Strength Of Density Dependence Research Articles

Population responses to temperature in 12‐year insect time series

Abstract Considerable work has examined the distribution of species under projected climate change, but empirical data are needed to support predicting change in abundance. I analysed populations of 17 saproxylic beetle species collected twice nightly from mid‐May to mid‐October from 2004 to 2015 at lights in Shizuoka, Japan, to investigate effects of two 100‐year hot summers (2010 and 2013) and correlated changes with June, July and August maximum temperatures. The first analysis examined the effect of heat stress and the second examined how populations tracked changes in temperature. I hypothesised that the responses would be species‐specific and could be related to interspecific variation in body size, voltinism, primary food source and the strength of density dependence. Heat stress was associated with population declines of five species during the same year and five other species in the following year. Conifer feeders were more likely to decline during the same year as the heat stress. Only one species tracked variation in maximum June temperature. Larger, later emerging species were more strongly detrimentally affected by high June temperatures than smaller, early emerging species. There was no relation between the response to heat stress and tracking June temperatures across species. The influence of summer temperature on saproxylic beetles was detectable from relatively short time series and occurred in a species‐specific manner. The effect of heat stress was more readily detected than correlated changes in abundance and temperature. Short time series may help reveal the response of insect populations to changing temperatures.

The consequences of density‐dependent individual growth for sustainable harvesting and management of fish stocks

AbstractDensity dependence is likely to act as a regulatory mechanism in fish stocks that are recovering from overfishing. In general, density dependence in fish stocks is assumed to only occur in reproduction and early life stages and is therefore usually modelled as a stock‐recruitment relationship. Recent research shows that density dependence can also reduce individual growth in body size later in life. In this study, we show how optimal fishing effort changes with the strength of density dependence in individual growth for four stocks of North Sea flatfish species. Using size‐structured population models we show that density dependence arises due to a mechanistic link between the resource availability and life history processes at the individual level. We furthermore show that the stock response to harvesting is either driven by changes in individual reproduction when density dependence in individual growth is weak or by changes in individual growth rate when individual growth is strongly affected by density dependence. These two types or regimes are separated by a sudden shift in dynamics. It is therefore of great importance to account for density dependence in growth when managing fish stocks.

Opposing effects of spatiotemporal variation in resources and temporal variation in climate on density dependent population growth in seabirds.

Understanding how ecological processes combine to shape population dynamics is crucial in a rapidly changing world. Evidence has been emerging for how fundamental drivers of density dependence in mobile species are related to two differing types of environmental variation-temporal variation in climate, and spatiotemporal variation in food resources. However, to date, tests of these hypotheses have been largely restricted to mid-trophic species in terrestrial environments and thus their general applicability remains unknown. We tested if these same processes can be identified in marine upper trophic level species. We assembled a multi-decadal data set on population abundance of 10 species of colonial seabirds comprising a large component of the UK breeding seabird biomass, and covering diverse phylogenies, life histories and foraging behaviours. We tested for evidence of density dependence in population growth rates using discrete time state-space population models fit to long time-series of observations of abundance at seabird breeding colonies. We then assessed if the strength of density dependence in population growth rates was exacerbated by temporal variation in climate (sea temperature and swell height), and attenuated by spatiotemporal variation in prey resources (productivity and tidal fronts). The majority of species showed patterns consistent with temporal variation in climate acting to strengthen density dependent feedbacks to population growth. However, fewer species showed evidence for a weakening of density dependence with increasing spatiotemporal variation in prey resources. Our findings extend this emerging theory for how different sources of environmental variation may shape the dynamics and regulation of animal populations, demonstrating its role in upper trophic marine species. We show that environmental variation leaves a signal in long-term population dynamics of seabirds with potentially important consequences for their demography and trophic interactions.

Spatial patterns in the contribution of biotic and abiotic factors to the population dynamics of three freshwater fish species.

