Judith H. Myers

Population Cycles in Small Mammals

Publisher Summary This chapter summarizes the current information on population cycles in small rodents. It first looks at the general questions about cycles, and then discusses the demographic machinery which drives the changes in numbers. And finally, analyzes the current theories which explain population cycles in rodents. Population cycles in voles and lemmings are accompanied by a series of changes. A few of them include fluctuations occurring in a variety of genera and species from arctic to temperate areas, from Mediterranean to continental climates, from snowy areas to snow-free areas. Populations living in a wide variety of plant communities in a small geographic area all fluctuate in the same way, often in phase. Survival of adult males fluctuates independently of that of adult females, when viewed on a weekly time scale. Males may suffer heavy losses in the decline for a few weeks when females are surviving very well, and vice versa.

Eradication revisited: dealing with exotic species

Invasions of nonindigenous species threaten native biodiversity, ecosystem functioning, animal and plant health, and human economies. The best solution is to prevent the introduction of exotic organisms but, once introduced, eradication might be feasible. The potential ecological and social ramifications of eradication projects make them controversial; however, these programs provide unique opportunities for experimental ecological studies. Deciding whether to attempt eradication is not simple and alternative approaches might be preferable in some situations.

Multiple agents in biological control: improving the odds?

Abstract The current interest in risks associated with classical biological control led us to review the literature to determine whether the introduction of multiple biological control agents has been more effective than the introduction of a single control agent. We analyzed 59 projects against weeds and 108 projects against insect pests. Establishment of control agents was significantly higher in single-agent projects than in multiple-agent projects against insect pests, but not in projects against weeds. The success of biological control against weeds increased with the number of agents released. However, no relationship was found between the number of agents released and biological control success for insect pests. These results suggest that negative interactions may play a significant role amongst biological control agents of insects. Thus, multiple releases may be effective against weeds only. In over 50% of the successful multiple-agent projects against weeds and insect pests, a single agent was shown to be responsible for success. This result indicates that, in a majority of biological control projects, multiple agents are not released for a cumulative control effect but to increase the likelihood that the right control species is released (lottery model). Considering the possibility of negative interactions among biological control agents and taking into account the risks associated with natural enemy introductions, we recommend restraint in the introduction of multiple agents for biological control.

Ecology and control of introduced plants

1. Introduction 2. Planet of weeds: exotics in the landscape 3. Ecosystem, community composition, richness and dynamics 4. Life history characteristics and predicting invasiveness 5. Population ecology 6. Introduced plant diseases 7. Classical biological control of exotic weeds 8. Models of invasive plant populations 9. Cultural control of invasive species 10. Conserving and restoring natural communities 11. Some tools for studying plant populations 12. Conclusions: what the future holds.

Genetic, Behavioral, and Reproductive Attributes of Dispersing Field Voles Microtus pennsylvanicus and Microtus ochrogaster

To investigate experimentally the relationship between dispersal and population regulation in small mammals, voles were removed continuously from two plots in southern Indiana for 2 years. Three control populations of two Microtus species were monitored over the same period, and animals dispersing onto the experimental areas were compared with resident control animals for the following characteristics: (1) age, weight, and sex; (2) genotype for two polymorphic plasma proteins, leucine aminopeptidase (LAP) and transferrin (Tf); and (3) exploratory, aggressive, and general activity behavior of males. Dispersal was most common during the fall and winter, and in the phase of population increase 59% of male and 69% of female Microtus pennsylvanicus loss from two control populations could be accounted for by dispersal. In contrast, little of the high loss during the population decline could be associated with dispersal. In the late peak and decline periods male M. pennsylvanicus of the Tf—E and LAP—S phenotypes...

Can a General Hypothesis Explain Population Cycles of Forest Lepidoptera

Publisher Summary In this chapter, the characteristics of population cycles of forest Lepidoptera is described and mechanisms proposed to explain them are evaluated. Evidence for population cycles in forest lepidoptera is also discussed in the chapter. Characteristics of cyclic populations of forest lepidoptera is reviewed, wherein, characteristics of cyclic species, patterns of population change, the beginning of the decline, insect fecundity and population fluctuations, parasitoids and population fluctuations, cyclic and non-cyclic populations, and the impact of forest defoliators on the forests are described. The chapter then turns to the discussion of hypotheses to explain population cycles, wherein, it has described variation in insect quality, climatic release hypothesis, and variation in plant quality, disease susceptibility, and mathematical models. Evaluation of hypotheses is also done in the chapter. Population cycles of other organisms are also summarized in the chapter. Finally, the chapter closes with conclusions and speculations.

