Influences of the feeding ecology on body mass and possible implications for reproduction in the edible dormouse (<Emphasis Type="Italic">Glis glis</Emphasis>)

The edible dormouse (Glis glis) is a small rodent and an obligate hibernator. Dormice undergo strong fluctuations of reproductive output during years that seem to be timed to coincide with future food supply. This behaviour enables them to avoid producing young that will starve with a high probability due to food shortage, and to increase their lifetime reproductive success. Aims of this study were to elucidate the extent to which feeding ecology in the edible dormouse has an impact on body mass and the fatty acid (FA) pattern of the white adipose tissue (WAT) before and after hibernation, which in turn might influence reproductive status in spring. Dormice show strong seasonal fluctuations of the body mass, which is reduced by one third during hibernation. Body mass and its changes depend on autumnal food availability as well as on the dietary FA pattern. During the pre-hibernation fattening period, dormice eat lipid rich food with a high content of linoleic acid. During hibernation, linoleic acid content is slightly but significantly reduced and body mass loss during winter is negatively correlated with the pre-hibernation linoleic acid content in the WAT. No relation between reproductive status and body mass, body condition or the FAs pattern of the WAT could be detected. However, in a year of high reproduction, dormice commence the shift to seed eating earlier than in a year of low reproduction. These seeds could be either a predictor for future food supply in autumn, or represent a high-energy food compensating high energetic costs of sexual activity in male edible dormice.     

Fitness consequences of alternative life histories in water striders,Aquarius remigis (Heteroptera: Gerridae)

Using field and laboratory observations and experiments over 3 years, I investigated whether reproductive trade-offs shape individual life histories in two natural populations of the water strider, Aquarius remigis, in which univoltine and bivoltine life cycles coexist. Both later eclosion dates and food shortages, even after adult eclosion, induced diapause in females, thus deferring reproduction to the following spring. Adult body size was positively affected by food availability during juvenile development. Higher food levels also increased the reproductive output of females, but not their longevity or oviposition period. When compared to spring breeders (univoltine life cycle), direct (summer) breeders (bivoltine life cycle) experienced reduced lifetime egg numbers and longevity, as well as reduced survivorship of their second-summer-generation offspring; these reproductive costs offset, at least in part, the advantage in non-decreasing populations of having two generations per year. Fecundity was correlated with body size, and among summer-generation females direct breeders were larger than non-breeders. The time remaining before the onset of winter and/or the time since adult eclosion augmented cumulative energy uptake, and consequently the lipid reserves and winter survival probability of non-breeding (diapausing) summer adults approaching hibernation. Overwintered spring reproductives died at faster rates than non-reproductive summer individuals despite greater food availability in spring, indicating a mortality cost of reproduction. Body length correlated with absolute and not with proportional lipid content but showed no consistent relationship with survivorship in the field. These results are in agreement with current theory on the evolution of insect voltinism patterns, and further indicate high degrees of life history flexibility (phenotypic plasticity) in the study populations in response to variable environmental factors (notably photoperiod and food availability). This may be related to their location in a geographic transition zone from uni- to bivoltine life cycles.     

Habitat differences affect life history tactics of a pulsed resource consumer,the edible dormouse (<Emphasis Type="Italic">Glis glis</Emphasis>)

We studied flexibility of life history tactics, in terms of habitat-dependent survival and reproduction, in a pulsed resource consumer, the edible dormouse (Glis glis). We compared capture–mark–recapture data from three subpopulations of dormice: one in a homogeneous beech forest (Forest), and two in patches of woodland (Grove/Hedge), with more constant but less energy-rich food availability (e.g., fleshy fruits), over a period of 5 years. The general seasonal pattern of hibernation and reproduction was similar in all three subpopulations. Juveniles were born in only 3 out of 5 years at all study sites, which was paralleled by the occurrence of strong seed production in beech. Reproductive output (number of juveniles/female) was lower at the two sites with low seed tree abundance (Grove: 1.3; Hedge: 2.0) than in the Forest (4.4). Yearly survival probability of adults was significantly lower in the Forest (0.57, CI = 0.42–0.70) than in the areas Grove and Hedge (0.83, CI = 0.70–0.91). Despite their shortened lifespan, estimated lifetime reproductive success of females in the Forest was higher (6.2 young) than in the areas Grove/Hedge (4.8 young). Together, these data indicate that, in more constant habitats (Grove/Hedge), lower reproductive investment was associated with increased longevity. However, dormice apparently maximise lifetime reproductive success by a ‘sit tight’ strategy that synchronises reproduction with energy rich seed pulses in deciduous forests. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users.     