BackgroundPopulation dynamics are driven by a number of biotic (e.g., density-dependence) and abiotic (e.g., climate) factors whose contribution can greatly vary across study systems (i.e., populations). Yet, the extent to which the contribution of these factors varies across populations and between species and whether spatial patterns can be identified has received little attention.MethodsHere, we used a long-term (1982–2011), broad scale (182 sites distributed across metropolitan France) dataset to study spatial patterns in the population’s dynamics of three freshwater fish species presenting contrasted life-histories and patterns of elevation range shifts in recent decades. We used a hierarchical Bayesian approach together with an elasticity analysis to estimate the relative contribution of a set of biotic (e.g., strength of density dependence, recruitment rate) and abiotic (mean and variability of water temperature) factors affecting the site-specific dynamic of two different size classes (0+ and >0+ individuals) for the three species. We then tested whether the local contribution of each factor presented evidence for biogeographical patterns by confronting two non-mutually exclusive hypotheses: the “range-shift” hypothesis that predicts a gradient along elevation or latitude and the “abundant-center” hypothesis that predicts a gradient from the center to the edge of the species’ distributional range.ResultsDespite contrasted life-histories, the three species displayed similar large-scale patterns in population dynamics with a much stronger contribution of biotic factors over abiotic ones. Yet, the contribution of the different factors strongly varied within distributional ranges and followed distinct spatial patterns. Indeed, while abiotic factors mostly varied along elevation, biotic factors—which disproportionately contributed to population dynamics—varied along both elevation and latitude.ConclusionsOverall while our results provide stronger support for the range-shift hypothesis, they also highlight the dual effect of distinct factors on spatial patterns in population dynamics and can explain the overall difficulty to find general evidence for geographic gradients in natural populations. We propose that considering the separate contribution of the factors affecting population dynamics could help better understand the drivers of abundance-distribution patterns.

Strength and consistency of density dependence in marine fish productivity

AbstractThe correct prediction of the shape and strength of density dependence in productivity is key to predicting future stock development and providing the best possible long‐term fisheries management advice. Here, we identify unbiased estimators of the relationship between somatic growth, recruitment and density, and apply these to 80 stocks in the Northeast Atlantic. The analyses revealed density‐dependent recruitment in 68% of the stocks. Excluding pelagic stocks exhibiting significant trends in spawning stock biomass, the probability of significant density dependence was even higher at 78%. The relationships demonstrated that at the commonly used biomass limit of 0.2 times maximum spawning stock size, only 32% of the stocks attained three quarters of their maximum recruitment. This leaves 68% of the stocks with less than three quarters of their maximum recruitment at this biomass limit. Significantly lower recruitment at high stock size than at intermediate stock size was seen in 38% of the stocks. Density dependence in late growth occurred in 54% of the stocks, whereas early growth was generally density‐independent. Pelagic stocks were less likely to exhibit density dependence in recruitment than demersal and benthic stocks. We recommend that both the degree to which productivity is related to density and the degree to which the relationship changes over time should be investigated. Both of these aspects should be considered in evaluations of whether sustainability and yield can be improved by including density dependence in forecasts of the effects of different management actions.

Age-dependent patterns of spatial autocorrelation in fish populations.

The degree of spatial autocorrelation in population fluctuations increases with dispersal and geographical covariation in the environment, and decreases with strength of density dependence. Because the effects of these processes can vary throughout an individual's lifespan, we studied how spatial autocorrelation in abundance changed with age in three marine fish species in the Barents Sea. We found large interspecific differences in age-dependent patterns of spatial autocorrelation in density. Spatial autocorrelation increased with age in cod, the reverse trend was found in beaked redfish, while it remained constant among age classes in haddock. We also accounted for the average effect of local cohort dynamics, i.e. the expected local density of an age class given last year's local density of the cohort, with the goal of disentangling spatial autocorrelation patterns acting on an age class from those formed during younger age classes and being carried over. We found that the spatial autocorrelation pattern of older age classes became increasingly determined by the distribution of the cohort during the previous year. Lastly, we found high degrees of autocorrelation over long distances for the three species, suggesting the presence of far-reaching autocorrelating processes on these populations. We discuss how differences in the species' life history strategies could cause the observed differences in age-specific variation in spatial autocorrelation. As spatial autocorrelation can differ among age classes, our study indicates that fluctuations in age structure can influence the spatio-temporal variation in abundance of marine fish populations.

