Multiple sources of evidence for density dependence in the endangered Hawaiian stilt (Himantopus mexicanus knudseni)

Hawaiian stilts (Himantopus mexicanus knudseni) are an endangered subspecies of the Black-necked stilt endemic to the Hawaiian Islands. Despite long-term study, the main drivers of Hawaiian stilt population dynamics are poorly understood. We tested for density dependence using two sources of evidence: a 30-year time series of annual estimated range-wide abundance, and two 15+ year time series of reproductive success. Using separate methods with independent data, sources allowed us to make up for the potentially positive bias of one approach with the more conservative nature of the second. We compared nonlinear density-dependent and density-independent population model fits to our time-series data, using both frequentist and Bayesian state-space approaches. Across both approaches, density-dependent models best fit observed population dynamics, with lower AICc and cross-validation statistics compared to density-independent models. Among density-dependent models, a conditional model in which density-independent dynamics occur below a population size threshold (~850–1,000 birds), and then density-dependent dynamics occur above that threshold, performed best across Bayesian and frequentist model comparisons, with the Ricker model ranked next or equivalently. Our analysis of reproduction data revealed a strong negative effect of local adult density on nest success (proportion of nests hatching at least one chick) at Kealia National Wildlife Refuge on Maui, where few alternative breeding habitats are available, but no such effect at another site where many nearby alternative wetlands are available. These congruent results across independent datasets and analytical approaches support the hypothesis that Hawaiian stilts exhibit density dependence across their range.     

The role of group size and environmental factors on survival in a cooperatively breeding tropical passerine

1. Variation in survival, a major determinant of fitness, may be caused by individual or environmental characteristics. Furthermore, interactions between individuals may influence survival through the negative feedback effects of density dependence. Compared to species in temperate regions, we have little knowledge about population processes and variation in fitness in tropical bird species. 2. To investigate whether variation in survival could be explained by population size or climatic variables we used capture-recapture models in conjunction with a long-term data set from an island population of the territorial, cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). The lack of migration out of the study population means that our results are not confounded by dispersal. 3. Annual survival was high, both for adults (84%) and juveniles (61%), and did not differ between the sexes. Although there was significant variation in survival between years, this variation could not be explained by overall population size or weather variables. 4. For territorial species, resource competition will work mainly on a local scale. The size of a territory and number of individuals living in it will therefore be a more appropriate measure of density than overall population density. Consequently, both an index of territory quality per individual (food availability) and local density, measured as group size, were included as individual covariates in our analyses. 5. Local density had a negative effect on survival; birds living in larger groups had lower survival probabilities than those living in small groups. Food availability did not affect survival. 6. Our study shows that, in a territorial species, although density-dependent effects might not be detectable at the population level they can be detected at the individual territory level - the scale at which individuals compete. These results will help to provide a better understanding of the small-scale processes involved in the dynamics of a population in general, but in particular in tropical species living in relatively stable environments.     

Should ecological factors affect the evolution of age at maturity in freshwater clams?

We studied the evolution of age at maturity in freshwater clams of the genus Anodonta in relation to their ecology. We analysed an age-structured density-dependent population dynamics model, which we developed for freshwater clams, using several different options for density dependence. As evolutionary optimality criteria we applied both the maximization of a fitness measure (either intrinsic rate of increase or expected lifetime fecundity) and the concept of evolutionarily stable strategies (ESSs). All three evolutionary criteria yielded estimates which were too high for the optimal age at maturity in a deterministic model with a constant survival rate. The predictions are improved when size-selective predation is included in the model. Mature clams also face a risk of infection by castrating parasites, which would select for delayed maturity. Variable newborn survival selects for earlier reproduction, though the observed levels of stochasticity probably have a negligible effect.     

The evolution of intermittent breeding

A central issue in life history theory is how organisms trade off current and future reproduction. A variety of organisms exhibit intermittent breeding, meaning sexually mature adults will skip breeding opportunities between reproduction attempts. It’s thought that intermittent breeding occurs when reproduction incurs an extra cost in terms of survival, energy, or recovery time. We have developed a matrix population model for intermittent breeding, and use adaptive dynamics to determine under what conditions individuals should breed at every opportunity, and under what conditions they should skip some breeding opportunities (and if so, how many). We also examine the effect of environmental stochasticity on breeding behavior. We find that the evolutionarily stable strategy (ESS) for breeding behavior depends on an individual’s expected growth and mortality, and that the conditions for skipped breeding depend on the type of reproductive cost incurred (survival, energy, recovery time). In constant environments there is always a pure ESS, however environmental stochasticity and deterministic population fluctuations can both select for a mixed ESS. Finally, we compare our model results to patterns of intermittent breeding in species from a range of taxonomic groups.     

