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22 - Evolution of Exploitation and Defense in Tritrophic Interactions
- Edited by Ulf Dieckmann, International Institute for Applied Systems Analysis, Austria, Johan A. J. Metz, Universiteit Leiden, Maurice W. Sabelis, Universiteit van Amsterdam, Karl Sigmund, Universität Wien, Austria
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- Book:
- Adaptive Dynamics of Infectious Diseases
- Published online:
- 15 January 2010
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- 11 April 2002, pp 297-322
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Summary
Introduction
Why do plants cover the earth and give the world a green appearance? This question is not as trivial as it might seem at first sight. Hairston et al. (1960) hypothesized that herbivores cannot ransack the earth of its green blanket because they are kept low in number by predators. They tacitly ignored the possibility that plants defend themselves directly against a suite of herbivores and together exhibit such great diversity in defense mechanisms that “super” herbivores able to master all plant defenses did not evolve and those that overcome the defenses of some plants are limited by the availability of these plants. Strong et al. (1984) recognized both possibilities in their review on the impact of herbivorous arthropods on plants, but they also favored the view that predators suppress the densities of herbivores, and thereby reduce the threat of plants being eaten.
The two explanatory mechanisms (plant defense versus predator impact), however, may well act in concert. Ever since the seminal review paper by Price et al. (1980) ecologists have become increasingly aware that plant defenses include more than just trickery to reduce the herbivore's capacity for (population) growth. For example, the plant may provide facilities to promote the foraging success of the herbivore's enemies. This form of defense is termed indirect as opposed to direct defense against herbivores.
7 - Contact Networks and the Evolution of Virulence
- Edited by Ulf Dieckmann, International Institute for Applied Systems Analysis, Austria, Johan A. J. Metz, Universiteit Leiden, Maurice W. Sabelis, Universiteit van Amsterdam, Karl Sigmund, Universität Wien, Austria
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- Book:
- Adaptive Dynamics of Infectious Diseases
- Published online:
- 15 January 2010
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- 11 April 2002, pp 85-103
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Summary
Introduction
Virulence management can be defined as that set of policies that not only aims to minimize the short-term impact of parasites on their host population (e.g., incidence, mortality, and morbidity), but also to account for the longer-term consequences of the evolutionary responses of these parasites, for example by adopting measures that select for less virulent strains.
An important question pertaining to the scope of virulence management concerns the effect of contact structures in the host population. For successful transmission many parasites require close contact between the host they are infecting and new susceptible hosts. Consequently, the network of social contact in their host population is of paramount importance. It has already become clear that differently structured networks lead to different types of epidemiology (Keeling 1999). For example, a sparsely connected host population is more difficult to invade than a densely connected host population. But to what extent will the contact structure of their host population affect the evolution of the parasites, in particular of their virulence? Can we change the selective pressures on the parasites by modifying these contact structures? Claessen and de Roos (1995) and Rand et al. (1995) carried out computer simulations of evolving parasites in spatially structured host populations and concluded that less virulent (hypovirulent) parasites are favored with respect to well-mixed systems. Clearly, parasite evolution does depend on host population structure. Qualitative insight into the pertinent aspects of population structure, in the form of social networks, is still lacking, however (Wallinga et al. 1999).
5 - Dilemmas in Virulence Management
- Edited by Ulf Dieckmann, International Institute for Applied Systems Analysis, Austria, Johan A. J. Metz, Universiteit Leiden, Maurice W. Sabelis, Universiteit van Amsterdam, Karl Sigmund, Universität Wien, Austria
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- Book:
- Adaptive Dynamics of Infectious Diseases
- Published online:
- 15 January 2010
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- 11 April 2002, pp 60-70
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Introduction
Both the patient who is infected with a communicable disease and the doctor treating the patient share a common interest: the eradication of the infection. That the treatment chosen by the doctor may have detrimental consequences for the population at large is not the primary concern of the doctor or the patient. Such matters are the concern of the larger-scale medical and political organizations that deal with the development of public health policies such as vaccination programs and possibly, as investigated in this book, “virulence management” strategies. Development of such policies is not only a complicated issue because of the intricacies of host–parasite interactions themselves, but also because the common aims of the public health authority and the population do not always overlap very well (Anderson et al. 1997). Of course, the community benefits when an individual ceases to be infective. However, parasites are not inert players in the game, and will adapt to any measures that are taken on a sufficiently large scale. Therefore, the development of some public health policies may not be beneficial to the community as a whole. The global resurgence of tuberculosis (TB) and the fact that many malaria parasites have become resistant against most preventive treatments are just two examples of the detrimental consequences of the large-scale application of individually beneficial medical treatment. The insight that strategies to fight parasites should be based not only on shortterm effects, but also on evolutionary considerations, is gaining ground (Ewald 1993, 1994a).