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.     

Comparing the seasonal survival of resident and migratory oystercatchers: carry‐over effects of habitat quality and weather conditions

Events happening in one season can affect life‐history traits at (the) subsequent season(s) by carry‐over effects. Wintering conditions are known to affect breeding success, but few studies have investigated carry‐over effects on survival. The Eurasian oystercatcher Haematopus ostralegus is a coastal wader with sedentary populations at temperate sites and migratory populations in northern breeding grounds of Europe. We pooled continental European ringing‐recovery datasets from 1975 to 2000 to estimate winter and summer survival rates of migrant and resident populations and to investigate long‐term effects of winter habitat changes. During mild climatic periods, adults of both migratory and resident populations exhibited survival rates 2% lower in summer than in winter. Severe winters reduced survival rates (down to 25% reduction) and were often followed by a decline in survival during the following summer, via short‐term carry‐over effects. Habitat changes in the Dutch wintering grounds caused a reduction in food stocks, leading to reduced survival rates, particularly in young birds. Therefore, wintering habitat changes resulted in long‐term (>10 years) 8.7 and 9.4% decrease in adult annual survival of migrant and resident populations respectively. Studying the impact of carry‐over effects is crucial for understanding the life history of migratory birds and the development of conservation measures.     

Food availability as a major driver in the evolution of life‐history strategies of sibling species

Life‐history theory predicts trade‐offs between reproductive and survival traits such that different strategies or environmental constraints may yield comparable lifetime reproductive success among conspecifics. Food availability is one of the most important environmental factors shaping developmental processes. It notably affects key life‐history components such as reproduction and survival prospect. We investigated whether food resource availability could also operate as an ultimate driver of life‐history strategy variation between species. During 13 years, we marked and recaptured young and adult sibling mouse‐eared bats (Myotis myotis and Myotis blythii) at sympatric colonial sites. We tested whether distinct, species‐specific trophic niches and food availability patterns may drive interspecific differences in key life‐history components such as age at first reproduction and survival. We took advantage of a quasi‐experimental setting in which prey availability for the two species varies between years (pulse vs. nonpulse resource years), modeling mark‐recapture data for demographic comparisons. Prey availability dictated both adult survival and age at first reproduction. The bat species facing a more abundant and predictable food supply early in the season started its reproductive life earlier and showed a lower adult survival probability than the species subjected to more limited and less predictable food supply, while lifetime reproductive success was comparable in both species. The observed life‐history trade‐off indicates that temporal patterns in food availability can drive evolutionary divergence in life‐history strategies among sympatric sibling species.     

Seasonal differences in the feeding ecology and behavior of male edible dormice (Glis glis)

Mammalian hibernators undergo dramatic seasonal changes of food intake and the use of their gastrointestinal tract. During several months of hibernation fat-storing hibernators do not use their intestinal tract for nutritional intake. However, during the rest of the year they have to increase their energy intake in order to compensate high reproductive investment and store sufficient body fat to survive the following hibernation period. Edible dormice (Glis glis) are obligate fat-storing hibernators which hibernate in Germany from September until June. Males incur high energetic costs during mating and as soon as reproduction is terminated they have to accumulate high quantities of fat to survive hibernation. In order to understand how fat-storing hibernators like edible dormice cope with these energetically demanding situations, we measured body mass changes of captured male edible dormice in the field and studied their feeding ecology. Furthermore, we measured seasonal changes in food ingestion and assimilation rates by feeding experiments carried out in captivity.Results of this study revealed that during the mating season males significantly lowered their body mass, while food ingestion and assimilation rates remained constant. The body mass reduction showed that they used their body fat reserves to pay at least part of the energetic costs of reproduction. During the pre-hibernation fattening period males increased their body mass but held their assimilation rates on a constant level. Nevertheless, they increased the amount of ingested food and subsequently the amount of energy intake. Furthermore, they changed their dietary spectrum in the field by turning to lipid-rich seeds. These behavioral adaptations enable them to restore their energy losses during reproduction and to accumulate sufficient body fat to survive hibernation.     

