Stephen D. Fretwell

Bio: Stephen D. Fretwell is an academic researcher. The author has contributed to research in topics: Population & Competition (biology). The author has an hindex of 8, co-authored 12 publications receiving 4728 citations.

The Optimal Balance between Size and Number of Offspring

Abstract: The relationship between the energy expended per offspring, fitness of offspring, and parental fitness is presented in a two-dimensional graphical model. The validity of the model in determining an optimal parental strategy is demonstrated analytically. The model applies under various conditions of parental care and sibling care for the offspring but is most useful for species that produce numerous small offspring which are given no parental care.

Exploitation Ecosystems in Gradients of Primary Productivity

Abstract: Based on the assumption that each trophic level acts as a single exploitative population, a model relating the trophic structure of ecosystems to their potential primary productivity is developed. According to the model, herbivory pressure should be most severe in relatively unproductive environments. With increased potential productivity, the role of predation in herbivore regulation should become more important and the impact of herbivory upon plant communities should decrease. In very productive environments, increase in herbivory pressure is again probable, at least in aquatic ecosystems. The predicted pattern of phytomass and predicted results of manipulations are compared with available data. A reasonable fit between predictions and observations is found, although the sparsity of data and methodological uncertainties weaken the corroboration in several cases. In terrestrial ecosystems, the present version of the model seems best applicable to the vertebrate branch of the grazing chain, whereas the a...

Delayed Maturation in Passerine Plumages and the Deceptive Acquisition of Resources

Abstract: We propose that the female-like plumage worn by some male birds in their first potential breeding season has evolved to facilitate breeding when 1-yr old through the deception of older males. By mimicking females 1-yr-old males exploit the tendency of old males not to attack females and, thus, are able to enter better quality habitats. Once subadults settle in such habitats, they hold territories in them by site dominance. An hypothesis invoking deception seems necessary because the cryptic hypothesis, which suggests that subadults are hiding from predators, cannot explain why subadult males of sexually dichromatic species always resemble females in every way by which they differ from older males. If the cryptic hypothesis were sufficient, subadult males should resemble juveniles in species for which the juvenile plumage is more cryptic than that of breeding females. Female mimicry should evolve (1) when competition for breeding resources is rigged against 1-yr-old males, (2) when obtaining a mate depends...

Surplus Killing in the Hunting Strategy of Small Predators

Abstract: Studies on the impact of vertebrate predators on a prey population are often based on estimates of the numbers of predators and prey (e.g., Pearson 1966, 1971; Goszczynski 1977; Phelan and Robertson 1978; King 1980; Erlinge et al. 1983). This approach includes assumptions about the functional response of predators, that is, about the changes in hunting effort for different prey species and about the factors that determine the handling time (Holling 1965). Changes in the preference for different prey species, often called switching, have an important effect on the functional response curve, especially at relatively low prey densities (Greenwood and Elton 1979). The lower part of the functional response curve also will probably be influenced by transit time (Murdoch 1977; Oaten 1977) and by the costs of activity (Abrams 1982). The upper part of the functional response curve is largely determined by handling time, that is, the time between the capture and the resumption of searching behavior (Holling 1965). The extent of switching can be estimated by using scat analysis (e.g., Day 1968; Phelan and Robertson 1978; Tapper 1979; Erlinge 1981), and weight loss of predators can be used for estimating inadequate feeding rates. Handling time could be estimated readily if all predators were characterized by rigid hunting behavior in which a prey is attacked when seen within a fixed attack radius and consumed before the predator starts to search for new prey items. If, however, predatory behavior is more flexible, handling time becomes more difficult to estimate. Such flexibility may be created, for example, by selective feeding and consequently decreased consumption per prey with increased prey density (Mysterud 1980; Stenseth 1981; Abrams 1982). It is also possible for predators to kill prey without immediately consuming them (Nyholm 1961; Kruuk 1972; Curio 1976; Mysterud 1980; Elgmork 1982). Such behavior substantially shortens the handling sequence by eliminating the time used for feeding activities. This can make the functional response curve nearly linear or only slightly convex at even high prey densities. Consequently, the numerical predator-to-prey ratio advocated by Pearson (1966, 1971) might not reflect the intensity of predation, and estimates of predation based on identifiable prey remains found from predator scats could be far too low.

Distress Screams as a Measure of Kinship in Birds

Abstract: The piercing distress screams given by winter birds are hypothesized to be cries for help (usually in the form of mobbing from altruists) and to have been evolved by kin selection. The screams are apparently directed at the screamer's associates, not the predator, a point which supports our hypothesis of selection for altruistic behavior and weakens self-interest selection as a viable mechanism for their evolution. The screams are highly localizable, and attracted individuals are known both to mob predators and to experience risk in doing so; these facts argue that it is the attracted individuals which are altruistic, not the screamer. For two species, the proportion of individuals that screamed when mist-netted diminished as winter progressed; this suggests that kin selection and not selection favoring reciprocal altruism resulted in the evolution of such behavior. Our most powerful arguments that altruism, rather than self-interest selection, is the mode by which distress-screaming behavior was evolved are: (1) that permanent residents scream more than winter residents, and (2) that among winter residents, diurnal migrants scream more than nocturnal migrants. The very important implication of this work is that measuring the frequencies of such screams in samples of mist-netted birds provides an ordinal index of kinship (or at least, stability of interindividual associations).

