Density-dependent Population Dynamics Research Articles

Testing Density-Dependent and Path-Dependent Population Dynamics in Greece with Spatial Quantile and Geographically Weighted Regressions

Regional variability in the spatial distribution of resident population and across-country density divides have consolidated heterogeneous demographic patterns at the base of modern urban systems in Europe. Although economic, historical, institutional, and cultural factors have demonstrated to affect the spatial distribution of resident population, density-dependence and path-dependence are mechanisms persistently shaping demographic dynamics at both local and regional scale. Analysis of density-dependent patterns of population growth (and decline) over sufficiently long time intervals allows a refined comprehension of socioeconomic processes underlying demographic divides. Despite a long settlement history, empirical investigation of the role of density-dependence in the long-term evolution of human populations along urban–rural gradients is relatively scarce especially in Mediterranean countries. The present study performs a comparative analysis of population distribution in 1033 Greek municipalities identifying (and testing the significance of) density-dependent and path-dependent mechanisms of population growth between 1961 and 2011, using spatially implicit and explicit econometric approaches. Results highlight a positive impact of density on population growth where settlements are concentrated. Assuming goodness of fit of the tested models as a proxy of density-dependence, the empirical findings clarify how density-dependent mechanisms were not significant all over the study period, being instead associated with specific phases of the city life cycle—basically urbanization with population concentrating in central locations. Density-dependence was less intense with suburbanization and counter-urbanization—when population sprawled over larger areas at medium–low density. An improved understanding of density-dependent and path-dependent mechanisms of population growth contributes to rethink spatial planning, regional development strategies, and socio-demographic policies adapting to heterogeneous (and rapidly changing) local contexts.

Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management

AbstractExtensive applications of fishery stock enhancement worldwide bring up broad concerns about its negative effects, creating a pivotal need for science‐based assessment and planning of enhancement strategies. However, the lack of mechanistic understanding of enhanced population dynamics, particularly the density‐dependent processes, leads to compromise in model development and limits the capacity in predicting enhancement effects. Here, we developed an individual‐based model based on dynamic energy budget theory and full life‐history processes, to understand the mechanism of density dependence in population dynamics that emerge from individual‐level processes. We demonstrated the utility of the model framework by applying it to an extensively enhanced species, Chinese prawn (Fenneropenaeus chinensis, Penaeidae). The model could yield projections reflecting the observed trajectory of population biomass and yields. The model also delineated the key effects of density dependence on the vital rates of growth, fecundity and starvation mortality. Regarding the manifold effects of stock enhancement, we demonstrated a dampened shape in population biomass and yields with increasing magnitude of enhancement, and trade‐offs between the ecological and economic objectives, that is, pursuing high benefit might compromise the wild population without proper management. Furthermore, we illustrated the possibility of combining stock enhancement and harvest regulation in promoting population recovery while maintaining fisheries yields. We highlight the potential of the proposed model for understanding density dependence in enhancement programme, and for designing integrated management strategies. The approach developed herein may serve as a general approach to assess the population dynamics in stock enhancement and inform enhancement management.

Inferring density-dependent population dynamics mechanisms through rate disambiguation for logistic birth-death processes.

Density dependence is important in the ecology and evolution of microbial and cancer cells. Typically, we can only measure net growth rates, but the underlying density-dependent mechanisms that give rise to the observed dynamics can manifest in birth processes, death processes, or both. Therefore, we utilize the mean and variance of cell number fluctuations to separately identify birth and death rates from time series that follow stochastic birth-death processes with logistic growth. Our nonparametric method provides a novel perspective on stochastic parameter identifiability, which we validate by analyzing the accuracy in terms of the discretization bin size. We apply our method to the scenario where a homogeneous cell population goes through three stages: (1) grows naturally to its carrying capacity, (2) is treated with a drug that reduces its carrying capacity, and (3) overcomes the drug effect to restore its original carrying capacity. In each stage, we disambiguate whether the dynamics occur through the birth process, death process, or some combination of the two, which contributes to understanding drug resistance mechanisms. In the case of limited sample sizes, we provide an alternative method based on maximum likelihood and solve a constrained nonlinear optimization problem to identify the most likely density dependence parameter for a given cell number time series. Our methods can be applied to other biological systems at different scales to disambiguate density-dependent mechanisms underlying the same net growth rate.

