Density-dependent Fecundity Research Articles

Density dependent survival drives variation in density dependent population growth of an insect pest

Several ecological processes – from population dynamics to species co‐existence – are driven by density dependence (DD) in population growth rate. Thus, to predict and manage ecological outcomes, we need a deep understanding of which factors and demographic traits drive variation in DD. In the insect pest Tribolium castaneum, we found large variation in DD across habitats but not across source populations. We modeled DD in population growth as the product of DD in fecundity and survival, experimentally estimating each parameter. Across habitats, survival parameters varied more than fecundity, including in simulations with varying parameter values. Thus, DD in survival drives variation in density‐dependent population growth. Hence, under strong density‐dependent selection, we expect evolutionary change in density‐dependent survival, provided sufficient genetic variance. Our general framework combining detailed experiments and simulations with a simple model can be used for other species to better understand the causes and consequences of density dependent population growth.

Mathematical Modeling of Mating Probability and Fertile Egg Production in Helminth Parasites.

In this work, we obtained a general formulation for the mating probability and fertile egg production in helminth parasites, focusing on the reproductive behavior of polygamous parasites and its implications for transmission dynamics. By exploring various reproductive variables in parasites with density-dependent fecundity, such as helminth parasites, we departed from the traditional assumptions of Poisson and negative binomial distributions to adopt an arbitrary distribution model. Our analysis considered critical factors such as mating probability, fertile egg production, and the distribution of female and male parasites among hosts, whether they are distributed together or separately. We show that the distribution of parasites within hosts significantly influences transmission dynamics, with implications for parasite persistence and, therefore, with implications in parasite control. Using statistical models and empirical data from Monte Carlo simulations, we provide insights into the complex interplay of reproductive variables in helminth parasites, enhancing our understanding of parasite dynamics and the transmission of parasitic diseases.

Diagnosis of helminths depends on worm fecundity and the distribution of parasites within hosts.

Helminth transmission and morbidity are dependent on the number of mature parasites within a host; however, observing adult worms is impossible for many natural infections. An outstanding challenge is therefore relating routine diagnostics, such as faecal egg counts, to the underlying worm burden. This relationship is complicated by density-dependent fecundity (egg output per worm reduces due to crowding at high burdens) and the skewed distribution of parasites (majority of helminths aggregated in a small fraction of hosts). We address these questions for the carcinogenic liver fluke Opisthorchis viverrini, which infects approximately 10 million people across Southeast Asia, by analysing five epidemiological surveys (n = 641) where adult flukes were recovered. Using a mechanistic model, we show that parasite fecundity varies between populations, with surveys from Thailand and Laos demonstrating distinct patterns of egg output and density-dependence. As the probability of observing faecal eggs increases with the number of mature parasites within a host, we quantify diagnostic sensitivity as a function of the worm burden and find that greater than 50% of cases are misdiagnosed as false negative in communities close to elimination. Finally, we demonstrate that the relationship between observed prevalence from routine diagnostics and true prevalence is nonlinear and strongly influenced by parasite aggregation.

Long-term demographic dynamics of a keystone scavenger disrupted by human-induced shifts in food availability.

Scavenging is a key ecological process controlling energy flow in ecosystems and providing valuable ecosystem services worldwide. As long-lived species, the demographic dynamics of vultures can be disrupted by spatiotemporal fluctuations in food availability, with dramatic impacts on their population viability and the ecosystem services provided. In Europe, the outbreak of bovine spongiform encephalopathy (BSE) in 2001 prompted a restrictive sanitary regulation banning the presence of livestock carcasses in the wild on a continental scale. In long-lived vertebrate species, the buffering hypothesis predicts that the demographic traits with the largest contribution to population growth rate should be less temporally variable. The BSE outbreak provides a unique opportunity to test for the impact of demographic buffering in a keystone scavenger suffering abrupt but transient food shortages. We studied the 42-year dynamics (1979-2020) of one of the world's largest breeding colonies of Eurasian griffon vultures (Gyps fulvus). We fitted an inverse Bayesian state-space model with density-dependent demographic rates to the time series of stage-structured abundances to investigate shifts in vital rates and population dynamics before, during, and after the implementation of a restrictive sanitary regulation. Prior to the BSE outbreak the dynamics was mainly driven by adult survival: 83% of temporal variance in abundance was explained by variability in this rate. Moreover, during this period the regulation of population size operated through density-dependent fecundity and subadult survival. However, after the onset of the European ban, a 1-month delay in average laying date, a drop in fecundity, and a reduction in the number of fledglings induced a transient increase in the impact of fledgling and subadult recruitment on dynamics. Although adult survival rate remained constantly high, as predicted by the buffering hypothesis, its relative impact on the temporal variance in abundance dropped to 71% during the sanitary regulation and to 54% after the ban was lifted. A significant increase in the relative impact of environmental stochasticity on dynamics was modeled after the BSE outbreak. These results provide empirical evidence on how abrupt environmental deterioration may induce dramatic demographic and dynamic changes in the populations of keystone scavengers, with far-reaching impacts on ecosystem functioning worldwide.

