Density-dependent Reproduction Research Articles

Is negative density-dependent reproduction regulated by density-induced stress in root voles? Two field experiments.

Density dependence in reproduction plays an important role in stabilizing population dynamics via immediate negative feedback from population density to reproductive output. Although previous studies have shown that negative density‐dependent reproduction is associated with strong spacing behavior and social interaction between individuals, the proximal mechanism for generating negative density‐dependent reproduction remains unclear. In this study, we investigated the effects of density‐induced stress on reproduction in root voles. Enclosed founder populations were established by introducing 6 (low density) and 30 (high density) adults per sex into per enclosure (four enclosures per density in total) during the breeding season from April to July 2012 and from May to August 2015. Fecal corticosterone metabolite (FCM) levels, reproductive traits (recruitment rate and the proportion of reproductively active individuals), and founder population numbers were measured following repeated live trapping in both years. The number of founders was negatively associated with recruitment rates and the proportion of reproductively active individuals, displaying a negative density‐dependent reproduction. FCM level was positively associated with the number of founders. The number of founder females directly affected the proportion of reproductive females, and directly and indirectly through their FCM levels affected the recruitment rate; the effect of the number of male founders on the proportion of reproductive males was mediated by their FCM level. Our results showed that density‐induced stress negatively affected reproductive traits and that density‐induced stress is one ecological factor generating negative density‐dependent reproduction.

Measuring Multiple Demographic Rates in Two Populations of Northern Bobwhite

Demographic rates of northern bobwhite (Colinus virginianus; hereafter, bobwhite) may vary spatially and temporally, and understanding the significance of these individual rates to population performance is critically important to bobwhite management. We present descriptive evidence from 2 populations that were simultaneously monitored from 2015–2020 that suggests different demographic rates can be more important to population performance than other demographic rates within the same region. Our objective was to understand the relative importance of various demographic rates to population performance in separate and seemingly stable populations. We monitored bobwhite seasonal survival and reproductive demographics on 2,475 bobwhites via radio-telemetry and estimated fall density using fall covey counts. Both sites maintained high densities (i.e., ≥3.45 birds/hectare) and remained relatively stable throughout the study period. On one site in the Red Hills region near Monticello, Florida, USA, bobwhite experienced comparatively low seasonal survival, but higher reproduction, including more frequent multiple-brood production. One hundred and twenty-nine kilometers away on a study site near Albany, Georgia, USA, bobwhite demonstrated consistently higher survival and lower reproductive output, including less multiple-brooding compared to the Red Hills population. This suggests, at a minimum, that compensatory or density-dependent reproduction may be occurring in these populations and regional population dynamics can vary locally even among stable populations.

Extinction time of the logistic process

AbstractThe logistic birth and death process is perhaps the simplest stochastic population model that has both density-dependent reproduction and a phase transition, and a lot can be learned about the process by studying its extinction time, $\tau_n$ , as a function of system size n. A number of existing results describe the scaling of $\tau_n$ as $n\to\infty$ for various choices of reproductive rate $r_n$ and initial population $X_n(0)$ as a function of n. We collect and complete this picture, obtaining a complete classification of all sequences $(r_n)$ and $(X_n(0))$ for which there exist rescaling parameters $(s_n)$ and $(t_n)$ such that $(\tau_n-t_n)/s_n$ converges in distribution as $n\to\infty$ , and identifying the limits in each case.

Anticipated effects of abiotic environmental change on intraspecific social interactions.