19 - Pair Approximations for Different Spatial Geometries
- Edited by Ulf Dieckmann, International Institute for Applied Systems Analysis, Austria, Richard Law, University of York, Johan A. J. Metz, Rijksuniversiteit Leiden, The Netherlands
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- Book:
- The Geometry of Ecological Interactions
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- 14 January 2010
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- 04 May 2000, pp 359-387
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Introduction
The standard assumption underlying the formulation of models for population dynamics (such as the logistic growth equation, the Lotka–Volterra predator–prey model, and the Kermack and McKendrick epidemiological equations, to name a few) is that populations spread homogeneously through space and that individuals mix rapidly. It is not a new insight that spatial structure is often an essential component of the ecological (and evolutionary) dynamics of populations, and there have been many approaches to understanding the various consequences of spatial structure. In this chapter I address one of the more recently developed techniques for modeling spatial population dynamics.
The oldest approach is to assume that populations are subdivided into different discrete subpopulations that are linked through migration (the “metapopulation” approach). This may be a reasonable assumption for certain systems (groups of parasites living in different hosts, for example), but space often has a more a continuous aspect. For example, a forest may be highly structured without having clear boundaries between subpopulations. Such situations are often modeled using a diffusion formalism, but this approach has its shortcomings as well. In particular, when one considers spatial spread of a population (or gene), individuality (discreteness) and its associated stochasticity may be important (Durrett and Levin 1994b). In a diffusion model, the rate of population growth is determined by the spread of “nano-individuals” at the wave front, whereas in reality it is often determined by the more erratic process of dispersal and subsequent successful settlement of individuals.
15 - The evolution of overexploitation and mutualism in plant–herbivore–predator interactions and its impact on population dynamics
- from Part IV - Genetic/evolutionary considerations
- Edited by Bradford A. Hawkins, University of California, Irvine, Howard V. Cornell, University of Delaware
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- Book:
- Theoretical Approaches to Biological Control
- Published online:
- 13 August 2009
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- 06 May 1999, pp 259-282
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Introduction
Populations of arthropod herbivores may show periodical outbreaks, large amplitude cycles, strongly bounded fluctuations or stable equilibria. At small spatial scales they may also show dynamics different from those at large spatial scales, and they may even go extinct. There is a need to experimentally test models predicting the dynamical consequences of the mechanisms underlying these patterns. In particular, what needs an explanation is the observation that plants retain a green appearance despite attacks by herbivores (Hairston et al., 1960; Strong et al., 1984). There are two possible explanations: (i) plants defend themselves effectively against herbivores, (ii) predators suppress herbivore populations to very low levels. Hairston et al. (1960) tacitly ignored the first and emphasized the latter in formulating their so-called ‘the world is green’ hypothesis. The two explanatory mechanisms, however, are not mutually exclusive; the plant may promote predator foraging success and the herbivore's enemies may use the facilities offered by the plant.
Price et al. (1980) stimulated a growing awareness that plants may defend themselves both directly against herbivorous arthropods and indirectly by promoting natural enemies, for example, by providing protection, food, and alarm signals to the enemies of the herbivore. Direct defenses include plant structures that hinder feeding by the herbivore, and secondary plant compounds that inhibit digestion, intoxicate the herbivore or deter feeding. Before Price et al. (1980) appeared, examples of indirect defenses were already well-known from ant–plant interactions (Janzen, 1966; Bentley, 1977; see also historical reviews by Beattie, 1985, and Jolivet, 1996).