Species-level selection reduces selfishness through competitive exclusion

Adaptation does not necessarily lead to traits which are optimal for the population. This is because selection is often the strongest at the individual or gene level. The evolution of selfishness can lead to a 'tragedy of the commons', where traits such as aggression or social cheating reduce population size and may lead to extinction. This suggests that species-level selection will result whenever species differ in the incentive to be selfish. We explore this idea in a simple model that combines individual-level selection with ecology in two interacting species. Our model is not influenced by kin or trait-group selection. We find that individual selection in combination with competitive exclusion greatly increases the likelihood that selfish species go extinct. A simple example of this would be a vertebrate species that invests heavily into squabbles over breeding sites, which is then excluded by a species that invests more into direct reproduction. A multispecies simulation shows that these extinctions result in communities containing species that are much less selfish. Our results suggest that species-level selection and community dynamics play an important role in regulating the intensity of conflicts in natural populations.     

Stage-structured cycles generate strong fitness-equalizing mechanisms

For many organisms, rates of reproduction, growth and mortality depend on the amount of resources that an individual consumes. When resource abundances fluctuate through space and time, the realized life-history of an individual can change dramatically depending on the dynamics experienced. Previous studies have investigated the influence of resource-dependent rates on population dynamics, but none have considered how the feedback between non-equilibrium resource dynamics and resource-dependent life-histories influence natural selection and the maintenance of genetic diversity within populations. Here we demonstrate that different patterns of resource dynamics have a strong impact on natural selection in organisms with resource-dependent life-histories. Small-amplitude consumer-resource cycles, lead to lower rates of natural selection than do large-amplitude consumer-resource cycles. Parameterizing the model for a Daphnia-algal system, we demonstrate that resource-dependent life-history can explain the recently published observation that selection among Daphnia genotypes changed depending on the pattern of algal resource fluctuations. The characteristically asexual reproduction of Daphnia allows us to draw a much-needed link to the large body of competition theory that has emerged from community ecology. Our results reveal that the common ecological features of resource-dependent life-history and ontogenetic size-structure generate strong fitness equalizing mechanisms that likely contribute to the maintenance of diversity in natural systems. Electronic Supplementary Material Supplementary material is available in the online version of this article at accessible for authorized users.     

Relatedness and competitive asymmetry – implications for growth and population dynamics

The members of natural populations often differ in size and relatedness to each other, which may affect the division of limited resources and have consequences on reproductive success and population dynamics. We modeled seasonal growth and dynamics in populations composed of different types of relatives (full-sibs, half-sibs and non-related individuals) under the continuum of competitive scenarios between complete symmetry and asymmetry. Growth was assumed logistic in proportion to individual biomass and the size-differences were weighted by the relatedness of individuals. The symmetric component of competition was experienced by all individuals in proportion to their biomass, whereas the asymmetric component was individual-specific, and influenced only by the individuals larger than the focal individual. Relatedness decreased and competitive asymmetry increased the variability of individual biomasses. Mortality of the smallest individuals and the size threshold of reproduction decreased population density. Population dynamics were stable when there was no size threshold for reproduction but the presence of the threshold led to cyclic dynamics under low competitive asymmetry. The effects of the threshold were greater among related than unrelated individuals. The results suggest that individual differences and the asymmetry of competition can greatly affect population dynamics. Full symmetry of competition may be evolutionarily unstable in populations of related individuals as it may increase the probability of extinction due to demographic stochasticity.     

Approximation of a physiologically structured population model with seasonal reproduction by a stage-structured biomass model

