Resource Supply Rates Research Articles

Optimal transition to backstop substitutes for nonrenewable resources

We analyze the optimal transition from a primary, nonrenewable resource to a backstop substitute for a class of problems characterized by the property that the backstop cost decreases continuously as learning from R&D efforts accumulates to increase the knowledge base. The transition policy consists of the R&D process and of the time profiles of the primary and backstop resource supply rates. We find that the optimal R&D process follows a most rapid approach path (MRAP): the knowledge process associated with R&D should approach some (endogenously derived) target process as rapidly as possible and proceed along it thereafter. Thus, if the initial knowledge level is sufficiently low and the cost structure justifies R&D activities, the R&D efforts should be initiated without delay at the highest affordable rate and slow down later on. This pattern contrasts previous findings that typically recommend a single-humped R&D process with a possible initial delay.

Resource quantity, not resource heterogeneity, maintains plant diversity

AbstractResource heterogeneity has often been proposed to explain the maintenance of plant species diversity and patterns of species diversity along productivity gradients. Resource heterogeneity should maintain biodiversity by preventing competitive exclusion because different species are superior competitors in different parts of a heterogeneous environment. In natural systems, however, resource heterogeneity covaries with average resource supply rate, making the effect of heterogeneity difficult to isolate. Using a novel experimental approach, we tested the independent effects of resource heterogeneity and average supply rate on plant species diversity. We show that the average supply rate of the most limiting resource controlled species diversity, whereas heterogeneity of this resource had virtually no effect. These findings also suggest that biodiversity declines with increasing productivity because at high enough levels of productivity one resource may always be driven to sufficiently short supply to exclude many species.

Megacities and the environment.

The world’s 25 largest cities comprise only 4% of the global population, but they have substantial impacts on the environment at multiple scales. Here we review what is known of the biogeochemistry of these megacities. Climatic, demographic, and economic data show no patterns across cities, save that wealthier cities have lower growth rates. The flows of water, fuels, construction materials, and food are examined where data are available. Water, which by mass dwarfs the other inputs, is not retained in urban systems, whereas construction materials and food predominate in the urban infrastructure and the waste stream. Fuels are transformed into chemical wastes that have the most far-reaching and global impacts. The effects of megacity resource consumption on geologic, hydrologic, atmospheric, and ecological processes are explored at local, regional, and global scales. We put forth the concepts of urban metabolism and urban succession as organizing concepts for data collection, analysis, and synthesis on urban systems. We conclude that megacities are not the final stage of urban evolution; rather, the climax of urban development will occur at a global scale when human society is at steady state with resource supply rates.

Increasing plant diversity does not influence productivity: empirical evidence and potential mechanisms

The relationship between species diversity and ecosystem functions has generated considerable debate among ecologists. Ecosystem functions (e.g. productivity, nutrient retention) are often positively correlated with species richness in experimental plant assemblages, but little or no correlation exists in natural communities.We examined the effects of species richness on productivity and available soil nitrate by experimentally manipulating richness using random draws from a pool of ten perennial grasses. Species richness had no significant effect on aboveground productivity or soil nitrate availability, suggesting that functional diversity may be more important than species richness in determining ecosystem functions. The relationship between diversity and ecosystem functions may also depend on resource limitation. A positive relationship is expected when below-ground resources are limiting, but the relationship is expected to weaken when below-ground resource supply rates are higher and competition for light becomes more important. Further experiments are required to determine the mechanisms underlying diversity-productivity relationships.

Root structure and function in an ecological context.

Ecological theories concerning the relationship of suites of traits with resource supply and plant potential growth rate have concentrated primarily on leaf structure and physiology - much less is known about interspecific patterns of variation in root structure and physiology. In this issue, Wahl & Ryser reduce this knowledge gap by demonstrating important correlations of root structure with potential growth rate and height among a range of perennial temperate grass species adapted to sites of a wide range of fertility (see pp. 459-471).

Should Plants in Resource-Poor Environments Invest More in Antiherbivore Defence?

