Variable age structure and apparent density dependence in survival of adult ungulates

Post-weaning survival in kangaroos is high and constant until senescence: Implications for population dynamics

Large herbivores typically have consistently high prime-aged adult survival and lower, more variable, juvenile, and senescent survival. Many kangaroo populations undergo greater fluctuations in density compared with other large herbivores, but age- and sex-specific survival of kangaroos and their response to environmental variation remain poorly estimated. We used long-term capture-mark-recapture data on 920 individuals to investigate the survival component of eastern grey kangaroo (Macropus giganteus) population dynamics. Forage availability and population density were monitored quarterly and included as predictors of survival in Bayesian Cormack-Jolly-Seber models. Annual survival probabilities were estimated for five age classes: 0 years (juveniles), 1-2 years (subadults), 3-6 years (prime-aged adults), 7-9 years (presenescent adults), and ≥10 years (senescent adults). Survival of juveniles varied widely during our 12-year study, ranging from 0.07 to 0.90 for females and 0.05-0.92 for males. Subadult survival was 0.80-0.93 for females and 0.75-0.85 for males, while that of prime-aged adults was ≥0.94 for females and ≥0.83 for males, despite large fluctuations in forage and density. The survival of presenescent adults spanned 0.86-0.93 for females and 0.60-0.86 for males. Senescent survival was variable, at 0.49-0.90 for females and 0.49-0.80 for males. Male survival was significantly lower than female survival in prime-aged and presenescent adults, but not in other age classes. Although most of the models supported by Watanabe-Akaike Information Criterion selection included at least one environmental covariate, none of these covariates individually had a discernible effect on survival. Temporal variability in overall survival appeared mostly due to changes in the survival of juvenile and senescent kangaroos. Kangaroo survival patterns are similar to those of ungulates, suggesting a strong role of sex-age structure on population dynamics.

Harvesting can stabilise population fluctuations and buffer the impacts of extreme climatic events

Harvesting can magnify the destabilising effects of environmental perturbations on population dynamics and, thereby, increase extinction risk. However, population-dynamic theory predicts that impacts of harvesting depend on the type and strength of density-dependent regulation. Here, we used logistic population growth models and an empirical reindeer case study to show that low to moderate harvesting can actually buffer populations against environmental perturbations. This occurs because of density-dependent environmental stochasticity, where negative environmental impacts on vital rates are amplified at high population density due to intra-specific resource competition. Simulations from our population models show that even low levels of harvesting may prevent overabundance, thereby dampening population fluctuations and reducing the risk of population collapse and quasi-extinction following environmental perturbations. Thus, depending on the species' life history and the strength of density-dependent environmental drivers, low to moderate harvesting can improve population resistance to increased climate variability and extreme weather expected under global warming.

Demographic responses of a threatened, low-density ungulate to annual variation in meteorological and phenological conditions

As global climate change progresses, wildlife management will benefit from knowledge of demographic responses to climatic variation, particularly for species already endangered by other stressors. In Canada, climate change is expected to increasingly impact populations of threatened woodland caribou (Rangifer tarandus caribou) and much focus has been placed on how a warming climate has potentially facilitated the northward expansion of apparent competitors and novel predators. Climate change, however, may also exert more direct effects on caribou populations that are not mediated by predation. These effects include meteorological changes that influence resource availability and energy expenditure. Research on other ungulates suggests that climatic variation may have minimal impact on low-density populations such as woodland caribou because per-capita resources may remain sufficient even in “bad” years. We evaluated this prediction using demographic data from 21 populations in western Canada that were monitored for various intervals between 1994 and 2015. We specifically assessed whether juvenile recruitment and adult female survival were correlated with annual variation in meteorological metrics and plant phenology. Against expectations, we found that both vital rates appeared to be influenced by annual climatic variation. Juvenile recruitment was primarily correlated with variation in phenological conditions in the year prior to birth. Adult female survival was more strongly correlated with meteorological conditions and declined during colder, more variable winters. These responses may be influenced by the life history of woodland caribou, which reside in low-productivity refugia where small climatic changes may result in changes to resources that are sufficient to elicit strong demographic effects. Across all models, explained variation in vital rates was low, suggesting that other factors had greater influence on caribou demography. Nonetheless, given the declining trajectories of many woodland caribou populations, our results highlight the increased relevance of recovery actions when adverse climatic conditions are likely to negatively affect caribou demography.

