Understanding the consequences of environmental change on ecological and evolutionary dynamics is inherently problematic because of the complex interplay between them. Using invertebrates in microcosms, we characterise phenotypic, population and evolutionary dynamics before, during and after exposure to a novel environment and harvesting over 20 generations. We demonstrate an evolved change in life-history traits (the age- and size-at-maturity, and survival to maturity) in response to selection caused by environmental change (wild to laboratory) and to harvesting (juvenile or adult). Life-history evolution, which drives changes in population growth rate and thus population dynamics, includes an increase in age-to-maturity of 76% (from 12.5 to 22 days) in the unharvested populations as they adapt to the new environment. Evolutionary responses to harvesting are outweighed by the response to environmental change (∼ 1.4 vs. 4% change in age-at-maturity per generation). The adaptive response to environmental change converts a negative population growth trajectory into a positive one: an example of evolutionary rescue.

Experimental and theoretical studies show that mortality imposed on a population can counter-intuitively increase the density of a specific life-history stage or total population density. Understanding positive population-level effects of mortality is advancing, illuminating implications for population, community, and applied ecology. Reconciling theory and data, we found that the mathematical models used to study mortality effects vary in the effects predicted and mechanisms proposed. Experiments predominantly demonstrate stage-specific density increases in response to mortality. We argue that the empirical evidence supports theory based on stage-structured population models but not on unstructured models. We conclude that stage-specific positive mortality effects are likely to be common in nature and that accounting for within-population individual variation is essential for developing ecological theory.

Summary 1.Population dynamics results from an interplay between the environmental state and population density. With many organisms there is structure to the life history, and this structure has important consequences for the population's density dependence and its interaction with environmental noise, and therefore its population dynamics. Perturbing population structure, such as through harvesting, may therefore affect the way that the populations respond to stochastic environmental variation. 2. We conducted three experiments on populations of soil mites kept under controlled conditions and harvested a constant proportion of eggs, juveniles or adults. The experiments (a) minimized environmental variability, (b) created environmental variability by randomizing food supplies, and (c) provided excess food and repeatedly subdivided the populations to maintain them below carrying capacity. 3. We find that harvesting different stages has marked effects on stage structure, which differ between constant and variable environments. For example, harvesting adults decreases the number of adults, harvesting eggs increases the number of adults and harvesting juveniles has no effect under near constant conditions, whereas in a variable environment harvesting adults and juveniles reduces adult numbers, but harvesting eggs has no effect. 4. As well as changing the mean age structure, harvesting can change the variance of the different stages. For example, in a constant environment harvesting juveniles does not change the variability in juvenile numbers or population size, but in a variable environment harvesting juveniles increases the variability in the size of the juvenile class and hence the total population variability. 5. For populations that are kept at one-third to one-half carrying capacity (approximately where the maximum sustainable yield should result) harvesting of the different age classes has marked positive effects on the population growth rates. Harvesting different age classes causes changes in the density dependence, which explains the way in which the population parameters respond. 6. In conclusion, the population response to harvesting depends on the stage/age structure and the way it changes with harvesting and environmental conditions. Managing economically important populations, subject to harvesting, should consider structured life histories.

Savanna ecosystems are dominated by two distinct plant life forms, grasses and trees, but the interactions between them are poorly understood. Here, we quantified the effects of isolated savanna trees on grass biomass as a function of distance from the base of the tree and tree height, across a precipitation gradient in the Kruger National Park, South Africa. Our results suggest that mean annual precipitation (MAP) mediates the nature of tree-grass interactions in these ecosystems, with the impact of trees on grass biomass shifting qualitatively between 550 and 737 mm MAP. Tree effects on grass biomass were facilitative in drier sites (MAP≤550 mm), with higher grass biomass observed beneath tree canopies than outside. In contrast, at the wettest site (MAP = 737 mm), grass biomass did not differ significantly beneath and outside tree canopies. Within this overall precipitation-driven pattern, tree height had positive effect on sub-canopy grass biomass at some sites, but these effects were weak and not consistent across the rainfall gradient. For a more synthetic understanding of tree-grass interactions in savannas, future studies should focus on isolating the different mechanisms by which trees influence grass biomass, both positively and negatively, and elucidate how their relative strengths change over broad environmental gradients.

Summary1. The patterns of density-dependent resource competition and the mechanisms leading to competitive exclusion in an experimental two-species insect age-structured interaction were investigated. 2. The modes of competition (scramble or contest) and strength of competition (underto overcompensatory) operating within and between the stages of the two species was found to be influenced by total competitor density, the age structure of the competitor community and whether competition is between stages of single or two species. 3. The effect of imposed resource limitation on survival was found to be asymmetric between stages and species. Environments supporting both dominant and subordinate competitors were found to increase survival of subordinate competitors at lower total competitor densities. Competitive environments during development within individual stage cohorts (i.e. small or large larvae), differed from the competitive environment in lumped age classes (i.e. development from egg → pupae). 4. Competition within mixed-age, stage or species cohorts, when compared with uniform-aged or species cohorts, altered the position of a competitive environment on the scramble-contest spectrum. In some cases the competitive environment switched from undercompensatory contest to overcompensatory scramble competition. 5. Such switching modes of competition suggest that the relative importance of the mechanisms regulating single-species population dynamics (i.e. resource competition) may change when organisms are embedded within a wider community.