Density-dependent reproduction in the European rabbit: a consequence of individual response and age-dependent reproductive performance

Density dependence of reproduction has generally been proposed to be caused by habitat heterogeneity and by the individual response of reproductive output. However, a further mechanism might generate density dependence of average reproductive rates. High density situations might be associated with a high proportion of first-season breeders which often show a principally lower reproductive performance. We tested for the existence of the latter mechanism as well as for density-dependent individual changes of reproductive effort in a population of European rabbits living in a homogeneous grassland habitat. The study was conducted over a period of eleven years. Overall, a strong relationship between mean reproductive rates and the breeding density of females was apparent. All necessary conditions for the presence of a density-dependent effect caused by age-dependent reproduction were fulfilled: Fluctuations of breeding density were paralleled by variations in the proportion of one-year-old females. These one-year-old, first-season breeders showed a consistently lower reproductive performance than older females, which might be caused by their lower body mass and their lower social rank. However, we also found strong evidence for density-dependent response of individual reproductive effort: Individual changes in fecundity over successive years were explained by changes in the breeding density of females. The results suggest that density dependence of reproduction in European rabbits is due to an interaction of age-dependent reproductive performance together with short-term fluctuations in breeding density, and a density-dependent, individual based response of reproductive rates. We further conclude that the lower reproductive performance of first-season breeders in age-structured animal populations may contribute substantially to interannual, and under particular circumstances to density-dependent variations of mean reproductive rates.     

Exploring density-dependent relationships in demographic parameters in populations of birds at a large spatial scale

The importance of density-dependent processes in natural populations is widely accepted, but the issue of the shape of density-dependent relationships (such as influenced by vagueness, or time-delay) remains unresolved. We explored the density-dependent relationships in demographic parameters for 12 species of birds in Britain using large-scale, long-term data sets. We predicted that a negative relation between density and demographic parameters should be observed for the stable species, whereas the decreasing or increasing species should display a positive relation if the environment changes progressively through time bringing about a continuous change in density dependence. Our prediction was verified for nine species out of 12; however, we observed, for the three remaining species, a significant decrease of survival rates through time that seems to be involved in a long-term population decline. In all cases where a density-dependent relation was found, we observed an important variance around the relation. In one case, we showed that this variance increased significantly with density. We found evidence for time-delayed effects of density dependence both for survival and breeding performance. In two species, our results suggest the existence of complex interactions (compensatory mechanisms) between survival and breeding performance or between the different components of breeding performance.     

Long-term demographic analysis in goshawk <Emphasis Type="Italic">Accipiter gentilis</Emphasis>: the role of density dependence and stochasticity

Density dependence and environmental stochasticity are both potentially important processes influencing population demography and long-term population growth. Quantifying the importance of these two processes for population growth requires both long-term population as well as individual-based data. I use a 30-year data set on a goshawk Accipiter gentilis population from Eastern Westphalia, Germany, to describe the key vital rate elements to which the growth rate is most sensitive and test how environmental stochasticity and density dependence affect long-term population growth. The asymptotic growth rate of the fully age-structured mean matrix model was very similar to the observed one (0.7% vs. 0.3% per annum), and population growth was most elastic to changes in survival rate at age classes 1-3. Environmental stochasticity led only to a small change in the projected population growth rate (between -0.16% and 0.67%) and did not change the elasticities qualitatively, suggesting that the goshawk life history of early reproduction coupled with high annual fertility buffers against a variable environment. Age classes most crucial to population growth were those in which density dependence seemed to act most strongly. This emphasises the importance of density dependence as a regulatory mechanism in this goshawk population. It also provides a mechanism that might enable the population to recover from population lows, because a mean matrix model incorporating observed functional responses of both vital rates to population density coupled with environmental stochasticity reduced long-term extinction risk of 30% under density-independent environmental stochasticity and 60% under demographic stochasticity to zero.     

