We found that the relative variability of populations tended to increase from the centre of the elevational range towards the margins because of an increase in the magnitude of stochastic fluctuations of growth rates. In fact, Exema and Trirhabda abundances correlated positively in a survey of the goldenrod fauna in central New York R. Biological control of scale insects and ecological theory. Physical and numerical experiments show that deterministic noise, or chaos, is ubiquitous. Foliar damage, parasitoids, and indirect competition: A test using herbivores of birch.
In a 38-year dataset of an age-structured great tit Parus major population, we find that the age classes including the youngest breeding females were the critical age classes for density regulation. The Varied Processes of Population Regulation: Testing Old Theories and Building New Ones The question of regulation has always been central to the study of population dynamics. Flowers in dense monocultures were visited less frequently than those on isolated plants surrounded by old-field grasses. We found that forest cover was a better measure of the amount of habitat for polyphagous species than for oligophagous species. Finally, using the spruce bark beetle, Ips typographus, as an example, we demonstrate how the theory can be used to explain the dynamics of specific herbivore populations and to develop general strategies for managing destructive herbivore outbreaks. It is a particular class of causally density-dependent processes that can ensure regulation more durable against such perturbations. We emphasize four inter-linked behavioral aspects-territoriality, dispersal, social inhibition of breeding, and infanticide-that together form a density-dependent syndrome with potentially regulatory effects on population growth.
Thus the notion that nonoutbreak species may be more sensitive to damage-induced changes in plant quality was not supported in this study. At the end of the density—manipulation experiment, all Eurosta were removed from the patches. Population processes in the vole and their relevance to general theory. Host-Plant Attributes Several studies have noted that eggs can be displaced by some host-plant species if they are placed on young, rapidly expanding leaves Mazanec, 1985; M. Two populations appeared to be density regulated.
The Enemy Release Hypothesis links exotic plant success to escape from enemies such as herbivores and pathogens. Even in such a case, cycles are possible only for a limited range of parameter values. We suggest that invasive plants may possess novel phytochemicals with anti-herbivore properties in addition to allelopathic and anti-microbial characteristics. Previous host experience led to a marginal increase, while competition led to a per-capita decrease, in the degree of host exploitation. Bold experiments, done on a larger scale than those attempted in the past, are enabling us to address directly the processes of population change.
In Chapter 8, Hanski and Kuussaari explain metapopulation theory and provide evidence that many butterfly populations, while locally unstable, are indeed regulated at the level of the metapopulation. The more nuanced grasp that emerges of local systems of tenure and access, of how these diverge from western property concepts, and of their envi- ronmental implications favours a better understanding of local realities, allowing for better management policy and consequently contributing more effectively towards poverty alleviation and environmental protection. An invulnerable age class and stability in delay-differential parasitoid-host models. Statistical dependence is defined as a converse of statistical independence, the latter being a process in which the rate of change in density has zero correlation with density; this is a very special class of processes and is unlikely to occur in natural population processes. Practical use of these models for univoltine insects, however, requires modification of the usual model structure to consider single-season epizootics. Genetics, Heritability, and the Evolution of Populations 398 J. Leek moth pupal mortality was significantly greater on uncaged than on caged leeks, indicating an impact by natural enemies, and this pattern was consistent over all three years of study.
Manipulations of aphid density in the field revealed that although patches with high densities of aphids accumulated more predators than patches with few aphids, predation pressure measured as number of predators per aphid was lower in dense patches. These contributions tie together several of the themes addressed in earlier chapters. Biodiversity of a Central American Heliconius community: Pattern, process, and problems. The frequency of over-population and of extinction is reviewed and both appear to occur frequently in Lepidoptera. In this review, I discuss how life-history strategies cause different nonlinear density-dependence patterns and I provide a simple modeling recommendation to incorporate nonlinear density dependence in population growth equations.
The ability to produce large numbers of selfed seeds is no doubt advantageous to this highly invasive asclepiad. Generally, carnivores play a relatively passive role in population dynamics, attacking where possible, but not changing the gross dynamics dictated by resource supply see Harrison and Cappuccino, Chapter 7, this volume. None of the series, all we could find to meet the necessary condition as well as being longer than 10 years, showed significant density dependence at the 0. Community structure, population control, and competition. This approach often sidesteps the question of regulation, concentrating instead on the magnitude of fluctuations, typically measured as the standard deviation of the log-transformed abundance. Ayres 339 Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755 Gary E.
This was always true of Varley and Gradwell's test. Amphibians are in decline in many parts of the world. What is a forest pest? In other words, under some conditions the model does an excellent job of quantitatively predicting virus epizootics. T h e first is that quantitative comparisons b e t w e e n theory and data can expose inadequacies in the theory clearly and rapidly, in turn exposing inadequacies in our understanding of an o r g a n i s m ' s ecology. We found a strong negative relationship between reproductive rate and the minimum habitat amount required for species presence.