Proponents of optimal foraging theory attempt to predict the behavior of animals while they are foraging; this theory is based on a number of assump­ tions ( 133 , 155 , 2 10, 23 1 ) . First, an individual's contribution to the next generation (i.e. its "fitness") depends on its behavior while foraging. This contribution may be measured genetically or culturally as the proportion of an individual's genes or "ideas", respectively, in the next generation. In the former case, the theory is simply an extension of Darwin's theory of evolution. Second, it is assumed that there should be a heritable component of foraging behavior, i.e. an animal that forages in a particular manner should be likely to have offspring that tend to forage in the same manner. This heritable compo­ nent can be either the actual foraging responses made by an animal or the rules by which an animal learns to make such responses. In other words, optimal foraging theory may apply regardless of whether the foraging behavior is learned or innate. Given these first two assumptions, it follows that the proportion of individuals in a population foraging in ways that enhance their fitness will tend to increase over time. Unless countervailed by sufficiently strong group selection (see 287, 242), foraging behavior will therefore evolve, and the average foraging behavior will increasingly come to be characterized by those characteristics that enhance individual fitness. The third assumption is that the relationship between foraging behavior and fitness is known. This relationship is usually referred to as the currency of fitness (23 1 ) . In general, any such currency will include a time scale, although in some cases it may be assumed that fitness is a function of some rate.

Optimal Foraging Theory: A Critical Review

La fragmentation de l'habitat (et des reserves) est la menace la plus serieuse pour la diversite biologique et est la cause fondamentale de la menace actuelle d'extinction

Conservation Strategy: The Effects of Fragmentation on Extinction

Theories concerning microtine population dynamics have relied increasingly on assumptions about social structure, but there has been no underlying theory for social biology itself. Among microtine species, patterns of sex-specific territoriality, the best-studied aspect of their social organization, are diverse, setting the scene for the fragmented nature of the contemporary approaches to this issue. Here I propose a set of hypotheses to predict under what conditions females and males should defend territories. The argument has three main components. 1. Microtines have a low-quality diet and should tend to be food-limited. Females invest a large portion of their energy in reproductive physiology, and their reproductive success should be limited by their ability to acquire food and convert it into weaned offspring. Males invest much less energy in reproductive effort and do not participate in parental care; thus, their reproductive success should be limited by access to females. Therefore, territoriality i...

Limiting Resources and Territoriality in Microtine Rodents

General activity, spatial movement patterns, grouping behaviour and antagonistic relationships, i.e., contest competition, as well as food selection and food utilization, i.e., intraspecific scramble competition, in the field vole, Microtus agrestis (L.), are discussed mainly on the basis of the author's own studies on the experimental and field populations. In the treatment of home range characteristics, use is made of the concept of a fixed home range centre (HRC) and the density function of the observed points of capture, HR-sigma. Both breeding males and females seem to have relatively fixed home ranges, but only the male ranges seem to be defended territories. The movement patterns of the juvenile voles are easily described in terms of their mother's HR-sigma, while the subadults behave nomadically without any fixed home range pattern. General activity, HR size, and HR overlap (neighbour density) show seasonal changes, which (at least in the females) cannot be related to the overall population density, but seemingly run parallel to scramble competition for preferred food. These characteristics also show some differences between cycle phases. Maternal care of the young lasts a fortnight, after which the fate of the young is determined by their maturation patterns and the functional role they adopt in the population. The majority of early spring-born female young stay in or near the mother's home range, while the bulk of the males is forced to disperse by the aggressiveness of the dominant males. The high dispersal rate of the subadult cohorts at mid summer can also probably be ascribed to contest competition. A phenomenon typical of M. agrestis is the "spontaneous" dispersal of the reproductive females, frequently observed during the short spell between weaning and birth of the next litter. M. agrestis is highly selective in its food search. The preferred items, e.g., the leaves of clover and dandelion, also appear to be the most nutritive. High selection pressure on the favourite food items, and the simultaneous cessation of their growth, lead to hard scramble competition during the "nutritional midsummer crisis", whose demographic effects are decreased litter size, abortions, increased nestling mortality, decreased growth and maturation rates, loss of weight in reproductive individuals, and even increased death rate of the reproductive adults. The adverse effects of scramble competition may be expected to continue during the winter following a peak year, when strong exploitation of winter-green food has been observed. It is concluded that scramble competition for food plays a more important role in the population dynamics of M. agrestis than is generally understood.

https://www.jstor.org/stable/pdfplus/3543594.pdf

Intraspecific competition and home range dynamics in the field vole Microtus agrestis

Small rodent populations are cyclic in north Fennoscandia and non-cyclic in southern Scandinavia. Populations of granivorous species respond positively to increased food supply in the south while predators seem to depress all declining populations in the north to very low densities. Such observations have led to a conceptual model where folivorous rodents in a mild climate with abundant alternative prey for predators are assumed to be regulated by generalist predators while similar rodent populations in snow-rich areas are permitted to expand towards food limitation but are finally depressed by specialist predators. The cyclic increase phase may be broken by food deficits, disease or even predation. Granivorous species sympatric with non-cyclic folivores are thought to be food-limited and there is also a declining gradient of potential cyclicity from folivores to granivores. This hypothesis is compared with similar ideas for other animal groups and general theoretical concepts in ecology. Old and new evidence for and against it are presented. The general conclusion is that it still deserves further testing.

