Range-body mass interactions of a northern ungulate – a test of hypothesis

Summer diet, summer temperature, length of the growth season and animal density appeared to best explain annual and regional differences in calf and yearling body mass in moose from southeastern Norway. In general animals inhabiting steep, alpine landscapes had less body mass than animals using flat, low-altitude habitats. Autumn body mass of calves and yearlings decreased with increasing snow depth during the preceding winter and spring. However, calf body mass was more influenced by the summer range and less by the winter range than was body mass of yearlings. There was no indication that the effect of snow depth on autumn body mass was greater in moose living on poor than on good summer ranges. Body mass decreased with increasing competition for summer forage, while the winter range mainly had an density-independent effect. Habitat quality, expressed as regression lines between calf and yearling body mass and animal density (hunting yield), differed between regions. On ranges of medium and high altitude where birch (Betula spp.) rowan (Sorbus aucuparia) and bilberry (Vaccinium myrtillus) dominated moose summer diet, body mass decreased at a rapid rate with increasing animal density. Body mass decreased at a slower rate at low-altitude ranges and at high-altitude ranges where willow (Salix spp.) and forbs dominated the diet. Body mass of lactating cows decreased with increasing animal density, but animal density did not affect body mass of non-lactating cows. There was no indication that the decrease in autumn body mass with increasing moose density over the last 25 years has caused a decrease in animal condition (ability to survive the winter). The results are discussed in relation to the effect of summer and winter range on population regulation in moose. It is concluded that a density-dependent effect is apparent on the summer range even at low and intermediate population densities. On the winter range, on the other hand, density-dependence is likely to occur only at high levels of population density. Received: 4 February 1997 / Accepted: 1 February 1999     

Dispersal of deer mice,Peromyscus maniculatus

Summary Dispersal of deer mice, Peromyscus maniculatus, was measured as immigration to and emigration from two control areas, and as immigration to a removal area. The number of mice dispersing was linearly related to the densities on the control areas, while the proportion of the population dispersing (rate of dispersal) was correlated primarily with the rate of increase of control populations. High rates of dispersal were also associated with a breakdown of the established social structure in the spring and fall. Dispersing animals were compared to residents with respect to sex ratio, weight, age, and breeding condition. The types of animals dispersing varied seasonally: light-weight, non-breeding males dispersed in the spring and summer; juveniles and breeding males dispersed at the end of the breeding season; and light-weight mice of both sexes dispersed over the winter. It is proposed that the animals that dominated the dispersal samples each season were moving in response to social pressure from residents, or local limitations of some resource, and thus, that dispersal was adaptive for the individuals concerned. Some tests of the hypotheses concerning resource limitation are suggested.     

Life history of the moose Alces alces: relationship between growth and reproduction

We studied the relationship between increase in body weight and reproductive performance in different populations of Norwegian moose to evaluate costs associated with early onset of reproduction, viz. whether early onset of reproduction was correlated with low adult body weight or reduced adult fecundity. The mean carcass weight of non-ovulating yearlings was significantly lower than for ovulating yearlings. However, those 2.5 yr old females that conceived as yearlings were lighter than non-reproducing females of the same age. Thus, to begin to reproduce as a yearling was assumed to be expensive because it reduces the possibilities for further growth. The cost associated with reproduction was further illustrated by the fact that the difference in mean carcass weight from age 2.5 to 3.5 yr of females that produced calves in both years, was less for the females from regions with lowest mean yearling weights, i.e. regions with probably the lowest resource availability. In populations with high mean yearling carcass weights, the proportion of cows with calf and the number of calves per pregnant female in the early reproductive phase (2.5 or 3.5 yr old) were higher than in populations in which the mean yearling weights were low. There was a negative correlation between growth rate in the population after 1.5 yr of age and the mean yearling carcass weight. Thus, low yearling weight was associated with a prolonged period of growth and thereby a reduced reproductive output during the first year of the female's life. For old females (≥ 9.5 yr old) the number of calves produced per pregnant female was highest in populations where yearling carcass weights were highest. Furthermore, mean yearling weight and the mean adult female weight were positively correlated in those regions. This relationship suggests that within this species early onset of reproduction is not related to retarded reproduction or lower weight later in life. We suggest that the moose has been selected for an early onset of reproduction.     

