Density-dependent Survival Research Articles

Population Level Impacts of Cooling Water Withdrawals on Harvested Fish Stocks

Trillions of gallons are withdrawn every year from U.S. rivers, estuaries, lakes, and coastal waters to cool the turbines of power plants and other equipment in manufacturing facilities. In the process, large numbers of aquatic organisms die from entrainment into the plant or impingement against the outer portion of the intake structure. In this paper, we develop a generalized age-structured population model with density dependent survival of sub-adult age classes, and we use the model to perform a screening analysis of the effects of entrainment and impingement for 15 harvested fish stocks off the California and Atlantic coasts. Stock sizes are estimated to be depressed by entrainment and impingement by less than 1% in 10 of the 15 cases considered, between 1 and 3% in two cases, and between 20 and 80% in three cases. A variety of sensitivity analyses are conducted to evaluate the influence of several sources of model and parameter uncertainties.

The demography of recovery of an overexploited bull trout, Salvelinus confluentus, population

An exploited bull trout, Salvelinus confluentus, population experienced a 28-fold increase in adult density during a 10-year period from a minimum of 60 individuals. This demonstrates the extent to which this population was overharvested. Its ability to respond in fewer than two generations to the implementation of zero-harvest regulations suggests this population was growth-overfished not recruitment-overfished. Examination of stock–recruitment relationships of various life stages indicates that recovery of this population was regulated by the density-dependent survival of juveniles in the rearing creek. This compensatory response occurred between egg deposition and age-1 and regulated the number of fish recruiting into the adult population. A second population bottleneck became apparent later in the recovery process when density-dependent survival of the adult population resulted in its approach to an asymptote, highlighting the necessity of long-term data sets for examining these compensatory responses. Results from this study demonstrate the importance of understanding the influence of individual life stages on the ability of overexploited populations such as threatened bull trout to recover and for their future management.

Analysis of Density-Dependent Survival of <I>Diabrotica</I> (Coleoptera: Chrysomelidae) in Cornfields

We analyzed published Þeld data concerning western corn rootworm (Diabrotica virgifera virgifera LeConte) and northern corn rootworm (Diabrotica barberi Smith and Lawrence) survival from egg to adult to create general relationships between density and survival. A series of equations were generated that best Þt the data, and these equations may be used to help understand density-dependent survival in future studies. In general, the datasets showed a decline in the pro- portion surviving to adulthood as egg density increased. The survival of northern corn rootworm was lower than that of western corn rootworm. Data for both natural and mechanical infestations of soil with eggs show decreasing proportion surviving as density increases. Several nonlinear equations Þt the data for both species very well. Density-dependent survival can account for differences in observations of insecticide survival at different initial western corn rootworm densities.

Purinergic signaling regulates neural progenitor cell expansion and neurogenesis

Neural stem and progenitor cells typically exhibit a density-dependent survival and expansion, such that critical densities are required below which clonogenic progenitors are lost. This suggests that short-range autocrine factors may be critical for progenitor cell maintenance. We report here that purines drive the expansion of ventricular zone neural stem and progenitor cells, and that purine receptor activation is required for progenitor cells to be maintained as such. Neural progenitors expressed P2Y purinergic receptors and mobilized intracellular calcium in response to agonist. Receptor antagonists suppressed proliferation and permitted differentiation into neurons and glia in vitro, while subsequent removal of purinergic inhibition restored progenitor cell expansion. Real-time bioluminescence imaging of extracellular ATP revealed that the source of extracellular nucleotides are the progenitor cells themselves, which appear to release ATP in episodic burst events. Enzyme histochemistry of the adult rat brain for ectonucleotidase activity revealed that NTDPase, which acts to degrade active ATP and thereby clears it from areas of active purinergic transmission, was selectively localized to the subventricular zone and the dentate gyrus, regions in which neuronal differentiation proceeds from the progenitor cell pool. These data suggest that purine nucleotides act as proliferation signals for neural progenitor cells, and thereby serve as negative regulators of terminal neuronal differentiation. As a result, progenitor cell-derived neurogenesis is thus associated with regions of both active purinergic signaling and modulation thereof.

The ideal free pike: 50 years of fitness-maximizing dispersal in Windermere.

