(1) Voles on three areas were provided with different levels of extra food in the form of laboratory chow and oats for approximately one year. (2) On areas with intermediate and high extra food, population density increased to twice the control density. (3) Extra food increased immigration and reproduction, and shortened the winter non-breeding season. (4) Males and females had smaller home ranges on areas with extra food. This may have facilitated immigration to these populations. (5) Voles were removed from three other areas and re-colonization was measured. Immigration was related to the density of extra food; three times as many voles colonized a high food area compared with a control. (6) These results, combined with those of previous research which has shown that a viable surplus of voles exists in natural populations, suggest that the vole's population size is limited by both behaviour and food. 126 Voles and extrafood Two sizes of grid were marked out in the grassland. Large grids had 100 trap stations at 7.6 m intervals, usually in a 10 x 10 checkerboard (0-47 ha). Small grids had 50 traps covering 0.21 ha. The trapping procedure was the same as that used for deermice (Taitt 1981), except that traps were checked in winter during the intervening day as well as the two overnight periods. In summer, the daytime trapping period was abandoned to avoid mortality from overheating. The type of data recorded on each mouse was identical to that recorded for deermice. A: Long-term extra food (grids C, I, L, H) Grid C (Fig. 1) was the control population trapped by Krebs et al. (1976) from May 1971 to September 1975. Grids L and H were set up in July 1972. At the end of March 1973, a high density of food stations (1 per 22 m2) was placed on grid H and a low density on grid L (1 per 73 m2). We had intended to have high and low food available throughout the study on these two grids. Unfortunately ploughing destroyed both in April 1974 after only one year of extra food. In May 1974 we set up grid I and provided it with food stations at an intermediate density (1 per 45 m2) at the end of August 1974. These were removed from grid I 10 weeks before the study ended in September 1975. B: Colonization andfood density (grids 1, 2, 3) These three small grids were set up on the area ploughed in April 1974. By the end of November 1974, grass was re-growing and some voles were immigrating into the area. All voles were removed from all grids on 18-20 November. On 25 November, a high density of food stations (1 per 11 m2) was placed on grid 1, a lower density (1 per 33 m2) FIG. 1. Location of vole grids at Ladner, B.C. Numbers and letters correspond to those assigned to each grid.
1976. Microtus population biology: dispersal in fluctuating populations of M. townsendii. Can. J. Zool .54: 79-95.If spacing behavior limits the breeding density of small mammals, the colonization of vacant areas by surplus animals ought to show how and when population regulation is achieved. Two 0.8-ha areas near Vancouver, British Columbia, were cleared of voles from May 1971 to December 1973 and the colonization of the cleared areas was monitored every 2nd week. All colonists were removed. The colonization rate of the experimental areas was most rapid when populations were increasing rapidly in the adjacent control areas, and much of the loss of individuals in increasing control populations was due to dispersal rather than death. In declining populations very little dispersal occurred. Voles of high body weight (>60 g) were characteristic of late increase and peak populations, but few of these heavy voles dispersed to the vacant experimental areas. Weight at sexual maturity was lower in colonizing voles, particularly among females. About 25% more males than females dispersed into the vacant areas. Colonizing voles were not a genetically random subsample from the control populations. Some leucine aminopeptidase (EC 3.4.11.1) genotypes were more prone to dispersal, particularly when populations were increasing. These results are in agreement with the results of earlier experiments on Microtus pennsylvanicus and M. ochrogaster, and they point to the need for more detailed studies of the dispersal process in voles. KREBS, C. J . , I. WINGATE, J . LEDUC, J . A. REDFIELD, M. TAI-IT et R. HILBORN. 1976. Microtus population biology: dispersal in fluctuating populations of M. townsendii. Can. J. Zool. 54: 79-95. S'il est vrai que le comportement d'espacement limite la densit6 reproductrice des petits mammifkres, la colonisation d'aires libres par les animaux de surplus devrait pouvoir dtmontrer de quelle f a~o n et a quel moment le contr6le de la population se manifeste. On a procede a une exptrience de mai 1971 a dtcembre 1973: on a evacuk les campagnols dans deux aires experimentales de 0.8 ha, pres de Vancouver, Colombie Britannique, et mesure la recolonisation des aires vidtes, toutes les 2 semaines. Tousles colonisateurs etaient ensuite retires de I'aire.La colonisation des aires experimentales se fait a un taux plus rapide lorsque les populations des aires temoins adjacentes augmentent rapidement; la perte d'individus dans les populations temoins en croissance est gen6ralement due a la dispersion plut6t qu'k la mortalitt. Lorsque les populations subissent une dtcroissance, il se produit tres peu de dispersion. La presence de campagnols de poids lourds (>60g) est caracttristique des populations en fin de croissance et des populations ayant atteint leur sommet de densite, mais peu de ces campagnols migrent vers les aires experimentales vacantes. Le poids a la maturite sexuelle est plus bas chez les campagnols colonisateurs, particulitrement chez les femelles. La dispersion dans les aires vides implique environ 25% ...
Supplemental food, in the form of millet seed, was provided to half of an island Song Sparrow population during the 1978-1979 winter to test if winter food influenced: (1) overwinter survival; (2) winter wights; (3) breeding density in 1979 and (4) 1979 breeding performance.Territorial males were most dominant at feeders and may have restricted access of young to feeders. Young females were most subordinate at feeders. Adult survival was not affected by supplementary food, but young survival was higher than in 6 previous years and young seen to visit feeders may have survived better than young not seen at feeders. Young females were more variable in weight on the unfed half of the island than on the fed end. The breeding population increased by 38% from 1978 to 1979, but it is not known how much of this increase resulted from food addition. Pairs of birds with feeders on their territories began to lay 25 days earlier in 1979 than control pairs, but delayed longer than controls before a second breeding attempt. One-year old females began to lay significantly later than adults on the control area, but not on the fed area. Other measures of breeding performance were not affected by supplemental food. Winter food may be more important and male territorial behaviour less important than previously supposed in limiting numbers in the Mandarte Island Song Sparrow population.
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