Aligning population models with data: Adaptive management for big game harvests

Models of population dynamics are a central piece for harvest management, allowing managers to evaluate alternative strategies and to identify uncertainty. Here we present a density-dependent population dynamics model that can be used in conjunction with adaptive management to optimize big game management, designed to use data commonly collected by state and provincial wildlife agencies. We review a case study for white-tailed deer (Odocoileus virginianus) in North Dakota, USA, where we evaluate how harvest composition and monitoring frequency affect the maximum sustainable yield (MSY). Data were obtained from winter aerial surveys and hunter questionnaires over six years between 2009 and 2019. Harvest composition moderately skewed towards antlered individuals (37.5% antlerless deer and 62.5% antlered deer, i.e., antlerless:antlered harvest ratio = 0.6) resulted in a harvest rate of 0.2, which translates to a long-term harvest that is more than double that obtained if the harvest composition matched the population composition. However, given environmental uncertainty, we recommend that managers adopt a harvest strategy that is at least 10–15% lower than the maximum sustainable yield to buffer against environmental variability. Despite the fact that contrasting monitoring schemes resulted in similar optimal harvest rates, we illustrated how adopting an adaptive harvest strategy (i.e., where harvests change with population size) affords lower risks of overexploitation than a static strategy in which populations are assessed only occasionally. An adaptive harvest strategy features resilience allowing harvested populations to return to equilibrium even after substantial perturbation events.

Density-dependent consequences of size-selective induced life-history changes to population fitness in medaka (Oryzias latipes)

There is an increasing concern about the potential for size-selective harvest to impair population persistence. Yet little is known about the relative contribution of the evolutionary (shifts in life history) and demographic effects (decreased population density and size truncation) of harvesting to changes in fitness. Using medaka (Oryzias latipes), we experimentally investigated the fitness consequences of antagonistic size-dependent selection under contrasted levels of density (low versus high) and size structure (uniform versus truncated). The size-dependent selection generated large- and small-breeder lines with fast growth and late maturity versus slow growth and early maturity life histories, respectively. A decrease in density had a positive effect on almost all fitness components, while size truncation only had a positive effect on fish growth. Small-breeder fish grew slower and had a greater probability of reproducing, at least when considering small-sized females. The number of larvae and juveniles did not differ between the two lines. Finally, the positive effect of decreased density on population asymptotic growth rate was less pronounced in small-breeders than in large-breeders of medaka. These findings stress the importance of considering the ramifications of fishing-adapted life history to population persistence in the light of density-dependent population dynamics.

Quantitative genetics of body size evolution on islands: an individual-based simulation approach.

According to the island rule, small-bodied vertebrates will tend to evolve larger body size on islands, whereas the opposite happens to large-bodied species. This controversial pattern has been studied at the macroecological and biogeographical scales, but new developments in quantitative evolutionary genetics now allow studying the island rule from a mechanistic perspective. Here, we develop a simulation approach based on an individual-based model to model body size change on islands as a progressive adaptation to a moving optimum, determined by density-dependent population dynamics. We applied the model to evaluate body size differentiation in the pigmy extinct hominin Homo floresiensis, showing that dwarfing may have occurred in only about 360 generations (95% CI ranging from 150 to 675 generations). This result agrees with reports suggesting rapid dwarfing of large mammals on islands, as well as with the recent discovery that small-sized hominins lived in Flores as early as 700 kyr ago. Our simulations illustrate the power of analysing ecological and evolutionary patterns from an explicit quantitative genetics perspective.

Density-dependent population model of effective release policy for Ayu fish

Ayu fish are actively released by many fishery cooperatives in Japan. These fish exhibit the density-dependent population dynamics within territorial competition. At low density, all fish can hold feeding territories. Once all territory sites are occupied, surplus fish become floaters. As the density further increases, all fish give up their own territories and then form a school. Territory holders are more valuable than floaters as a food source and a target of recreational fishing. Thus, the main purpose of the release of ayu is to increase the amount of territory holders. For these reasons, the release policy for ayu should take into account the influence of a change in density on the population dynamics. We develop a rate equation to describe the density-dependent population dynamics of ayu accompanied by the release of fingerlings. Our results indicate that the release of ayu demands unique policy different from other fish species. The release of ayu should be carried out so as not to break down territories due to the increase in density caused by released fingerlings. Specifically, the release policy that reduces the amount of fingerlings that are released at one time, and lengthens the time interval between each release, increases the catch of territory holders.

Spatial memory shapes density dependence in population dynamics.