Nest distribution shaping within-stream variation in Atlantic salmon juvenile abundance and competition over small spatial scales

1. Spatial heterogeneity in population density is predicted to have important effects on population characteristics, such as competition intensity and carrying capacity. Patchy breeding distributions will tend to increase spatial heterogeneity in population density, whereas dispersal from breeding patches will tend to decrease it. The potential for dispersal to homogenize densities is likely to differ both among organisms (e.g. plants vs. mobile animals) and throughout ontogeny (e.g. larvae vs. adults). However, for mobile organisms, experimental studies of the importance of breeding distributions from the wild are largely lacking. 2. In the present study, experimental manipulations replicated over eight natural streams and 2 years enabled us to test for effects of the distribution of Atlantic salmon eggs over spatial scales which are relevant to local interactions among individuals. Artificial nests were placed along 250 m study reaches at one of two levels of nest dispersion - patchy (two nests per stream) and dispersed (10 nests per stream) - while holding total egg density (eggs m(-2) stream area) constant. 3. Nest dispersion had significant effects on the spatial distribution of the resulting juveniles in their first summer. Patchy nest distributions resulted in a highly right-skewed frequency distribution of local under-yearling densities (among 25 m sampling sections), as sample sections adjacent to the nest sites had relatively high densities. In contrast, dispersed nest distributions yielded approximately normal density distributions. Sections with high relative densities in the patchy nest distribution treatments also had relatively small juvenile body sizes, and patchy egg distribution appeared to produce a higher redistribution of individuals from the first to the second juvenile growth season than the dispersed distribution. 4. Because patchy breeding distribution combined with limited early dispersal can create spatial variation in density over scales directly relevant for individual interactions, this will be one important component in determining mean levels of early juvenile competition and its spatial variation within populations. Assuming random or ideal-free distribution of individuals may therefore underestimate the mean level of density experienced by juveniles over surprisingly small spatial scales (orders of magnitude smaller than total spatial extent of populations), even for mobile organisms.     

Spatial synchrony in vole population fluctuations – a field experiment

Three mechanisms have been proposed to induce spatial synchrony in fluctuations of small mammal populations: climate‐related environmental effects, predation and dispersal. We conducted a field experiment in western Finland to evaluate the relative roles of these mechanisms in inducing spatial synchrony among cyclic populations of field voles Microtus agrestis. The study was conducted during the increase and peak phases of a vole population cycle on four agricultural field sites situated 1.5–7.0 km apart. Each field contained two 0.5‐ha fenced enclosures and one 1‐ha unfenced control area. One enclosure per field allowed access by small mustelid predators and the other by avian predators; all enclosures prevented the dispersal of voles. The unfenced control areas allowed access by all predators as well as dispersal by voles. Enclosed vole populations were in a treatment‐wise asynchronous phase before the predator access treatments were applied. The growth rates of all enclosed populations were tightly synchronized during the course of the experiment. Conversely, synchrony both among the unfenced populations and between the fenced and unfenced populations was practically non‐existent. During winter, in the increase phase of the cycle, vole populations in all treatments declined to low densities due to a seasonal effect of winter food depletion. During summer, in the peak year of the vole cycle, all populations fluctuated in synchrony. At this time, both small mustelids and birds of prey appeared to be abundant enough to induce synchrony. Dispersal was identified as a potential contributor to synchronization, but the magnitude of its effects could not be reliably discerned. Our results indicate that no single mechanism can account for the observed patterns of spatial synchrony among cyclic northern vole populations. Rather, spatial synchronization is induced by different mechanisms, namely seasonality and predation, acting successively during different seasons and phases of the vole cycle.     