Early reproductive investment,senescence and lifetime reproductive success in female Asian elephants

The evolutionary theory of senescence posits that as the probability of extrinsic mortality increases with age, selection should favour early‐life over late‐life reproduction. Studies on natural vertebrate populations show early reproduction may impair later‐life performance, but the consequences for lifetime fitness have rarely been determined, and little is known of whether similar patterns apply to mammals which typically live for several decades. We used a longitudinal dataset on Asian elephants (Elephas maximus) to investigate associations between early‐life reproduction and female age‐specific survival, fecundity and offspring survival to independence, as well as lifetime breeding success (lifetime number of calves produced). Females showed low fecundity following sexual maturity, followed by a rapid increase to a peak at age 19 and a subsequent decline. High early life reproductive output (before the peak of performance) was positively associated with subsequent age‐specific fecundity and offspring survival, but significantly impaired a female's own later‐life survival. Despite the negative effects of early reproduction on late‐life survival, early reproduction is under positive selection through a positive association with lifetime breeding success. Our results suggest a trade‐off between early reproduction and later survival which is maintained by strong selection for high early fecundity, and thus support the prediction from life history theory that high investment in reproductive success in early life is favoured by selection through lifetime fitness despite costs to later‐life survival. That maternal survival in elephants depends on previous reproductive investment also has implications for the success of (semi‐)captive breeding programmes of this endangered species.     

Reproductive success and failure: the role of winter body mass in reproductive allocation in Norwegian moose

A life history strategy that favours somatic growth over reproduction is well known for long-lived iteroparous species, especially in unpredictable environments. Risk-sensitive female reproductive allocation can be achieved by a reduced reproductive effort at conception, or the subsequent adjustment of investment during gestation or lactation in response to unexpected environmental conditions or resource availability. We investigated the relative importance of reduced investment at conception compared with later in the reproductive cycle (i.e. prenatal, perinatal or neonatal mortality) in explaining reproductive failure in two high-density moose (Alces alces) populations in southern Norway. We followed 65 multiparous, global positioning system (GPS)-collared females throughout the reproductive cycle and focused on the role of maternal nutrition during gestation in determining reproductive success using a quasi-experimental approach to manipulate winter forage availability. Pregnancy rates in early winter were normal (≥0.8) in all years while spring calving rates ranged from 0.4 to 0.83, with prenatal mortality accounting for most of the difference. Further losses over summer reduced autumn recruitment rates to 0.23–0.69, despite negligible predation. Over-winter mass loss explained variation in both spring calving and autumn recruitment success better than absolute body mass in early or late winter. Although pregnancy was related to body mass in early winter, overall reproductive success was unrelated to pre-winter body condition. We therefore concluded that reproductive success was limited by winter nutritional conditions. However, we could not determine whether the observed reproductive allocation adjustment was a bet-hedging strategy to maximise reproduction without compromising survival or whether females were simply unable to invest more resources in their offspring.     

Trans‐generational effects on ageing in a wild bird population

Ageing, long thought to be too infrequent to study effectively in natural populations, has recently been shown to be ubiquitous, even in the wild. A major challenge now is to explain variation in the rates of ageing within populations. Here, using 49 years of data from a population of great tits (Parus major), we show that offspring life‐history trajectories vary with maternal age. Offspring hatched from older mothers perform better early in life, but suffer from an earlier onset, and stronger rate, of reproductive senescence later in life. Offspring reproductive lifespan is, however, unaffected by maternal age, and the different life‐history trajectories result in a similar fitness payoff, measured as lifetime reproductive success. This study therefore identifies maternal age as a new factor underlying variation in rates of ageing, and, given the delayed trans‐generational nature of this effect, poses the question as to proximate mechanisms linking age‐effects across generations.     