The resource availability hypothesis' proposes that low resource availability favours plants with inherently slow growth rates. which in turn favours large investments in antiherbivore defence. Here we present a simple model of evolution of plant defence allocation in a system incorporating plant-resource dynamics. and show that taking into account the feedback of plants on the amount of a limiting resource leads to very different predictions than those from the classical resource availability hypothesis. The latter is based on the assumption that fitness is determined by the potential growth rate, hence, implicitly, on the assumption of unlimited growth in a transient environment where plants do not deplete soil resources. In an environment where plants have accumulated enough biomass to control resource concentration, fitness is determined by the ability to deplete limiting resources if the environment is homogeneous or by other traits correlated with biomass or productivity if the environment is spatially structured. In this case, resource supply and maximum growth rate may either increase, decrease, or in most cases not affect al all, optimal defence investment. Much more detailed data about what determines fitness in climax' environments would be necessary to make even qualitative predictions about the direction of evolution of antiherbivore defence investment as a function of resource availability and maximum growth rate.

BIOLOGICAL WEED CONTROL VIA NUTRIENT COMPETITION: POTASSIUM LIMITATION OF DANDELIONS

Weedy plants are often controlled by the application of herbicides. Here we explore an alternative method of control. We suggest that the abundance of an undesired plant species (here dandelions: Taraxacum officinale) may be controlled by modifying interspecific competition via changes in resource supply rates. This hypothesis is supported by several lines of evidence. First, analyses of effects of different patterns of fertilization on plant-species abundances in the 140-yr-old Park Grass Experiment at Rothamsted, England, show that Taraxacum abundances were highly dependent on potassium fertilization and on liming, but not on addition of other nutrients. Potassium fertilization led to a 17- to 20-fold increase in Taraxacum abundances in the classical Park Grass data, and to a 4- to 7-fold increase in the modern data. Liming led to a 2- to 3-fold increase for classical data and to a 3- to 4-fold increase for modern data. Second, in a greenhouse study in Minnesota, Taraxacum had a higher requirement for potassium and had its biomass more limited by potassium than any of five common grass species of Park Grass. This suggests that Taraxacum may be a poorer competitor for potassium than these grasses, but this mechanism has not yet been tested. Third, in a series of Minnesota lawns that had not received fertilizer or herbicides, both Taraxacum density and abundance were significantly positively correlated with its tissue potassium levels. This demonstration that desired and weedy plant species can differ in their resource requirements suggests that adjustments in resource supply rates may determine the outcome of interspecific competition, allowing desired species to competitively control weedy species. In particular, for soils with low potassium levels, the use of potassium-free lawn fertilizer is predicted to decrease Taraxacum because of competition from grasses like Festuca rubra.

The Relationship between the Supply of Cardiac Catheterization Laboratories, Cardiologists and the use of Invasive Cardiac Procedures in Northern New England

Utilization rates of coronary angiography and cardiac revascularization have been found to vary between areas. This study addresses the relationship between resource supply and procedure rates. We compared the association of per capita catheterization laboratories, per capita cardiologists and multi-provider markets (where more than one hospital offers coronary angiography services) with the utilization rates for angiography and cardiac revascularization in northern New England, USA. Administrative data were used to capture invasive cardiac procedures. Small area analyses were used to create coronary angiography service areas. Linear regression methods were used to measure associations between the resource supply and utilization rates. Variation in the use of invasive cardiac procedures was strongly associated with the population-based availability of catheterization facilities and multi-provider markets and unrelated to cardiologist supply or need (as reflected in the hospitalization rates for myocardial infarction). In the multivariate model, an increase of 1 catheterization laboratory per 100,000 population was associated with an increase in the angiography rate of 1.62 per 1000 population; those service areas with multi-provider markets were associated with an additional increase in the angiography rate of 1.27 per 1000 population (R2 = 0.84, P = 0.0006). There was a moderately strong relationship between the catheterization laboratories per capita and the revascularization rates (R2 = 0.43, P = 0.029). Angiography rates were highly associated with cardiac revascularization rates: an increase in the angiography rate of 1 per 1000 population was associated with a 0.46 per 1000 increase in the cardiac revascularization rate (R2 = 0.85, P = 0.0001). Our work suggests that current efforts to address variation in cardiac procedures through activities such as appropriateness criteria, guidelines and utilization review are misdirected and should be redirected towards capacity, in this case the supply of catheterization facilities.