Movements of white-tailed deer in riparian habitat: Implications for infectious diseases

Movements of male white-tailed deer (Odocoileus virginianus) are of great concern with respect to spread of chronic wasting disease (CWD) across landscapes because most yearlings males disperse and adult males have higher prevalence of CWD than do females and younger deer. We radiocollared and monitored 85 male white-tailed deer in the middle Missouri River Valley of eastern Nebraska and western Iowa, USA from 2004 to 2008. Average size (±SE) of fixed-kernel annual home ranges (95%) and core areas (50%) for resident deer were 449 (±32) ha and 99 (±7) ha, respectively. Resident deer exhibited a high-degree of fidelity to their home ranges. Mean overlap between consecutive annual home ranges and core areas was 81% and 74%, respectively. Average dispersal distance was 17.7 ± 4.5 km (range = 3–121 km) for 22 radio-marked and 6 ear-tagged yearlings. Mean spring dispersal distance (25 km) was 150% greater than fall (10 km). Dispersal direction from Desoto National Wildlife Refuge (DNWR) was bimodal on a northwest to southeast axis that followed the Missouri River corridor. Of 22 yearlings that dispersed, 18 (82%) established adult home ranges within the river valley. Dispersal movements of yearling males represent the greatest risk for rapid spread of diseases from infected source populations. Disease management efforts in riparian habitats should target male fawns and yearling males for removal in areas within or immediately adjacent to river corridors. © 2011 The Wildlife Society.     

Determinants of reproductive success in female Columbian ground squirrels

Summary We studied the reproductive success of female Columbian ground squirrels (Spermophilus columbianus) in southwestern Alberta for nine years. We defined reproductive success as the number of offspring surviving their first hibernation, classified as yearlings. The number of weaned juveniles explained one third of the variance in number of yearlings at emergence from their first hibernation the following spring, and much of the variance in individual reproductive success originated after weaning. Weight of adult females at emergence from hibernation was correlated with annual reproductive success. The mother's survival beyond weaning and the subsequent winter's snow accumulation had positive effects on annual reproductive success, whereas population density and summer temperature had negative effects. We found no effects on annual reproductive success of date of litter emergence, weight at emergence as a yearling, presence or absence of adult kin, distance from the natal site, location within the study area, winter temperature or summer precipitation. Age of first breeding did not affect lifetime reproductive success, which ranged from 0 to 19 yearlings produced over a lifetime. The greatest source of variation in lifetime reproductive success for females surviving to breeding age was offspring survival, followed by reproductive lifespan.     

Family-based winter territoriality in western bluebirds, Sialia mexicana: the structure and dynamics of winter groups

Winter residency is characteristic of the majority of cooperatively breeding birds, but the composition and dynamics of winter groups have been examined in relatively few. In 1996-1998, we examined winter territoriality in the western bluebird, a year-round resident that shows a limited degree of helping behaviour in central coastal California, U.S.A. In spring, most western bluebirds breed as socially monogamous pairs, but a small proportion of pairs (3-16%) have additional breeding-age males helping at the nest, usually assisting parents or brothers. We found that year-round residents commonly wintered in family groups that defended territories similar to those used in spring. Winter groups had an even sex ratio and formed early in the autumn, when hatch-year birds dispersed. More females than males left their natal groups to be replaced by an influx of immigrant hatch-year birds. Winter groups typically consisted of breeders and one or two sons from the prior breeding season along with one or more immigrant females. A second period of dispersal occurred in spring when winter groups broke up and most birds other than the breeding pair left the winter territory. When they bred, yearling males and females often bred with unrelated individuals from their winter groups. Sons were more likely to remain on the study area as yearlings when they wintered with both parents than when they wintered with just one parent. We suggest that young males stay the winter due to benefits of remaining in family groups on mistletoe-based winter territories. Subsequent localized dispersal of sons then leads to opportunistic kin-based interactions later in life. Copyright 2001 The Association for the Study of Animal Behaviour.     