A 13-year population study of the edible dormouse <Emphasis Type="Italic">Glis glis</Emphasis> in Britain

Edible dormice Glis glis Linnaeus, 1766 were monitored from 1996–2008 inclusive. This has been the first population study of this non-native species since its introduction to Britain in 1902. Dormice were present in nest boxes from May or June until October or November. Numbers recorded varied greatly from year to year, and breeding failure occurred in 6 of the 13 years, apparently in response to environmental cues such as food availability. Litter sizes declined with growth of the young but some of the largest litters resulted from females aggregating their offspring. Males used more nest boxes than females, some of whom showed clear site fidelity from year to year. Survival rates were high and some individual lifespans exceeded 9 years. Longevity, compensating for periodic reproductive failure, and an extremely short active season, make the life history of edible dormice exceptional among small rodents, perhaps unique. Population density, rate of population increase and evidence for local emigration in non-breeding years are discussed, issues of particular significance in populations of recently-established introduced species such as this.     

The biology and life history of arctic populations of the littoral mite Ameronothrus lineatus (Acari, Oribatida)

The present study attempts to elucidate possible microevolutionary adaptations of life-history traits of high-latitude populations of the holarctic, littoral oribatid mite Ameronothrus lineatus by comparing arctic and temperate populations. Additionally, the paper provides an overview of the limited research on general ecology and population biology of arctic populations. In the Arctic the larviparous A. lineatus has a 5-year life cycle (larva-to-larva), and adults survive a further 2–3 years. High survival to maturity is consistent with a low lifetime reproductive output of ca. 20 larvae. The life history can be regarded as an extreme version of the typical oribatid life history. However, several life-history features suggest specific adaptations of arctic populations. In particular, the pre-moult resting stage is synchronized with the warmest part of the arctic summer, which shortens this vulnerable part of development. High reproductive investment by females at relatively low temperatures may represent a physiological adaptation to the cool arctic summer. Finally, prolonged cold exposure positively affects reproduction and survival the following summer, suggesting adaptation of the species to the highly seasonal arctic environment. On the other hand, the ability of all life-cycle stages to overwinter, and a flexible life history with the species being able to take advantage of favourable climatic conditions to accelerate development and larviposition, seem to be ancestral features. Thus, the success of A. lineatus in arctic habitats is probably attributable to a combination of derived and ancestral life-history traits. Studies of conspecific temperate populations are required to elucidate further local adaptations of arctic populations.     

The biology and life history of arctic populations of the littoral mite Ameronothrus lineatus (Acari, Oribatida)

The present study attempts to elucidate possible microevolutionary adaptations of life-history traits of high-latitude populations of the holarctic, littoral oribatid mite Ameronothrus lineatus by comparing arctic and temperate populations. Additionally, the paper provides an overview of the limited research on general ecology and population biology of arctic populations. In the Arctic the larviparous A. lineatus has a 5-year life cycle (larva-to-larva), and adults survive a further 2-3 years. High survival to maturity is consistent with a low lifetime reproductive output of ca. 20 larvae. The life history can be regarded as an extreme version of the typical oribatid life history. However, several life-history features suggest specific adaptations of arctic populations. In particular, the pre-moult resting stage is synchronized with the warmest part of the arctic summer, which shortens this vulnerable part of development. High reproductive investment by females at relatively low temperatures may represent a physiological adaptation to the cool arctic summer. Finally, prolonged cold exposure positively affects reproduction and survival the following summer, suggesting adaptation of the species to the highly seasonal arctic environment. On the other hand, the ability of all life-cycle stages to overwinter, and a flexible life history with the species being able to take advantage of favourable climatic conditions to accelerate development and larviposition, seem to be ancestral features. Thus, the success of A. lineatus in arctic habitats is probably attributable to a combination of derived and ancestral life-history traits. Studies of conspecific temperate populations are required to elucidate further local adaptations of arctic populations.     