Simulating density-dependent relationships in south Texas northern bobwhite populations

Density dependence influences northern bobwhite (Colinus virginianus) reproduction and overwinter mortality. However, the functional forms of these density-dependent relationships or the factors that influence them during the annual life cycle events of this bird are not clear. We used a systems analysis approach with a compartment model based on difference equations (Δt = 3 months) for bobwhites in South Texas to simulate population behavior using 16 different functional forms of density-dependent production and overwinter mortality. During the reproductive season, a weak linear density-dependent relationship resulted in the longest population persistence (up to 100.0 yr), whereas a reverse-sigmoid density-dependent relationship had the worst population persistence (2.5–3.5 yr). Regarding overwinter mortality, a sigmoid or weak linear density-dependent relationship and a weak linear or no density-dependent reproduction relationship had the longest population persistence (87.5–100.0 yr). Weak linear density-dependent reproduction with either sigmoid or weak linear overwinter mortality produced stable fall population trends. Our results indicated that density dependence may have a greater influence on overwinter survival of bobwhites than previously thought. Inclusion of density-dependent functional relationships that represent both density-dependent reproduction and overwinter mortality, were critical for our simulation model to function properly. Therefore, integrating density-dependent relationships for both reproductive and overwinter periods of the annual cycle of bobwhite life history events is essential for conducting realistic bobwhite population simulation analyses that can be used to test different management scenarios in an integrated and interdisciplinary manner. © 2012 The Wildlife Society.     

REPRODUCTIVE COST,AGE-SPECIFIC SURVIVAL AND A COMPARISON OF THE REPRODUCTIVE STRATEGY IN TWO EUROPEAN TITS (GENUS PARUS)

Theoretical analyses of optimal reproductive rates usually assume a trade-off between offspring production and parental survival. This study verified a survival cost for willow tit males; nonbreeding males survived better than males attending a brood. Theory also predicts a smaller clutch size in birds that are less successful in transforming reproductive investments into mature offspring. As predicted, we found that crested tits, suffering a higher nest predation rate, laid smaller clutches than willow tits. The generally lower survival rate of willow tit adults may largely be attributed to their higher reproductive commitment (larger willow tit clutch size), because no significant interspecific survival difference remained between nonbreeding males. Finally, in willow tits we found a positive correlation between average clutch size and juvenile survival rate (density-dependent) the ensuing year, suggesting that willow tits may adjust clutch size in response to changing survival prospects for their young by using the breeding density as a cue.     

A basic equation for population dynamics with destruction of breeding habitats and its application to outbreak of cyprinid herpesvirus 3

In reproduction, many animal species migrate to local habitats that are appropriate for reproduction and for growth of newly born offspring. The examples are ubiquitous among crabs, freshwater fishes, amphibians, migratory birds, and sea animals. We propose a basic equation for population dynamics of such animals, assuming that the number of offspring is proportional to the area of the local breeding habitats as a first approximation. This equation is very simple to be solved analytically, and useful for representing environmental issues of habitat destruction and degradation. According to the equation, the adult density in breeding habitats increases temporarily during habitat destruction and returns to the original density afterwards. The temporal peak value is higher for a larger proportion of area with destruction, a higher temporal rate of destruction, and a higher survival probability of the adults. In contrast, habitat degradation results simply in a decrease of the adult density in breeding habitats. Using this equation, we will discuss the vulnerability of populations to epidemic diseases due to temporal local high densities with decreasing breeding habitats by human activities, exemplifying an outbreak of cyprinid herpesvirus 3 for wild carps in Lake Biwa.     

Density-dependent vital rates and their population dynamic consequences

We explore a set of simple, nonlinear, two-stage models that allow us to compare the effects of density dependence on population dynamics among different kinds of life cycles. We characterize the behavior of these models in terms of their equilibria, bifurcations, and nonlinear dynamics, for a wide range of parameters. Our analyses lead to several generalizations about the effects of life history and density dependence on population dynamics. Among these are: (1) iteroparous life histories are more likely to be stable than semelparous life histories; (2) an increase in juvenile survivorship tends to be stabilizing; (3) density-dependent adult survival cannot control population growth when reproductive output is high; (4) density-dependent reproduction is more likely to cause chaotic dynamics than density dependence in other vital rates; and (5) changes in development rate have only small effects on bifurcation patterns. Received: 12 April 1999 / Published online: 3 August 2000     

Food supply and breeding occurrences: the West European population of the lekking great snipe Gallinago media (Latham, 1787) (Aves)