Detecting Resource Limitation in a Large Herbivore Population Is Enhanced With Measures of Nutritional Condition

Resource limitation at the population level is a function of forage quality and its abundance relative to its per capita availability, which in turn, determines nutritional condition of individuals. Effects of resource limitation on population dynamics in ungulates often occur through predictable and sequential changes in vital rates, which can enable assessments of how resource limitation influences population growth. We tested theoretical predictions of bottom-up (i.e., resource limitation) forcing on moose (Alces alces) through the lens of vital rates by quantifying the relative influence of intrinsic measures of nutritional condition and extrinsic measures of remotely sensed environmental data on demographic rates. We measured rates of pregnancy, parturition, juvenile, and adult survival for 82 adult females in a population where predators largely were absent. Life stage simulation analyses (LSAs) indicated that interannual fluctuations in adult survival contributed to most of the variability in λ. We then extended the LSA to estimate vital rates as a function of bottom-up covariates to evaluate their influence on λ. We detected weak signatures of effects from environmental covariates that were remotely sensed and spatially explicit to each seasonal range. Instead, nutritional condition strongly influenced rates of pregnancy, parturition, and overwinter survival of adults, clearly implicating resource limitation on λ. Our findings depart from the classic life-history paradigm of population dynamics in ungulates in that adult survival was highly variable and generated most of the variability in population growth rates. At the surface, lack of variation explained by environmental covariates may suggest weak evidence of resource limitation in the population, when nutritional condition actually underpinned most demographics. We suggest that variability in vital rates and effects of resource limitation may depend on context more than previously appreciated, and density dependence can obfuscate the relationships between remotely sensed data and demographic rates.

Impact of Mycoplasma ovipneumoniae on juvenile bighorn sheep (Ovis canadensis) survival in the northern Basin and Range ecosystem

Determining the demographic impacts of wildlife disease is complex because extrinsic and intrinsic drivers of survival, reproduction, body condition, and other factors that may interact with disease vary widely. Mycoplasma ovipneumoniae infection has been linked to persistent mortality in juvenile bighorn sheep (Ovis canadensis), although mortality appears to vary widely across subspecies, populations, and outbreaks. Hypotheses for that variation range from interactions with nutrition, population density, genetic variation in the pathogen, genetic variation in the host, and other factors. We investigated factors related to survival of juvenile bighorn sheep in reestablished populations in the northern Basin and Range ecosystem, managed as the formerly-recognized California subspecies (hereafter, "California lineage"). We investigated whether survival probability of 4-month juveniles would vary by (1) presence of M. ovipneumoniae-infected or exposed individuals in populations, (2) population genetic diversity, and (3) an index of forage suitability. We monitored 121 juveniles across a 3-year period in 13 populations in southeastern Oregon and northern Nevada. We observed each juvenile and GPS-collared mother semi-monthly and established 4-month capture histories for the juvenile to estimate survival. All collared adult females were PCR-tested at least once for M. ovipneumoniae infection. The presence of M. ovipneumoniae-infected juveniles was determined by observing juvenile behavior and PCR-testing dead juveniles. We used a known-fate model with different time effects to determine if the probability of survival to 4 months varied temporally or was influenced by disease or other factors. We detected dead juveniles infected with M. ovipneumoniae in only two populations. Derived juvenile survival probability at four months in populations where infected juveniles were not detected was more than 20 times higher. Detection of infected adults or adults with antibody levels suggesting prior exposure was less predictive of juvenile survival. Survival varied temporally but was not strongly influenced by population genetic diversity or nutrition, although genetic diversity within most study area populations was very low. We conclude that the presence of M. ovipneumoniae can cause extremely low juvenile survival probability in translocated bighorn populations of the California lineage, but found little influence that genetic diversity or nutrition affect juvenile survival. Yet, after the PCR+ adult female in one population died, subsequent observations found 11 of 14 ( 79%) collared adult females had surviving juveniles at 4-months, suggesting that targeted removals of infected adults should be evaluated as a management strategy.

