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

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

The effects of herbivores on landscape patterns and ecosystem processes have generally been inferred only from small-plot or exclosure experiments. However, it is important to directly determine the interactions between herbivores and landscape pat- terns, because herbivores range over large portions of the landscape to meet requirements for food and shelter. In two valleys on Isle Royale, Michigan, USA, soil nitrogen availability and its temporal variance decreased rapidly as consumption of browse by moose (Alces alces) increased up to 2 g·m 22 ·yr 21 ; with greater amounts of consumption, nitrogen avail- ability was uniformly low and constant from year to year. We tested three geostatistical models of the spatial distribution of available browse, annual browse consumption, conifer basal area, and soil nitrogen availability across the landscape: (1) no spatial autocorrelation (random spatial distribution); (2) short-range spatial autocorrelation within a patch, but random distribution of patches at larger scales (spherical model); and (3) both short-range autocorrelation within a patch and regular arrangement of patches at larger scales (harmonic oscillator model). Conifer basal area and soil nitrogen availability fit the harmonic oscillator model in both valleys. Annual consumption and available browse showed oscillations in one of the valleys and only short-range autocorrelation in the other. In both valleys, however, the spatial pattern of annual consumption followed that of available browse. The predom- inance of spatially oscillatory patterns suggests that the interactions of moose with the forest ecosystem cause the development of both local patches of vegetation and associated nitrogen cycling rates, as well as the development of higher order patterns across the larger landscape. We suggest a coupled diffusion model of herbivore foraging and plant seed dispersal that may account for these patterns.

Spatial patterns in the moose–forest–soil ecosystem on isle royale, michigan, usa

Dispersal in rodents : a resident fitness hypothesis / Paul K. Anderson.

The theme of this review is ecological and evolutionary consequences of habitat heterogeneity (interpreted as different degrees of structural complexity of the habitat). A series of population dynamics models of predator-prey systems are considered. These models are developed in order to analyse changes with respect to stability as a result of (1) altering the difficulties with which predators are able to capture prey individuals (assumed to increase with increasing structural complexity of the habitat), (2) the existence of hiding places for prey individuals, and/or (3) the occurrence of dispersal between patches with differing availability of hiding places for prey individuals. From these analyses it is concluded that increased habitat heterogeneity leads to increased population stability. Applying the Red Queen Hypothesis, this conclusion, linking habitat heterogeneity with population stability, is used for arriving at the conclusion that an increased number of species is expected to exist in a spatially heterogeneous habitat as compared with a homogeneous one. A series of models of optimal life histories (e.g., reproductive rates and sex ratios) in habitats of different degrees of habitat heterogeneity are then reviewed. The prediction is that current reproductive rate is assumed to be higher, and to increase faster to its maximal level in a homogeneous habitat than in a more heterogeneous habitat; some qualifications to this prediction are, however, necessary. The sex ratio at birth is further predicted to be female biased in heterogeneous and patchy habitats; however, the condition for the female biased sex ratio to be optimal is that enough males are produced to ensure fertilization of all (or most) females. Thus, it is concluded that the biases may be only slight and consequently difficult to detect in field samples unless very large sample sizes are available; this is so because in heterogeneous habitats, both small family sizes and female biased sex ratios are expected to be optimal. In spatially homogeneous habitats, an approximately even sex ratio is predicted. The female biased sex ratio is assumed to be a result of closely related individuals mating with each other; therefore, models for analyzing under which circumstances inbreeding is expected to be optimal, are discussed. Throughout data from a variety of taxonomic groups relating to these issues are discussed.

Spatial Heterogeneity and Population Stability: Some Evolutionary Consequences

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

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