Body temperature patterns and use of torpor in an alpine glirid species,woolly dormouse

Woolly dormice, Dryomys laniger Felten and Storch (Senckenbergiana Biol 49(6):429–435, 1968), are a small (20–30 g), omnivorous (mainly insectivorous), nocturnal glirid species endemic to Turkey. Although woolly dormice have been assumed to hibernate during winter, no information exists on body temperature patterns and use of torpor in the species. In the present study, we aimed to determine body temperature patterns and use of torpor in woolly dormice under controlled laboratory conditions. Accordingly, body temperature (Tb) of woolly dormice was recorded using surgically implanted Thermochron iButtons, small and inexpensive temperature-sensitive data loggers. Woolly dormice exhibited robust, unimodal daily Tb rhythmicity during the euthermic stage before the beginning of hibernation. They displayed short torpor before they began hibernation, although the tendency to enter short torpor was different among individuals. Woolly dormice began hibernation within 1–3 days after exposure to cold and darkness, i.e., on October 22–24, and ended hibernation in the first half of April. Hibernation consisted of a sequence of multiday torpor bouts, interrupted by euthermic intervals. Thus, the patterns of hibernation in woolly dormice were similar to those observed in classical hibernating mammals.     

Selection for life‐history traits to maximize population growth in an invasive marine species

Species establishing outside their natural range, negatively impacting local ecosystems, are of increasing global concern. They often display life‐history features characteristic for r‐selected populations with fast growth and high reproduction rates to achieve positive population growth rates (r) in invaded habitats. Here, we demonstrate substantially earlier maturation at a 2 orders of magnitude lower body mass at first reproduction in invasive compared to native populations of the comb jelly Mnemiopsis leidyi. Empirical results are corroborated by a theoretical model for competing life‐history traits that predicts maturation at the smallest possible size to optimize r, while individual lifetime reproductive success (R₀), optimized in native populations, is near constant over a large range of intermediate maturation sizes. We suggest that high variability in reproductive tactics in native populations is an underappreciated determinant of invasiveness, acting as substrate upon which selection can act during the invasion process.     

Seasonal reproductive costs contribute to reduced survival of female greater sage‐grouse

Tradeoffs among demographic traits are a central component of life history theory. We investigated tradeoffs between reproductive effort and survival in female greater sage‐grouse breeding in the American Great Basin, while also considering reproductive heterogeneity by examining covariance among current and future reproductive success. We analyzed survival and reproductive histories from 328 individual female greater sage‐grouse captured between 2003 and 2011, and examined the effect of reproductive effort on survival and future reproduction. Monthly survival of females was variable within years, and this within‐year variation was associated with distinct biological seasons. Monthly survival was greatest during the winter (November–March; Φ_W= 0.99 ± 0.001 SE), and summer (June–July; Φ_S= 0.98 ± 0.01 SE), and lower during nesting (April–May; Φ_N= 0.93 ± 0.02 SE) and fall (August–October; Φ_F= 0.92 ± 0.02 SE). Successful reproduction was associated with reduced monthly survival during summer and fall, and this effect was greatest during fall. Females that successfully fledged chicks had lower annual survival (0.47 ± 0.05 SE) than females who were not successful (0.64 ± 0.04 SE). Annual survival did not vary across years, consistent with a slow‐paced life history strategy in greater sage‐grouse. In contrast, reproductive success varied widely, and was positively correlated with annual rainfall. We found evidence for heterogeneity among females with respect to reproductive success; compared with unsuccessful females, females that raised a brood successfully in year t were more than twice as likely to be successful in year t+ 1. Female greater sage‐grouse incur costs to survival associated with reproduction, however, variation in quality among females may override costs to subsequent reproductive output.     