Seasonal reproduction causes, due to the periodic inflow of young small individuals in the population, seasonal fluctuations in population size distributions. Seasonal reproduction furthermore implies that the energetic body condition of reproducing individuals varies over time. Through these mechanisms, seasonal reproduction likely affects population and community dynamics. While seasonal reproduction is often incorporated in population models using discrete time equations, these are not suitable for size-structured populations in which individuals grow continuously between reproductive events. Size-structured population models that consider seasonal reproduction, an explicit growing season and individual-level energetic processes exist in the form of physiologically structured population models. However, modeling large species ensembles with these models is virtually impossible. In this study, we therefore develop a simpler model framework by approximating a cohort-based size-structured population model with seasonal reproduction to a stage-structured biomass model of four ODEs. The model translates individual-level assumptions about food ingestion, bioenergetics, growth, investment in reproduction, storage of reproductive energy, and seasonal reproduction in stage-based processes at the population level. Numerical analysis of the two models shows similar values for the average biomass of juveniles, adults, and resource unless large-amplitude cycles with a single cohort dominating the population occur. The model framework can be extended by adding species or multiple juvenile and/or adult stages. This opens up possibilities to investigate population dynamics of interacting species while incorporating ontogenetic development and complex life histories in combination with seasonal reproduction.     

放牧退化群落中冷蒿种群生物量资源分配的变化

对放牧退化群落中冷蒿种群生物量及生物量资源分配的变化进行了研究.结果表明,在放牧干扰下,随着放牧退化程度的增加,冷蒿种群叶、茎、根的生物量及总生物量增加.其中根的重量增加幅度较大,但生殖构件(花序、果实)的生物量在轻度退化群落中增加,中度退化群落中迅速减少,重度退化群落中未发现生殖构件.随着放牧退化程度增加,冷蒿种群生物量的资源分配发生变化,对根的分配增加,对茎、叶的分配减少,根冠比增加;对无性繁殖的分配增加,对有性生殖的分配减少.在重度退化群落,冷蒿有性生殖严重受阻,繁殖格局发生变化.从资源分配的动态来看,随着放牧退化程度的增加,生长初期至盛期,冷蒿种群资源优先分配给地上部分,尤其是光合器官叶;而生长盛期至末期,资源优先分配给有性生殖或贮藏器官.繁殖格局的转变是冷蒿种群耐牧,在重度退化下成为建群种的关键.资源分配格局的时空变化,使生长、维持和繁殖等方面的分配达到和谐,是冷蒿种群在重度退化下成为建群种的物质基础.     

A community model of ciliateTetrahymena and bacteriaE. coli: Part I. Individual-based models ofTetrahymena andE. coli populations

The dynamics of a microbial community consisting of a eucaryotic ciliateTetrahymena pyriformis and procaryoticEscherichia coli in a batch culture is explored by employing an individual-based approach. In this portion of the article, Part I, population models are presented. Because both models are individual-based, models of individual organisms are developed prior to construction of the population models. The individual models use an energy budget method in which growth depends on energy gain from feeding and energy sinks such as maintenance and reproduction. These models are not limited by simplifying assumptions about constant yield, constant energy sinks and Monod growth kinetics as are traditional models of microbal organisms. Population models are generated from individual models by creating distinct individual types and assigning to each type the number of real individuals they represent. A population is a compilation of individual types that vary in a phase of cell cycle and physiological parameters such as filtering rate for ciliates and maximum anabolic rate for bacteria. An advantage of the developed models is that they realistically describe the growth of the individual cells feeding on resource which varies in density and composition. Part II, the core of the project, integrates models into a dynamic microbial community and provides model analysis based upon available data.     

Community-level education accelerates the cultural evolution of fertility decline

Explaining why fertility declines as populations modernize is a profound theoretical challenge. It remains unclear whether the fundamental drivers are economic or cultural in nature. Cultural evolutionary theory suggests that community-level characteristics, for example average education, can alter how low-fertility preferences are transmitted and adopted. These assumptions have not been empirically tested. Here, we show that community-level education accelerates fertility decline in a way that is neither predicted by individual characteristics, nor by the level of economic modernization in a population. In 22 high-fertility communities in Poland, fertility converged on a smaller family size as average education in the community increased—indeed community-level education had a larger impact on fertility decline than did individual education. This convergence was not driven by educational levels being more homogeneous, but by less educated women having fewer children than expected, and more highly educated social networks, when living among more highly educated neighbours. The average level of education in a community may influence the social partners women interact with, both within and beyond their immediate social environments, altering the reproductive norms they are exposed to. Given a critical mass of highly educated women, less educated neighbours may adopt their reproductive behaviour, accelerating the pace of demographic transition. Individual characteristics alone cannot capture these dynamics and studies relying solely on them may systematically underestimate the importance of cultural transmission in driving fertility declines. Our results are inconsistent with a purely individualistic, rational-actor model of fertility decline and suggest that optimization of reproduction is partly driven by cultural dynamics beyond the individual.     

