Population dynamics of the collared lemming and the tundra vole at Pearce Point,Northwest Territories,Canada

From 1987 to 1989 we monitored population changes during summer of the collared lemming (Dicrostonyx groenlandicus) and the tundra vole (Microtus oeconomus) at Pearce Point, Northwest Territories, Canada (69° 48 N, 122° 40 W). Populations on four study areas did not cycle but remained at low density (<3/ha) each year and continued at low numbers for the following 3 years (Reid et al. 1995). Lemming numbers often declined throghout the summer in spite of continous reproduction, and population recovery occurred overwinter. Heavy predation losses of radio-collared lemmings occurred each summer and this lemming population may be trapped in a predator-pit. Collared lemmings breed in winter and only because of winter population growth do these populations persist. Tundra vole numbers increased rapidly in most summers but usually declined overwinter. Tundra voles do not seem able to sustain winter reproduction in this extreme environment and this prevents them from reaching high density because of the short summer. Population growth in both these rodents could be prevented by poor food or by predation losses, and landscape patchiness may also help to prevent population growth. For lemmings we do not think that a shortage of shelter or intrinsic limitations could be restricting population increase at Pearce Point. This is the first detailed study of a non-cyclic collared lemming population.     

Functional and numerical responses of four lemming predators in high arctic Greenland

The high‐arctic tundra ecosystem has the world's simplest vertebrate predator–prey community, with only four predators preying upon one rodent species, the collared lemming (Dicrostonyx groenlandicus). We document the functional and numerical responses of all the four predators in NE Greenland. Using these data, we assess the impact of predation on the dynamics of the collared lemming with a 4 yr cycle and >100‐fold difference between maximum and minimum densities. All predator species feed mostly (>90%) on lemmings when lemming density is >1 ha^?1, but the shapes of the predators’ responses vary greatly. The snowy owl (Nyctea scandiaca) is present and breeds only when lemming densities at snowmelt are >2 ha^?1, giving rise to a step‐like numerical response. The long‐tailed skua (Stercorarius longicaudus) has a type III functional response and shifts from alternate food (mainly berries and insects) to lemmings with increasing lemming density. The skua surpasses all the other predators in summer by its total response. The type III functional response of the Arctic fox (Alopex lagopus) starts to increase at much lower lemming densities than the responses of the avian predators, but it has only a weak numerical response. Finally, the stoat (Mustela erminea) is the most specialized predator and the only one with a clearly delayed numerical response. According to their specific functional and numerical responses, each predator plays a key role at some point of the lemming cycle, but only the stoat has the potential to drive the lemming cycle. Stoat predation is greatly reduced in the winter preceding the lemming peak, and it reaches a maximum in the winter preceding the lowest lemming summer density. Stoat predation appears to maintain low lemming densities for at least two successive years. Our study provides empirical support for the specialist predator hypothesis about small mammal population cycles.     

Does lemming winter grazing impact vegetation in the Canadian Arctic?

In low-productivity environments such as the tundra, it has been proposed that regular, multi-annual population cycles of lemmings could be driven by winter food depletion in years of peak abundance. If lemming population dynamics is controlled by food resources, we predict that (1) winter grazing should negatively impact the abundance of food plants, (2) this impact should be proportional to lemming density and (3) high lemming winter grazing pressure should result in reduced plant growth during the following summer. We tested these predictions on Bylot Island, Nunavut, Canada, where two species of lemmings are present: the brown (Lemmus trimucronatus) and collared lemming (Dicrostonyx groenlandicus). We installed 16 exclosures in their preferred wintering habitat (snowbeds) and annually sampled plant biomass inside and outside exclosures at snow melt and at peak growth during the summers of 2009–2012, covering a full population cycle. Winter grazing had no impact on total vascular plant or moss biomass at snow melt in all years. Among plant families, only Caryophyllaceae, which was uncommon, showed a decline. In moss taxa, a negative effect was found on Polytrichum in only 1 year out of three. Overall, plant regrowth during the subsequent summer showed annual variation and tended to be reduced in the 2 years of high lemming abundance. However, this could be a consequence of summer grazing. Overall, the impact of lemming winter grazing on plants was weak and short-lived, even in years of high lemming abundance. Therefore, our results are not consistent with the hypothesis that food depletion during winter was the cause of the lemming decline following peak abundance at our study site. Other factors may limit lemming populations and prevent them from reaching densities high enough to exhaust their food resources.     