Most population dynamics studies assume that individuals use space uniformly, and thus mix well spatially. In numerous species, however, individuals do not move randomly, but use spatial memory to visit renewable resource patches repeatedly. To understand the extent to which memory-based foraging movement may affect density-dependent population dynamics through its impact on competition, we developed a spatially explicit, individual-based movement model where reproduction and death are functions of foraging efficiency. We compared the dynamics of populations of with- and without-memory individuals. We showed that memory-based movement leads to a higher population size at equilibrium, to a higher depletion of the environment, to a marked discrepancy between the global (i.e. measured at the population level) and local (i.e. measured at the individual level) intensities of competition, and to a nonlinear density dependence. These results call for a deeper investigation of the impact of individual movement strategies and cognitive abilities on population dynamics.

Orienting the Interaction Compass: Resource Availability as a Major Driver of Context Dependence

Life on earth is enormously diverse, in part because each individual engages in countless interactions with its biotic and abiotic environment during its lifetime. Not only are there many such interactions, but any given interaction of each individual with, say, its neighbor or a nutrient could lead to a different effect on its fitness and on the dynamics of the population of which it is a member. Predicting those effects is an enduring challenge to the field of ecology. Using a simple laboratory system, Hoek and colleagues present evidence that resource availability can be a primary driver of variation between interactions. Their results suggest that a complex continuum of interaction outcomes can result from the simple combined effects of nutrient availability and density-dependent population dynamics. The future is rich with potential to integrate tractable experimental systems like theirs with hypotheses derived from studies of interactions in natural communities.

Stability of NonNegative Lur'e Systems

A stability/instability trichotomy for a class of nonnegative continuous-time Lur'e systems is derived. Asymptotic, exponential, and input-to-state stability concepts are considered. The presented trichotomy rests on Perron--Frobenius theory, absolute stability theory, and recent input-to-state stability results for Lur'e systems. Applications of the results derived arise in various fields, including density-dependent population dynamics, and two examples are discussed in detail.

Restoring population structure and dynamics in translocated species: learning from wild populations

Conservation of Leucojum aestivum, a wetland-dependent species distributed in Europe and west Asia, should aim to reduce the fragmentation of wild stands, through the establishment of new populations. However, density-dependent dynamics occur in L. a estivum. For instance, fruit set and seed set increase with increasing plant density. In this study, we evaluate the effect of plant density on translocation success of two recently established populations of L. a estivum. Twenty-six populations of L. a estivum were investigated in northern Italy to find out differences in population traits (size, density, age structure, and reproductive performance) between populations from different habitats. Data obtained were used to establish two new populations of the species differing for population density (high H, mirroring the typical plant density of a wild population in Salix alba woods and low L, in which plant density was halved compared to H), to evaluate the role of density-dependent dynamics on the translocation success. 4 years after the translocation, H produced seedlings, while L did not. Moreover, H produced a significantly higher number of fruits per fruiting plant and higher fruit set. Seed set was also greater in H than in L, while mortality was greater in L than in H, but differences were not significant. Our results suggest that population density is an important factor to account for in newly established populations, especially in those species showing density-dependent population dynamics. Moreover, the imitation of successful within-population dynamics occurring in natural stable populations may increase the translocation success.

Local density effects on individual production are dynamic: insights from natural stands of a perennial savanna grass.

Perennial grasses are a dominant component of grasslands, and provide important ecosystem services. However, most knowledge of grasslands' functioning and production comes from plot-level studies, and drivers of individual-level production remain poorly explored. Extrapolation from existing experiments is hampered by the fact that these are mostly concentrated on even-aged cohorts, and/or on the early stages of a plant's life cycle. Here we explored how local density regulates individual production in mono-specific natural grassland, focusing on adult individuals of a perennial savanna grass (Stipagrostis uniplumis). We found individual production to increase with individuals' size, but to decrease with neighbour abundance. A metric of neighbour abundance that considered size was superior to a metric based solely on the number of individuals. This finding is particularly important for studying competitive effects in natural populations, where plants are normally not even-sized. The inferred competition kernel, i.e. the function describing how competitive strength varies with spatial distance from a target plant, was hump-shaped, indicating strongest intraspecific competition at intermediate distances (10-30 cm). The spatial signature of competitive effects changed with time since fire; peak effects moved successively away from the target plant. Our results suggest that inferred competition kernels of long-lived plant populations may have shapes that differ from exponential or sigmoidal decreases. More generally, results underline that competition among neighbouring plants is dynamic. Studies that address density-dependent and density-independent (fire-related) population dynamics of perennial grasses in their fire-prone environment may thus shed new light on the functioning and production of grasslands.