Social interactions are ubiquitous across the animal kingdom. A variety of ecological and evolutionary processes are dependent on social interactions, such as movement, disease spread, information transmission, and density-dependent reproduction and survival. Social interactions, like any behaviour, are context dependent, varying with environmental conditions. Currently, environments are changing rapidly across multiple dimensions, becoming warmer and more variable, while habitats are increasingly fragmented and contaminated with pollutants. Social interactions are expected to change in response to these stressors and to continue to change into the future. However, a comprehensive understanding of the form and magnitude of the effects of these environmental changes on social interactions is currently lacking. Focusing on four major forms of rapid environmental change currently occurring, we review how these changing environmental gradients are expected to have immediate effects on social interactions such as communication, agonistic behaviours, and group formation, which will thereby induce changes in social organisation including mating systems, dominance hierarchies, and collective behaviour. Our review covers intraspecific variation in social interactions across environments, including studies in both the wild and in laboratory settings, and across a range of taxa. The expected responses of social behaviour to environmental change are diverse, but we identify several general themes. First, very dry, variable, fragmented, or polluted environments are likely to destabilise existing social systems. This occurs as these conditions limit the energy available for complex social interactions and affect dissimilar phenotypes differently. Second, a given environmental change can lead to opposite responses in social behaviour, and the direction of the response often hinges on the natural history of the organism in question. Third, our review highlights the fact that changes in environmental factors are not occurring in isolation: multiple factors are changing simultaneously, which may have antagonistic or synergistic effects, and more work should be done to understand these combined effects. We close by identifying methodological and analytical techniques that might help to study the response of social interactions to changing environments, highlight consistent patterns among taxa, and predict subsequent evolutionary change. We expect that the changes in social interactions that we document here will have consequences for individuals, groups, and for the ecology and evolution of populations, and therefore warrant a central place in the study of animal populations, particularly in an era of rapid environmental change.

Beyond the trends: The need to understand multiannual dynamics in aquatic ecosystems

Beyond the trends: The need to understand multiannual dynamics in aquatic ecosystems

Coupling the Yoccoz–Birkeland population model with price dynamics: chaotic livestock commodities market cycles

We propose a new model for the time evolution of livestock commodities prices which exhibits endogenous deterministic stochastic behaviour. The model is based on the Yoccoz–Birkeland integral equation, a model first developed for studying the time-evolution of single species with high average fertility, a relatively short mating season and density-dependent reproduction rates. This equation is then coupled with a differential equation describing the price of a livestock commodity driven by the unbalance between its demand and supply. At its birth the cattle population is split into two parts: reproducing females and cattle for butchery. The relative amount of the two is determined by the spot price of the meat. We prove the existence of an attractor (theorem A) and of a non-trivial periodic solution (theorem B) and we investigate numerically the properties of the attractor: the strange attractor existing for the original Yoccoz–Birkeland model is persistent but its chaotic behaviour depends also on the time evolution of the price in an essential way.

Conservation strategy for the sea cucumber (Isostichopus badionotus) fishery in Cuba

The biological characteristics of the sea cucumber, Isostichopus badionotus (Selenka, 1867), which include slow movement, late sexual maturity, density-dependent reproduction, and a low recruitment rate, make it vulnerable to overexploitation. Since 1999, Cuba has been harvesting this species, and maintaining this activity at a profitable level. Of the six fishing zones in which the species is caught, the yield in the southern Camaguey zone in the southeast region produces the greatest biomass. Fisheries- dependent and -independent indices were combined to evaluate status and develop strategies for fisheries management off the southern coast of Camaguey. This entailed establishing quantitative benchmarks using catch and effort data from the Santa Cruz del Sur enterprise for the period 2003–2014. There was a reduction in sea cucumber biomass and abundance during this period, and catch rates have been above maximum sustainable yield for the last 3 yrs. We propose that the quotas based on 15%–25% of the fishing potential be reduced to 10%. We recommend applying quotas only in those places where the densities are higher than 0.45 cucumbers per 100 m2 and closing fisheries where the density is lower. The application of the dynamic surplus production model, together with independent indicators of the fishery, are useful tools for management of the species and should be applied to the sea cucumber fisheries around the island.

Strong territoriality and weak density‐dependent reproduction in Lesser Spotted Eagles Clanga pomarina

We assessed whether populations of the migratory, philopatric, territorial Lesser Spotted Eagle Clanga pomarina are regulated through territoriality and density‐dependent reproduction rates. In the north‐western part of the distribution to the east of the Baltic Sea, territories were spaced regularly and consistently between years. We did not find strong support for an improvement of reproductive output with an increase in distance to the nearest neighbour or with a decrease of the number of conspecifics within a 2‐km radius around nest‐sites. Eagles differed greatly in territory spacing across three studied geographical areas, but breeding performance did not follow the same pattern. Performance of birds at the northern limit of their distribution did not differ from that in an adjacent southern area. Moreover, breeding performance fluctuated synchronously across different geographical areas, perhaps indicating climatic spatial autocorrelation or trophic interactions with synchronous vole population fluctuations over large areas. Our data suggest population regulation through territoriality but do not identify density‐dependent breeding performance in this internationally protected raptor. Population density may be a good criterion for the selection of priority sites for conservation, but synchronous fluctuations in reproductive success over large geographical areas suggest that habitat conservation should similarly be focused at large scales.