Effects of density dependence in an evergreen-deciduous broadleaf karst forest in southwest China

There is increasing evidence that density dependence is an important mechanism for the coexistence of species in natural forest communities. Although density dependence among neighbors has been widely studied in tropical, subtropical, and temperate forests, the effects thereof remain poorly understood for non-zonal forest types (e.g., karst forests). Within this context, we conducted a census of tree species (DBH ≥ 1 cm) in a 25 ha plot within the Mulun forest, southwest China. Based on the data collected, a spatial statistical technique was used to detect the prevalence of density dependence after accounting for the effects of habitat heterogeneity. Furthermore, we investigated the relationships between species characteristics and the strength of density dependence. Our results indicate that 19 of the 93 tree species recorded in the species-rich subtropical karst forest showed a decline in strength of clustering from juveniles to adults, as expected from self-thinning, while 61 species exhibited density-dependent effects. The strength of density dependence was closely related to specific species characteristics. Trees with a smaller species abundance and higher aggregation had stronger density dependence. Additionally, the strength of density dependence of evergreen and canopy species was greater than that of deciduous and understory species. Our study demonstrates that density dependence is a prevalent mechanism for regulating population structures of the majority of tree species in forest ecosystems. Our findings further improve the understanding of species coexistence and forest ecosystem stability, and provide guidance for forest management.

Elk population dynamics when carrying capacities vary within and among herds

Population and land management relies on understanding population regulation and growth, which may be impacted by variation in population growth parameters within and among populations. We explored the interactions between variation in carrying capacity (K), intrinsic population growth rate (r), and strength of density dependence (β) within and among elk (Cervus elaphus) herds in a small part of the geographic range of the species. We also estimated stochastic fluctuations in abundance around K for each herd. We fit linear Ricker growth models using Bayesian statistics to seven time series of elk population survey data. Our results indicate that K and β varied among herds, and that r and β varied temporally within herds. We also found that herds with smaller K had less stochastic fluctuation in abundances around K, but higher temporal variation in β within herds. Population regulation and the rate of return to the equilibrium abundance is often understood in terms of β, but ecological populations are dynamic systems, and temporal variation in population growth parameters may also influence regulation. Population models which accommodate variation both within and among herds in population growth parameters are necessary, even in mild climates, to fully understand population dynamics and manage populations.

Breeding transients in capture\u2013recapture modeling and their consequences for local population dynamics

Standard procedures for capture–mark–recapture modelling (CMR) for the study of animal demography include running goodness-of-fit tests on a general starting model. A frequent reason for poor model fit is heterogeneity in local survival among individuals captured for the first time and those already captured or seen on previous occasions. This deviation is technically termed a transience effect. In specific cases, simple, uni-state CMR modeling showing transients may allow researchers to assess the role of these transients on population dynamics. Transient individuals nearly always have a lower local survival probability, which may appear for a number of reasons. In most cases, transients arise due to permanent dispersal, higher mortality, or a combination of both. In the case of higher mortality, transients may be symptomatic of a cost of first reproduction. A few studies working at large spatial scales actually show that transients more often correspond to survival costs of first reproduction rather than to permanent dispersal, bolstering the interpretation of transience as a measure of costs of reproduction, since initial detections are often associated with first breeding attempts. Regardless of their cause, the loss of transients from a local population should lower population growth rate. We review almost 1000 papers using CMR modeling and find that almost 40% of studies fitting the searching criteria (N = 115) detected transients. Nevertheless, few researchers have considered the ecological or evolutionary meaning of the transient phenomenon. Only three studies from the reviewed papers considered transients to be a cost of first reproduction. We also analyze a long-term individual monitoring dataset (1988–2012) on a long-lived bird to quantify transients, and we use a life table response experiment (LTRE) to measure the consequences of transients at a population level. As expected, population growth rate decreased when the environment became harsher while the proportion of transients increased. LTRE analysis showed that population growth can be substantially affected by changes in traits that are variable under environmental stochasticity and deterministic perturbations, such as recruitment, fecundity of experienced individuals, and transient probabilities. This occurred even though sensitivities and elasticities of these parameters were much lower than those for adult survival. The proportion of transients also increased with the strength of density-dependence. These results have implications for ecological and evolutionary studies and may stimulate other researchers to explore the ecological processes behind the occurrence of transients in capture–recapture studies. In population models, the inclusion of a specific state for transients may help to make more reliable predictions for endangered and harvested species.