Differences in Life History Traits in Rural vs. Urban Populations of a Specialist Ground Beetle, Carabus convexus.

Simple SummaryUrbanization is an important driver of global change, with negative consequences for biodiversity. Specialist species living in isolated urban forest fragments may be the most impacted by urbanization-driven environmental modifications. We compared various life history parameters of a forest specialist ground beetle in its original forest habitat and in urban forest fragments. Abundance was more than five times higher in the rural forest stands than in the urban forest fragments. We found no significant differences in body size or condition between the rural and urban individuals of either sex. Despite higher temperatures in urban habitats, the beginning of the reproductive period did not start earlier in the urban than the rural habitat. The number of ripe eggs was significantly higher in urban than rural females. The urban environmental conditions, however, seemed to cause high mortality of the immature stages, preventing the growth of urban populations.Urbanization is increasing worldwide and causes substantial changes in environmental parameters, generating various kinds of stress on arthropods, with several harmful consequences. We examined a forest specialist ground beetle, Carabus convexus, in forested habitats to evaluate the changes in four important life history traits between rural and urban populations. Analyzing beetles from the overwintered cohort in their first breeding season, we found no significant differences in body length or body mass between the rural and urban individuals. Body condition, judged by fat reserve scores, was similarly poor in both habitats, indicating that beetles were not able to accumulate substantial fat reserves at either habitat. Females with ripe eggs in their ovaries were first captured at the same time in both areas. The number of ripe eggs, however, was significantly higher in females of the low-density urban population (6.13 eggs/female) than in those of the high-density rural population (4.14 eggs/female), indicating density-dependent fecundity. Altered environmental and habitat conditions by urbanization, however, seemed to cause high mortality during egg hatching and/or larval development, preventing the growth of the urban population to the level of rural one.

Revisiting density-dependent fecundity in schistosomes using sibship reconstruction

The stability of parasite populations is regulated by density-dependent processes occurring at different stages of their life cycle. In dioecious helminth infections, density-dependent fecundity is one such regulatory process that describes the reduction in egg production by female worms in high worm burden within-host environments. In human schistosomiasis, the operation of density-dependent fecundity is equivocal and investigation is hampered by the inaccessibility of adult worms that are located intravascularly. Current understanding is almost exclusively limited to data collected from two human autopsy studies conducted over 40 years ago, with subsequent analyses having reached conflicting conclusions. Whether egg production is regulated in a density-dependent manner is key to predicting the effectiveness of interventions targeting the elimination of schistosomiasis and to the interpretation of parasitological data collected during monitoring and evaluation activities. Here, we revisit density-dependent fecundity in the two most globally important human Schistosoma spp. using a statistical modelling approach that combines molecular inference on the number of parents/adult worms in individual human hosts with parasitological egg count data from mainland Tanzania and Zanzibar. We find a non-proportional relationship between S. haematobium egg counts and inferred numbers of female worms, providing the first clear evidence of density-dependent fecundity in this schistosome species. We do not find robust evidence for density-dependent fecundity in S. mansoni because of high sensitivity to some modelling assumptions and the lower statistical power of the available data. We discuss the strengths and limitations of our model-based analytical approach and its potential for improving our understanding of density dependence in schistosomiasis and other human helminthiases earmarked for elimination.