The ideal free distribution (IFD) theory is one of the most influential theories in evolutionary ecology. It predicts how animals ought to distribute themselves within a heterogeneous habitat in order to maximize lifetime fitness. We test the population level consequence of the IFD theory using 40-year worth data on pike (Esox lucius) living in a natural lake divided into two basins. We do so by employing empirically derived density-dependent survival, dispersal and fecundity functions in the estimation of basin-specific density-dependent fitness surfaces. The intersection of the fitness surfaces for the two basins is used for deriving expected spatial distributions of pike. Comparing the derived expected spatial distributions with 50 years data of the actual spatial distribution demonstrated that pike is ideal free distributed within the lake. In general, there was a net migration from the less productive north basin to the more productive south basin. However, a pike density-manipulation experiment imposing shifting pike density gradients between the two basins managed to switch the net migration direction and hence clearly demonstrated that the Windermere pike choose their habitat in an ideal free manner. Demonstration of ideal free habitat selection on an operational field scale like this has never been undertaken before.

Modeling Larval Survival and Movement to Evaluate Seed Mixtures of Transgenic Corn for Control of Western Corn Rootworm (Coleoptera: Chrysomelidae)

I expanded the population dynamics and genetics model published in 2005 by Crowder and Onstad to include larval survival and movement to evaluate the role of mixtures of transgenic and nontransgenic corn, Zea mays L., seed for resistance management of western corn rootworm. I studied both density-independent and density-dependent toxin survival. In all but the worst-case scenarios, resistance did not evolve within 30 yr when the resistance allele, R, was recessive. The standard model with density-independent toxin survival based on the expression of a medium dose of toxin indicated that 50% R allele frequency will be reached by years 5 and 7, respectively, with dominant and partially recessive expression and 20% nontransgenic seed. The standard model with density-dependent toxin survival indicates that resistance will occur in year 5 under the same conditions. These results are similar to the published results of Crowder and Onstad who studied a model with adjacent block refuges and mostly nonrandom mating in the landscape (random only within each block). Results depended on the heterozygote advantage (differential survival between SS and RS) and the degree of random mating provided by the seed mixture.

Experimental evidence for density-dependent survival in mallard (Anas platyrhynchos) ducklings

It is unresolved to what extent waterfowl populations are regulated by density-dependent processes. By doing a 2-year crossover perturbation experiment on ten oligotrophic boreal lakes we addressed the hypothesis that breeding output is density dependent. Wing-clipped mallard (Anas platyrhynchos) hens were introduced with their own brood and then monitored for 24 days. Predicted responses were that per capita duckling and hen survival would be lower in high-density than in low-density treatments. Survival was evaluated by model fitting in program MARK. Density, year, and lake were used as main effects, while day after introduction, a weather harshness index, and presence of hens were covariates. Daily survival in ducklings was lower in the high-density treatment, but this effect was year dependent. The highest-ranking model for duckling survival also included a positive effect of duckling age and presence of hens, and a negative effect of harsh weather. Density did not affect female survival although there was a prominent year effect. The highest-ranking model for female survival also included negative effects of day after introduction and harsh weather. This is the first study to report density-dependent survival in experimentally introduced ducklings in a natural setting. Implications for population dynamics and management of harvested populations are far-reaching if such regulation occurs in some years, but not in others.

Exposure, postexposure, and density-mediated effects of atrazine on amphibians: breaking down net effects into their parts.

Most toxicology studies focus on effects of contaminants during exposure. This is disconcerting because subsequent survival may be affected. For instance, contaminant-induced mortality can be later ameliorated by reduced competition among the survivors, a concept we refer to as “density-mediated compensation.” Alternatively, it can be exacerbated by toxicant effects that persist or appear after exposure, a phenomenon we term “carryover effects.” We developed a laboratory framework for testing the contribution of exposure, density-mediated, and carryover effects to net survival, by exposing embryos and larvae of the streamside salamander (Ambystoma barbouri) to atrazine (0, 4, 40, 400 ppb; 3 ppb is the U.S. drinking water maximum) and quantifying survival during and 14 months after exposure. Atrazine is the most commonly used herbicide in the United States and a documented endocrine disruptor. We show that atrazine-induced mortality during exposure was ameliorated by density-dependent survival after exposure, but complete density-mediated compensation was precluded by significant carryover effects of atrazine. Consequently, salamanders exposed to ≥4 ppb of atrazine had significantly lower survival than did control animals 14 months postexposure. The greatest change in survival occurred at low exposure concentrations. These nonlinear, long-term, postexposure effects of atrazine have similarities to effects of early development exposure to other endocrine disruptors. Together with evidence of low levels of atrazine impairing amphibian gonadal development, the results here raise concerns about the role of atrazine in amphibian declines and highlight the importance of considering persistent, postexposure effects when evaluating the impact of xenobiotics on environmental health.