Weak effects of a microsporidian parasite on mottled sculpin in Michigan streams

Despite the widespread occurrence of microsporidian parasites in vertebrates and many invertebrates, research regarding their effects on wild populations remains uncommon. We used several metrics of individual growth, condition, and population characteristics to evaluate the effects of a microsporidian infection (Glugea) on six infected populations of mottled sculpin (Cottus bairdii) compared with 10 nearby uninfected populations. Infected individuals (n = 129) were significantly heavier in each age class when compared with uninfected fish (n = 331), yet other measures of growth and condition indicated little influence of the parasite. No significant parasite-associated effects were found when comparing mortality rates, mean age, sex ratios, and history of population genetic bottleneck between infected and uninfected populations. Additionally, we did not detect significant differences in environmental variables or fish assemblages between streams with and without the parasite. Given the disfiguring nature of infection and the strong effects of microsporidian parasites on other hosts, the lack of significant effects caused by parasitism was unexpected and is likely attributed to strong host density-dependent population dynamics or relatively low host infection intensities.

A generic target for species recovery

Recovery targets for threatened species are typically developed on a species- or population-specific basis. Such narrow taxonomic specificity stands in contrast with widely applied species-independent metrics of conservation status. Here, we propose a generic protocol that can be used to specify broadly applicable targets intended to recover the ecological and evolutionary functionality of threatened species. The method is based on basic density-dependent population dynamics, draws on first principles related to population growth, and explicitly incorporates habitat by accounting for changes in carrying capacity. It offers a consistently applied, methodologically transparent, and predictable biological benchmark for recovery purposes. The benefits of a generic method for articulating recovery targets, particularly from a policy- and statute-implementation perspective, are substantive.

Uncertainty in a Measurement of Density Dependence on Population Fluctuations

This article discusses the question of how elasticity of the system is intertwined with external stochastic disturbances. The speed at which a displaced system returns to its equilibrium is a measure of density dependence in population dynamics. Population dynamics in random environments, linearized around the equilibrium point, can be represented by a Langevin equation, where populations fluctuate under locally stable (not periodic or chaotic) dynamics. I consider a Langevin model in discrete time, driven by time-correlated random forces, and examine uncertainty in locating the population equilibrium. There exists a time scale such that for times shorter than this scale the dynamics can be approximately described by a random walk; it is difficult to know whether the system is heading toward the equilibrium point. Density dependence is a concept that emerges from a proper coarse-graining procedure applied for time-series analysis of population data. The analysis is illustrated using time-series data from fisheries in the North Atlantic, where fish populations are buffeted by stochastic harvesting in a random environment.

LINKING POPULATION DYNAMICS AND DISEASE MODELS FOR MULTI-HOST PATHOGEN SYSTEMS: IMPLICATIONS FOR PATHOGEN AND SPECIES INVASION

We consider models for the spread of disease in one or two species where density-dependent population dynamics can range from affecting primarily reproduction and juvenile survival to affecting death rates of adults. We show that disease and competition (both intra- and inter-specific) can interact to change the dynamics of a system, resulting in changes in species density, disease invasion and prevalence. Additionally, we show that the mechanism of competition between and within species can affect the ability of a disease to spread and persist. If density dependence acts only on reproduction and juvenile survival rates, disease is more likely to persist, while for the same system if competition acts solely on the survival rate of adults the disease is less likely to successfully invade. This has implications both for conservation and public health, especially as climate change and invasive species change community structure and as zoonotic diseases become more important.

Population decline in semi-migratory caribou (Rangifer tarandus): intrinsic or extrinsic drivers?

Many populations of caribou (Rangifer tarandus (L., 1758)) across North America, including Newfoundland, are in a state of decline. This phenomenon may reflect continental-scale changes in either the extrinsic or the intrinsic factors affecting caribou abundance. We hypothesized that caribou decline reflected marked resource limitation and predicted that fluctuations should correspond to time-delayed density dependence associated with a decline in range quality and decadal trends in winter severity. By conducting time-series analysis using 12 populations and evaluating correlations between caribou abundance and trends in (i) vegetation available at calving (normalized difference vegetation index, NDVI), (ii) winter weather severity (index of North Atlantic Oscillation, NAO), and (iii) caribou morphometrics, we observed strong evidence of density dependence in population dynamics (i.e., a negative relationship between caribou population size and caribou morphometrics). Caribou population trajectories were time-delayed relative to winter severity, but not relative to calving-ground greenness. These island-wide correlations could not be traced to dispersal between herds, which appears rare at least for adult females. Our results suggest that trends in winter severity may synchronize broad-scale changes in caribou abundance that are driven by time-delayed density dependence, although it remains possible that calving-ground deterioration also may contribute to population limitation in Newfoundland. Our findings provide the basis for additional research into density dependence and caribou population decline.