Decomposing fecundity and evaluating demographic influence of multiple broods in a migratory bird

Relevance of breeding season fecundity as a driver of population dynamics has been highlighted by many studies. Despite that, knowledge about how brood type specific (i.e. first, second or replacement) fecundity affects demography of multiple-brooded species is limited. In fact, estimation of brood type specific fecundity is often challenging due to imperfect detection of nesting attempts. We examined the demographic contribution and the feedback on population density of different components of fecundity, along with other vital rates, in a facultative multiple-brooded migratory bird. We used a novel formulation of a fecundity model that allows incorporating reproductive data for which information on the type of brood was unknown in some cases, and embedded it into an integrated population model (IPM) to obtain consensual estimates of all demographic rates, including brood type specific fecundities, reproductive success probabilities and proportion of breeding pairs that performed a second or replacement brood. We then conducted transient life table response experiments on IPM estimates to account for non-stationary environments. We applied the model to two 20-year datasets collected in a Swiss and a German local population of wrynecks Jynx torquilla. Brood type specific fecundities and temporal patterns of brood type specific probabilities of success, number of successful and unsuccessful first broods, probability of starting a second or a replacement brood and proportion of pairs that performed a second or a replacement brood differed between the two populations. However, changes in immigration rate and apparent survival were the dominant contributors to temporal variation and large sequential changes in realized population growth rates in both populations. In the Swiss population we also found that second brood fecundity declined when population size increased. Our study provides insight into the reproductive processes that affect population dynamics and mediate density-dependent fecundity in a migratory bird. In addition, the analytical approach proposed can be used in other studies of multiple-brooded species to maximize the use of available fecundity data through the estimation of unknown brood types, thus favouring a better understanding of the demographic contribution of brood type specific fecundity.

Effect of floral traits mediated by plant-soil feedback on the relationship between plant density and fecundity: Case study of Tamarix chinensis in the Yellow River Delta, China

Density-dependent fecundity is important for the successful recruitment and reproduction of plant populations, especially for plants inhabiting saltmarshes with highly varied environmental gradients. The highest fecundity of plants at intermediate densities is usually correlated with high pollinator visitation rates mediated by floral displays of medium density. However, little data explains the mechanism that floral traits and pollinator visitation optimize at intermediate densities. Here, we investigated how the density of Tamarix chinensis (in the Yellow River Delta) influence soil conditions, floral traits, pollinator visitation rates, and plant fecundity in environments with different soil salinity and moisture. At all sites, soil moisture and floral traits (number and length of racemes, amounts of floral volatiles) increased parabolically with increasing density, whereas soil salinity declined. Soil conditions and floral traits of T. chinensis were optimal at intermediate plant densities, which, in turn, maximized pollinator visitation and seed set. Reduced fecundity at plant densities that were higher than optimal was related with higher soil salinity, shorter raceme length, and lower amounts of floral volatiles in T. chinensis. Regular freshwater restoration showed a positive impact on pollinator visitation and fecundity of T. chinensis. Consequently, positive plant-soil feedback enhanced floral traits and pollinator visitation, which are related with the optimal fecundity of T. chinensis at intermediate densities. Thus, the density of T. chinensis should be managed in restoration projects of the T. chinensis population through artificial transplantation. The transplanting density of T. chinensis should be strictly controlled in harsh environment, while this requirement could be relaxed in mild conditions.

Fecundity regulation in a spatial birth-and-death process

We study a Markov birth-and-death process on a space of locally finite configurations, which describes an ecological model with a density-dependent fecundity regulation mechanism. We establish existence and uniqueness of this process and analyze its properties. In particular, we show global time-space boundedness of the population density and, using a constructed Foster–Lyapunov-type function, we study return times to certain level sets of tempered configurations. We also find sufficient conditions that the degenerate invariant distribution is unique for the considered process.

Reproductive inequalities in the acanthocephalan Corynosoma cetaceum: looking beyond \u2018crowding\u2019 effects

BackgroundAt present, much research effort has been devoted to investigate overall (“average”) responses of parasite populations to specific factors, e.g. density-dependence in fecundity or mortality. However, studies on parasite populations usually pay little attention to individual variation (“inequality”) in reproductive success. A previous study on the acanthocephalan Corynosoma cetaceum in franciscana dolphins, Pontoporia blainvillei, revealed no overall intensity-dependent, or microhabitat effects, on mass and fecundity of worms. In this study, we investigated whether the same factors could influence mass inequalities for this species of acanthocephalan.MethodsA total of 10,138 specimens of C. cetaceum were collected from 10 franciscanas accidentally caught in Buenos Aires Province between 1988 and 1990. To investigate mass inequalities, all the specimens were sexed, and females were classified according to their developmental stage and weighed. Additionally, the relationship between biomass and fecundity (estimated as the number of acanthors) was investigated for some females. Inequalities in fecundity and biomass were assessed using standard methods, i.e. the Lorenz curve and the Gini coefficient (G).ResultsWe found a modest, but highly significant linear relationship between mass and fecundity. The G was very low (0.314) compared with that from other helminth species. G values were significantly lower in gravid females, which presumably exhibit a slow rate of growth. Also, G values significantly increased with total intensity, but only for gravid females, and the effect was more predictable considering only the intensity of gravid females.ConclusionsApparently, high intensity infections increase inequality of reproducing females without producing “crowding” effects. Although several processes could generate this pattern, gravid females, at higher intensities, expanded their distribution and occupied gut chambers with contrasting environmental conditions, which might result in greater variability in body size. The observed inequalities are not expected to strongly influence the population genetics of C. cetaceum, but they reveal subtle individual effects beyond an overall population impact.