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.

Hierarchical analysis of taxonomic variation in intraspecific competition across fish species.

The nature and intensity of intraspecific competition can vary greatly among taxa, yet similarities in these interactions can lead to similar population dynamics among related organisms. Variation along the spectrum of intraspecific competition, with contest and scramble competition as endpoints, leads to vastly different responses to population density. Here we investigated the diversity of intraspecific competition among fish species, predicting that functional forms of density-dependent reproduction would be conserved in related taxa. Using a hierarchical model that links stock-recruitment parameters among populations, species, and orders, we found that the strength of overcompensation, and therefore the type of intraspecific competition, is tightly clustered within taxonomic groupings, as species within an order share similar degrees of compensation. Specifically, species within the orders Salmoniformes and Pleuronectiformes exhibited density dependence indicative of scramble competition (overcompensation) while the orders Clupeiformes, Gadiformes, Perciformes, and Scorpaeniformes exhibited dynamics consistent with contest competition (compensation). Maximum potential recruitment also varied among orders, but with less clustering across species. We also tested whether stock-recruitment parameters correlated with maximum body length among species, but found no strong relationship. Our results suggest that much of the variation in the form of density-dependent reproduction among fish species may be predicted taxonomically due to evolved life history traits and reproductive behaviors.

Positive density-dependent reproduction regulated by local kinship and size in an understorey tropical tree.

Global pollinator declines and continued habitat fragmentation highlight the critical need to understand reproduction and gene flow across plant populations. Plant size, conspecific density and local kinship (i.e. neighbourhood genetic relatedness) have been proposed as important mechanisms influencing the reproductive success of flowering plants, but have rarely been simultaneously investigated. We conducted this study on a continuous population of the understorey tree Miconia affinis in the Forest Dynamics Plot on Barro Colorado Island in central Panama. We used spatial, reproductive and population genetic data to investigate the effects of tree size, conspecific neighbourhood density and local kinship on maternal and paternal reproductive success. We used a Bayesian framework to simultaneously model the effects of our explanatory variables on the mean and variance of maternal viable seed set and siring success. Our results reveal that large trees had lower proportions of viable seeds in their fruits but sired more seeds. We documented differential effects of neighbourhood density and local kinship on both maternal and paternal reproductive components. Trees in more dense neighbourhoods produced on average more viable seeds, although this positive density effect was influenced by variance-inflation with increasing local kinship. Neighbourhood density did not have significant effects on siring success. This study is one of the first to reveal an interaction among tree size, conspecific density and local kinship as critical factors differentially influencing maternal and paternal reproductive success. We show that both maternal and paternal reproductive success should be evaluated to determine the population-level and individual traits most essential for plant reproduction. In addition to conserving large trees, we suggest the inclusion of small trees and the conservation of dense patches with low kinship as potential strategies for strengthening the reproductive status of tropical trees.

Effects of sexual reproduction of the inferior competitor Brachionus calyciflorus on its fitness against Brachionus angularis

Sexual reproduction adversely affects the population growth of cyclic parthenogenetic animals. The density-dependent sexual reproduction of a superior competitor could mediate the coexistence. However, the cost of sex may make the inferior competitor more vulnerable. To investigate the effect of sexual reproduction on the inferior competitor, we experimentally paired the competition of one Brachionus angularis clone against three Brachionus calyciflorus clones. One of the B. calyciflorus clones showed a low propensity for sexual reproduction, while the other two showed high propensities. The results show that all B. calyciflorus clones were excluded in the competition for resources at low food level. The increased food level promoted the competition persistence, but the clones did not show a clear pattern. Both the cumulative population density and resting egg production increased with the food level. The cumulative population density decreased with the mixis investment, while the resting egg production increased with the mixis investment. A trade-off between the population growth and sexual reproduction was observed in this research. The results indicate that although higher mixis investment resulted in a lower population density, it would not determinately accelerate the exclusion process of the inferior competitor. On the contrary, higher mixis investment promoted resting egg production before being excluded and thus promised a long-term benefit. In conclusion, our results suggest that mixis investment, to some extent, favored the excluded inferior competitor under fierce competition or some other adverse conditions.