Competition and Coexistence: The Effects of Resource Transport and Supply Rates

Classical resource competition theory can be generalized to apply to a variety of specific resource types and specific supply media (e.g., soil, water, or air). We develop a general model that relaxes the assumptions that (1) resources and organisms are sufficiently mixed that all organisms experience the same resource concentration and (2) the organisms themselves regulate the resource concentration of their shared environment. These assumptions are shown to apply to a limited subset of conditions in which the resource input rate is low and the resource transport rate in the environment is high. Under such conditions, the coexistence criteria of our general model converge with those of classical resource competition models. Such conditions may be met in some aquatic environments, but under other conditions, in which resource transport rates may be low or input fluxes high, the general model makes predictions that differ radically from those of the classical models. Specifically, our model predicts that, ...

Convergence and Divergence of Old‐Field Plant Communities Along Experimental Nitrogen Gradients

Data from experiments in three old fields were analyzed using similarity indices and population trajectories to determine if similarity of resource supply rates influenced the similarity of plant community composition. In two of three fields, similarity indices indicated that experimental plots having different resource supply rates diverged in species composition with time. There was some evidence of convergence both within and between fields on plots with similar resource supply rates; however, divergence was also common, especially at high rates of nutrient supply. Much of the divergence indicated by similarity indices can be explained by plot—to—plot differences in the initial abundances of three dominant species: Agropyron repens, Poa pratensis, and Schizachyrium scoparium. High nitrogen treatments favored Agropyron repens if it was present, but led to major increases in P. pratensis or S. scoparium if A. repens was absent. The trajectories through time of these three species suggest that plots will converge at high nitrogen levels.

Optimal Patch Time Under Exploitative Competition

An important theoretical problem in behavioral ecology is the issue of how animals should use patchily distributed resources in order to maximize their rate of resource intake. Charnov (1976) provided an optimal-time solution (the marginal-value theorem) whereby a predator leaves a patch as the rate of prey capture decreases. Parker (1978) similarly analyzed the optimal copulation time for a male, with females considered as resource patches. In these works and most of the subsequent theoretical and experimental studies on optimal foraging, a single animal has been considered the optimizer of time investment (Pyke et al. 1977; Iwasa et al. 1981; Kamil and Sargent 1981; Krebs and McCleery 1984). In natural habitats, however, many animals appear to use their resources competitively, and therefore, the optimal strategy of each animal may depend on the strategies of other animals (Maynard Smith 1974). Here we present a simple game-theoretical model in which animals attempt to maximize their own rate of resource intake. Resource patches considered here are small enough that only one consumer can occupy a given patch. The patch may be a small area where prey is found, as in several experimental studies on small birds (Krebs et al. 1974; Cowie 1977); or it may be a single prey item (Cook and Cockrell 1978), a flower for bumblebees (Whitham 1977), or a female as a mate (Parker 1978). Competition for patches among consumers is assumed to exist as a natural consequence of the exploitation of limited resources, not because of direct interference (Miller 1967). Incorporation of interference competition in the model will be discussed later. The primary purposes of this paper are twofold: to demonstrate a method for finding the optimal strategy in competitive foraging; and to show how the optimal strategy depends on such parameters as the resource supply rate, the number of competitors, and searching efficiency. We also analyze a cooperative case, in which a number of animals attempt to maximize their total rate of resource intake, and we compare the results of the cooperative case and the competitive case. Finally, we discuss the implications of the model for application to actual experimental and observational studies.

Global stability of partially closed food-chains with resources.

We consider the existence and global stability of aq-member equilibrium (1≤q≤n) in partially closed food-chains of lengthn having an abiotic component as resource. We observe that such existence demands bounds of resource supply rate and these bounds are weighted sums of interaction coefficients. Particular results of global sector-stability of partially feasible equilibria of simple food-chains obeying Lotka-Volterra dynamics are shown. Lastly the elasticity of such food-chains when a new species is introduced at the highest trophic level is investigated.

Global stability of partially closed food-chains with resources

We consider the existence and global stability of a q-member equilibrium (1⩽ q⩽ n) in partially closed food-chains of length n having an abiotic component as resource. We observe that such existence demands bounds of resource supply rate and these bounds are weighted sums of interaction coefficients. Particular results of global sector-stability of partially feasible equilibria of simple food-chains obeying Lotka-Volterra dynamics are shown. Lastly the elasticity of such food-chains when a new species is introduced at the highest trophic level is investigated.