For species with very high energetic costs during reproduction we expect occurence during the reproduction season to be dramatically affected by the availability of energy. Recent studies have shown very high energetic costs of lekking for great snipe males, Gallinago media (Latham, 1787) and that breeding great snipes prefer to feed in soft soil with a high abundance of earthworms. We here evaluate the hypothesis that the breeding occurrence of great snipe is restricted to areas with very high availability of food. All the 125 registered great snipe leks in Scandinavia were situated in open habitats along the tree line. The occurrence of leks were analysed in relation to bedrock quality, soil chemistry and earthworm biomass. There was a strong positive relationship between soil pH and earthworm biomass. High pH values were found on or in the close vicinity of base-rich bedrocks. No great snipe leks were documented in areas with acid soil, even if the extent of acid bedrocks along the tree line predicts that 31% of the leks should be situated on acid soils. Hypotheses, including both natural and sexual selection, for why breeding occurrence of great snipe is restricted to areas with high abundance of high quality food are evaluated, and we find indications for this to be a consequence of the very high energetic costs for the lekking males. These costs have probably evolved through female mate choice and indicate that sexual selection may have important consequences for a species distribution. Population-level effects of sexual selection have previously received little attention. The stringent habitat demands here documented may also make great snipes vulnerable to environmental changes and can contribute to explain the dramatic reduction in breeding range of this species in western Europe during the past 150 years (Løfaldli et al ., 1989).     

Interspecific differences in stochastic population dynamics explains variation in Taylor's temporal power law

Taylor’s power law, i.e. that the slope for the increase in variance with mean population size is between 1 and 2 at a logarithmic scale, provides one of the few quantitative relationships in population ecology, yet the underlying ecological mechanisms are only poorly understood. Stochastic theory of population dynamics predicts that demographic and environmental stochasticity will affect the slope differently. In a stable environment under the influence of demographic stochasticity alone the slope will be equal to 1. In large populations in which demographic variance will have a negligible effect on the dynamics the slope will approach 2. In addition, the slope will also be influenced by how the strength of density dependence is related to mean population size. To disentangle the relative contribution of these processes we estimate the mean‐variance relationship for a large number of populations of British birds. The variance in population size of most species decreased with the mean due to decreased influence of demographic stochasticity at larger population sizes. Interspecific differences in demographic stochasticity was the main factor influencing variation in slopes of Taylor’s power law among species through a significant negative relationship between the slope and demographic variance. In addition, slopes were influenced by interspecific variation in life history parameters such as adult survival and clutch size. These analyses show that Taylor’s power law is generated from an interplay between stochastic and density dependent factors, modulated by life history.     

Bird behaviour and environmental planning: approaches in the study of wader populations

Predicting the effect of changes in land-use on bird populations requires a degree of understanding of their population dynamics which is seldom available. Such knowledge is especially difficult to acquire if the birds occupy a variety of habitats of differing quality and they are also migratory. The effects of habitat loss at a particular time of year on the year-round population dynamics are then difficult to predict. The problem is discussed using wintering waders as examples. Unless a species can extend its present range, the initial effect of habitat loss is to increase bird density. Whether this affects population size will depend on whether bird density affects either winter survival or subsequent breeding success. Measuring such density-dependent relationships is in practice extremely difficult in migratory populations. However, behavioural studies help in testing the key hypothesis that birth and death rates are affected by bird density, and may even allow the form of any density-dependent functions to be deduced. Simulation modelling then allows the effects of habitat loss on the overall population dynamics to be explored. The general point is that behavioural studies play an important part in Environmental Impact Studies because behaviour is the main way in which birds respond to environmental changes and compete for limited resources, such as diminishing habitat. They are therefore likely to provide insights when making predictions about the responses of birds and populations to habitat loss.     

Density-dependent nest predation – an experiment with simulated Mallard nests in contrasting landscapes

Breeding success is a key element of animal population dynamics. In many taxa including birds, nest success, or the proportion of laid clutches that actually hatch, is mainly determined by predation. Previous research gives an inconsistent picture of the prevalence of density-dependent nest predation and one reason for this is the general lack of well-designed replicated experiments. Using simulated Mallard Anas platyrhynchos nests and a crossover design for 20 lakes in the nemoral and boreal biotic zones, we tested the predictions that nest survival is negatively density-dependent and that nest predation is higher in agricultural than in forested landscapes. Study day and daily abundance of waterfowl, other waterbirds, as well as avian predators were included as covariates in the analysis. Model fitting in program mark revealed a general negative effect of nest density on nest survival. In addition, nest survival rate was higher at forest lakes than at lakes in agricultural landscapes, irrespective of nest density. The only covariate producing model improvement was study day; older nests had higher survival rates than recently initiated ones. This is the first replicated lake-level experimental study showing that nest predation is density-dependent in waterfowl. The pattern was consistent between landscape types, implying that density-dependent nest predation may affect habitat choice and population dynamics over large parts of the Mallard's range.     