Effect of Dedifferentiation on Time to Mutation Acquisition in Stem Cell-Driven Cancers

Accumulating evidence suggests that many tumors have a hierarchical organization, with the bulk of the tumor composed of relatively differentiated short-lived progenitor cells that are maintained by a small population of undifferentiated long-lived cancer stem cells. It is unclear, however, whether cancer stem cells originate from normal stem cells or from dedifferentiated progenitor cells. To address this, we mathematically modeled the effect of dedifferentiation on carcinogenesis. We considered a hybrid stochastic-deterministic model of mutation accumulation in both stem cells and progenitors, including dedifferentiation of progenitor cells to a stem cell-like state. We performed exact computer simulations of the emergence of tumor subpopulations with two mutations, and we derived semi-analytical estimates for the waiting time distribution to fixation. Our results suggest that dedifferentiation may play an important role in carcinogenesis, depending on how stem cell homeostasis is maintained. If the stem cell population size is held strictly constant (due to all divisions being asymmetric), we found that dedifferentiation acts like a positive selective force in the stem cell population and thus speeds carcinogenesis. If the stem cell population size is allowed to vary stochastically with density-dependent reproduction rates (allowing both symmetric and asymmetric divisions), we found that dedifferentiation beyond a critical threshold leads to exponential growth of the stem cell population. Thus, dedifferentiation may play a crucial role, the common modeling assumption of constant stem cell population size may not be adequate, and further progress in understanding carcinogenesis demands a more detailed mechanistic understanding of stem cell homeostasis.

Population dynamics of stage-structured sequential hermaphrodites

The population dynamics of sequential hermaphrodites were investigated using three-stage matrix population models. A simple hermaphrodite model with density-dependent reproduction exhibited monotonic asymptotic growth or decline depending on parameter values. After incorporating density-dependent survival and sex change, the model was parameterized for red grouper (Epinephelus morio), which exhibit protogynous life history, and the model was used to investigate the effects of fishing mortality on offspring production and population density. The sensitivity of offspring production to fishing mortality was less when sex change depended on the relative density of female to male (ratio-dependent sex change) or male density alone compared with either female density alone or no density dependence. Although strong ratio-dependent sex change also resulted in less sensitivity of equilibrium abundance to fishing mortality, it caused the population to fluctuate chaotically. This suggests that the strength and type of density dependence on sex change affect reproductive output, equilibrium abundance, and the types of dynamics exhibited.

Density‐dependent life history and the dynamics of small populations

1. Small population dynamics depend importantly on the strength and shape of density dependence. Unfortunately, the lack of reliable life-history data often prevents to make accurate demographic predictions for populations regulated by density dependence. 2. We created a gradient from low to high densities in small experimental populations of common lizards (Zootoca vivipara) and investigated the shape and strength of the density dependence of life-history traits during a yearly cycle. We then analysed stochastic population dynamics using one-sex and two-sex age-structured matrix models. 3. Body growth and reproductive performances decreased with density, yearling and adult survival and body size at birth were density-independent, and juvenile survival increased with density. The density dependence of reproduction was partly explained by positive effects of body size on age at first reproduction and clutch size. 4. Parturition date decreased with density in sparse populations and then increased, providing one of the first empirical evidence of a component Allee effect in the phenology of reproduction. 5. Population growth rate (λ) was most affected by variations in juvenile and yearling survival. However, density at equilibrium was most affected by juvenile access to reproduction and yearling clutch size. 6. Stochastic simulations revealed that negative density dependence buffers the effects of initial density on extinction probability, has positive effects on the persistence of sparse populations and interacts with sex ratio fluctuations to shape extinction dynamics. 7. This study demonstrates that negative density dependence modifies the dynamics of small populations and should be investigated together with Allee effects to predict extinction risks.