Demography of two lemming species on Bylot Island, Nunavut, Canada

Lemmings play a key role in the tundra food web and their widely reported cyclic oscillations in abundance may have a strong effect on other components of the ecosystem. We documented seasonal and annual variations in population density, reproductive activity, survival, and body mass of two sympatric species, the brown (Lemmus trimucronatus) and collared lemmings (Dicrostonyx groenlandicus), over a 2-year period on Bylot Island, Nunavut, Canada. We live trapped and marked lemmings on two grids throughout the summer and we estimated demographic parameters using three different capture–recapture methods. All three methods are based on robust estimators and they yielded similar population density estimates. The density of brown lemmings declined markedly between the 2 years whereas that of collared lemmings was relatively constant. For brown lemmings, 2004 was a peak year in their cycle and 2005 a decline phase. Density of brown lemmings also decreased during the summer, but not that of collared lemmings. The recruitment of juvenile brown lemmings in the population increased during the summer and was higher in the peak year than in the year after, but no change was detected in collared lemmings. Survival rates of both species tended to be lower during the peak year than in the following year and body mass of brown lemmings was higher in the peak year than in the following year. We conclude that both changes in adult survival and juvenile recruitment occur during the population decline of brown lemmings.     

Habitat selection,reproduction and predation of wintering lemmings in the Arctic

Snow cover has dramatic effects on the structure and functioning of Arctic ecosystems in winter. In the tundra, the subnivean space is the primary habitat of wintering small mammals and may be critical for their survival and reproduction. We have investigated the effects of snow cover and habitat features on the distributions of collared lemming (Dicrostonyx groenlandicus) and brown lemming (Lemmus trimucronatus) winter nests, as well as on their probabilities of reproduction and predation by stoats (Mustela erminea) and arctic foxes (Vulpes lagopus). We sampled 193 lemming winter nests and measured habitat features at all of these nests and at random sites at two spatial scales. We also monitored overwinter ground temperature at a subsample of nest and random sites. Our results demonstrate that nests were primarily located in areas with high micro-topography heterogeneity, steep slopes, deep snow cover providing thermal protection (reduced daily temperature fluctuations) and a high abundance of mosses. The probability of reproduction increased in collared lemming nests at low elevation and in brown lemming nests with high availability of some graminoid species. The probability of predation by stoats was density dependent and was higher in nests used by collared lemmings. Snow cover did not affect the probability of predation of lemming nests by stoats, but deep snow cover limited predation attempts by arctic foxes. We conclude that snow cover plays a key role in the spatial structure of wintering lemming populations and potentially in their population dynamics in the Arctic.     

Prolonging the arctic pulse: long-term exploitation of cached eggs by arctic foxes when lemmings are scarce

1. Many ecosystems are characterized by pulses of dramatically higher than normal levels of foods (pulsed resources) to which animals often respond by caching foods for future use. However, the extent to which animals use cached foods and how this varies in relation to fluctuations in other foods is poorly understood in most animals. 2. Arctic foxes Alopex lagopus (L.) cache thousands of eggs annually at large goose colonies where eggs are often superabundant during the nesting period by geese. We estimated the contribution of cached eggs to arctic fox diets in spring and autumn, when geese were not present in the study area, by comparing stable isotope ratios (delta(13)C and delta(15)N) of fox tissues with those of their foods using a multisource mixing model in Program IsoSource. 3. The contribution of cached eggs to arctic fox diets was inversely related to collared lemming Dicrostonyx groenlandicus (Traill) abundance; the contribution of cached eggs to overall fox diets increased from < 28% in years when collared lemmings were abundant to 30-74% in years when collared lemmings were scarce. 4. Further, arctic foxes used cached eggs well into the following spring (almost 1 year after eggs were acquired) - a pattern that differs from that of carnivores generally storing foods for only a few days before consumption. 5. This study showed that long-term use of eggs that were cached when geese were superabundant at the colony in summer varied with fluctuations in collared lemming abundance (a key component in arctic fox diets throughout most of their range) and suggests that cached eggs functioned as a buffer when collared lemmings were scarce.     

