Sam Erlinge

CAN VERTEBRATE PREDATORS REGULATE THEIR PREY

Whether vertebrate predators can regulate their prey or not has long been a controversial question. At the one extreme it has been claimed that predators have no impact on prey numbers but consume only a doomed surplus (Errington 1946), and at the other that predators strongly interact with their prey causing either stable equilibria or cycles (Tanner 1975; Keith et al. 1977). However,.there are no field studies showing a regulatory effect of predation among vertebrates. Here we report on such a study.

Predation and noncyclicity in a microtine population in southern Sweden

Field data on density changes in field voles and consumption by eight predator populations suggested that the noncyclic pattern in the vole population in southern Sweden was attributable to predation. I tested experimentally whether the spring decline in this non-cyclic population of Microtus agrestis, could be ascribed to predation. In a grassland area of 0.5 ha, avian and mammalian predators (apart from weasels) were excluded by fencing. Voles could move freely in and out of the exclosure. Changes in vole numbers were estimated by mark-recapture. The spring decline was significantly less pronounced in the exclosure than in the control area and this difference was produced by a higher survival rate of voles in the exclosure. The small decline that occurred in the exclosure could be explained by weasel predation and by the fact that voles at the border of the fenced area were exposed to all predators as they moved outside. The results confirm earlier observations that predation is a primary cause of the spring decline in vole populations in southern Sweden. From a graphic analysis of a predator-prey model, Rosenzweig and MacArthur predicted that the following conditions would promote stability in a predator-prey system: 1) access to alternative food that can sustain predators when preferred prey becomes scarce, 2) predators characterized by low prey-capture efficiency, and 3) predator populations limited by other factor(s) than food (e.g., territorial behaviour). These predictions were in agreement with data from our study area. The vole population showed a high degree of stability (no multi-annual cyclicity). Predators of primary importance (certain generalist predators) had a rich supply of alternative food and these generalist predators fed on voles mainly when these were abundant and easily available. Finally, the size of these generalist predator populations was probably limited by territorial behaviour.

Seasonal Changes in the Social Organization of Male Stoats, Mustela erminea: An Effect of Shifts between Two Decisive Resources

A change in male social organization is expected when the most important resource during the breeding season (receptive females) has different dispersion and predictability characteristics as compared with the most important resource during the nonbreeding season (usually food). Male stoats showed a marked seasonal shift in their social organization from a pattern of intrasexual territories during the non-breeding season (autumn and winter) to a non-territorial pattern with extensive and overlapping ranges during the mating season (spring and summer). This shift could not be explained by changes in food availability or distribution, but was correlated instead with the onset of breeding. The proposed hypothesis, that a change in social structure is caused by a shift of decisive resources, will explain seasonal shifts in the social organization of several other mammalian species.

Density-Related Home-Range Size and Overlap in Adult Field Voles (Microtus agrestis) in Southern Sweden

Home-range dynamics and the relation between population density, home-range size, and spacing pattern in a Microtus agrestis population were examined in a 2-year-long mark-recapture study. Home-range size varied with vole density. At high densities during the nonbreeding season, ranges were half the size of those measured at low densities. Home ranges during the breeding season were considerably larger than those established during the nonbreeding season with high densities, but did not differ in size from those in the nonbreeding season with low densities. Ranges of males generally were larger than those of females. Established voles generally stayed in the same restricted area throughout winter. At low densities, males showed a tendency to change home ranges. With onset of breeding in spring, overwintered individuals usually settled in areas adjacent to their winter ranges. Adult females became distributed according to a territorial pattern in spring, whereas ranges of males still overlapped extensively during the early part of the breeding season. During the nonbreeding season, vole ranges overlapped extensively, especially at high densities. There was, however, no evidence of group structure with aggregation behavior in any of the sexes.

Terrestrial trophic dynamics in the Canadian Arctic

The Swedish Tundra Northwest Expedition of 1999 visited 17 sites throughout the Canadian Arctic. At 12 sites that were intensively sampled we estimated the standing crop of plants and the densities of herbivores and predators with an array of trapping, visual surveys, and faecal-pellet transects. We developed a trophic-balance model using ECOPATH to integrate these observations and determine the fate of primary and secondary production in these tundra ecosystems, which spanned an 8-fold range of standing crop of plants. We estimated that about 13% of net primary production was consumed by herbivores, while over 70% of small-herbivore production was estimated to flow to predators. Only 9% of large-herbivore production was consumed by predators. Organization of Canadian Arctic ecosystems appears to be more top-down than bottom-up. Net primary production does not seem to be herbivore-limited at any site. This is the first attempt to integrate trophic dynamics over the entire Canadian Arctic.