Continuous Structured Population Models for Daphnia magna.

We continue our efforts in modeling Daphnia magna, a species of water flea, by proposing a continuously structured population model incorporating density-dependent and density-independent fecundity and mortality rates. We collected new individual-level data to parameterize the individual demographics relating food availability and individual daphnid growth. Our model is fit to experimental data using the generalized least-squares framework, and we use cross-validation and Akaike Information Criteria to select hyper-parameters. We present our confidence intervals on parameter estimates.

Role of parasite transmission in promoting inbreeding: I. Infection intensities drive individual parasite selfing rates.

Among parasitic organisms, inbreeding has been implicated as a potential driver of host-parasite co-evolution, drug-resistance evolution and parasite diversification. Yet, fundamental topics about how parasite life histories impact inbreeding remain to be addressed. In particular, there are no direct selfing-rate estimates for hermaphroditic parasites in nature. Our objectives were to elucidate the mating system of a parasitic flatworm in nature and to understand how aspects of parasite transmission could influence the selfing rates of individual parasites. If there is random mating within hosts, the selfing rates of individual parasites would be an inverse power function of their infection intensities. We tested whether selfing rates deviated from within-host random mating expectations with the tapeworm Oochoristica javaensis. In doing so, we generated, for the first time in nature, individual selfing-rate estimates of a hermaphroditic flatworm parasite. There was a mixed-mating system where tapeworms self-mated more than expected with random mating. Nevertheless, individual selfing rates still had a significant inverse power relationship to infection intensities. The significance of this finding is that the distribution of parasite infection intensities among hosts, an emergent property of the transmission process, can be a key driver in shaping the primary mating system, and hence the level of inbreeding in the parasite population. Moreover, we demonstrated how potential population selfing rates can be estimated using the predicted relationship of individual selfing rates to intensities and showed how the distribution of parasites among hosts can indirectly influence the primary mating system when there is density-dependent fecundity.

Corrigendum

Corrigendum

Sensitivity and elasticity analysis of a Lur’e system used to model a population subject to density-dependent reproduction

Sensitivity and elasticity analyzes have become central to the analysis of models in population biology and ecology. While much work has been done applying sensitivity and elasticity analysis to study density-independent (linear) matrix and integral projection models, little work has been done to study the sensitivity and elasticity of density-dependent models, especially integral projection models. In this paper we derive sensitivity and elasticity formulas for the equilibrium population n* of a structured population modeled by a Lur’e system, which consists of a linear system plus a nonlinearity modeling density-dependent fecundity. Sensitivity and elasticity formulas are easy to interpret ecologically, and we apply these formulas to published models for Chinook Salmon and Platte thistle (Cirsium canescens). In the C. canescens example we show that models with identical equilibrium populations can have sensitivities that are an order-of-magnitude apart, depending on the functional form for the nonlinearity.

Refined stratified-worm-burden models that incorporate specific biological features of human and snail hosts provide better estimates of Schistosoma diagnosis, transmission, and control.