Asynchronous population dynamics of Siberian lemmings across the Palaearctic tundra

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. Received: 19 August 1998 / Accepted: 1 March 1999     

Breeding of waders (Charadrii) and Brent Geese Branta bernicla bernicla at Pronchishcheva Lake, northeastern Taimyr, Russia, in a peak and a decreasing lemming year

During summer 1991, lemmings occurred at high densities in Arctic tundra at Pronchishcheva Lake in the northeastern Taimyr Peninsula, whereas, in 1992, lemming densities were substantially lower and decreased further during the summer. In 1991, avian predators such as Snowy Owls Nyctea scandiaca, gulls and skuas bred well; Arctic foxes Alopex lagopus were rarely observed in the study area but bred in the immediate vicinity. In both years there was a late thaw, but this did not deter breeding by birds. The insect food supply for waders showed similar patterns of abundance in both years. In 1991, 73 nests of nine species of wader were found within a 14-km² study area, and Dark-bellied Brent Geese Branta bernicla bernicla nested in association with Snowy Owls. The overall density of wader nests was estimated to be 7 per km². Clutches disappeared at only two wader nests and no Brent Goose nests, and the Mayfield estimate of the daily probability of predation for waders was 0.0022. In contrast, the daily probability of predation was 0.20 in 1992, when there was a similar breeding density of waders. Arctic foxes were seen searching for food daily within the study area, and fox droppings were found associated with nests taken by predators. The predicted scenarios for peak and decreasing lemming years (the Roselaar-Summers hypothesis), i.e. low predation and high nest success in the peak year and high predation and low nest success in the decreasing year, therefore occurred.     

Bee population returns and cherry yields in an orchard pollinated with Osmia lignaria (Hymenoptera: Megachilidae)

During 1998-2003, we used populations of the solitary bee Osmia lignaria Say to pollinate a commercial sweet cherry orchard in northern Utah. Bee densities released each year ranged from 1290 to 1857 females/ha, with approximately twice as many males. Female progeny produced each year were greater than parental populations released, except in 2003, when nesting was poor due to bird predation. Despite poor weather during bloom, and in contrast to most other local producers, the study orchard produced harvestable crops in 1999 (2,964 kg/ha) and 2001 (3,154 kg/ha). In 1998 and 2000, record yields were obtained (10,625 and 12,096 kg/ha, respectively). Including only those years with harvestable crops, average production was 2.2 times higher in 1998-2003 (when O. lignaria populations were used) compared with 1992-1997 (when 10 Apis mellifera hives were used). This is the first study reporting multiyear cherry yields in an orchard pollinated with O. lignaria in North America.     

Landscapes of fear or competition? Predation did not alter habitat choice by Arctic rodents

In systems where predation plays a key role in the dynamics of prey populations, such as in Arctic rodents, it is reasonable to assume that differential patterns of habitat use by prey species represent adaptive responses to spatial variation in predation. However, habitat selection by collared (Dicrostonyx groenlandicus) and brown (Lemmus trimucronatus) lemmings depends on intra- and inter-specific densities, and there has been little agreement on the respective influences of food abundance, predators, and competition for habitat on lemming dynamics. Thus, we investigated whether predation affected selection of sedge-meadow versus upland tundra by collared lemmings in the central Canadian Arctic. We first controlled for the effects of competition on lemming habitat selection. We then searched for an additional signal of predation by comparing habitat selection patterns between 12 control plots and one large grid where lemmings were protected from predators by fencing in 1996 and 1997, but not during 5 subsequent years when we monitored habitat use in the grid as well as in the control plots. Dicrostonyx used upland preferentially over meadows and was more numerous in 1996 and 2011 than in other sample years. Lemmus was also more abundant in 1996 than in subsequent years, but its abundance was too low in the exclosure to assess whether exclusion of predators influenced its habitat selection. Contrary to the effects of competition, predation had a negligible impact on the spatial dynamics of Dicrostonyx, at least during summer. These results suggest that any differences in predation risk between the two habitats have little direct influence on the temporal dynamics of Dicrostonyx even if induced through predator–prey cycles.     