The accuracy and precision of age determination by dental cementum annuli in four northern cervids

Individual age is an important element in models of population demographics, but the limitations of the methods used for age determination are not always clear. We used known-age data from moose (Alces alces), red deer (Cervus elaphus), semi-domestic reindeer (Rangifer tarandus tarandus) and Svalbard reindeer (Rangifer tarandus platyrhynchus) to evaluate the accuracy and repeatability of age estimated by cementum annuli analysis of longitudinally sectioned permanent incisors. Four observers with varying experience performed blind duplicate age estimation of 37 specimens from each cervid. The relationship between known age and estimated age was linear, except for Svalbard reindeer where a quadratic model gave a slightly better fit. After correcting for observer ID and animal ID, there was a slightly declining probability to assess the correct age with increasing age for moose, red deer and Svalbard reindeer. Across cervids and observers, estimated age equalled known age in 69% of all readings, while 95% age ± 1 year. Predicted probability of correct age assessment for experienced observers was 93% for red deer, 89% for Svalbard reindeer, 84% for moose and 73% for semi-domestic reindeer. Regardless of observer experience and cervid, there was a high agreement between repeated assessments of a given animal’s tooth sections. The accuracy varied between cervids but was generally higher for observers with former ageing experience with a given cervid. We conclude that the accuracy of estimated age using longitudinally sectioned incisors is generally high, and even more so if performed by observers with former ageing experience of a given species. To ensure consistency over time, a reference material from known-age individuals for each species analysed should be available for calibration and training of observers.

Divergent estimates of herd-wide caribou calf survival: Ecological factors and methodological biases

Population monitoring is a critical part of effective wildlife management, but methods are prone to biases that can hinder our ability to accurately track changes in populations through time. Calf survival plays an important role in ungulate population dynamics and can be monitored using telemetry and herd composition surveys. These methods, however, are susceptible to unrepresentative sampling and violations of the assumption of equal detectability, respectively. Here, we capitalized on 55 herd-wide estimates of woodland caribou (Rangifer tarandus caribou) calf survival in Newfoundland, Canada, using telemetry (n = 1,175 calves) and 249 herd-wide estimates of calf:cow ratios (C:C) using herd composition surveys to investigate these potential biases. These data included 17 herd-wide estimates replicated from both methods concurrently (n = 448 calves and n = 17 surveys) which we used to understand which processes and sampling biases contributed to disagreement between estimates of herd-wide calf survival. We used Cox proportional hazards models to determine whether estimates of calf mortality risk were biased by the date a calf was collared. We also used linear mixed-effects models to determine whether estimates of C:C ratios were biased by survey date and herd size. We found that calves collared later in the calving season had a higher mortality risk and that C:C tended to be higher for surveys conducted later in the autumn. When we used these relationships to modify estimates of herd-wide calf survival derived from telemetry and herd composition surveys concurrently, we found that formerly disparate estimates of woodland caribou calf survival now overlapped (within a 95% confidence interval) in a majority of cases. Our case study highlights the potential of under-appreciated biases to impact our understanding of population dynamics and suggests ways that managers can limit the influence of these biases in the two widely applied methods for estimating herd-wide survival.

Line-transect data may not produce reliable estimates of interannual sex-ratio and age structure variation in West African savannah ungulates

Adult sex ratios and age structures are important wildlife population parameters, but they have been poorly investigated in ungulate species in West African savannahs. We used line transects to investigate these parameters in 11 ungulates from a protected area in south-western Burkina Faso during the period 2010-2018. We created an empirical model of “detectability” for each species based on its main ecological characteristics (habitat and group size) and body size, and then compared the observed interannual inconsistency in sex ratios and age structures with the a priori detectability score. Six out of 11 species showed low interannual inconsistency in sex ratio and age structure. In 82% of the study species, however, the predicted detectability score matched the observed score, with two exceptions being Tragelaphus scriptus and Sincerus caffer.