Toward a metabolic theory of ecology

Abstract: Metabolism provides a basis for using first principles of physics, chemistry, and biology to link the biology of individual organisms to the ecology of populations, communities, and ecosystems. Metabolic rate, the rate at which organisms take up, transform, and expend energy and materials, is the most fundamental biological rate. We have developed a quantitative theory for how metabolic rate varies with body size and temperature. Metabolic theory predicts how metabolic rate, by setting the rates of resource uptake from the environment and resource allocation to survival, growth, and reproduction, controls ecological processes at all levels of organization from individuals to the biosphere. Examples include: (1) life history attributes, including devel- opment rate, mortality rate, age at maturity, life span, and population growth rate; (2) population interactions, including carrying capacity, rates of competition and predation, and patterns of species diversity; and (3) ecosystem processes, including rates of biomass production and respiration and patterns of trophic dynamics. Data compiled from the ecological literature strongly support the theoretical predictions. Even- tually, metabolic theory may provide a conceptual foundation for much of ecology, just as genetic theory provides a foundation for much of evolutionary biology.

The metacommunity concept: a framework for multi-scale community ecology

Abstract: The metacommunity concept is an important way to think about linkages between different spatial scales in ecology. Here we review current understanding about this concept. We first investigate issues related to its definition as a set of local communities that are linked by dispersal of multiple potentially interacting species. We then identify four paradigms for metacommunities: the patch-dynamic view, the species-sorting view, the mass effects view and the neutral view, that each emphasizes different processes of potential importance in metacommunities. These have somewhat distinct intellectual histories and we discuss elements related to their potential future synthesis. We then use this framework to discuss why the concept is useful in modifying existing ecological thinking and illustrate this with a number of both theoretical and empirical examples. As ecologists strive to understand increasingly complex mechanisms and strive to work across multiple scales of spatio-temporal organization, concepts like the metacommunity can provide important insights that frequently contrast with those that would be obtained with more conventional approaches based on local communities alone.

Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis

Abstract: Although the effect of biodiversity on ecosystem functioning has become a major focus in ecology, its significance in a fluctuating environment is still poorly understood. According to the insurance hypothesis, biodiversity insures ecosystems against declines in their functioning because many species provide greater guarantees that some will maintain functioning even if others fail. Here we examine this hypothesis theoretically. We develop a general stochastic dynamic model to assess the effects of species richness on the expected temporal mean and variance of ecosystem processes such as productivity, based on individual species’ productivity responses to environmental fluctuations. Our model shows two major insurance effects of species richness on ecosystem productivity: (i) a buffering effect, i.e., a reduction in the temporal variance of productivity, and (ii) a performance-enhancing effect, i.e., an increase in the temporal mean of productivity. The strength of these insurance effects is determined by three factors: (i) the way ecosystem productivity is determined by individual species responses to environmental fluctuations, (ii) the degree of asynchronicity of these responses, and (iii) the detailed form of these responses. In particular, the greater the variance of the species responses, the lower the species richness at which the temporal mean of the ecosystem process saturates and the ecosystem becomes redundant. These results provide a strong theoretical foundation for the insurance hypothesis, which proves to be a fundamental principle for understanding the long-term effects of biodiversity on ecosystem processes.

Habitat, the template for ecological strategies?

Abstract: The very etymology of Ecology, from the greek 'Qikos', 'the household', implies that ecologists should devote some attention to the 'house' or habitat of the population or community they are studying. However, as Charles Elton (1966) has so forcibly pointed out, 'definition of habitats, or rather lack of it, is one of the chief blind spots in Zoology'. Elton himself has provided us with a qualitative classification of habitats, while another past President, Alex Watt (1947) highlighted the dynamic nature of habitats by his phrase, 'pattern and process'. Elton referred to the need to quantify habitat characteristics. In this Address I will attempt some quantification;however, you will all be aware that in doing this I will not be able to emulate those former Presidents who have been able to provide a definitative synthesis of a field or of their own studies, my offering can be but a small beginning, an indication of the type of characteristics we should quantify. In considering ecosystem patterns and environment R. M. May (1974) writes 'it is to be emphasized that although patterns may underlie the rich and varied tapestry of the natural world, there is no single simple pattern. Theories must be pluralistic'. Indeed, the complexity of the subject is daunting and in any attempt to formulate some type of general framework, one is continually beset with exceptions. In stressing the need for a framework I am echoing a plea of my predecessor Amyan Macfadyen (1975) who cited K. E. F. Watt's (1971) vivid image 'if we do not develop a strong theoretical core that will bring all parts of ecology back together we shall all be washed out to sea in an immense tide of unrelated information'. In some ways I think we may see ourselves at a similar point to the inorganic chemist before the development of the periodic table; then he could not predict, for example, how soluble a particular sulphate would be, or what was the likelihood of a particular reaction occurring. Each fact had to be discovered for itself and each must be remembered in isolation. It is noteworthy that from Dobereiner's early efforts in 1816 it took more than fifty years before Mendeleeff ormulated his Periodic Law (1869) and even after this there were various attempts at rearrangement. Another parallel may be drawn with astronomy before the development of the Hertzsprung-Russell diagram that relates the evolution and the properties of stars. Again in our own subject biology, the situation is somewhat analagous to that before the formulation of the Linnean system of classification; but now from this system of classification, we are able to organize our knowledge of, for example, the functional morphology of organisms and we can even make assumptions, with a high probability