Genetic and demographic implications of aquaculture in white sturgeon (Acipenser transmontanus) conservation

This study uses a genetic individual-based model of white sturgeon (Acipenser transmontanus) populations in a river to examine the genetic and demographic trade-offs associated with operating a conservation hatchery. Simulation experiments evaluated three management practices: (i) setting quotas to equalize family contributions in an effort to prevent genetic swamping, (ii) an adaptive management scheme that interrupts stocking when introgression exceeds a specified threshold, and (iii) alternative broodstock selection strategies that influence domestication. The first set of simulations, designed to evaluate equalizing the genetic contribution of families, did not show the genetic benefits expected. The second set of simulations showed that simulated adaptive management was not successful in controlling introgression over the long term, especially with uncertain feedback. The third set of simulations compared the effects of three alternative broodstock selection strategies on domestication for hypothetical traits controlling early density-dependent survival. Simulated aquaculture selected for a density-tolerant phenotype when broodstock were taken from a genetically connected population. Using broodstock from an isolated population (i.e., above an upstream barrier or in a different watershed) was more effective at preventing domestication than using wild broodstock from a connected population.

Economic Analysis of Dynamic Management Strategies Utilizing Transgenic Corn for Control of Western Corn Rootworm (Coleoptera: Chrysomelidae)

We studied management strategies for western corn rootworm, Diabrotica virgifera virgifera LeConte, using transgenic corn, Zea mays L., from both a biological and an economic perspective. In areas with and without populations adapted to a 2-yr rotation of corn and soybean (rotation-resistant), the standard management strategy was to plant 80% of a cornfield (rotated and continuous) to a transgenic cultivar each year. In each area, we also studied dynamic management strategies where the proportion of transgenic corn increased over time in a region. We also analyzed management strategies for a single field that is the first to adopt transgenic corn within a larger unmanaged region. In all areas, increasing the expression of the toxin in the plant increased economic returns. In areas without rotation-resistance, planting 80% transgenic corn in the continuous cornfield each year generated the greatest returns with a medium toxin dose or greater. In areas with alleles for rotation-resistance at low initial levels, a 2-yr rotation of nontransgenic corn and soybean, Glycine max (L.) Merr., may be the most economical strategy if resistance to crop rotation is recessive. If resistance to crop rotation is additive or dominant, planting transgenic corn in the rotated cornfield was the most effective strategy. In areas where rotation-resistance is already a severe problem, planting transgenic corn in the rotated cornfield each year was always the most economical strategy. In some cases the strategies that increased the proportion of transgenic corn in the region over time increased returns compared with the standard strategies. With these strategies the evolution of resistance to crop rotation occurred more rapidly but resistance to transgenic corn was delayed compared with the standard management strategy. In areas not managed by a regional norm, increasing the proportion of transgenic corn and increasing toxin dose in the managed field generally increased returns. In a sensitivity analysis, among the parameters investigated, only density-dependent survival affected the results.

A new perspective on the importance of marine-derived nutrients to threatened stocks of Pacific salmon (Oncorhynchus spp.)

Considerable research has highlighted the important role of anadromous salmon in importing marine-derived nutrients to freshwater and riparian ecosystems. These subsidies are thought to support diverse food webs and increase the growth and survival of juvenile salmon during their freshwater residency. Quite recently, however, salmon smolts have been identified as important exporters of nutrients from freshwater ecosystems. Using a mass-balance approach, we examined the phosphorus-transport dynamics by spring/summer Chinook salmon (Oncorhynchus tshawytscha) in the Snake River basin and estimated that net phosphorus transport into the basin over the past 40 years was <2% of historical levels. Furthermore, a nonlinear relationship existed between nutrient import by adults and subsequent export by smolts, such that smolts exported proportionally more phosphorus as spawner abundance decreased. In 12% of years, smolts exported more than adults imported, resulting in a net loss of phosphorus from the ecosystem. This loss of marine subsidies may have caused a state shift in the productivity of the freshwater ecosystem, resulting in strong density-dependent survival observed in juvenile salmon. These results suggest that conserving this threatened stock of salmon requires the need to explicitly address the important role of marine-derived nutrients and energy in sustaining salmon populations.