The Demographic Buffering Hypothesis: Evidence and Challenges

In (st)age-structured populations, the long-run population growth rate is negatively affected by temporal variation in vital rates. In most cases, natural selection should minimize temporal variation in the vital rates to which the long-run population growth is most sensitive, resulting in demographic buffering. By reviewing empirical studies on demographic buffering in wild populations, we found overall support for this hypothesis. However, we also identified issues when testing for demographic buffering. In particular, solving scaling problems for decomposing, measuring, and comparing stochastic variation in vital rates and accounting for density dependence are required in future tests of demographic buffering. In the current context of climate change, demographic buffering may mitigate the negative impact of environmental variation and help populations to persist in an increasingly variable environment.

Genetic decline, restoration and rescue of an isolated ungulate population

Isolation of small populations is expected to reduce fitness through inbreeding and loss of genetic variation, impeding population growth and compromising population persistence. Species with long generation time are the least likely to be rescued by evolution alone. Management interventions that maintain or restore genetic variation to assure population viability are consequently of significant importance. We investigated, over 27 years, the genetic and demographic consequences of a demographic bottleneck followed by artificial supplementation in an isolated population of bighorn sheep (Ovis canadensis). Based on a long-term pedigree and individual monitoring, we documented the genetic decline, restoration and rescue of the population. Microsatellite analyses revealed that the demographic bottleneck reduced expected heterozygosity and allelic diversity by 6.2% and 11.3%, respectively, over two generations. Following supplementation, first-generation admixed lambs were 6.4% heavier at weaning and had 28.3% higher survival to 1 year compared to lambs of endemic ancestry. Expected heterozygosity and allelic diversity increased by 4.6% and 14.3% after two generations through new alleles contributed by translocated individuals. We found no evidence for outbreeding depression and did not see immediate evidence of swamping of local genes. Rapid intervention following the demographic bottleneck allowed the genetic restoration and rescue of this bighorn sheep population, likely preventing further losses at both the genetic and demographic levels. Our results provide further empirical evidence that translocation can be used to reduce inbreeding depression in nature and has the potential to mitigate the effect of human-driven environmental changes on wild populations.

More frequent extreme climate events stabilize reindeer population dynamics

Extreme climate events often cause population crashes but are difficult to account for in population-dynamic studies. Especially in long-lived animals, density dependence and demography may induce lagged impacts of perturbations on population growth. In Arctic ungulates, extreme rain-on-snow and ice-locked pastures have led to severe population crashes, indicating that increasingly frequent rain-on-snow events could destabilize populations. Here, using empirically parameterized, stochastic population models for High-Arctic wild reindeer, we show that more frequent rain-on-snow events actually reduce extinction risk and stabilize population dynamics due to interactions with age structure and density dependence. Extreme rain-on-snow events mainly suppress vital rates of vulnerable ages at high population densities, resulting in a crash and a new population state with resilient ages and reduced population sensitivity to subsequent icy winters. Thus, observed responses to single extreme events are poor predictors of population dynamics and persistence because internal density-dependent feedbacks act as a buffer against more frequent events.

Density dependence governs when population responses to multiple stressors are magnified or mitigated

Population endangerment typically arises from multiple, potentially interacting anthropogenic stressors. Extensive research has investigated the consequences of multiple stressors on organisms, frequently focusing on individual life stages. Less is known about population-level consequences of exposure to multiple stressors, especially when exposure varies through life. We provide the first theoretical basis for identifying species at risk of magnified effects from multiple stressors across life history. By applying a population modeling framework, we reveal conditions under which population responses from stressors applied to distinct life stages are either magnified (synergistic) or mitigated. We find that magnification or mitigation critically depends on the shape of density dependence, but not the life stage in which it occurs. Stressors are always magnified when density dependence is linear or concave, and magnified or mitigated when it is convex. Using Bayesian numerical methods, we estimated the shape of density dependence for eight species across diverse taxa, finding support for all three shapes.