An Interpretation of Rodent Dynamics as Due to Trophic Interactions

This is the application of a model on food selection in grazing microtine rodents. Net gain functions for any particular food value, the currency to be optimized, are constructed by evaluating the abundance, distribution, and cost of acquisition of different food items. In the specific application of the model, digestible energy content is used as the currency. The basic premise in the hypothesis is that microtines select their diet to ensure that the net energy gain per effort is maximized. The theory is used for evaluating the food selection of field voles Microtus agrestis (L.) in two localities in Scandinavia (northern Sweden and southern Finland). There is close agreement between predictions and observations. Alternative hypotheses (e.g. food selection for nutrients) are evaluated, but found less likely than the one discussed here. Nutritional consequences of food selection for energy-per-effort as well as consequences at the population level of organization are evaluated.

Food selection of the field vole Microtus agrestis

Mathematical models have been developed for different aspects of the population dynamics of the field vole. The models are presumed to cover the most essential phenomena influencing the demography and habitat utilization of the field vole in Scandinavia. Their presentation permits a simulation model for the field vole's population dynamics to be developed. The models include: Energy requirement and utilization; a generalized version of the "functional response curve"; impact on vegetation and scramble competition; reproduction and survival schedules; predation; habitat utilization and contest competition. Our basic hypothesis is that the major features of the population dynamics of the field vole may be understood by analyzing the "condition" of individual organisms, particularly their nutritional balance. Emphasis is placed on evaluating the models with respect to realism, thus showing that many observations on free-living field vole populations may be explained by these models. Critical areas for further research are indicated. Most important are: To study food acquisition and nutritional effects on demography; to evaluate other factors which have been supposed to affect demography; to study the effects of predation; and to examine the field vole's use of heterogeneous environments including demic structures and dispersal. Any information which may reject particular hypotheses is indicated.

General Models for the Population Dynamics of the Field Vole Microtus agrestis in Central Scandinavia

Theoretical and practical purposes of mathematical modelling are discussed. Of these, special attention is given to large scale simulation models and regression models. It is pointed out that, for proper use of simulation models for predictive purposes, it is necessary to realize the uncertainties associated with such model predictions. These uncertainties are mainly due to insufficient knowledge of parameter values included in the simulation model. As these model predictions are made iteratively (i.e. by short time steps), biases in parameter estimates have, with increasing length of simulated time, an accumulating effect on the biases in the output variables.



Regression models are described and methods for finding the best mathematical model are discussed. Assumptions regarding statistical properties of the data from which the regression model is constructed are discussed. It is pointe out that valid predictions are made only within the domain defined by the data Examples of regression models for forecasting rodent outbreaks are discussed. Some further developments are proposed.



Results from a model emphasizing spatio-temporal heterogeneity in a heterogeneous region are discussed. Based on this model conclusions regarding what type of landscapes are likely to cause rodent outbreaks are deduced. Furthermore, proposed control treatments are suggested by analyzing the model. It is concluded that larger regions should be composed of suitable and non-suitable rodent habitats interminded with each other. It is suggested that treatments for increasing the local extinction rate be applied uniformly over the whole region, but varyjng in time as much as possible and not correlated with the density in local habitats nor in the entire region. Within these limits, treatments should be applied during periods of low mobility and low reproduction.

Forecasting of Rodent Outbreaks: Models and the Real World1

The basic hypothesis underlying a population dynamics model for the field vole (Microtus agrestis [L.]) in central Scandinavia is described and discussed. The hypothesis is that most aspects of population dynamics of the field vole may be understood by analyzing the nutritional (energy and matter) balance of individual animals and their differential allocation of available nutrients. Digested nutrients are assumed to be utilized for maintenance cost, M; growth, A S; reproduction, R; and dispersion behaviour (including dispersal), D. The simulation model whose main ideas are described verbally is being developed at present. The model is intended to simulate density, age structure, reproductive activity of different categories of the population and spatial distribution; grazing impact apportioned among the most important potential food species (or group); interaction with sympatric rodents, their predators and parasites. Special emphasis is placed on the spatial heterogeneity of the landscape. The population dynamics model is of the Monte Carlo type and considers the realization of a series of events by calculating average probabilities and “drawing” random numbers. The model simulates individual animals in a large heterogeneous area.

Qualitative predictions based on our basic hypothesis are discussed and compared with available field information.

Population Dynamics of the Field Vole (Microtus agrestis [L]): a Model

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Food selection of the field vole Microtus agrestis

General Models for the Population Dynamics of the Field Vole Microtus agrestis in Central Scandinavia

Forecasting of Rodent Outbreaks: Models and the Real World1

Population Dynamics of the Field Vole (Microtus agrestis [L]): a Model