ANNUAL AND SPATIAL VARIATION IN SEEDFALL AND SEEDLING RECRUITMENT IN A NEOTROPICAL FOREST

An economy of scale may lead to selection to increase interannual variation in seed production when the per seed probability of seedling establishment increases with seed production. Variable annual seedfall will, however, reduce this probability when postdispersal seed fate is negatively density dependent on the local density of seeds, and seed dispersal and density dependence act identically across years. Intuitively, more variable annual seedfall causes the representative seed to experience a greater density of conspecific seeds and suffer greater density-dependent effects. This handicap must be overcome for the per seed probability of recruitment to be greater in years with greater seed production. We quantified spatial and annual variation in seedfall and seedling recruitment, evaluated density dependence and economies of scale during the seed-to-seedling transition, and investigated the synergistic consequences of density dependence and variable annual seedfall for seedling recruitment on Barro Colorado Island (BCI), Panama. Weekly censuses of 200 0.5-m2 seed traps documented seedfall for 15 years and 108 plant species. Annual censuses of 600 1-m2 seedling plots documented recruitment for nine years and 32 species. The density of seedling recruits tended to increase with the density of seeds; however, the per seed probability of recruitment invariably decreased with seedfall density. Negative density dependence characterized the seed-to-seedling transition. Observed levels of spatial and interannual variation in seedfall density would reduce long-term recruitment by up to 28% if negatively density-dependent survival acted identically across years; however, the strength of negative density dependence varied significantly among years for 12 of 32 species. Negative density dependence occurred in all years for these species but was significantly weaker during the one or two years of greatest seedfall than during the remaining years of lower seedfall. The per seed probability of recruitment increased significantly with annual seedfall for eight of these species. These eight species realized postdispersal economies of scale despite the reduction in long-term recruitment expected from the synergism between variable annual seed production and negatively density-dependent seed fate.

Local-scale density-dependent survival of mobile organisms in continuous habitats: an experimental test using Atlantic salmon

For organisms with restricted mobility, density dependence may occur on spatial scales much smaller than that of the whole population. Averaging densities over whole populations in such organisms gives a more or less inaccurate description of the real variation in competitive intensity over time and space. The potential for local density dependence in more mobile organisms is less well understood, particularly for organisms living in continuous habitats. To test for local density-dependent processes in such an organism, we manipulated egg density (the number of eggs nest(-1)) among ten artificial nests of Atlantic salmon along an 1,848-m long river during two consecutive years. Eggs in different nests were given unique thermal otolith-banding patterns to allow identification of juvenile nest origin. At capture, 1-2 months after emergence, the spatial distribution of juveniles reflected nest locations, with the median absolute dispersal distance being 92 and 41 m in the 2 years. Estimated nest-specific survival rates were strongly negatively related to hatched-egg density in both years (r(2)=0.72 and 0.62), despite dramatic differences in overall mean survival (0.22 and 0.02). Thus, density-dependent survival following emergence in Atlantic salmon juveniles occurs on spatial scales much smaller than that of whole populations. The consistency across years suggests that the phenomenon is likely to occur over most environmental conditions. Our observation of local-scale density dependence is consistent with strong juvenile territoriality, which forces individuals emerging in high-initial density areas to disperse farther, and a high cost (metabolic or predation) of dispersal. We conclude that for mobile organisms with patchy distributions of propagules and constrained juvenile dispersal, increased emphasis on local-scale dynamics should enable a more mechanistic understanding of population regulation even in continuous habitats, and hence increase the predictive power of population models.

NONADDITIVE EFFECTS OF THE ENVIRONMENT ON THE SURVIVAL OF A LARGE MARINE FISH POPULATION

Climate can affect population dynamics in indirect ways via nonadditive forcing by external variables on internal demographic rates. Current analytical techniques, employed in population ecology, fail to explicitly include nonadditive interactions between internal and external variables, and therefore cannot efficiently address indirect climate effects. Here, we present the results of an analysis, employing specifically developed statistical methodology, on density-dependent survival of walleye pollock (Theragra chalcogramma) prerecruitment stages in relation to background environmental variables in the Gulf of Alaska. We found that spring winds and water temperature mediate the intensity of density-dependent survival from the eggs to the age-0 stage. Fall water temperature and juvenile pollock predator abundance mediate density dependence from the age-0 to the age-1 stage. The inclusion of such nonadditive and nonlinear effects in a population dynamics model improved our ability to simulate pollock recruitment. Our results point to the importance of understanding nonadditive and nonlinear interactions between external (climate) and internal factors in the presence of underlying environmental variation. These topics are discussed in the context of current research priorities in population ecology and conservation biology.