Supplemental feeding reduces natural selection in juvenile red deer

In red deer, variation in winter and spring weather conditions encountered by the mothers during pregnancy and during the first year of life are a main determinant for individual life-history as well as population dynamics. We tested the hypothesis that supplementary feeding which provides constant food supply throughout winter removes the selective pressure of winter harshness on nutrition-mediated phenotypic traits. We analysed cohort variation in body weight in calves in October, before their first winter, and in yearlings in June, after their first winter, in a food-supplemented population in the Eastern Austrian Alps. Over eleven years, cohort body weight varied between years in calves and yearlings. Contrary to studies on non-supplemented red deer populations we found neither short- nor long-term effects of winter weather on body weight. In calves, autumn body weight was negatively related to April–May and June temperatures, suggesting that cool weather during the main growth period retarded plant senescence and thereby prolonged the period of high protein content of summer forage. In yearlings, variation in June body weight, shortly after the end of the feeding period, was lower after a wet April–May, suggesting a negative effect of a prolonged period of supplemental feeding. For both calves and yearlings intra-cohort variation in body weight was higher, inter-cohort variation was lower as compared to non-supplemented red deer, suggesting that in their first year of life supplemented red deer are under reduced natural selection pressure.     

Divergent patterns of impact of environmental conditions on life history traits in two populations of a long-distance migratory bird

Some areas have experienced recent dramatic warming due to climate change, while others have shown no change at all, or even recent cooling. We predicted that patterns of selection on life history would differ between southern and northern European populations of a long-distance migratory bird, the barn swallow Hirundo rustica, because global patterns of weather as reflected by large-scale weather phenomena such as the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO) have different effects on environmental conditions in different parts of the world frequented during the annual cycle. We investigated relationships between mean arrival date, dispersal rate and yearling survival rate among years, using two long-term population studies in Spain and Denmark. We found evidence of a difference in the effects of normalized difference vegetation index in North and West Africa on mean arrival date of male barn swallows, with the effect differing significantly between populations. Second, there was a significant interaction between ENSO and population on dispersal rate, showing that conditions in Africa during winter differentially affected dispersal in the two populations. Finally, the NAO index in winter had an effect on yearling survival that differed between populations. These findings highlight the divergent patterns of response to climate change among populations, and they suggest that climate change can differentially affect important life history traits with potential implications for maintenance of viable populations and gene flow among populations.     

Fluctuating food supply affects the clutch size of Tengmalm's owl independent of laying date

Summary In western Finland, yearly median laying dates of Tengmalm's owls varied from 14 March to 27 April during 1973–1989 and were negatively correlated with the winter densities of voles. Yearly mean clutch sizes varied from 4.0 to 6.7 and were more closely related to the spring than to the winter densities of voles. The yearly mean clutch size decreased with yearly median laying date. The 3-year vole population cycle is typical of the study area. The start of egg-laying was earliest in the peak phase of the cycle (median laying date 22 March), when vole numbers are high during egg-laying, but decline rapidly to low numbers in the next autumn or winter. In the increase phase (1 April) vole abundances are moderate at the time of laying, but increase to a peak in the next autumn or winter. In the low phase (15 April) voles are scarce in spring and in the preceding winter, starting to increase in late summer. Clutch size and female body mass were independent of laying date in the low phase, decreased slowly but significantly in the increase phase, and declined abruptly in the peak phase. These trends also held when the effects of territory quality, female age and male age were ruled out. When comparing the same laying periods, clutch sizes were significantly larger in the increase than in other phases of the cycle, but there was no difference between the peak and low phases. Supplementary feeding prior to and during egg-laying increased clutch size independent of laying date. These results agreed with the income model (the rate of energy supply during laying determines clutch size). Tengmalm's owls invest most in a clutch in the increase phase, as the reproductive value of eggs is largest because of high survival of yearlings. A high reproductive effort may be adaptive during this phase, because the availability of voles is predictable during the laying period.     