Climate warming is associated with smaller body size and shorter lifespans in moose near their southern range limit

Despite the importance of body size for individual fitness, population dynamics and community dynamics, the influence of climate change on growth and body size is inadequately understood, particularly for long-lived vertebrates. Although temporal trends in body size have been documented, it remains unclear whether these changes represent the adverse impact of climate change (environmental stress constraining phenotypes) or its mitigation (via phenotypic plasticity or evolution). Concerns have also been raised about whether climate change is indeed the causal agent of these phenotypic shifts, given the length of time-series analysed and that studies often do not evaluate - and thereby sufficiently rule out - other potential causes. Here, we evaluate evidence for climate-related changes in adult body size (indexed by skull size) over a 4-decade period for a population of moose (Alces alces) near the southern limit of their range whilst also considering changes in density, predation, and human activities. In particular, we document: (i) a trend of increasing winter temperatures and concurrent decline in skull size (decline of 19% for males and 13% for females) and (ii) evidence of a negative relationship between skull size and winter temperatures during the first year of life. These patterns could be plausibly interpreted as an adaptive phenotypic response to climate warming given that latitudinal/temperature clines are often accepted as evidence of adaptation to local climate. However, we also observed: (iii) that moose with smaller skulls had shorter lifespans, (iv) a reduction in lifespan over the 4-decade study period, and (v) a negative relationship between lifespan and winter temperatures during the first year of life. Those observations indicate that this phenotypic change is not an adaptive response to climate change. However, this decline in lifespan was not accompanied by an obvious change in population dynamics, suggesting that climate change may affect population dynamics and life-histories differently.

Survival of adult Steller sea lions in Alaska: senescence, annual variation and covariation with male reproductive success

Population dynamics of long-lived vertebrates depend critically on adult survival, yet factors affecting survival and covariation between survival and other vital rates in adults remain poorly examined for many taxonomic groups of long-lived mammals (e.g. actuarial senescence has been examined for only 9 of 34 extant pinniped species using longitudinal data). We used mark-recapture models and data from 2795 Steller sea lion (Eumetopias jubatus) pups individually marked at four of five rookeries in southeastern Alaska (SEAK) and resighted for 21 years to examine senescence, annual variability and covariation among life-history traits in this long-lived, sexually dimorphic pinniped. Sexes differed in age of onset (approx. 16-17 and approx. 8-9 years for females and males, respectively), but not rate (-0.047 and -0.046/year of age for females and males) of senescence. Survival of adult males from northern SEAK had greatest annual variability (approx. ±0.30 among years), whereas survival of adult females ranged approximately ±0.10 annually. Positive covariation between male survival and reproductive success was observed. Survival of territorial males was 0.20 higher than that of non-territorial males, resulting in the majority of males alive at oldest ages being territorial.

Robustness of adult female survival maintains a high-density sika deer (Cervus nippon) population following the initial irruption

Context Irruption of large herbivore populations is characterised by three distinct phases: (1) an exponential increase in population to a peak abundance; (2) a population crash; and (3) a second increase to another population peak, typically lower than the first peak of abundance. However, there has been little study of age- and sex-specific factors that affect the post-initial irruption interactions with food sources. Aims We aimed to investigate annual survival rates of sika deer (Cervus nippon Temminck, 1838) in the sequent irruption of a population on Nakanoshima Island, Lake Toya, Japan, with a chronically high density during the period 2002–12. Methods Survival monitoring data were obtained for 219 individuals (93 males and 126 females) using radio-collars. Annual survival was quantified, and related factors, i.e. deer abundance and winter severity, were determined by model selection using Akaike information criterion values. Key Results The results showed that annual survival rates across sexes and age classes (fawn, yearling, prime-aged, old) decreased with increasing population density, snow depth and winter precipitation. Winter severity had a greater effect on adult survival than density regulation. Nevertheless, female adult survival was maintained at a high level, with a mean of 0.84 (95% CI: 0.80–0.88). Key conclusions Robust survival rates for adult females might contribute to the maintenance of a high-density sika deer population in the post-initial irruption. Implications We suggest that in the absence of predation and hunting, sika deer population is not able to self-regulate to the density level that avoids an irreversible impact on plants.

