative importance of these two classes of factors in controlling within-patch species richness is particularly important when patches are created via ecosystem engineering. In such cases, factors affecting the population dynamics or behavior of a single species could indirectly affect species richness if richness is controlled primarily by landscape-level factors. We used a combination of experimental mesocosms and field observations to determine whether species richness in beaver wetlands in the Adirondack Mountains (New York) is more strongly controlled by the position of the wetland in the landscape or by within-wetland hydrology. Drainage rate had a significant effect on both richness and composition in mesocosms, with well-drained treatments having significantly higher richness than poorly drained treatments. Seed germinated from the seed bank in sediments collected from different ponds showed relatively small differences in richness or community composition in mesocosms, suggesting a comparatively small effect of dispersal limitation on species richness. Experimental results were mirrored in a survey of 14 meadows over two years, which indicated that variability in water table depth was consistently a significant predictor of species richness, while meadow area and isolation showed little relation to richness. The survey also suggested that the number of years since beaver had abandoned a site was a significant predictor of the number of species found in beaver meadows. The results indicate that species richness in beaver meadows is strongly controlled by local factors, but that the population dynamics of beaver could also potentially affect species richness by altering the age distribution of meadows across the landscape.

The putative niche requirements and landscape dynamics of Microstegium vimineum: an invasive Asian grass

The theoretical foundations of population and community ecology stress the importance of identifying crucial niche requirements and life history stages of invasive species and, in doing so, give insight into research and management. We focus on Microstegium vimineum, an invasive grass which is causing marked changes in the structure and function of US forests. We describe M. vimineum’s life history and habitat characteristics, infer its niche requirements and synthesize this information in the context of population dynamics and management. Based on the results synthesized here, M. vimineum’s crucial niche requirements appear to be light (reproductive output), soil moisture (reproductive output, seedling recruitment) and aboveground coverage by leaf-litter and competing species (seedling recruitment and survival). These data suggest a source-sink dynamic might allow M. vimineum to disperse and thrive along sunny, and sometimes wet, edge habitats and, in turn, these populations might act as source populations for adjacent shady forest habitats. By evaluating M. vimineum in the context of its stage-specific requirements, we highlight potential weaknesses in its life history that provide strategies for effective management.

Does the leaf economic spectrum hold within local species pools across varying environmental conditions

Summary Understanding patterns of trait variation across environmental variability is necessary for development of ecological predictions. The leaf economic spectrum (LES) has demonstrated global trade-offs in leaf traits, but it is unclear whether such patterns are robust in local communities exposed to varying environments. We conducted separate greenhouse experiments to examine the effects of varying water-table depth and nitrogen availability on leaf-level trait values among a suite of co-occurring wetland species. We then assessed the effects of species-specific trait value responses on relationships predicted by LES and whether species responded similarly to variations in water-table depth and nitrogen availability. We found that both water-table depth and nitrogen availability had significant species by treatment interactions for specific leaf area, leaf nitrogen and photosynthetic rates, indicating species-specific responses to environmental variability. The responses of individual traits to different treatment levels were relatively consistent across species, but multivariate responses were more variable. We found that apart from significant relationships between specific leaf area and photosynthetic rate under some treatments, there was little support for the relationships predicted by the LES. These results suggest that, before trait-based ecology will be able to make progress towards using plant traits to predict responses of communities and ecosystems to changes in environmental drivers, considerable attention needs to be paid to the processes that control intraspecific trait variation.