Determinants of intra‐annual population dynamics in a tropical soil arthropod

AbstractStudies on inter‐annual dynamics of populations from temperate regions have shown that density dependence and climate effects are relatively common, albeit weak. Yet, for short‐lived organisms, intra‐annual variation may be at least as important. Furthermore, tropical species commonly experience temperatures close to their upper thermal limit and thus may be more likely to experience heat stress. Here, we used the soil mite Rostrozetes ovulum to investigate the drivers of intra‐annual population dynamics in an Amazonian rain forest. We sampled 3,600 soil cores from 20 transects during 13 months, obtaining 180 mite counts. Next, we built a dynamic N‐mixture model accounting for different detection probabilities between soil types. In a Bayesian framework, we used this model to estimate (a) the strength of density dependence and (b) per capita growth rates, which were then tested against environmental variables. We found that the intra‐annual population dynamics of R. ovulum were weakly density dependent. Further, per capita growth rates increased with resource supply (litterfall) and decreased with maximum temperature over much of the observed thermal range, although these effects explained relatively little variance. Yet, the seasonal correlation between these factors created a trade‐off, so that realized population growth was highest when neither resource supply nor thermal suitability was optimal. Overall, our results suggest that the mechanisms shaping soil animal population dynamics may be surprisingly similar across latitudes. Our model offers a starting point for analyses of soil animal counts when extraction from soil samples is imperfect.

A note on the Lasota discrete model for blood cell production

In an attempt to explain experimental evidence of chaotic oscillations in blood cell population, A. Lasota suggested in 1977 a discrete-time one-dimensional model for the production of blood cells, and he showed that this equation allows to model the behavior of blood cell population in many clinical cases. Our main aim in this note is to carry out a detailed study of Lasota's equation, in particular revisiting the results in the original paper and showing new interesting phenomena. The considered equation is also suitable to model the dynamics of populations with discrete reproductive seasons, adult survivorship, overcompensating density dependence, and Allee effects. In this context, our results show the rich dynamics of this type of models and point out the subtle interplay between adult survivorship rates and strength of density dependence (including Allee effects).

Interactions between species attributes explain population dynamics in stream fishes under changing climate

AbstractSpecies responses to climate change have been shown to vary in both direction and magnitude. Understanding these idiosyncratic responses is crucial if we are to predict extinction risk and set up efficient conservation strategies. The variations observed across species have been related to several species attributes including intrinsic traits such as physiological tolerances or life‐history strategies but also to niche characteristics (e.g., niche breadth [NB], niche position [NP]). However, although previous studies have successfully linked species attributes to population dynamics or range shifts, few have considered synergistic effects to explain responses to climate variations. Here, we assessed whether five species attributes (fecundity, thermal safety margin, trophic position [TP], NB, and NP) explained interspecific differences in four parameters influencing population dynamics of 35 stream fish species at the French scale. We used Bayesian N‐mixture models to estimate posterior distributions of the growth rate, the strength of density dependence, and the influence of both mean temperature and temperature variability on populations for each species. We then used phylogenetic generalized least squares (PGLS) models to investigate the influence of species attributes and their interactions on interspecific differences in each of the four parameter driving population dynamics. The percentage of variance explained by the PGLS models was relatively high (around 40% on average), indicating that species attributes are good predictors of species population dynamics. Furthermore, we showed that the influence of these single attributes was mediated by other attributes, especially NP and TP. Importantly, we found that models including interaction terms had greater support over simple additive models in explaining interspecific differences in population dynamics. Taken together, these results point to the importance of considering the interplay between species attributes in unraveling the mechanisms involved in population dynamics and understanding the vulnerability of species to global change.