Reproductive behaviour of female Siberian lemmings during the increase and peak phase of the lemming cycle

The reproduction of female Siberian lemmings in the increase and peak phases of the lemming cycle was investigated in connection with a ship-borne expedition along the Siberian arctic tundra. The cycle phase of each studied lemming population was determined using demographic analyses, i.e. current density indices (captured lemmings per 100 traps per 24 h), information on previous density (frequency of old lemming faeces and runways), and information from dendrochronological analyses revealing the most recent winters with a high intensity of willow-stem scarring caused by lemmings. The cycle phase determination was corroborated with data on the age profiles of the populations. The reproductive behaviour of female lemmings differed markedly in relation to cycle phase. In increase-phase populations, all captured females (including young and winter born) were reproducing (had embryos or were lactating), and females started to reproduce early in life, i.e. when <2 months old. By contrast, in peak-phase populations, only 6% of the young females and 63% of the winter-born ones were reproducing, and females did not start to reproduce until they were 5–6 months old. The average number of embryos per reproducing female was significantly higher in increase-phase populations than in peak-phase ones. It is concluded that the rapid population growth in lemmings during the increase phase can largely be explained by the early (young age) reproductive start and, consequently, the shorter generation time, the high proportion of females taking part in reproduction, and the large litters produced. Similarly, a delay in the start of reproduction, a lower proportion of reproducing females, and smaller litter sizes produced by peak-phase lemming populations can contribute substantially to the deceleration in the population increase and possibly lead to a decline. Received: 14 June 1999 / Accepted: 15 November 1999     

Response of an arctic predator guild to collapsing lemming cycles

Alpine and arctic lemming populations appear to be highly sensitive to climate change, and when faced with warmer and shorter winters, their well-known high-amplitude population cycles may collapse. Being keystone species in tundra ecosystems, changed lemming dynamics may convey significant knock-on effects on trophically linked species. Here, we analyse long-term (1988–2010), community-wide monitoring data from two sites in high-arctic Greenland and document how a collapse in collared lemming cyclicity affects the population dynamics of the predator guild. Dramatic changes were observed in two highly specialized lemming predators: snowy owl and stoat. Following the lemming cycle collapse, snowy owl fledgling production declined by 98 per cent, and there was indication of a severe population decline of stoats at one site. The less specialized long-tailed skua and the generalist arctic fox were more loosely coupled to the lemming dynamics. Still, the lemming collapse had noticeable effects on their reproductive performance. Predator responses differed somewhat between sites in all species and could arise from site-specific differences in lemming dynamics, intra-guild interactions or subsidies from other resources. Nevertheless, population extinctions and community restructuring of this arctic endemic predator guild are likely if the lemming dynamics are maintained at the current non-cyclic, low-density state.     

Population cycles of lemmings near Barrow,Alaska: a historical review

Current hypotheses regarding the causes of population cycling of brown lemmingsLemmus trimucronatus (Richardson, 1828), developed during long-term studies from 1950–1974. We maintain that three factors largely determine the timing and amplitude of population cycles in brown lemmings. First, a basic interaction between lemmings and vegetation sets the stage because dense populations of lemmings severely damage the vegetation, at which point lemming populations decline and remain low until the vegetation recovers. Second, opportunistic predators, mainly jaegers and owls, assemble as the snow melts during peak years and drive already declining populations to extremely low densities. Weasels are effective predators under the snow, but they appear irregularly. If weasel populations increase early in the cycle, lemming populations that normally increase dramatically during the winter in a peak year can be decimated before the snow melts. Finally, both wet summers that result in extensive flooding of the preferred habits of lemmings and freezing rains or winter thaws that cause ice formation reduce food availability, disrupt the cycle and extend the period between peak densities. Numerous observations, experimental results and simulation models support these views. Similar results regarding the importance of the available food supply, particularly in winter, and of the predation regime also have been reported for arvicoline populations at lower latitudes.     