BackgroundSchistosoma parasites sustain a complex transmission process that cycles between a definitive human host, two free-swimming larval stages, and an intermediate snail host. Multiple factors modify their transmission and affect their control, including heterogeneity in host populations and environment, the aggregated distribution of human worm burdens, and features of parasite reproduction and host snail biology. Because these factors serve to enhance local transmission, their inclusion is important in attempting accurate quantitative prediction of the outcomes of schistosomiasis control programs. However, their inclusion raises many mathematical and computational challenges. To address these, we have recently developed a tractable stratified worm burden (SWB) model that occupies an intermediate place between simpler deterministic mean worm burden models and the very computationally-intensive, autonomous agent models.MethodsTo refine the accuracy of model predictions, we modified an earlier version of the SWB by incorporating factors representing essential in-host biology (parasite mating, aggregation, density-dependent fecundity, and random egg-release) into demographically structured host communities. We also revised the snail component of the transmission model to reflect a saturable form of human-to-snail transmission. The new model allowed us to realistically simulate overdispersed egg-test results observed in individual-level field data. We further developed a Bayesian-type calibration methodology that accounted for model and data uncertainties.ResultsThe new model methodology was applied to multi-year, individual-level field data on S. haematobium infections in coastal Kenya. We successfully derived age-specific estimates of worm burden distributions and worm fecundity and crowding functions for children and adults. Estimates from the new SWB model were compared with those from the older, simpler SWB with some substantial differences noted. We validated our new SWB estimates in prediction of drug treatment-based control outcomes for a typical Kenyan community.ConclusionsThe new version of the SWB model provides a better tool to predict the outcomes of ongoing schistosomiasis control programs. It reflects parasite features that augment and perpetuate transmission, while it also readily incorporates differences in diagnostic testing and human sub-population differences in treatment coverage. Once extended to other Schistosoma species and transmission environments, it will provide a useful and efficient tool for planning control and elimination strategies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1681-4) contains supplementary material, which is available to authorized users.

Negative frequency-dependent selection or alternative reproductive tactics: maintenance of female polymorphism in natural populations

BackgroundSex-limited polymorphisms have long intrigued evolutionary biologists and have been the subject of long-standing debates. The coexistence of multiple male and/or female morphs is widely believed to be maintained through negative frequency-dependent selection imposed by social interactions. However, remarkably few empirical studies have evaluated how social interactions, morph frequencies and fitness parameters relate to one another under natural conditions. Here, we test two hypotheses proposed to explain the maintenance of a female polymorphism in a species with extreme geographical variation in morph frequencies. We first elucidate how fecundity traits of the morphs vary in relation to the frequencies and densities of males and female morphs in multiple sites over multiple years. Second, we evaluate whether the two female morphs differ in resource allocation among fecundity traits, indicating alternative tactics to maximize reproductive output.ResultsWe present some of the first empirical evidence collected under natural conditions that egg number and clutch mass was higher in the rarer female morph. This morph-specific fecundity advantage gradually switched with the population morph frequency. Our results further indicate that all investigated fecundity traits are negatively affected by relative male density (i.e. operational sex ratio), which confirms male harassment as selective agent. Finally, we show a clear trade-off between qualitative (egg mass) and quantitative (egg number) fecundity traits. This trade-off, however, is not morph-specific.ConclusionOur reported frequency- and density-dependent fecundity patterns are consistent with the hypothesis that the polymorphism is driven by a conflict between sexes over optimal mating rate, with costly male sexual harassment driving negative frequency-dependent selection on morph fecundity.

Higher survival at low density counteracts lower fecundity to obviate Allee effects in a perennial plant

Summary 1. At low densities, plants may produce fewer seeds than at high densities, due to reduced pollinator visits or reduced receipt of compatible pollen. In principle, lower seed production could lead to an Allee effect (a decline in the population growth rate with declining density), which, if severe, could cause the population to decline to extinction when recruitment is insufficient to replace dying individuals. 2. However, plants at low density may also grow or survive better than those at high densities, due to release from intraspecific competition. Few studies have examined the combined effects on plant population dynamics of positive density dependence in fecundity and negative density dependence in survival or growth. 3. To do so, we parameterized a density-dependent population projection matrix model for Piriqueta caroliniana, a widespread sandhill plant that often occurs at low density, using data from a demographic study of natural populations and from field experiments testing for effects of density on fecundity, growth and survival. We then tested for an Allee effect when various combinations of positive and negative density dependence are included in the model. 4. The model exhibits an Allee effect when only positive density dependence in fecundity is included, but when negative density dependence in survival is added, the Allee effect disappears. Thus the increase in survival as density declines overwhelms the effect of declining fecundity on the population growth rate. This occurs because survival has a greater influence on the population growth rate than fecundity (as measured by vital rate elasticities), increasingly so as density declines. 5. Synthesis. Our results caution against considering only declining fecundity at low density. We show that survival may increase at low densities, and will often have stronger effects on population growth than does fecundity, especially for perennial plants. Of course, reproductive failure and population collapse must eventually occur as the density of any non-selfing species approaches zero, or if pollinator abundance falls to low levels.