Spatiotemporal variation in the strength of density dependence: implications for biocontrol of Centaurea solstitialis

Invasive plants often occupy large ranges in the introduced region and consequently, local population dynamics vary in ways that affect the potential for biological control. We used matrix models to describe how density and population growth rate of Centaurea solstitialis varies in time and space. Matrix models were parameterized with data collected over 4 years from invasions at the coast, interior valleys and Sierra Nevada Mountains in California (USA). Asymptotic population growth rates (λ) varied dramatically across all populations and years (0.24–6.45), density varied by an order of magnitude and had a measurable effect on survival and λ in all populations. We used simulations to estimate the degree to which a biocontrol agent would need to reduce plant survival to control the weed. Because seedling survival was dependent on density, an agent that reduced seedling density had the effect of increasing the probability that the remaining plants survived to flowering. Interestingly, this meant that in the highest density populations the plant had the largest compensatory response to agent attack and experienced decline (λ ≤ 1.0) only after heavy losses (≥90%) to the agent. Conversely, in populations where density was so low that it had only a weak effect on survival, the agent was able to control the plant (λ ≤ 1.0) at much lower levels of attack (≤50%). In other words, the impact of a biocontrol agent is predicted to be lower where the plant reaches its highest densities because the surviving plants, now experiencing less intraspecific competition, are more likely to survive to flowering and produce more seeds. This may also be true for other invasive species in which strong density dependent processes are operating. For this reason, prospective agents ought to target density-independence vital rates.

Ecological drift and the distribution of species diversity.

Ecological drift causes species abundances to fluctuate randomly, lowering diversity within communities and increasing differences among otherwise equivalent communities. Despite broad interest in ecological drift, ecologists have little experimental evidence of its consequences in nature, where competitive forces modulate species abundances. We manipulated drift by imposing 40-fold variation in the size of experimentally assembled annual plant communities and holding their edge-to-interior ratios comparable. Drift over three generations was greater than predicted by neutral models, causing high extinction rates and fast divergence in composition among smaller communities. Competitive asymmetries drove populations of most species to small enough sizes that demographic stochasticity could markedly influence dynamics, increasing the importance of drift in communities. The strong effects of drift occurred despite stabilizing niche differences, which cause species to have greater population growth rates when at low local abundance. Overall, the importance of ecological drift appears greater in non-neutral communities than previously recognized, and varies with community size and the type and strength of density dependence.

Structural uncertainty limits generality of fall harvest strategies for wild turkeys

ABSTRACTWild turkey (Meleagris gallopavo) populations are broadly distributed, occupy a variety of habitats, and have demographic rates that are heterogeneous through space and time. Dynamics of turkey populations are sensitive to the magnitude of fall either‐sex harvest, yet there have been few attempts to study performance of fall harvests systematically across a range of plausible demographic scenarios. Thus robustness of existing recommendations to structural uncertainty in population dynamics is marginally understood. We used a stochastic, sex‐specific theta‐logistic model to simulate performance of fall harvests (0–15%) across scenarios representing uncertainty about current rates of population productivity (3 levels), female losses during spring hunting (2 levels), and spring male harvest (3 levels), with uncertainty in the strength of density dependence as a common attribute. We demonstrated that performance of previously recommended fall harvests was not robust to changes in demographic parameters that occur within and among populations, and thus previous management recommendations may not be appropriate for all regions. Fall harvest rates capable of maintaining large populations with high probability varied from 0–6% with changes to population productivity, when median male and female spring harvests were 30% and 5%, respectively. In general, risks and management tradeoffs accompanying fall harvests were tightly linked to assumed values of population parameters, where changes to productivity and female loss had particularly strong effects on management outcomes. Specifically, reduced productivity or increased female loss decreased the ability to maintain large populations for a given fall harvest rate. Thus, fall harvest recommendations deduced from models that considered only a small portion of the demographic parameter space may not meet modern management objectives over a broader range of conditions. Moreover, our results suggest that future management could be improved by reducing structural uncertainty about turkey demography to allow for region‐specific harvest strategies, or by using decision‐theoretic approaches to identify harvest strategies that are robust to uncertainty about population parameters. © 2017 The Wildlife Society.