Hunter feedback of individually marked wild boar <Emphasis Type="Italic">Sus scrofa</Emphasis> L.: dispersal and efficiency of hunting in northeastern Germany

Increasing wild boar (Sus scrofa L.) population densities all over Europe cause severe economic problems. For understanding mechanisms of epidemics, the knowledge of dispersal is required. Thus, we investigated dispersal rates and distances with regard to sex and age of wild boar in southwestern Mecklenburg-Western Pomerania. From 152 marked wild boar, 105 have been registered as dead, of which, 51% were males and 49% females. Forty-five percent were shot as piglets, 41% as yearlings, and 14% as adults. The distance between capture site and site of death ranged between 184 m and 41.5 km. Piglets were shot closer to their capture site (mean distance 1 km) than older animals (mean 4 km), although this difference was only significant for males. In general, males tended to disperse further before being shot (3.8 km) than females (1.6 km). Only 3.8% of all animals were shot at distances larger than 10 km. As most animals (84.6%) were shot inside their natal home range, only a small proportion (15.4%) did actually disperse (shot outside mothers home range), which is 32% of all animals surviving to the age of yearlings. Of those dispersed animals, 25% were females. The low dispersal rate is biased by female philopatry and allows actual dispersal only at very high population densities or in sparsely populated regions. In consideration for the low natural mortality proved by radio-tagged animals, the harvest rate is lower than the net reproduction. We did not detect any sex-biased hunting. The dominating hunting method was single hunt at bait, although drive hunts are highly effective. However, hunting rates on piglets and females were too low for regulating the population.     

Dispersal of yearling yellow-bellied marmots (Marmota flaviventris)

Field studies of the social organization of yellow-bellied marmots (Marmota flaviventris) in the vicinity of Gothic, Colorado, were initiated in 1962. Data collected during the subsequent 14 years are sufficient to analyse the behaviours that correlate with the time yearling male and yearling female marmots leave their natal home sites. Dispersal of yearling male yellow-bellied marmots was delayed when there were many yearling males in a harem, when yearling males were underweight, and when the rates of amicable interactions between yearling males and adults were high. Dispersal of yearling females was delayed when adults behaved amicably toward them and when the rates of adult aggression were low. Dispersal of yearling females was independent of the number of females in a harem. These observations are in accord with a model of dispersal that relates sexual differences in dispersal to maternal reproductive tactics, and to responses of yearlings that would maximize their own fitness.     

Phase dependence in winter physiological condition of cyclic voles

Lack of food resources has been suggested as a factor which limits the growth of cyclic vole populations. During peak phases of the cycle, vole population growth typically ceases during late autumn or early winter, and is followed by a decrease in density over the winter. To investigate whether this decrease is due to increased mortality induced by a depletion of food resources, we studied overwinter food consumption and physiological condition of field voles ( Microtus agrestis ) in western Finland in both an increase and a decrease phase of a three-year population cycle. The growth rate of vole populations was negatively related both to prevailing vole densities and to densities six months earlier. The condition index of voles, as well as their blood levels of haematocrit, proteins, free fatty acids and immunoglobulin G, were positively related to population growth rate when populations were declining. When populations were increasing, these parameters tended to be negatively related to population growth rate. The overall physiological condition of voles was lower in the winter of the decrease phase as compared to the increase phase. The return rate of voles, a proxy of survival, was also lower in the decrease than in the increase phase of the cycle and positively related to haematocrit levels. Almost 90% of all green vegetation shoots were consumed by voles during the winter of the decrease phase while only two thirds were eaten in the increase phase. Our results suggest that the winter decrease phase of cyclic vole populations is associated with both a deterioration in the physiological condition of voles and a significant depletion of winter food resources. This implies that malnutrition induces poor physiological condition in voles, which in turn may increase mortality either directly through starvation or indirectly through increased susceptibility to predators and pathogens.     