A baseline model for the co-evolution of hosts and pathogens

The basic reproduction ratio (R (0)) is the expected number of secondary cases per primary in a totally susceptible population. In a baseline model, faced with an individual host strain pathogen virulence evolves to maximise R (0) which yields monomorphism. The basic depression ratio (D (0)) is the amount by which the total population is decreased, per infected individual, due to the presence of infection. Again, in a baseline model, faced with an individual pathogen strain host resistance evolves to minimise D (0) which yields monomorphism. With this in mind we analyse the community dynamics of the interaction between R (0) and D (0) and show that multi-strain co-existence (polymorphism) is possible and we discuss the possibility of stable cycles occuring within the co-existence states. We show for co-existence, the number of host and pathogen strains present need to be identical in order to achieve stable equilibria. For polymorphic states we observe contingencies (outcome dependent on initial conditions) between both point equilibrium and sustained oscillations. Invasion criteria for host and pathogen strains are identified.     

Sexual reproduction and diversity: Connection between sexual selection and biological communities via population dynamics

Sexual reproduction is a mysterious phenomenon. Most animals and plants invest in sexual reproduction, even though it is more costly than asexual reproduction. Theoretical studies suggest that occasional or conditional use of sexual reproduction, involving facultative switching between sexual and asexual reproduction, is the optimal reproductive strategy. However, obligate sexual reproduction is common in nature. Recent studies suggest that the evolution of facultative sexual reproduction is prevented by males that coerce females into sexual fertilization; thus, sexual reproduction has the potential to enforce costs on a given species. Here, the effect of sex on biodiversity is explored by evaluating the reproductive costs arising from sex. Sex provides atypical selection pressure that favors traits that increase fertilization success, even at the expense of population growth rates, that is, sexual selection. The strength of sexual selection depends on the density of a given species. Sexual selection often causes strong negative effects on the population growth rates of species that occur at high density. Conversely, a species that reduces its density is released from this negative effect, and so increases its growth rate. Thus, this negative density-dependent effect on population growth that arises from sexual selection could be used to rescue endangered species from extinction, prevent the overgrowth of common species and promote the coexistence of competitive species. Recent publications on sexual reproduction provide several predictions related to the evolution of reproductive strategies, which is an important step toward integrating evolutionary dynamics, demographic dynamics and community dynamics.     

Effects of Growth Curve Plasticity on Size-Structured Population Dynamics

The physiological-structured population models assume that a fixed fraction of energy intake is utilized for individual growth and maintenance while the remaining for adult fertility. The assumption results in two concerns: energy loss for juveniles and a reproduction dilemma for adults. The dilemma results from the possibility that adults have to breed even if metabolic costs fail to be covered. We consider a size-structured population model, where standard metabolism is given top priority for utilizing energy intake and the surplus energy, if there is any, is distributed to individual growth and reproduction. Moreover, the portion of surplus energy for reproduction is size-dependent and increases monotonically with size. Using the newly developed parameter continuation, we demonstrate their disparate effects on population dynamics. Results show that the size-dependent mechanism of energy allocation primarily exerts destabilizing effects on the system but considerably promotes species coexistence, in comparison with the size-independent mechanism. We conclude that the size-dependent mechanism is, to a large extent, a dispensable component of model ingredients when ontogeny is explicitly taken into consideration.     