Effects of Broad-Scale Conservation on Northern Bobwhite Populations in Agricultural Landscapes

Private land initiatives such as the Conservation Reserve Enhancement Program (CREP) are avenues for broad-scale northern Bobwhite (Colinus virginianus) conservation. The CREP in Kentucky established 40,468 ha of native prairie grasses and riparian corridors in the Green River Basin. Northern bobwhite responses to similar conservation measures at local scales (i.e., the site of implementation) have been positive; however, the geographic extent of the influence of private land initiatives on populations is less understood. Our objectives were to investigate landscape-scale effects of CREP on northern bobwhite populations. Using a stratified random sampling design, 254 roadside point counts were performed over 5 years throughout the Green River Basin along a gradient of landscape-scale CREP density. Local-scale (500 m radius) CREP density was held constant at monitoring points. We analyzed data using an openpopulation distance sampling model that included estimators of appropriate landscape scale and strength of density dependence. Population response to the CREP was positive and outweighed conservation footprint. Our results suggest that broad-scale conservation can influence wildlife populations outside of targeted areas. Concurrently, because the majority of land in the Eastern U.S. is privately owned, private land conservation initiatives present an effective strategy for promoting wildlife population recovery across large areas. Our future directions with this research include improving model estimators, determining mechanisms behind landscape-scale effects of CREP, and determining the influence of the spatial arrangement of landscape features on local populations.

An Analysis of a Semelparous Population Model with Density-Dependent Fecundity and Density-Dependent Survival Probabilities

A discrete age-structured semelparous Leslie matrix model where density dependence is included both in the fecundity and in the survival probabilities is analysed. Depending on strength of density dependence, we show in the precocious semelparous case that the nonstationary dynamics may indeed be rich, ranging from SYC (a dynamical state where the whole population is in one age class only) dynamics to cycles of low period where all age classes are populated. Quasiperiodic and chaotic dynamics have also been identified. Moreover, outside parameter regions where SYC dynamics dominates, we prove that the transfer from stability to instability goes through a supercritical Neimark−Sacker bifurcation, and it is further shown that when the population switches from possessing a precocious to a delayed semelparous life history both stability properties and the possibility of periodic dynamics become weaker.

Effects of density dependence in a temperate forest in northeastern China.

Negative density dependence may cause reduced clustering among individuals of the same species, and evidence is accumulating that conspecific density-dependent self-thinning is an important mechanism regulating the spatial structure of plant populations. This study evaluates that specific density dependence in three very large observational studies representing three successional stages in a temperate forest in northeastern China. The methods include standard spatial point pattern analysis and a heterogeneous Poisson process as the null model to eliminate the effects of habitat heterogeneity. The results show that most of the species exhibit conspecific density-dependent self-thinning. In the early successional stage 11 of the 16 species, in the intermediate successional stage 18 of the 21 species and in the old growth stage all 21 species exhibited density dependence after removing the effects of habitat heterogeneity. The prevalence of density dependence thus varies among the three successional stages and exhibits an increase with increasing successional stage. The proportion of species showing density dependence varied depending on whether habitat heterogeneity was removed or not. Furthermore, the strength of density dependence is closely related with species abundance. Abundant species with high conspecific aggregation tend to exhibit greater density dependence than rare species.