Antler length of yearling red deer is determined by population density, weather and early life-history

In red deer, yearling antler length is a largely nutrition-mediated phenotypic character, and is therefore sometimes used as an indirect estimate of range quality and population condition. However, the parameters affecting yearling antler length have been little studied. We analyse the contributions of density, weather and maternal effects on yearling antler length of 581 individual stags born 1970-1996 on the Isle of Rum (Scotland). We show that antler length is a good measure of yearling condition: the probability of overwinter survival in yearlings that developed antlers was 3 times higher than for yearlings that did not develop antlers, and yearling antler length was correlated with the number of antler points the following year. Between years, variation in yearling antler length was best explained by variation in red deer density and June temperature at 12 months of age. Both of these variables were negatively correlated with antler length, and most likely this effect is due to changes in nutrient availability. Population density affects biomass availability for the individual, while low temperatures in early summer prolong the availability of high forage quality. At the individual level, antler length increased with birth weight and decreased with birth date, reflecting the persistent and pervasive influence of conditions in early life.     

Scale of Management for Mature Male White-Tailed Deer as Influenced by Home Range and Movements

Abstract: The scale at which populations use landscapes influences ecological processes and management. We used dispersal and home-range data of 3 age groups of male white-tailed deer (Odocoileus virginianus) to determine the scale at which management will be effective. Home-range size at 5.5 years of age (182 ha ± 24.9 SE) was 56% smaller (P < 0.001) than home-range size of the same 13 males as yearlings (416 ± 59.4 ha). Percent overlap of yearling and 5.5-year-old home ranges was 62.7 6 10.3% (n = 13). Distance between home-range centers of yearling and mature deer was 1,264.9 ± 407.4 m, including 3 deer that dispersed after 2.5 years of age. Average 95% fixed-kernel home-range size was 207.4 ± 20.4 ha and 225.7 ± 30.1 ha for all mature males in years 1 and 2 of our study, respectively. We found that properties >10,000 ha were needed to manage >50% of original yearling males found on the property, whereas properties of 4,500 ha would maintain 50% of original middle-aged (2.5-4.5 yr of age) and mature males (≥4.5 yr of age). Movements after dispersal were minimal, with deer shifting their center of activity <600 m and <350 m each year for middle-aged and mature males, respectively. These data could be used by managers developing management plans, recommending harvest rates, and interpreting harvest data of male white-tailed deer and by biologists attempting to understand ecological processes such as spread of disease.     

Seroconversion rates to Jamestown Canyon virus among six populations of white-tailed deer (Odocoileus virginianus) in Indiana

The annual seroconversion of fawns, yearlings, and adult white-tailed deer (Odocoileus virginianus) to Jamestown Canyon virus (California group) was followed at six Indiana sites from 1981 through 1984. In all, sera from 1,642 deer (515 fawns, 618 yearlings, and 509 adults) were tested for neutralizing antibody to three California serogroup viruses: Jamestown Canyon, La Crosse, and trivittatus. Virtually all deer with specific neutralizing antibody showed evidence of a prior infection with Jamestown Canyon virus; only three deer showed evidence of a prior infection with only La Crosse virus and none showed evidence of an infection with only trivittatus virus. While there were no significant differences in antibody prevalence to Jamestown Canyon virus between yearling and adult deer at any site, fawns had significantly lower antibody prevalences than either of the two older age groups. Significant differences in antibody prevalence were found between northern versus southern populations of white-tailed deer in Indiana, however, no significant differences were found among the four northern populations or between the two southern populations. The mean antibody prevalences in the two southern fawn, yearling, and adult populations were 15%, 38%, and 41% respectively, while the prevalences in the four northern fawn, yearling, and adult populations were 5%, 67%, and 67% respectively. These different prevalences (northern vs. southern) correlate with the higher Jamestown Canyon virus antibody prevalence in human residents of northern Indiana (2-15%) compared to residents of southern Indiana (less than 2%) found in other studies. The significantly lower prevalence of antibody to Jamestown Canyon virus in fawns is attributed to maternal antibody protecting them from a primary infection their first summer. Yearling deer showed high rates of seroconversion following their second summer of life. These results suggest that infection of white-tailed deer in Indiana with Jamestown Canyon virus is a common phenomenon.     