Harvesting, census timing and “hidden” hydra effects

Population control in some form of harvesting might be expected to reduce population size, but quite the opposite can happen due to the hydra effect. This phenomenon describes an increase in population size with increased mortality. One mechanism causing hydra effects is the temporal separation of (i) harvesting and (ii) density-dependent reproduction. Here we consider discrete-time models of these two processes. It is commonly believed that harvesting needs to precede reproduction for a hydra effect to occur. We show that, by contrast, hydra effects also take place for harvest after reproduction. Due to the timing of population census, however, the hydra effect will not be measured and thus remains ‘hidden’. As a consequence, managers may miss out on the opportunity to increase both the yield and the remaining stock of renewable resources. If harvesting aims at controlling pest species, management interventions may backfire in the sense that the pest increases rather than decreases—and, to make things even worse, this may actually go unnoticed. To remedy these undesirable consequences, we propose a modelling framework that can reveal hidden hydra effects. Our results are based on rigorous mathematical proofs that the order of two events does not matter for standard harvesting/hunting strategies.

Harvesting and predation of a sex- and age-structured population

We study a discrete-time system of equations for a structured ungulate population exploited by human harvesting or a dynamic predator. The population is divided into juveniles, and female and male adults. Harvesting is concentrated on adults (trophy hunting of males or population control measures on females), whereas predation occurs in juveniles. Though the model consists of four nonlinear equations, we find explicit expressions for the steady states. We use these explicit expressions to investigate harvesting rates that allow population persistence, rates that ensure population control, and optimal harvesting efforts. Several reductions of complexity allow for a detailed analysis of the dynamics of the model. Most notably, we find that even compensatory density dependence can lead to a period doubling bifurcation, that the model does not support consumer–resource cycles, and that an Allee effect can emerge from the interplay of stage-specific predation and density-dependent prey reproduction.

LIFE HISTORY, HABITAT SATURATION AND THE EVOLUTION OF FECUNDITY AND SURVIVAL ALTRUISM

Hamilton's rule provides a general description of the conditions for the evolution of altruism. But altruism can take different forms depending on which life-history trait is affected by the helping behavior (fecundity vs. survival helping). In particular, these different forms of helping may have very different demographic consequences, which may feed back on evolution. We examine the interplay between various forms of helping and demography in viscous populations with empty sites. A key component of our analysis is the local density of empty sites experienced by a focal individual, which provides a measure of habitat saturation. Habitat saturation is shown to have contrasting effects depending on (1) whether the physiological costs and benefits of helping affect fecundity, survival or both; and (2) whether the costs of helping are paid in a density-dependent or density-independent manner. For a given level of habitat saturation and with density-dependent reproduction, we find that the conditions for the evolution of helping should be more favorable in the survival altruism life cycle with a cost on fecundity, and more stringent in the fecundity altruism life cycle with a cost on survival. More generally, our analysis stresses the importance of taking into account the feedback between population demography, life history, and kin selection when investigating the selective pressures on altruism.

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.

Experimental evidence for density‐dependent reproduction in a cooperatively breeding passerine

Temporal variation in survival, fecundity, and dispersal rates is associated with density-dependent and density-independent processes. Stable natural populations are expected to be regulated by density-dependent factors. However, detecting this by investigating natural variation in density is difficult because density-dependent and independent factors affecting population dynamics may covary. Therefore, experiments are needed to assess the density dependence of demographic rates. In this study, we investigate the effect of density on demographic rates of the Seychelles Warbler (Acrocephalus sechellensis). This species, endemic to a few islands in the Indian Ocean, went through a severe population bottleneck in the middle of the last century, with only approximately 30 individuals left on one small island, but has since recovered. Our monitoring shows that since reaching the island's carrying capacity, population density has remained stable. However, we detected neither density-dependent reproduction nor survival on the basis of natural density variation during this stable period. For conservation reasons, new populations have been established by transferring birds to nearby suitable islands. Using the change of numbers during the process of saturation as a natural experiment, we investigated whether we can detect regulation of numbers via density-dependent survival and reproduction within these new populations. We found that populations were mainly regulated by density-dependent reproduction, and not survival. Variation in density between islands can be explained by food abundance, measured as insect density. Islands with the highest insect densities also had the highest bird densities and the largest breeding groups. Consequently, we suggest that the density-dependent effect on reproduction is caused by competition for food.