Sex Ratio Adjustment Under Food Stress: Maximization of Quality or Numbers of Offspring?

Trivers and Willard (1973) proposed that females in poor condition should produce female offspring. The suggested reason for this is that poor-condition female young are more likely to be reproductively successful than are poor-condition male young. Evidence from the literature does not support this hypothesis, and cited studies can be interpreted in terms of differential mortality between the sexes. I have proposed as an alternative the hypothesis that poor environmental conditions will lead to the production of the cheaper sex. This allows for maximization of the number of young produced. A study of redwing blackbirds illustrates the hypothesis, and the information necessary for a rigorous testing is outlined. Confusion should be avoided between adjustment of the primary sex ratio, which allows modification without the loss of potential offspring, and sex ratio adjustment through differential mortality of the two sexes after conception. The latter requires a loss in reproductive potential, which is particularly serious in organisms which produce only one young a year.

SYNCHRONY IN OUTBREAKS OF FOREST LEPIDOPTERA: A POSSIBLE EXAMPLE OF THE MORAN EFFECT

I compiled information from the literature on 140 population outbreaks of 26 species of forest Lepidoptera that occurred between the years 1932 and 1992 in the Northern Hemisphere, including Asia, North America, and Europe. Considerable variation in the number of outbreaks occurred among years, with a tendency for outbreaks to be synchronous. Years of numerous outbreaks include 1948, 1956, 1964, the mid-1970s, and the mid-1980s. Published summaries of weather in the Northern Hemisphere indicate the occurrence of above-normal precipitation and cool temperatures between 1954 and 1957, and a high proportion of populations were at peak density in 1954 and 1956. Cool springs in 1964, 1965, and 1976 were also associated with outbreaks in a number of species of forest Lepidoptera. Periods of synchrony of population outbreaks and cool temperatures appear to be associated with troughs in the sunspot cycle. While the details of population dynamics of caterpillars are determined through endogenous processes that influence fecundity and mortality, exogenous forces, such as cool springs, may occasionally play a role in synchronizing populations in the Northern Hemisphere. Thus weather conditions may act as a Moran effect and synchronize fluctuating populations of forest caterpillars over large geographic areas.

THE INFLUENCE OF PREDATORS ON THE MOVEMENT OF APTEROUS PEA APHIDS BETWEEN PLANTS

SUMMARY (1) This paper examines the influence of predators on the dispersal of apterous pea aphids in the laboratory and field. (2) Individual pea aphids can be placed in one of two categories, searchers and runners, depending upon the behaviour they exhibit after dropping from their host plant. (3) In field cages, pea aphids readily disperse to new host plants in the presence of predators. Dispersal of apterous aphids is rare when active predators are absent. (4) Evidence is presented which suggests fecundity is reduced the day after an aphid disperses. (5) The distance dispersed by aphid nymphs is positively correlated with the density of aphids on the plant the aphid leaves. (6) The significance of dispersal by apterous aphids is discussed in the light of evidence presented.

Previous herbivore attack of red alder may improve food quality for fall webworm larvae

SummaryThree hypotheses of insect-plant interactions were tested by rearing fall webworm larvae in the laboratory on foliage from red alder trees with different histories of western tent caterpillar herbivory. Fall webworm larvae raised on foliage from trees which had been attacked previously for two summers by moderate densities of western tent caterpillars grew faster and attained heavier pupal weights than did those fed foliage from unattacked trees. This contradicts the hypothesis that moderate levels of previous herbivory induces the production of plant defensive chemicals in red alders. Growth of webworms, when fed foliage from unattacked trees adjacent to alders that were attacked by fall webworm larvae, was the same as when fed foliage from trees isolated by distance from attacked trees. This contradicts the hypothesis that attacked trees stimulate the production of defensive chemicals in neigh-boring trees. Young and mature alder foliage was equally good for fall webworm growth and survival, and foliage from trees heavily attacked by both fall webworm and western tent caterpillars for three years produced slow growth rates and small pupal sizes. This supports the hypothesis that continued heavy insect attack can cause the deterioration of the food quality of attacked trees.