Immunosenescence patterns differ between populations but not between sexes in a long-lived mammal

In animals, physiological mechanisms underlying reproductive and actuarial senescence remain poorly understood. Immunosenescence, the decline in the ability to display an efficient immune response with increasing age, is likely to influence both reproductive and actuarial senescence through increased risk of disease. Evidence for such a link has been reported from laboratory animal models but has been poorly investigated in the wild, where variation in resource acquisitions usually drives life-history trade-offs. We investigated immunosenescence patterns over 7 years in both sexes of two contrasting roe deer populations (Capreolus capreolus). We first measured twelve immune markers to obtain a thorough identification of innate and adaptive components of immunity and assessed, from the same individuals, the age-dependent variation observed in parasitic infections. Although the level of innate traits was maintained at old age, the functional innate immune traits declined with increasing age in one of two populations. In both populations, the production of inflammatory markers increased with advancing age. Finally, the adaptive response declined in late adulthood. The increasing parasite burden with age we reported suggests the effective existence of immunosenescence. Age-specific patterns differed between populations but not between sexes, which indicate that habitat quality could shape age-dependent immune phenotype in the wild.

Rare catastrophic events drive population dynamics in a bat species with negligible senescence

Bats are remarkably long-lived with lifespans exceeding even those of same-sized birds. Despite a recent interest in the extraordinary longevity of bats very little is known about the shape of mortality over age, and how mortality rates are affected by the environment. Using a large set of individual-based data collected over 19 years in four free-ranging colonies of Bechstein's bats (Myotis bechsteinii), we found no increase in the rate of mortality and no decrease in fertility demonstrating no senescence until high ages. Our finding of negligible senescence is highly unusual for long-lived mammals, grouping Bechstein's bats with long-lived seabirds. The most important determinant of adult mortality was one particular winter season, which affected all ages and sizes equally. Apart from this winter, mortality risk did not differ between the winter and the summer season. Colony membership, a proxy for local environmental conditions, also had no effect. In addition to their implications for understanding the extra-ordinary longevity in bats, our results have strong implications for the conservation of bats, since rare catastrophic mortality events can only be detected in individual based long-term field studies. With many bat species globally threatened, such data are crucial for the successful implementation of conservation programs.

Fecal DNA, hormones, and pellet morphometrics as a noninvasive method to estimate age class: an application to wild populations of Central Mountain and Boreal woodland caribou (Rangifer tarandus caribou)

Determining age structure of populations is a valuable parameter in wildlife management, but is often difficult to obtain. Here, we tested a noninvasive method via fecal DNA, hormones, and pellet morphometrics to distinguish calf from adult in Central Mountain and Boreal woodland caribou (Rangifer tarandus caribou (Gmelin, 1788)) populations. Annual surveys of fall-sampled Central Mountain caribou were done in Jasper National Park, Alberta, between 2006 and 2011 and winter-sampled Boreal caribou were surveyed in the North Interlake area, Manitoba, between 2004 and 2010. Samples were amplified at 10 microsatellite loci to identify unique individuals and capture histories were used to identify putative calves and adults. Fecal pellets were measured for length, width, depth, dry mass, and analyzed for progesterone, estrogen, and testosterone concentrations. Results showed significant differences in fecal pellet size between putative calves and adults for both sexes and populations–seasons. Progesterone concentration was significantly higher in Jasper–fall and North Interlake–winter adult females. Testosterone was significantly higher in Jasper–fall adult males. North Interlake–winter males exhibited no significant difference in hormone concentrations between age classes. When applied to the entire Jasper data set, 98% of females and 88% of males were assigned to an age class. This study illustrates the possibilities of using noninvasive methods to determine an age class in wild ungulate populations.

Disease introduction is associated with a phase transition in bighorn sheep demographics

Ecological theory suggests that pathogens are capable of regulating or limiting host population dynamics, and this relationship has been empirically established in several settings. However, although studies of childhood diseases were integral to the development of disease ecology, few studies show population limitation by a disease affecting juveniles. Here, we present empirical evidence that disease in lambs constrains population growth in bighorn sheep (Ovis canadensis) based on 45 years of population-level and 18 years of individual-level monitoring across 12 populations. While populations generally increased (λ = 1.11) prior to disease introduction, most of these same populations experienced an abrupt change in trajectory at the time of disease invasion, usually followed by stagnant-to-declining growth rates (λ = 0.98) over the next 20 years. Disease-induced juvenile mortality imposed strong constraints on population growth that were not observed prior to disease introduction, even as adult survival returned to pre-invasion levels. Simulations suggested that models including persistent disease-induced mortality in juveniles qualitatively matched observed population trajectories, whereas models that only incorporated all-age disease events did not. We use these results to argue that pathogen persistence may pose a lasting, but under-recognized, threat to host populations, particularly in cases where clinical disease manifests primarily in juveniles.