Population regulation of territorial species: both site dependence and interference mechanisms matter

Spatial patterns of site occupancy are commonly driven by habitat heterogeneity and are thought to shape population dynamics through a site-dependent regulatory mechanism. When examining this, however, most studies have only focused on a single vital rate (reproduction), and little is known about how space effectively contributes to the regulation of population dynamics. We investigated the underlying mechanisms driving density-dependent processes in vital rates in a Mauritius kestrel population where almost every individual was monitored. Different mechanisms acted on different vital rates, with breeding success regulated by site dependence (differential use of space) and juvenile survival by interference (density-dependent competition for resources). Although territorial species are frequently assumed to be regulated through site dependence, we show that interference was the key regulatory mechanism in this population. Our integrated approach demonstrates that the presence of spatial processes regarding one trait does not mean that they necessarily play an important role in regulating population growth, and demonstrates the complexity of the regulatory process.     

Impact of density and environmental factors on population fluctuations in a migratory passerine

1. Populations of plants and animals typically fluctuate because of the combined effects of density-dependent and density-independent processes. The study of these processes is complicated by the fact that population sizes are typically not known exactly, because population counts are subject to sampling variance. Although the existence of sampling variance is broadly acknowledged, relatively few studies on time-series data have accounted for it, which can result in wrong inferences about population processes. 2. To increase our understanding of population dynamics, we analysed time series from six Central European populations of the migratory red-backed shrike Lanius collurio by simultaneously assessing the strength of density dependence, process and sampling variance. In addition, we evaluated hypotheses predicting effects of factors presumed to operate on the breeding grounds, at stopover sites in eastern Africa during fall and spring migration and in the wintering grounds in southern Africa. We used both simple and state-space formulations of the Gompertz equation to model population size. 3. Across populations and modelling approaches, we found consistent evidence for negative density-dependent population regulation. Further, process variance contributed substantially to variation in population size, while sampling variance did not. Environmental conditions in eastern and southern Africa appear to influence breeding population size, as rainfall in the Sahel during fall migration and in the south African wintering areas were positively related to population size in the following spring in four of six populations. In contrast, environmental conditions in the breeding grounds were not related to population size. 4. Our findings suggest negative density-dependent regulation of red-backed shrike breeding populations and are consistent with the long-standing hypothesis that conditions in the African staging and wintering areas influence population numbers of species breeding in Europe. 5. This study highlights the importance of jointly investigating density-dependent and density-independent processes to improve our understanding of factors influencing population fluctuations in space and time.     

Growth and decline of a penguin colony and the influence on nesting density and reproductive success

Colonial breeding is characteristic of seabirds but nesting at high density has both advantages and disadvantages and may reduce survival and fecundity. African penguins (Spheniscus demersus) initiated breeding at Robben Island, South Africa in 1983. The breeding population on the island increased in the late 1990s and early 2000s before decreasing rapidly until 2010. Before the number breeding peaked, local nest density in the areas where the colony was initiated plateaued, suggesting that preferred nests sites were mostly occupied, and the area used by breeding birds expanded. However, it did not contract again as the population decreased, so that nesting density varied substantially. Breeding success was related positively to the prey available to the breeding birds and negatively to local nest density, particularly during the chick-rearing period, suggesting a density-dependence operating through social interactions in the colony, possibly exacerbated by poor prey availability when the breeding population was large. Although nest density at Robben Island was not high, nesting burrows, which probably reduce the incidence of aggressive encounters in the colony, are scarce and our results suggest that habitat alteration has modified the strength of density-dependent relationships for African penguins. Gaining a better understanding of how density dependence affects fecundity and population growth rates in colonial breeders is important for informing conservation management of the African penguin and other threatened taxa.     

Interactions between density-dependent processes, population dynamics and control of an invasive plant species, Tripleurospermum perforatum (scentless chamomile)

Tripleurospermum perforatum is an invasive weedy species which exhibits strong over-compensating density dependence. Interactions between density-dependent survival, probability of flowering and fecundity were modelled and their impact on the population dynamics were examined. When only fecundity was density-dependent, the dynamics were similar to those observed in the model containing all three density-dependent terms. Density-dependent survival was a stabilizing process when acting in combination with density-dependent fecundity and probability of flowering; removing density-dependent survival from the model produced two-point cycles. The addition of a seed bank was also stabilizing. Simulations of control strategies at different life-history stages indicated that full control would be difficult due to the strong over-compensating density dependence, with severe reductions in fecundity and late season survival necessary in order to reduce equilibrium seed density and biomass.