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This study is focused on the population ecology of water voles (<I>Arvicola terrestris</I>) in linear habitats in north-east Scotland. The three aims of this study were (a) to assess one ecological method for estimating population size, (b) to identify ecological factors acting on individual spacing patterns, and (c) to make and test predictions about the mating system based on the observed spacing pattern. Data from latrine counts were used to validate a method previously proposed to estimate the population size of water voles. The relationship between latrine counts and vole abundance from sites with a broad range of population size was used in the calibration, and equations were derived from six different regressions with appropriate confidence limits. Due to the uncertainties involved, the calibration appeared to be unreliable for estimating the true population size. Two hypotheses, food-defence and pup-defence, previously proposed to explain territoriality of adult female microtines, were tested experimentally by manipulation of food during the reproductive cycles. Female home range length and daily distance moved were monitored by radio-tracking. The changes observed in home range length, movement, and distance between neighbours, suggested that both food-defence and pup-defence influence the spatio-temporal pattern of territoriality in female water voles. Spacing patterns of adult males and females, and the movement of males in relation to oestrus females, were studied by radio-tracking. A male removal experiment was also conducted to establish the method of home range marking by males. The study showed that intersexual overlap was more pronounced than intrasexual overlap, with males showing more intrasexual overlap. Male with heavier weight and larger distances moved were able to monopolise large numbers of receptive females. Results from the male removal experiment suggested that home range boundaries of males were not maintained by daily contact between neighbours. The pattern of space use by both sexes, and the timing of oestrus in females, predicted that mating system is promiscuity.

A natural population of Pleuroptya ruralis has been studied for three years with the following aims: (1) to elucidate its life history; (2) to study the variations in the larval and adult densities; (3) to identify the mortality factors acting upon them; (4) to investigate possible relationships between the intensity of the mortality factors and the spatial distribution (horizontal and vertical) of P. ruralis ; (5) to describe the role of the major mortality factors in determining the size of the population studied; (6) to assess the potential of the system for further studies about the mechanism of population regulation. P. ruralis caterpillars were individually labelled and followed throughout their development on a natural growth of their food plant, Urtica dioica (the nettle), near Leeds, Yorkshire, United Kingdom. Parasitism and predation as well as the distribution and movements of the larvae on the host plant were assessed. Caterpillars were also collected and reared in the laboratory for identification of the parasitoid species and to provide support for the field results. Adults were studied in the field by the mark- release- recapture method. Potential female fertility was investigated in the laboratory. The population suffered a dramatic decline in numbers during the period of this study. The main factors determining the observed population levels were the interactions between the weather, time spent in development, deviations in the sex ratio, bird predation towards the completion of pre-adult development and parasitization by a braconid (Macrocentrus grandii). Other parasitoids also had some effect, notably Diadegma sp. Predation also had a strong impact on the population of M. grandii. Birds appear to concentrate their attack in the part of the plant in which they are more likely to find the prey. Weather also influenced adult activity and availability of nectar sources in the field, and starvation of females reduced fecundity in the laboratory. Thus, egg shortfall may also have been important in the decrease of population size. The field method used with the caterpillars provided detailed and valuable information about individual and spatial variation and it is suggested that the system 'nettles / P. ruralis / natural enemies' can be a powerfull tool for studies on the mechanisms of population regulation.