Are goose nesting success and lemming cycles linked? Interplay between nest density and predators

The suggested link between lemming cycles and reproductive success of arctic birds is caused by potential effects of varying predation pressure (the Alternative Prey Hypothesis, APH) and protective association with birds of prey (the Nesting Association Hypothesis, NAH). We used data collected over two complete lemming cycles to investigate how fluctuations in lemming density were associated with nesting success of greater snow geese ( Anser caerulescens atlanticus ) in the Canadian High Arctic. We tested predictions of the APH and NAH for geese breeding at low and high densities. Goose nesting success varied from 22% to 91% between years and the main egg predator was the arctic fox ( Alopex lagopus ). Nesting associations with snowy owls ( Nyctea scandiaca ) were observed but only during peak lemming years for geese nesting at low density. Goose nesting success declined as distance from owls increased and reached a plateau at 550 m. Artificial nest experiments indicated that owls can exclude predators from the vicinity of their nests and thus reduce goose egg predation rate. Annual nest failure rate was negatively associated with rodent abundance and was generally highest in low lemming years. This relationship was present even after excluding goose nests under the protective influence of owls. However, nest failure was inversely density-dependent at high breeding density. Thus, annual variations in nest density influenced the synchrony between lemming cycles and oscillations in nesting success. Our results suggest that APH is the main mechanism linking lemming cycles and goose nesting success and that nesting associations during peak lemming years (NAH) can enhance this positive link at the local level. The study also shows that breeding strategies used by birds (the alternative prey) could affect the synchrony between oscillations in avian reproductive success and rodent cycles.     

Predator–prey relationships: arctic foxes and lemmings

1. The number of breeding dens and litter sizes of arctic foxes Alopex lagopus were recorded and the diet of the foxes was analysed during a ship-based expedition to 17 sites along the Siberian north coast. At the same time the cyclic dynamics of co-existing lemming species were examined.
2. The diet of arctic foxes was dominated by the Siberian lemming Lemmus sibiricus (on one site the Norwegian lemming L. lemmus ), followed by the collared lemming Dicrostonyx torquatus .
3. The examined Lemmus sibiricus populations were in different phases of the lemming cycle as determined by age profiles and population densities.
4. The numerical response of arctic foxes to varying densities of Lemmus had a time lag of 1 year, producing a pattern of limit cycles in lemming–arctic fox interactions. Arctic fox litter sizes showed no time lag, but a linear relation to Lemmus densities. We found no evidence for a numerical response to population density changes in Dicrostonyx .
5. The functional or dietary response of arctic foxes followed a type II curve for Lemmus , but a type III response curve for Dicrostonyx .
6. Arctic foxes act as resident specialist for Lemmus and may increase the amplitude and period of their population cycles. For Dicrostonyx , on the other hand, arctic foxes act as generalists which suggests a capacity to dampen oscillations.     

Regulation of reproduction rate in a cyclic population of the red-backed vole,Clethrionomys rufocanus bedfordiae

Changes of the components of reproduction were analyzed quantitatively in a two-year cyclic population (which has two peaks in alternate years during a five-year census) of the red-backed vole, Clethrionomys rufocanus bedfordiae, with reference to its regulatory mechanism: (1) Variation in sex ratios was not associated with population phase or density, although a higher percentage of females in mature individuals was observed in the increase phase. (2) Females attained to sexual maturity at younger age and at lighter body weight than did males. All the youngest mature individuals were found in the low and the increase phases. Age and size at maturity became older and larger as the population went toward the peak phase. (3) Maturation rate was strongly associated with population phase and density; this component is an important and good parameter to predict population trend. Maturation rates were in the order, the low phase>the increase phase>the peak phase>the decline phase; the differences in the rates among these phases were significant. Maturation rate was somewhat depressed when the population density exceeded about 40 individuals/ha. Changes in age at maturity and in maturation rate are interpreted as derivative phenomena related to the population density and the capacity of the number of mature voles per unit area. (4) The maximum number of mature individuals were 26 males/ha and 29 females/ha; there was almost no increase of the number of mature voles at higher population densities over about 40 individuals/ha. The number of exclusive home ranges per hectare calculated from the observed range lengths did not differ much from the maximum number of mature voles of either sex. (5) Length of breeding period was shorter in the high-density years than in the low-density years; the breeding started earlier and ended earlier in the former than that in the latter. In the increase phase a few voles reproduced in winter. (6) The percentage of pregnant females was significantly lower in the peak phase than those in the other phases.     