Individual-based chaos: Extensions of the discrete logistic model

Simple models of density-dependent population growth such as the discrete logistic map provide powerful demonstrations of complex population dynamics. Yet it is unclear whether the dynamics observed in such idealized systems would be present, under realistic conditions, in the context of demographic stochasticity, which is well known to exist in finite natural populations. Here, using a set of simple, individual-based models (IBM's) and their population-level iterative map counterparts, we computationally investigate the contribution of demographic stochasticity to density-dependent population dynamics in a simple model of seed production and recruitment. Notably, for a sufficiently large lattice, even in the presence of demographic stochasticity, many of the qualitative features of these idealized maps – including bifurcations – are still present. Demographic stochasticity and the constraints imposed by a finite lattice size appear to produce mixed dynamics that are partially stochastic, yet qualitatively similar to the deterministic models. The mechanistic assumptions and lattice sizes required to generate these dynamics cast doubt on whether they might be observable in annual plant populations. Nevertheless, we cannot rule out the theoretical possibility that such dynamics might be observable in ecological communities having similar mechanistic properties.

Consumer–resource dynamics of indirect interactions in a mutualism–parasitism food web module

Food web dynamics are well known to vary with indirect interactions, classic examples including apparent competition, intraguild predation, exploitative competition, and trophic cascades of food chains. Such food web mod- ules entailing predation and competition have been the focus of much theory, whereas modules involving mutualism have received far less attention. We examined an empirically common food web module involving mutualistic (N2) and parasitic (N3) consumers exploiting a resource of a basal mutualist (N1), as illustrated by plants, pollinators, and nectar robbers. This mutualism-parasitism food web mod- ule is structurally similar to exploitative competition, suggesting that the module of two consumers exploiting a resource is unstable. Rather than parasitic consumers destabilizing the module through (−,−) indirect interactions, two mechanisms associated with the mutualism can actually enhance the persistence of the module. First, the positive feedback of mutualism favors coexistence in stable limit cycles, whereby (+,−) indirect interactions emerge in which increases in N2 have positive effects on N3 and increases in N3 have negative effects on N2 .T his (+,−) indirect interaction arising from the saturatingpositivefeedbackof mutualism has broad feasibility across many types of food web modules entailing mutualism. Second, optimization of resource exploi- tation by the mutualistic consumer can lead to persistence of the food web module in a stable equilibrium. The mutualism- parasitism food web module is a basic unit of food webs in which mutualism favors its persistence simply through density-dependent population dynamics, rather than parasit- ism destabilizing the module.

Agricultural irrigation mediates climatic effects and density dependence in population dynamics of Chinese striped hamster in North China Plain

Several studies show that climatic (extrinsic) factors can interact with density-dependent (intrinsic) factors to alter long-term population dynamics, yet there is a surprising lack of investigations of how anthropogenic disturbance modifies such dynamics. Such interactions could be especially important in agricultural systems subject to climate change. We investigated the effects of density dependence, climate, recurrent disturbance from flood irrigation and their interactions on the population dynamics of an important rodent pest, the Chinese striped hamster (Cricetulus barabensis), over 27years in the croplands of the North China Plain. Strong density-dependent feedbacks occurred at both annual and seasonal scales. While warmer weather increased population sizes in nonbreeding seasons, this effect was counteracted by the negative effect of flood irrigation in breeding seasons. Precipitation showed significant positive effects in nonbreeding seasons, but negative effects in breeding seasons. There were important interactions between intrinsic dynamics, extrinsic dynamics and disturbance. Low temperature significantly increased the strength of density dependence in nonbreeding seasons, whereas intensification of flood irrigation area significantly increased the strength of density dependence but reduced the effect of summer precipitation in breeding seasons. Overall climate change is expected to increase population levels, but anthropogenic disturbance from flood irrigation will help prevent long-term population increases. The interactions between anthropogenic disturbance and both intrinsic and extrinsic (weather-driven) population dynamics caution that we need to consider anthropogenic disturbance as an integral component of population responses to climate change.