Summary1. Mathematical models are frequently used to make predictions of the response of a population to management interventions or environmental perturbations, but it is rarely possible to make controlled or replicated tests of the accuracy of these predictions. 2. We report results from replicated laboratory experiments on populations of a soil mite, Sancassania berlesei, living in 'constant' or 'variable' environments. We experimentally perturbed vital rates, via selective harvesting, and examined the population-level responses. The response depends on the stage manipulated and whether there is environmental variability. Increased mortality usually decreased population size and increased population variability. However, egg mortality in a variable environment increased total population size. 3. We used time-series analysis to construct a stage-based population model of this system, incorporating the responses to both density and variation in food supply. 4. The time-series model qualitatively captures the population dynamics, but does not predict well the way the populations will respond to the change in mortality. Elasticity analysis, conducted on the model's output, therefore did not lead to accurate predictions. 5. The presence of indirect positive population effects of a negative perturbation, but only in a variable environment, suggests that predicting the population response will require the incorporation of density dependence and environmental stochasticity. That the considerable biological complexity of our time-series model did not allow accurate predictions suggests that accurate prediction requires modelling processes within a stage class rather than trying to make do with simple functions of total density.

Environmental change continually perturbs populations from a stable state, leading to transient dynamics that can last multiple generations. Several long-term studies have reported changes in trait distributions along with demographic response to environmental change. Here we conducted an experimental study on soil mites and investigated the interaction between demography and an individual trait over a period of nonstationary dynamics. By following individual fates and body sizes at each lifehistory stage, we investigated how body size and population density influenced demographic rates. By comparing the ability of two alternative approaches, a matrix projection model and an integral projection model, we investigated whether consideration of trait-based demography enhances our ability to predict transient dynamics. By utilizing a prospective perturbation analysis, we addressed which stage-specific demographic or trait-transition rate had the greatest influence on population dynamics. Both body size and population density had important effects on most rates; however, these effects differed substantially among life-history stages. Considering the observed trait-demography relationships resulted in better predictions of a population's response to perturbations, which highlights the role of phenotypic plasticity in transient dynamics. Although the perturbation analyses provided comparable predictions of stagespecific elasticities between the matrix and integral projection models, the order of importance of the life-history stages differed between the two analyses. In conclusion, we demonstrate how a trait-based demographic approach provides further insight into transient population dynamics. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. Here we conducted an experimental study on soil mites and investigated the interaction between demography and an individual trait over a period of nonstationary dynamics. By following individual fates and body sizes at each life-history stage, we investigated how body size and population density influenced demographic rates. By comparing the ability of two alternative approaches, a matrix projection model and an integral projection model, we investigated whether consideration of trait-based demography enhances our ability to predict transient dynamics. By utilizing a prospective perturbation analysis, we addressed which stage-specific demographic or trait-transition rate had the greatest influence on population dynamics. Both body size and population density had important effects on most rates; however, these effects differed substantially among life-history stages. Considering the observed trait-demography relationships resulted in better predictions of a population's response to pert...

Summary1. Cyclic dynamics of various periods are pervasive in many insect populations where interactions with natural enemies are known to be important. How stage-structured processes within the host population, such as competition and cannibalism, affect these interactions has received little attention so far. 2. Using the well-studied laboratory host-parasitoid system of Plodia interpunctella and Venturia canescens , we explore a series of host-parasitoid models of increasing complexity. Specifically, we identify the circumstances under which stage-structured processes both within the host and parasitoid populations generate dynamical behaviour ranging from generation to true consumer-resource (multi-generation) cycles. 3. We find that both within-host interactions (strong competition and egg cannibalism by late instar larvae) and parasitoid recruitment structure (a developmental lag in the parasitism of larvae) can suppress long period cycles and promote host generation cycles. In short, we show that simple stage-structured processes in both host and parasitoid can modulate the strength of the consumer-resource interaction. 4. For some parameters we find more than one stable cyclic attractor. The presence of multiple attractors means that the same mechanism can give rise to cycles of different periods depending on initial population numbers. Because the host-alone system may exhibit transient dynamics for a substantial period, the timing of a parasitoid invasion can be crucial to the dynamical outcome. 5. We discuss the consequences of using a single descriptor of an ecological time series, the cycle period, to infer properties about the underlying system and its food web interactions.