The topography of Gomori positive neurosecretory elements of the hypothalamus of the lemmings Dicrostonyx torquatus and Lemmus sibiricus]

Under study was the structure of the hypothalamo-hypophyseal neurosecretory system in two species of lemmings of the Wrangel island: the collared lemming, (Dicrostonyx torquatus) and the Ob lemming (Lemmus sibiricus). Inspite of the similarity in the general organization of this system in all rodents, certain species features were found in the topography of neurosecretory elements in the animals studied. In the Ob lemming there is a great similarity in the structures of the neurosecretory system with the albino rat than in the collared lemming. In both species of lemmings, and in the collared lemming in particular, the supraoptic nucleus is located more dorso-caudally and the paraventricular one-more dorsally. A greater amount of neurosecretory cells is found which from additional groups. The hypothalamo-hypophyseal and extrahypothalamic pathways are well pronounced. The postoptical nucleus is considerably developed. In the collared lemming it is almost equal to the supraoptic one in the amount of cells composing it. The ratio of the cells in the supraoptic and postoptic nuclei is suggested to be related to the level of the water-salt metabolism and the specific ecology of lemmings.     

Study on the current population and habitat of the wild Chinese alligator ( Al ligator sinensis )

A survey of wild Chinese Alligator was conducted from July to August 1999 and from August to September 2000 by direct counting after dark (20 z00pm) using headlamps or a 12v portable spotlight. At each site visited , the authors mapped the ponds using laser rangefinder and compass , characterized the physical nature of the ponds and vegetation , and conducted interviews with local residents. The location of all sites was recorded using a hand2held GPS. There are 26 sites in 5 counties that were surveyed where wild Chinese Alligators still exist in Anhui province , including 13 designated conservation sites of the National Chinese Alligator Reserve. The current wild alligator populations mainly exist in the best type of habitat , which totals 17.4 ha with estimated populations of 50.7 % in 1999 and 40.0 % in 2000. This can be compared with the other two types of habitat , which total 22. 0 ha and 19. 0 ha , with their estimated population numbers of 24.0 % in 1999 and 30.9 % in 2000 , and 25.3 % in 1999 and 29.1 % in 2000 respectively. The average ecological densities of the alligators in the two years were 1.3 ind. / ha and 1.8 ind. / ha , with an estimated population of 145 individuals. However , the population has been evidently divided into 18 isolated small local populations with different numbers of individuals. We suggest that habitat should be restored and captive-reared alligators should be reint roduced into the area to rebuild a viable population.     

野生扬子鳄种群及栖息地现状研究

1999年7～8月及2000年8～9月,利用GPS、激光测距仪等,采用夜间灯光照射计数方法,对 有野生扬子鳄(Alligator sinensis)存在的安徽省宣州、泾县、广德、郎溪、南陵等5 县市的26个地点进行了调查,包括扬子鳄国家级自然保护区的13个指定保护点。结果发现： 目前野生扬子鳄主要生存在第一类栖息地（1999年50.7%、2000年40.0%）,面积为17.38 hm²;其他两类栖息地的野生扬子鳄分布比率较小（各为1999年24.0%、2000年30.9%、1999 年25.3% 、2000年29.1%）,面积分别为22.04hm²、19.03hm²。两年的平均生态密度分别为1.28条/hm²和1.79条/hm²,野生扬子鳄种群数量为145条。其种群已明显分为至少18个数 量不等且相互隔离的地方种群。建议恢复足够大的栖息地,并放饲养鳄于其中以重新 建立有效野生种群。     

Social organization of confined male collared lemmings (Dicrostonyx groenlandicus Traill)

This study describes and quantifies the social organization and agonistic behaviour of adult male collared lemmings (Dicrostonyx groenlandicus) in an indoor enclosure. Five groups of four lemmings were observed during separate 8-day tests. Experimental design permitted an increase in group size while density remained constant. When in groups of two, males established stable dominant-subordinate relationships and, when in groups of four, they formed stable non-linear dominance hierarchies. For either group size, frequency of agonistic behaviour declined over time. Habituation, formation of dominance relationships and spatio-temporal partitioning of space are suggested as possible explanations of this decline. Percentage of initial body weight lost during the experiments varied inversely with social rank. Possible causes of this weight loss are discussed. Social relationships in this study are discussed in light of field observations of male lemming home range distribution.