Space use of a reintroduced population of Iberian ibex (Capra pyrenaica) in a protected natural area

In Europe, wild ungulates have undergone major expansion and population growth during recent decades. In certain cases, the high density achieved by these populations has led to excessive pressure on the environment, which eventually becomes a limiting factor for the population itself. One of these reintroductions was performed with the Iberian ibex (Capra pyrenaica Schinz, 1838) in the Sierra de Guadarrama National Park (Spain). This reintroduced population was monitored during six field seasons (2000, 2003, 2005, 2007, 2010, and 2014) by direct observation of the animals along transects using the distance sampling method to determine the degree of expansion over the years and the use of different habitats according to different seasons. The abundances obtained for each field season showed a significant increase from 4.16 to 8.65 individuals/km, showing a linear relationship between abundance and extent of the area occupied by the species. We observed that differences between habitat availability and use were significant for all seasons. Our data can be used as an example of the colonization process of a population of wild ungulates and their impact on vegetation to better manage future reintroductions.

Quantifying the influence of measured and unmeasured individual differences on demography

1. Demographic rates can vary not only with measured individual characters like age, sex and mass but also with unmeasured individual variables like behaviour, genes and health. 2. Predictions from population models that include measured individual characteristics often differ from models that exclude them. Similarly, unmeasured individual differences have the potential to impact predictions from population models. However, unmeasured individual differences are rarely included in population models. 3. We construct stage- and age-structured models (where stage is mass) of a roe deer population, which are parameterized from statistical functions that either include, or ignore, unmeasured individual differences. 4. We found that mass and age structures substantially impacted model parameters describing population dynamics, as did temporal environmental variation, while unmeasured individual differences impacted parameters describing population dynamics to a much smaller extent once individual heterogeneity related to mass and age has been included in the model. We discuss how our assumptions (unmeasured individual differences only in mean trait values) could have influenced our findings and under what circumstances unmeasured individual differences could have had a larger impact on population dynamics. 5. There are two reasons explaining the relative small influence of unmeasured individual differences on population dynamics in roe deer. First, individual body mass and age both capture a large amount of individual differences in roe deer. Second, in large populations of long-lived animals, the average quality of individuals (independent of age and mass) within the population is unlikely to show substantial variation over time, unless rapid evolution is occurring. So even though a population consisting of high-quality individuals would have much higher population growth rate than a population consisting of low-quality individuals, the probability of observing a population consisting only of high-quality individuals is small.

Vegetation changes associated with a population irruption by Roosevelt elk

Interactions between large herbivores and their food supply are central to the study of population dynamics. We assessed temporal and spatial patterns in meadow plant biomass over a 23-year period for meadow complexes that were spatially linked to three distinct populations of Roosevelt elk (Cervus elaphus roosevelti) in northwestern California. Our objectives were to determine whether the plant community exhibited a tolerant or resistant response when elk population growth became irruptive. Plant biomass for the three meadow complexes inhabited by the elk populations was measured using Normalized Difference Vegetation Index (NDVI), which was derived from Landsat 5 Thematic Mapper imagery. Elk populations exhibited different patterns of growth through the time series, whereby one population underwent a complete four-stage irruptive growth pattern while the other two did not. Temporal changes in NDVI for the meadow complex used by the irruptive population suggested a decline in forage biomass during the end of the dry season and a temporal decline in spatial variation of NDVI at the peak of plant biomass in May. Conversely, no such patterns were detected in the meadow complexes inhabited by the nonirruptive populations. Our findings suggest that the meadow complex used by the irruptive elk population may have undergone changes in plant community composition favoring plants that were resistant to elk grazing.