Synchrony in lemming and vole populations in the Canadian Arctic

Population fluctuations may occur in synchrony among several rodent species at a given site, and they may occur in synchrony over large geographical areas. We summarize information on synchrony in lemmings and voles from the Canadian Arctic for the past 20 years. The most detailed available information is from the central Canadian Arctic, where snap-trap samples have been taken annually at several sites for periods of up to 15 years. Geographical synchrony in the same species among different sites was strong, especially for the central and eastern Canadian Arctic. Synchrony among different species at a given site was also generally high. When one species is at high density, densities of all species at that site tend to be high. These results do not easily fit the mobile-predator hypothesis proposed to explain regional synchrony, and are more consistent with the weather hypothesis, which we suggest both entrains synchrony among sites and enforces synchrony among species within a site. We tentatively support the weather hypothesis for geographical synchrony in lemmings, and recommend the establishment of a circumpolar program to monitor lemming cycles and predator movements that would advance our understanding of these large-scale patterns of cyclic synchrony.

Delayed density-dependence in a small-rodent population.

The role of delayed density-dependent processes in the dynamics of animal populations poses a problem for ecologists; although generally assumed important in populations that show cyclic or chaotic fluctuations, little experimental evidence for such processes exist. Through manipulation of vole densities within enclosed areas it was shown that reproduction, recruitment, and body growth rate in introduced populations were negatively affected by high previous density. In addition, female movement patterns shifted, and territoriality as well as home-range size was increased after high density. The observed changes in female spacing-behaviour suggested that negative effects of previous density were partly mediated by social interactions, and agreed with the finding that smaller (less competitive) females were the ones suffering most from increased competition. Contrary to expectations from recent work, predation could be excluded as the cause of delayed density-dependence in this study. Instead, chemical analyses of a dominating food plant suggested that herbivory at high vole-density had delayed negative effects on food quality.

Natal dispersal in relation to population density and sex ratio in the field vole, Microtus agrestis

SummaryIn a sample of 240 juvenile field voles 8% of the males and 22% of the females reached sexual maturity within their natal home range. Among individuals retrapped as adults, 58% of males and 23% of females had dispersed, i.e. had moved more than one home range diameter. The mean distance moved for males (58.5 m) exceeded that for females (28.6 m). Male movement distances were negatively associated with total density, and with density of adult females, but not with male density. Female movements were not related to population density. There were no relation between sex ratio and distance moved. The distribution of distances moved for both males and females fit a geometrical distribution, suggesting the importance of competitive processes.

Asynchronous population dynamics of Siberian lemmings across the Palaearctic tundra

Abstract The synchrony of Siberian lemming (Lemmus sibiricus L.) population dynamics was investigated during a ship-borne expedition along the Palaearctic tundra coast in the summer of 1994. On 12 sites along the coast from the Kola Peninsula to Wrangel Island, relative densities of lemmings were recorded using a standardised snap-trapping programme. The phase position of the lemming cycle in each of the studied populations was determined based on current density estimates, signs of previous density and the age profile of each population (ageing based on eye lens mass). In addition, dendrochronological methods were used to determine when the last peak in the density of microtine populations occurred at each site. The examined lemming populations were in different phases of the lemming cycle. Some populations were in the peak phase, as indicated by high current densities, an age profile in which older individuals were well represented, and signs of high previous density (abundant old lemming faeces). Other populations were in the decline phase, as reflected in a moderate current density, a predominance of older individuals and signs of high previous density. Populations in the low phase had an extremely low current density and showed signs of high previous density, while populations in the increase phase had a moderate current density, a predominance of younger individuals and showed signs of low previous density. The results of phase determinations based on dendrochronological methods support the findings based on lemming demography. Recent Russian studies carried out on some of the sites also agreed with our phase determination results. Thus, on a regional scale (across the whole Palaearctic tundra), the population dynamics of Siberian lemmings can be considered asynchronous. However, sites situated adjacent to each other were often phase synchronous, suggesting a more fine-grained pattern of dynamics with synchrony over distances as long as 1000 km or so, e.g. the Yamal and Taymyr Peninsulas.

Adult philopatry and dispersal in the field vol Microtus agrestis

SummaryUsing mark-recapture data, we related the movements of adult field voles to population density, sex ratio and population growth. Dispersal movements (defined as distances larger than 1 home range diameter) were few in both sexes; 4 out of 197 (2.0%) in males and 8 of 316 (2.5%) in females. The distance moved between sequential trapping periods was similar for males and females; the mean being 10.2 m and 9.0 m respectively. Both males and females moved larger distances during the breeding season than during the nonbreeding period. The distance moved between sequential trapping periods showed a strong negative relation to density, i.e. both sexes moved shorter distances at higher densities, but there were no differences between periods of increasing and declining population densities. These results contradict the dispersal predictions of all major hypotheses proposed to explain population fluctuations in small mammals. The dispersal patterns fit a geometric distribution, suggesting that competition is the primary factor determining the dispersal characteristics of this population.