Heather L. Reynolds

Consequences of changing biodiversity

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.

What is the observed relationship between species richness and productivity

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 ...

GRASSROOTS ECOLOGY: PLANT–MICROBE–SOIL INTERACTIONS AS DRIVERS OF PLANT COMMUNITY STRUCTURE AND DYNAMICS

A growing body of research on plant-microbe interactions in soil is con- tributing 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 pro- cesses that may exert key influences on plant community structure and dynamics are mi- crobial 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 plant- microbe 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.

The ecological significance of plasticity in root weight ratio in response to nitrogen: Opinion

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.

GRASSLAND INVASIBILITY AND DIVERSITY: RESPONSES TO NUTRIENTS, SEED INPUT, AND DISTURBANCE

The diversity and composition of a community are determined by a com- bination of local and regional processes. We conducted a field experiment to examine the impact of resource manipulations and seed addition on the invasibility and diversity of a low-productivity grassland. We manipulated resource levels both by a disturbance treatment that reduced adult plant cover in the spring of the first year and by addition of fertilizer every year. Seeds of 46 native species, both resident and nonresident to the community, were added in spring of the first year to determine the effects of recruitment limitation from local (seed limitation) and regional (dispersal limitation) sources on local species richness. Our results show that the unmanipulated community was not readily invasible. Seed addition increased the species richness of unmanipulated plots, but this was primarily due to increased occurrence of resident species. Nonresident species were only able to invade following a cover-reduction disturbance. Cover reduction resulted in an increase in nitrogen availability in the first year, but had no measurable effect on light availability in any year. In contrast, fertilization created a persistent increase in nitrogen availability that increased plant cover or biomass and reduced light penetration to ground level. Initially, fertilization had an overall positive effect on species richness, but by the third year, the effect was either negative or neutral. Unlike cover reduction, fertilization had no observable effect on seedling recruitment or occurrence (number of plots) of invading resident or nonresident species. The results of our experiment demonstrate that, although resource fluctuations can increase the invasibility of this grass- land, the community response depends on the nature of the resource change.

SOIL HETEROGENEITY AND PLANT COMPETITION IN ANANNUAL GRASSLAND

Variation in competitive ability due to variation in soil characteristics is one possible mechanism allowing the local coexistence of plant species. We measured soil water, depth, and nitrogen pools and fluxes in distinct patches of three serpentine grassland species to determine whether soil heterogeneity existed and was correlated with plant species abundance. Through experimental manipulation of species’ abundances, we also examined the relative importance of inherent site characteristics vs. plant species’ effects in generating heterogeneity in the measured soil characteristics; and measured species’ competitive abilities in different patch types. The three common grassland annuals, Calycadenia multiglandulosum, Plantago erecta, and Lasthenia californica, were segregated with respect to the measured soil characteristics. Differences in soil water, soil depth, soil microbial nitrogen, and soil carbon to nitrogen ratio were due to inherent site characteristics, while differences in nitrate availability ...

Rapid assay for amidohydrolase (urease) activity in environmental samples

The use of microplate technology for enzyme assays has made it economical to measure a wide range of activities in environmental samples. Urease is one of the most widely measured soil enzyme activities, but current methods are cumbersome. We have developed a rapid, safe and sensitive assay that can be performed on microplates.

Root foraging for patchy resources in eight herbaceous plant species

The root foraging strategy of a plant species can be characterized by measuring foraging scale, precision, and rate. Trade-offs among these traits have been predicted to contribute to coexistence of competitors. We tested for trade-offs among root foraging scale (total root mass and length of structural roots), precision (ln-ratio of root lengths in resource-rich and resource-poor patches), and rate (days required for roots to reach a resource-rich patch, or growth rate of roots within a resource-rich patch) in eight co-occurring species. We found that root foraging scale and precision were positively correlated, as were foraging scale and the rate of reaching patches. High relative growth rate of a species did not contribute to greater scale, precision, or rate of root foraging. Introduced species had greater foraging scale, precision, and rate than native species. The positive correlations between foraging scale and foraging precision and rate may give larger species a disproportionate advantage in competition for patchy soil resources, leading to size asymmetric competition below ground.

Nitrate and ammonium uptake for single-and mixed-species communities grown at elevated CO2

Sustained increases in plant production in elevated CO2 depend on adequate belowground resources. Mechanisms for acquiring additional soil resources include increased root allocation and changes in root morphology or physiology. CO2 research to date has focused almost exclusively on changes in biomass and allocation. We examined physiological changes in nitrate and ammonium uptake in elevated CO2, hypothesizing that uptake rates would increase with the amount of available CO2. We combined our physiological estimates of nitrogen uptake with measurements of root biomass to assess whole root-system rates of nitrogen uptake. Surprisingly, physiological rates of ammonium uptake were unchanged with CO2, and rates of nitrate uptake actually decreased significantly (P<0.005). Root boomass increased 23% in elevated CO2 (P<0.005), but almost all of this increase came in fertilized replicates. Rates of root-system nitrogen uptake in elevated CO2 increased for ammonium in nutrient-rich soil (P<0.05) and were unchanged for nitrate (P>0.80). Root-system rates of nitrogen uptake were more strongly correlated with physiological uptake rates than with root biomass in unamended soil, but the reverse was true in fertilized replicates. We discuss nitrogen uptake and changes in root biomass in the context of root nutrient concentrations (which were generally unchanged with CO2) and standing pools of belowground plant nitrogen. In research to date, there appears to be a fairly general increase in root biomass with elevated CO2, and little evidence of up-regulation in root physiology.

PERTURBATIONS ALTER COMMUNITY CONVERGENCE, DIVERGENCE, AND FORMATION OF MULTIPLE COMMUNITY STATES

Environmental perturbations (e.g., disturbance, fertilization) commonly shift communities to a new mean state, but much less is known about their effects on the variability (dispersion) of communities around the mean, particularly when perturbations are combined. Community dispersion may increase or decrease (representing a divergence or convergence among communities) if changing environmental conditions alter species interactions or magnify small initial differences that develop during community assembly. We used data from an experimental study of disturbance and fertilization in a low-productivity grassland to test how these two perturbations affect patterns of species composition and abundance. We found that a one-time biomass reduction decreased community dispersion, which persisted over four growing seasons. Conversely, continuous fertilization increased community dispersion and, when combined with disturbance, led to the formation of three distinct community states. These results illustrate that perturbations can have differing effects on community dispersion. Attention to the variance in community responses to perturbations lends insight into how ecological interactions determine community structure, which may be missed when focusing only on mean responses. Furthermore, multiple perturbations may have complex effects on community dispersion, yielding convergence or divergence patterns that are difficult to predict based on analysis of single factors.