Trends and cohort resonant effects in age‐structured populations

Summary 1. Trends and fluctuations in populations are determined by complex interactions between extrinsic forcing and intrinsic dynamics. As an example, the dynamics of many marine fish are characterized by age-structured dynamics forced by stochastic recruitment. 2. In this study we develop stochastic age-structured models for two case studies, the Atlantic bluefin tuna and the Atlantic cod. The former exemplifies intracohort interactions and density-dependent reproduction, the latter exemplifies density-dependent survival and intercohort interactions. 3. We use transfer functions and delay-coordinate models to study how the combination of age-structured interactions and stochastic recruitment can induce low-frequency variability. ‘Cohort resonance’, as we dub this effect, can induce apparent trends in abundance and may be common in age-structured populations. 4. Our study complements the theory of structured populations that focuses on cycles and chaos (high-frequency dynamics). 5. The innate low-frequency fluctuations we describe can potentially mimic or cloak critical variation in abundance linked to environmental change, over-exploitation or other types of anthropogenic forcing. 6. From a management and conservation viewpoint, it will be important to find ways to separate anthropogenic forcing from cohort resonant effects and/or to understand the way they interact.

Red environmental noise and the appearance of delayed density dependence in age-structured populations.

Previous work suggests that red environmental noise can lead to the spurious appearance of delayed density dependence (DDD) in unstructured populations regulated only by direct density dependence. We analysed the effect of noise reddening on the pattern of spurious DDD in several variants of the density-dependent age-structured population model. We found patterns of spurious DDD in structured populations with either density-dependent fertility or density-dependent survival of the first age class, inconsistent with predictions from unstructured population models. Moreover, we found that nonspurious negative DDD always emerges in populations with deterministic chaotic dynamics, regardless of population structure or the type of environmental noise. The effect of noise reddening in generating spurious DDD is often negligible in the chaotic region of population deterministic dynamics. Our findings suggest that differences in species' life histories may exhibit different patterns of spurious DDD (owing to noise reddening) than predicted by unstructured models.

Economics Versus Alleles: Balancing Integrated Pest Management and Insect Resistance Management for Rotation-Resistant Western Corn Rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the central United States. We expanded a simple model of adult behavior and population genetics to explain how rotation resistance may have developed and to study ways to manage the western corn rootworm in a landscape of corn, soybean, and winter wheat where evolution of resistance may occur. We modeled six alternative management strategies over a 15-yr time horizon, as well as a strategy involving a 2-yr rotation of corn and soybean in 85% of the landscape, to investigate their effectiveness from both a biological and economic perspective. Generally, resistance to crop rotation evolves in fewer than 15 yr, and the rate of evolution increases as the level of rotated landscape (selection pressure) increases. When resistance is recessive, all six alternative strategies were effective at preventing evolution of rotation resistance. The two most successful strategies were the use of transgenic rotated corn in a 2-yr rotation and a 3-yr rotation of corn, soybean, and wheat with unattractive wheat (for oviposition) preceding corn. Results were most sensitive to increases in the initial allele frequency and modifications of the density-dependent survival function. Economically, three alternative strategies were robust solutions to the problem, if technology fees were not too high. Repellant soybean, attractive rotated corn, and transgenic rotated corn, all in 2-yr rotations, were economically valuable approaches. However, even the currently common 2-yr rotation was economical when resistance was recessive and the actual costs of resistance would not be paid until far in the future.

Economics Versus Alleles: Balancing Integrated Pest Management and Insect Resistance Management for Rotation-Resistant Western Corn Rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, has overcome crop rotation in several areas of the central United States. We expanded a simple model of adult behavior and population genetics to explain how rotation resistance may have developed and to study ways to manage the western corn rootworm in a landscape of corn, soybean, and winter wheat where evolution of resistance may occur. We modeled six alternative management strategies over a 15-yr time horizon, as well as a strategy involving a 2-yr rotation of corn and soybean in 85% of the landscape, to investigate their effectiveness from both a biological and economic perspective. Generally, resistance to crop rotation evolves in fewer than 15 yr, and the rate of evolution increases as the level of rotated landscape (selection pressure) increases. When resistance is recessive, all six alternative strategies were effective at preventing evolution of rotation resistance. The two most successful strategies were the use of transgenic rotated corn in a 2-yr rotation and a 3-yr rotation of corn, soybean, and wheat with unattractive wheat (for oviposition) preceding corn. Results were most sensitive to increases in the initial allele frequency and modifications of the density-dependent survival function. Economically, three alternative strategies were robust solutions to the problem, if technology fees were not too high. Repellant soybean, attractive rotated corn, and transgenic rotated corn, all in 2-yr rotations, were economically valuable approaches. However, even the currently common 2-yr rotation was economical when resistance was recessive and the actual costs of resistance would not be paid until far in the future.