Impact of Environmental Covariation in Growth and Mortality on Evolving Maturation Reaction Norms

Maturation age and size have important fitness consequences through their effects on survival probabilities and body sizes. The evolution of maturation reaction norms in response to environmental covariation in growth and mortality is therefore a key subject of life-history theory. The eco-evolutionary model we present and analyze here incorporates critical features that earlier studies of evolving maturation reaction norms have often neglected: the trade-off between growth and reproduction, source-sink population structure, and population regulation through density-dependent growth and fecundity. We report the following findings. First, the evolutionarily optimal age at maturation can be decomposed into the sum of a density-dependent and a density-independent component. These components measure, respectively, the hypothetical negative age at which an individual's length would be 0 and the delay in maturation relative to this offset. Second, along any growth trajectory, individuals mature earlier when mortality is higher. This allows us to deduce, third, how the shapes of evolutionarily optimal maturation reaction norms depend on the covariation between growth and mortality (positive or negative, linear or curvilinear, and deterministic or probabilistic). Providing eco-evolutionary explanations for many alternative reaction-norm shapes, our results appear to be in good agreement with current empirical knowledge on maturation dynamics.

Food supply influences offspring provisioning but not density-dependent fecundity in a marine fish

Replenishment of many marine populations occurs through the entry of juveniles to adult populations following a pelagic larval stage. Because mortality during the pelagic stage is thought to be high and density independent, larval abundance and traits of individual larvae should have strong effects on overall population dynamics in marine organisms. Surprisingly, few experiments have tested how localized interactions among breeding adults affect the quantity and phenotypic traits of larvae they produce. Here I experimentally test for the influence of food competition, mate limitation, and population density on somatic growth, fecundity, and offspring provisioning (larval length and energy reserves) in a planktivorous, territorial coral reef damselfish, Stegastes partitus. I manipulated food supply and adult S. partitus density on isolated patch reefs in the Bahamas and also made behavioral observations of S. partitus occurring on nearby natural reefs at a range of population densities. On the experimental reefs, females experienced density-dependent growth and fecundity; male reproductive success was density dependent, but male growth was not. Density-dependent growth and reproduction were not moderated by food supplementation, and density-dependent reproduction was not influenced by mate availability. On natural reefs, the frequency of aggressive interactions, particularly involving females, increased with population density, implicating aggression-related energetic costs as the source of both forms of density dependence in the experiments. Food supplementation increased female somatic growth and larval energy reserves, suggesting that females allocated surplus energy to future reproductive potential and enhanced offspring quality. Neither experimental treatment affected larval length. By altering patterns of reproduction, the interplay between spatial variation in food availability and population density may drive population dynamics in a broad range of benthic marine organisms.

Non‐independence of demographic parameters: positive density‐dependent fecundity in eagles

Summary 1 Using information on the Donana population of Spanish imperial eagles Aquila adalberti from 1959 to 2004, we present strong empirical support to theoretical models on the regulation of population trajectories by the relationships between breeder mortality and floater availability. 2 During the study period, the eagle population showed three distinct phases: (i) a population increase with negative density-dependent fecundity; (ii) a period of stability without any relationship between density and fecundity; and (iii) a population decrease with a positive relationship between density and fecundity. 3 A dramatic increase in annual adult mortality due to an increase in poisoning in hunting areas surrounding the Donana National Park was recorded. The use of poison against generalist predators accounted for more than 54% of the total number of breeding eagles found dead since 1990, increasing annual adult mortality from 6·07 to 12·01%. 4 This high mortality reduced the population annually by 6% during the 1992–2004 period. Also, the population changed from a negative to a positive relationship between density and fecundity (Allee effect). These trends made the population approach extinction due to the double effect of increasing breeder mortality and low availability of floaters. 5 A supplementary feeding programme established in 1990 did not increase fecundity because it was a consequence of high adult mortality rather than low food availability. The high mortality of adults increased the risk of breeding failure and also decreased the availability of floaters, decreasing the likelihood of mate substitution. 6 Synthesis and applications. The main target in species conservation management plans is often a single demographic parameter (typically, fecundity in raptor populations). Our research demonstrates, however, that demographic parameters must not be considered as independent variables when formulating management programmes. The essential relationship between adult mortality and the availability of floaters must be better understood to avoid the risk of implementing inefficient management strategies. Although fecundity was low in this eagle population, it was a result of high adult mortality rates. Consequently, management must aim to correct the unusual mortality and recover the floater population.