Human alteration of the global environment has triggered the sixth major extinction event in the history of life and caused widespread changes in the global distribution of organisms. These changes in biodiversity alter ecosystem processes and change the resilience of ecosystems to environmental change. This has profound consequences for services that humans derive from ecosystems. The large ecological and societal consequences of changing biodiversity should be minimized to preserve options for future solutions to global environmental problems.

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least‐squares (OLS) regression and generalized linear‐model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?). A meta‐analysis conducted on the distribution of standardized quadratic regression coefficients showed that the average quadratic coefficient was negative (i.e., the average species richness–productivity relationship was curvilinear and decelerating), and that the distribution of standardized quadratic regression coefficients was significantly heterogeneous (i.e., the studies did not sample the same underlying species richness–productivity relationship). Looking more closely at the patterns of productivity–diversity relationships, we found that, for vascular plants at geographical scales smaller than continents, hump‐shaped relationships occurred most frequently (41–45% of all studies). A positive relationship between productivity and species richness was the next most common pattern, and positive and hump‐shaped relationships co‐dominated at the continental scale. For animals, positive, negative, and hump‐shaped patterns were common at most geographical scales, and no one pattern predominated. For both plants and animals, hump‐shaped curves were relatively more common in studies that crossed community boundaries compared to studies conducted within a community type, and plant studies that crossed community types tended to span a greater range of productivity compared to studies within community types. Sample size and plot size did not affect the probability of finding a particular productivity–diversity relationship (e.g., positive, hump‐shaped, etc.). However, hump‐shaped curves were especially common (65%) in studies of plant diversity that used plant biomass as a measure of productivity, and in studies conducted in aquatic systems.

Understanding the relationship between species richness and productivity is fundamental to the management and preservation of biodiversity. Yet despite years of study and intense theoretical interest, this relationship remains controversial. Here, we present the results of a literature survey in which we examined the relationship between species richness and productivity in 171 published studies. We extracted the raw data from published tables and graphs and subjected these data to a standardized analysis, using ordinary least-squares (OLS) regression and generalized linear-model (GLIM) regression to test for significant positive, negative, or curvilinear relationships between productivity and species diversity. If the relationship was curvilinear, we tested whether the maximum (or minimum) of the curve occurred within the range of productivity values observed (i.e., was there evidence of a hump?).A meta-analysis conducted on the distribution of standardized quadratic regression coefficients showed that the average quadratic coefficient was negative (i.e., the average species richness-productivity relationship was curvilinear and decelerating), and that the distribution of standardized quadratic regression coefficients was significantly heterogeneous (i.e., the studies did not sample the same underlying species richness-productivity relationship).Looking more closely at the patterns of productivity-diversity relationships, we found that, for vascular plants at geographical scales smaller than continents, hump-shaped relationships occurred most frequently (41-45% of all studies). A positive relationship between productivity and species richness was the next most common pattern, and positive and hump-shaped relationships co-dominated at the continental scale. For animals, positive, negative, and hump-shaped patterns were common at most geographical scales, and no one pattern predominated. For both plants and animals, hump-shaped curves were relatively more common in studies that crossed community boundaries compared to studies conducted within a community type, and plant studies that crossed community types tended to span a greater range of productivity compared to studies within community types. Sample size and plot size did not affect the probability of finding a particular productivity-diversity relationship (e.g., positive, hump-shaped, etc.). However, hump-shaped curves were especially common (65%) in studies of plant diversity that used plant biomass as a measure of productivity, and in studies conducted in aquatic systems.

Abstract. A growing body of research on plant-microbe interactions in soil is contributing to the development of a new, microbially based perspective on plant community ecology. Soil-dwelling microorganisms are diverse, and interactions with plants vary with respect to specificity, environmental heterogeneity, and fitness impact. Two microbial processes that may exert key influences on plant community structure and dynamics are microbial mediation of niche differentiation in resource use and feedback dynamics between the plant and soil community. The niche differentiation hypothesis is based on observations that soil nutrients occur in different chemical forms, that different enzymes are required for plant access to these nutrients, and that soil microorganisms are a major source of these enzymes. We predict that plant nutrient partitioning arises from differential associations of plant species with microbes able to access different nutrient pools. Feedback dynamics result from changes in the soil community generated by the specificity of response in plantmicrobe interactions. We suggest that positive feedback between plants and soil microbes plays a central role in early successional communities, while negative feedback contributes both to species replacements and to diversification in later successional communities. We further suggest that plant-microbe interactions in the soil are an important organizing force for large-scale spatial gradients in species richness. The relative balance of positive feedback (a homogenizing force) and negative feedback (a diversifying force) may contribute to observed latitudinal (and altitudinal) diversity patterns. Empirical tests of these ideas are needed, but a microbially based perspective for plant ecology promises to contribute to our understanding of long-standing issues in ecology, and to reveal new areas of future research.