Phenotypic divergence in reproductive traits of a moth population experiencing a phenological shift

Allochrony that is reproductive isolation by time may further lead to divergence of reproductive adaptive traits in response to different environmental pressures over time. A unique “summer” population of the pine processionary moth Thaumetopoea pityocampa, reproductively isolated from the typical winter populations by allochronic differentiation, is here analyzed. This allochronically shifted population reproduces in the spring and develops in the summer, whereas “winter” populations reproduce in the late summer and have winter larval development. Both summer and winter populations coexist in the same pine stands, yet they face different climatic pressures as their active stages are present in different seasons. The occurrence of significant differences between the reproductive traits of the summer population and the typical winter populations (either sympatric or allopatric) is thus hypothesized. Female fecundity, egg size, egg covering, and egg parasitism were analyzed showing that the egg load was lower and that egg size was higher in the summer population than in all the studied winter populations. The scales that cover the egg batches of T. pityocampa differed significantly between populations in shape and color, resulting in a looser and darker covering in the summer population. The single specialist egg parasitoid species of this moth was almost missing in the summer population, and the overall parasitism rates were lower than in the winter population. Results suggest the occurrence of phenotypic differentiation between the summer population and the typical T. pityocampa winter populations for the life‐history traits studied. This work provides an insight into how ecological divergence may follow the process of allochronic reproductive isolation.     

Risk‐sensitive reproductive allocation: fitness consequences of body mass losses in two contrasting environments

For long‐lived organisms, the fitness value of survival is greater than that of current reproduction. Asymmetric fitness rewards suggest that organisms inhabiting unpredictable environments should adopt a risk‐sensitive life history, predicting that it is adaptive to allocate resources to increase their own body reserves at the expense of reproduction. We tested this using data from reindeer populations inhabiting contrasting environments and using winter body mass development as a proxy for the combined effect of winter severity and density dependence. Individuals in good and harsh environments responded similarly: Females who lost large amounts of winter body mass gained more body mass the coming summer compared with females losing less mass during winter. Additionally, females experienced a cost of reproduction: On average, barren females gained more body mass than lactating females. Winter body mass development positively affected both the females' reproductive success and offspring body mass. Finally, we discuss the relevance of our findings with respect to scenarios for future climate change.     

Late-born intermittently fasted juvenile garden dormice use torpor to grow and fatten prior to hibernation: consequences for ageing processes

Torpor is thought to slow age-related processes and to sustain growth and fattening of young individuals. Energy allocation into these processes represents a challenge for juveniles, especially for those born late in the season. We tested the hypothesis that late-born juvenile garden dormice (Eliomys quercinus) fed ad libitum (‘AL’, n = 9) or intermittently fasted (‘IF’, n = 9) use short torpor bouts to enhance growth and fat accumulation to survive winter. IF juveniles displayed more frequent and longer torpor bouts, compared with AL individuals before hibernation. Torpor frequency correlated negatively with energy expenditure and water turnover. Hence, IF juveniles gained mass at the same rate, reached similar pre-hibernation fattening and displayed identical hibernating patterns and mass losses as AL animals. We found no group differences in relative telomere length (RTL), an indicator of ageing, during the period of highest summer mass gain, despite greater torpor use by IF juveniles. Percentage change in RTL was negatively associated with mean and total euthermic durations among all individuals during hibernation. We conclude that torpor use promotes fattening in late-born juvenile dormice prior to hibernation. Furthermore, we provided the first evidence for a functional link between time spent in euthermy and ageing processes over winter.     

Determinants and consequences of age of primiparity in bighorn ewes

Because variation in age of first reproduction can have major effects on individual fitness and population dynamics, it is important to understand what maintains that variability. Although early primiparity is assumed to be costly, it is sometimes associated with high lifetime reproductive success. We used a long‐term study on bighorn sheep Ovis canadensis to determine what variables affect age at first reproduction, investigate the impact of primiparity on body resources and quantify the reproductive performance of primiparous ewes. We then examined the consequences of delayed primiparity on adult body mass, longevity and lifetime reproductive success. Environmental conditions during early development, body mass as a yearling, genotype and maternal effects affected age of primiparity. Primiparous ewes lost more mass in winter and gained less mass in summer than multiparous ewes. Small yearling ewes that postponed reproduction attained similar adult mass than heavy yearling ewes who reproduced at a younger age. Early primiparity did not reduce longevity and was positively associated with lifetime reproductive success. Starting to reproduce as soon as possible appears to maximize fitness of females. When early life conditions are unfavorable, however, delayed primiparity allows greater body growth and likely maximizes survival. The combination of a conservative reproductive strategy and maternal effects on age of primiparity may partly delay population recovery following density‐dependent declines.