Cross‐seasonal effects on waterfowl productivity: Implications under climate change

ABSTRACTPrevious efforts to relate winter‐ground precipitation to subsequent reproductive success as measured by the ratio of juveniles to adults in the autumn failed to account for increased vulnerability of juvenile ducks to hunting and uncertainty in the estimated age ratio. Neglecting increased juvenile vulnerability will positively bias the mean productivity estimate, and neglecting increased vulnerability and estimation uncertainty will positively bias the year‐to‐year variance in productivity because raw age ratios are the product of sampling variation, the year‐specific vulnerability, and year‐specific reproductive success. Therefore, we estimated the effects of cumulative winter precipitation in the California Central Valley and the Mississippi Alluvial Valley on pintail (Anas acuta) and mallard (Anas platyrhnchos) reproduction, respectively, using hierarchical Bayesian methods to correct for sampling bias in productivity estimates and observation error in covariates. We applied the model to a hunter‐collected parts survey implemented by the United States Fish and Wildlife Service and band recoveries reported to the United States Geological Survey Bird Banding Laboratory using data from 1961 to 2013. We compared our results to previous estimates that used simple linear regression on uncorrected age ratios from a smaller subset of years in pintail (1961–1985). Like previous analyses, we found large and consistent effects of population size and wetland conditions in prairie Canada on mallard productivity, and large effects of population size and mean latitude of the observed breeding population on pintail productivity. Unlike previous analyses, we report a large amount of uncertainty in the estimated effects of wintering‐ground precipitation on pintail and mallard productivity, with considerable uncertainty in the sign of the estimated main effect, although the posterior medians of precipitation effects were consistent with past studies. We found more consistent estimates in the sign of an interaction effect between population size and precipitation, suggesting that wintering‐ground precipitation has a larger effect in years of high population size, especially for pintail. When we used the estimated effects in a population model to derive a sustainable harvest and population size projection (i.e., a yield curve), there was considerable uncertainty in the effect of increased or decreased wintering‐ground precipitation on sustainable harvest potential and population size. These results suggest that the mechanism of cross‐seasonal effects between winter habitat and reproduction in ducks occurs through a reduction in the strength of density dependence in years of above‐average wintering‐ground precipitation. We suggest additional investigation of the underlying mechanisms and that habitat managers and decision‐makers consider the level of uncertainty in these estimates when attempting to integrate habitat management and harvest management decisions. Collection of annual data on the status of wintering‐ground habitat in a rigorous sampling framework would likely be the most direct way to improve understanding of mechanisms and inform management. © 2016 The Wildlife Society.

Simulation‐based assessment of dynamic N‐mixture models in the presence of density dependence and environmental stochasticity

Summary Estimating survival and recruitment over large spatial scales is extremely challenging because it requires ‘expensive’ demographic data from marked animals. Dynamic N‐mixture models (Dail & Madsen ) yield estimates of abundance, apparent survival, recruitment and detection probability from simple counts replicated in space and time, as frequently collected in monitoring programmes all over the world. However, the vital rates are assumed to be density‐independent and constant over time, arguably unrealistic assumptions in many situations. We evaluated the performance of the traditional dynamic N‐mixture model from simulated data generated with density dependence and environmental stochasticity in the vital rates. We then developed a series of more advanced models that take density‐dependent vital rates and environmental stochasticity into account and assessed whether these models provide accurate estimates of vital rates, strength of density dependence and levels of environmental stochasticity. We found that the traditional Dail–Madsen model produced accurate estimates of abundances and vital rates when the assumptions were met, but not when vital rates were subject to density dependence and environmental stochasticity. Accurate parameter estimates were generally obtained when the data generation matched the data analysis model, but accuracy was substantially reduced otherwise. Interestingly, accurate estimates of abundance and detection were obtained from all models, regardless of the data generation model. Shorter time series from more sites yielded estimates of similar accuracy as longer series from fewer sites. The new dynamic N‐mixture models represent a promising basis for developing integrated population models that combine data from different sources to get insights into population dynamics of species over large areas.