Population and evolutionary dynamics in spatially structured seasonally varying environments

Increasingly imperative objectives in ecology are to understand and forecast population dynamic and evolutionary responses to seasonal environmental variation and change. Such population and evolutionary dynamics result from immediate and lagged responses of all key life‐history traits, and resulting demographic rates that affect population growth rate, to seasonal environmental conditions and population density. However, existing population dynamic and eco‐evolutionary theory and models have not yet fully encompassed within‐individual and among‐individual variation, covariation, structure and heterogeneity, and ongoing evolution, in a critical life‐history trait that allows individuals to respond to seasonal environmental conditions: seasonal migration. Meanwhile, empirical studies aided by new animal‐tracking technologies are increasingly demonstrating substantial within‐population variation in the occurrence and form of migration versus year‐round residence, generating diverse forms of ‘partial migration’ spanning diverse species, habitats and spatial scales. Such partially migratory systems form a continuum between the extreme scenarios of full migration and full year‐round residence, and are commonplace in nature. Here, we first review basic scenarios of partial migration and associated models designed to identify conditions that facilitate the maintenance of migratory polymorphism. We highlight that such models have been fundamental to the development of partial migration theory, but are spatially and demographically simplistic compared to the rich bodies of population dynamic theory and models that consider spatially structured populations with dispersal but no migration, or consider populations experiencing strong seasonality and full obligate migration. Second, to provide an overarching conceptual framework for spatio‐temporal population dynamics, we define a ‘partially migratory meta‐population’ system as a spatially structured set of locations that can be occupied by different sets of resident and migrant individuals in different seasons, and where locations that can support reproduction can also be linked by dispersal. We outline key forms of within‐individual and among‐individual variation and structure in migration that could arise within such systems and interact with variation in individual survival, reproduction and dispersal to create complex population dynamics and evolutionary responses across locations, seasons, years and generations. Third, we review approaches by which population dynamic and eco‐evolutionary models could be developed to test hypotheses regarding the dynamics and persistence of partially migratory meta‐populations given diverse forms of seasonal environmental variation and change, and to forecast system‐specific dynamics. To demonstrate one such approach, we use an evolutionary individual‐based model to illustrate that multiple forms of partial migration can readily co‐exist in a simple spatially structured landscape. Finally, we summarise recent empirical studies that demonstrate key components of demographic structure in partial migration, and demonstrate diverse associations with reproduction and survival. We thereby identify key theoretical and empirical knowledge gaps that remain, and consider multiple complementary approaches by which these gaps can be filled in order to elucidate population dynamic and eco‐evolutionary responses to spatio‐temporal seasonal environmental variation and change.     

Nutrition, fertility and steady-state population dynamics in a pre-industrial community in Penrith, northern England

The effect of nutrition on fertility and its contribution thereby to population dynamics are assessed in three social groups (elite, tradesmen and subsistence) in a marginal, pre-industrial population in northern England. This community was particularly susceptible to fluctuations in the price of grains, which formed their basic foodstuff. The subsistence class, who formed the largest part of the population, had low levels of fertility and small family sizes, but women from all social groups had a characteristic and marked subfecundity in the early part of their reproductive lives. The health and nutrition of the mother during pregnancy was the most important factor in determining fertility and neonatal mortality. Inadequate nutrition had many subtle effects on reproduction which interacted to produce a complex web of events. A population boom occurred during the second half of the 18th century; fertility did not change but there was a marked improvement in infant mortality and it is suggested that the steadily improving nutritional standards of the population, particularly during crucial periods in pregnancy (i.e. the last trimester), probably made the biggest contribution to the improvement in infant mortality and so was probably the major factor in triggering the boom.     

克隆和有性亲体效应及其调控机制

植物表型受自身基因型、所处环境及其亲体所经历环境的共同影响;其中,亲体环境对子代表型的影响被称为亲体效应。亲体效应不仅可通过有性繁殖产生的种子传递给后代(即有性亲体效应),也可以通过克隆生长等无性繁殖产生的分株传递给后代(即克隆亲体效应)。亲体效应对植物种群,特别是对有性繁殖受限、缺乏遗传变异的克隆植物种群的长期进化可能发挥着极其重要的作用,因此,对亲体效应研究进展的梳理非常必要。对克隆亲体效应和有性亲体效应的内涵进行了阐释,并论述了克隆和有性亲体效应对子代表型、适合度、种内/种间竞争能力以及种群/群落结构和功能的潜在影响;阐述了亲体效应的潜在调控机制,包括供给机制、代谢物质调控机制、表观遗传机制等;论述了克隆亲体效应在克隆植物适应进化中的作用。未来可以就克隆亲体效应的遗传稳定性及其对克隆生活史性状变异的贡献程度,以及克隆和有性亲体效应引起的表型多样性对种内/种间关系、种群/群落多样性及生态系统结构、功能和稳定性的影响开展深入研究。     

Demographic consequences of greater clonal than sexual reproduction in Dicentra canadensis

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.     