SPATIAL DISPERSION AND DENSITY DEPENDENCE IN A PERENNIAL DESERT SHRUB (CHRYSOTHAMNUS NAUSEOSUS: ASTERACEAE)

Dogma asserts that desert shrubs should become uniformly dispersed through time through a process of self‐thinning caused by competition for water. We present long‐term data (1985–2001) on two nearby populations of the perennial shrub, Chrysothamnus nauseosus ssp. consimilis, in sand‐dune ecosystems near Mono Lake, California. Because previous studies established that water limited the growth, survival, and reproduction of this shrub, we searched for population‐level patterns of competition. A seven‐year history of survival, growth, and flowering of marked individuals delineated five demographic stage classes corresponding roughly to age and plant size: seedlings, nonflowering juveniles, juveniles that later flowered, adults that flowered intermittently, and “immortal” adults that flowered annually. For all stages except nonflowering juveniles, initial size was a strong predictor of life expectancy, and for the reproductive classes, flowering was associated with larger size and higher survival. Survival of nonflowering juveniles was strongly related to depth to groundwater (consistent with the biology of phreatophytes, defined as plants that use deep soil moisture) corresponding to a critical demographic shift at this phreatophytic threshold necessary for long‐term survival and reproduction. We therefore predicted that only the pre‐phreatophytic stage classes dependent on depletable upper soil moisture should compete with close neighbors.Neither mixed‐age population of C. nauseosus nor any single stage class within them was uniformly dispersed. Younger classes (seedling and juveniles) were aggregated, and older classes (from newly reproductive through oldest adults) were randomly dispersed with respect to their nearest neighbor. The younger the stage, the more aggregated was the dispersion. All five seedling cohorts became significantly less aggregated with time during the study, consistent with a hypothesis of competitive thinning. However, mortality was statistically density independent at a landscape scale in all classes and all years and resulted in thinning toward reduced aggregation, just as expected under competition. Two distinct stage‐specific patterns occurred. Whereas the phreatophytic stage classes experienced only density‐independent mortality, the pre‐phreatophytic stage classes experienced opposing density dependence at different spatial scales. Seedling cohorts in particular exhibited strong direct density dependence within the first 1–3 years in a local neighborhood of 20 cm or less, in both survival (higher mortality in dense aggregations) and growth (seedlings in groups were smaller and grew more slowly than those not in groups). In contrast, these classes exhibited inversely density‐dependent survival, and often growth, on a scale of 1–4 m caused by spatial heterogeneity and not facilitation such as by nurse plants. Although competition occurred for only the first 1–3 years of the life history, it caused size differences among individuals that increased through time and that significantly predicted long‐term survival and eventual recruitment to the reproductive class. Despite this potential for long‐lasting effects on population dynamics, competitive thinning was too brief and stage‐structured spatial processes were too complex to result in a uniform dispersion of individuals in this mixed‐age population.Corresponding Editor: S. D. Wilson

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

Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the proportion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival. To test for density dependence in adult survival, we used data from three populations that underwent considerable changes in density. Bighorn sheep (Ovis canadensis) on Ram Mountain, Alberta, ranged from 94 to 232, mountain goats (Oreamnos americanus) on Caw Ridge, Alberta, varied from 81 to 147, and estimates of roe deer (Capreolus capreolus) older than 1year at Chizé, France, ranged from 157 to 569. We used recent developments of capture-mark-recapture modelling to assess the response of adult survival to changes in density when age structure was and was not taken into account. Survival rates were 10-15% higher during the prime-age stage than during the senescent stage for all sex-species combinations. When adults were pooled into a single age class there was an apparent negative effect of density on female survival in bighorns and roe deer, and negative trends for female mountain goats, male roe deer and male bighorn sheep. When age class was taken into account, there were no significant effects of density on adult survival. Except for male mountain goats, the strength of density dependence was lower when age was taken into account. In ungulate populations, age structure is an important determinant of adult survival. Most reports of density dependence in adult survival may have been confounded by changes in age structure.