Summary• We investigated the effects of arbuscular mycorrhizal fungal (AMF) species richness and composition on plant community productivity and diversity, and whether AMF mediate plant species coexistence by promoting niche differentiation in phosphorus use.• Our experiment manipulated AMF species richness and identity across a range of P conditions in tallgrass prairie mesocosms.• We showed that increasing AMF richness promoted plant diversity and productivity, but that this AMF richness effect was small relative to the effects of individual AMF species. We found little support for AMF-facilitated complementarity in P use. Rather, the AMF richness effect appeared to be caused by the inclusion of particular diversity-and productivity-promoting AMF (a sampling effect). Furthermore, the identity of the diversity-promoting fungi changed with P environment, as did the relationship between the diversity-promoting and productivity-promoting benefits of AMF.• Our results suggest that plant diversity and productivity are more responsive to AMF identity than to AMF diversity per se , and that AMF identity and P environment can interact in complex ways to alter community-level properties.

Question Plant–soil feedback (PSF) has emerged as a ubiquitous phenomenon and a potentially important predictor of plant community structure and dynamics. However, the predictive power of PSF in field contexts is mixed, and ecologists do not yet understand its relative importance compared to other factors that structure communities. Further progress requires a more nuanced understanding of how PSF interacts with other biotic and abiotic factors. Environmental factors (e.g. natural enemies, moisture, light, nutrients) are known to affect plant interactions with soil and soil organisms, leading to an expectation of environmental context dependence in plant–soil feedback. Such context dependence could explain when PSF is expected to be an important driver of community dynamics, and under what conditions PSF is likely to be positive (destabilizing) vs negative (stabilizing). Methods We conducted a literature review of studies that examined PSF across biotic and abiotic gradients. Results Although few relevant studies have been conducted in this emerging research area, results to date suggest that plant–soil feedback is sensitive to biotic factors, such as above‐ and below‐ground herbivory, and abiotic factors, such as nutrients and light. Conclusions We develop a conceptual framework to predict variation in the direction and strength of PSF depending on the biotic or abiotic drivers of feedback (e.g. pathogens, nutrients) and the environmental context (e.g. intensity of herbivory, soil fertility). We explore the utility of our predictive framework through discussion of case studies from the literature on context dependence in PSF. We also consider how different experimental approaches might yield different insights about PSF–environment interactions, and suggest key future research directions.

We analyzed data on root weight ratio from a range of experimental studies documenting plant allocation changes in response to altered nitrogen availability. Our goal was to determine the degree to which plasticity in allocation between roots and shoots exists and to search for patterns in such plasticity among species. Our survey included 77 studies representing 206 cases and 129 species. As expected, we found that root weight ratio decreased with increased nitrogen availability in the majority of cases examined, and this response was most consistent when plants were grown individually or in intraspecific competition (versus interspecific competition). Surprisingly, however, we found no evidence to support existing hypotheses that fast-growing species adapted to high soil fertilities exhibit the highest levels of morphological plasticity, or that plasticity is positively associated with competitive ability. Rather, we found that average amounts of plasticity in root weight ratio in response to nitrogen availability were similar among species grouped by maximum relative growth rate and habitat fertility. Similar results were obtained for species categorized by life form, life history or root weight ratio itself, and plasticity in root weight ratio also had no consistent relationship with competitive ability. Numerous difficulties are associated with the attempt to search for pattern using independent studies, however our results lead to the conclusion that strong patterns in plasticity of root weight ratio in response to nitrogen availability among species do not exist. We discuss two reasons for this: (1) the costs of plasticity relative to its benefits are lower than previously predicted and (2) plasticity in traits other than root weight ratio is more important to plant foraging ability.

Summary• Arbuscular mycorrhizal fungi (AMF) are known to promote plant growth when phosphorus is limiting, but the role of AMF in plant growth under nitrogen (N) limiting conditions is unclear.• Here, we manipulated N (control vs inorganic and organic forms) and AMF species (control vs four AMF species) for five old-field perennials grown individually in a glasshouse under N-limiting conditions.• We found that AMF were at best neutral and that some AMF species depressed growth for some plant species (significant plant-fungus interaction). Native plant species growth was strongly depressed by all but one AMF species; exotic plant species were less sensitive to AMF. We found no evidence of plant N preferences. Both natives and exotics were able to acquire more N with N addition, but only exotics grew more with added N.• Our results suggest that AMF do not promote plant N acquisition at low N supply, and our results are consistent with other research showing that AMF can act as a parasitic carbon drain when phosphorus availability is relatively high.