Costs and benefits of natal dispersal in yearling mallards Anas platyrhynchos

Costs and benefits of natal dispersal have not been fully evaluated in birds. We compared timing of breeding and nesting success for yearling female mallards Anas platyrhynchos returning to or dispersing from their natal areas. Information about natal origins was discerned with feather‐isotopes and combined with detailed reproductive histories for 503 radio‐marked females monitored at 16 study sites across the Canadian Aspen Parklands, during 1993–2000. A natal origin assignment model based on feather‐ δ³⁴S, δD, δ¹⁵N, and δ¹³C values correctly assigned 81% (112 of 138) of known‐source yearlings to region of origin; region‐specific rates ranged from 70–90%. Timing of breeding and nesting success was not related to whether or not a female had dispersed from its region of natal origin in Aspen Parkland (i.e. short‐distance dispersal) versus the southern prairies or boreal forest regions (i.e. long‐distance dispersal). Rather, nesting success was best modeled to include effects of site‐specific wetland and breeding pair abundances and an interaction between local breeding pair and wetland densities. Nest success performance relative to dispersal distance varied among study sites but was unrelated to local upland nest cover, wetland habitat conditions, or conspecific density. Thus, we detected no strong costs of dispersal but some evidence that long‐distance dispersers presumably benefitted when they were able to acquire better nest sites.     

Sex, sex-ratios, and the dynamics of pelagic copepod populations

I examine how the population biology of pelagic copepods depends on their mating biology using field data and a simple demographic model. Among calanoid copepods, two distinct patterns emerge. Firstly, copepods that lack seminal receptacle and require repeated mating to stay fertilized have near equal adult sex ratios in field populations. Winter population densities are orders of magnitude less than the critical population density required for population persistence, but populations survive winter seasons as resting eggs in the sediment. Population growth in these species is potentially high because they have on average a factor of 2 higher egg production rates than other pelagic copepods. Secondly, other copepods require only one mating to stay fertile, and populations of these species have strongly female-skewed adult sex-ratios in field populations. Resting eggs have not been described within this group. Winter population sizes are well predicted by the critical density required for population persistence which, in turn, is closely related to the body-size-dependent mate-search capacity. Thus, the different requirements for mating lead in the first case to a more opportunistic reproductive strategy that implies rapid colonization of the pelagic during productive seasons, and in the second case to a strategy that allows maintenance of a pelagic populations during unproductive seasons. Positive density dependent population growth during periods of low population density (‘Allee effect’) amplifies population density variation during winter into the subsequent summer, thus explaining why summer densities appear to depend more on winter densities than on current growth opportunities in pelagic copepods.     

Abundance,population structure and seasonally changing social organization of argali <Emphasis Type="Italic">Ovis ammon karelini</Emphasis> in West-Central Tian-Shan of Kyrgyzstan

The main aim of this study was to determine the numbers, population structure and seasonal changes in group structure of argali Ovis ammon karelini Severtzov, 1873 in the Tian-Shan of Kyrgyzstan. The study was carried out within two adjoining areas: the Baralbas River region, and the Ak-Tash River region. Data were collected during three seasons: winter, spring and summer. This population consisted of 42.3% females, 22.2% males, 13.4% yearlings, and 22.1% lambs. Composition and numbers of groups were seasonally changing. Argali occurred predominantly in mixed groups during winter and exclusively in separated groups during summer. The maximum group size decreased from 25% from winter to spring, however, increased during summer.