Competitive exclusion and coexistence for a quasilinear size-structured population model

We present a quasilinear size-structured model which describes the dynamics of a population with n competing ecotypes. We assume that the vital rates of each subpopulation depend on the total population due to competition. We provide conditions on the individual rates which guarantee competitive exclusion in the case of closed reproduction (offspring always belongs to the same ecotype as the parent). In particular, our results suggest that the ratio of the reproduction and mortality rates is a good measure to determine the winning ecotype. Meanwhile, we show that in the case of open reproduction all ecotypes coexist.     

A bit of sex stabilizes host-parasite dynamics

To date, only a few studies have focused on the effects of sex on population dynamics. Previous models have typically found that sexual reproduction dampens population fluctuations. Although asexual and sexual reproduction are just the two endpoints along a continuum of varying rates of sex, previous work has ignored the effects of intermediate degrees of sex on population dynamics. Here we study the effects of partial sexual reproduction (i.e. sex occurs only every few generations or with small probability in each generation) on the coupled population dynamics of a Nicholson-Bailey host-parasite model. We show that complex dynamics are simplified for high host population growth rates if the frequency of sex is sufficiently high in both host and parasite: sex decreases fluctuations in population density, and leads to non-chaotic dynamics for population growth rates that would result in chaotic dynamics in the absence of sexual reproduction. However, the simplification does not increase gradually with an increasing frequency of sex but appears abruptly at low-to-intermediate frequencies of sex. For some parameter settings, intermediate frequencies of sexual reproduction can simplify the dynamics more than lower or higher frequencies. Thus, in agreement with earlier results, sexual reproduction typically stabilizes complex population dynamics in our models. Additionally, our results suggest that low-to-intermediate frequencies of sex may often be as (or even more) stabilizing as high frequencies.     

What individual life histories can (and cannot) tell about population dynamics

We present an overview of a long-term research programme that is aimed at revealing the relations between individual feeding, growth, reproduction and mortality in Daphnia pulex and the state and dynamics of the population. We analyse a physiologically structured population model, in which individual performance is described using an energy budget model that incorporates a food dependence. The model predictions are shown to be at odds with experimental observations on populations of Daphnia. We argue that these discrepancies are primarily due to insufficient knowledge about the precise size-scaling of the food ingestion rate, which plays a central role in the competitive interaction among individuals. To a lesser extent, the discrepancies arise because details about the energy budget of individual Daphnia are not sufficiently known for the food conditions prevailing in population experiments.     

Physiologically structured models – from versatile technique to ecological theory

A ubiquitous feature of natural communities is the variation in size that can be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that ecological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) constitute a modelling approach especially designed to analyse these size‐dependent interactions as they explicitly link individual level processes such as consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size‐structured populations. In the first case, a model of a size‐structured consumer population feeding on a non‐structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth, survival, cycle amplitude) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competition is a general outcome of size‐structured consumer–resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with individuals ending up as giants. These growth trajectories, which have also been observed in field data, could not be predicted from individual level information, but are emergent properties of the population feedback on individual processes. In contrast to the size‐structured consumer–resource model, the dynamics in this case cannot be reduced to simpler lumped stage‐based models, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both population level processes and individual level processes under conditions of population feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the individual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality than purely individual‐based models. At the same time, PSPMs offer a much higher degree of realism, precision and testing ability than lumped stage‐based or non‐structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanisms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual‐based, size‐dependent community theory using PSPMs.     

On the Population Dynamics of the Malaria Vector

A deterministic differential equation model for the population dynamics of the human malaria vector is derived and studied. Conditions for the existence and stability of a non-zero steady state vector population density are derived. These reveal that a threshold parameter, the vectorial basic reproduction number, exist and the vector can established itself in the community if and only if this parameter exceeds unity. When a non-zero steady state population density exists, it can be stable but it can also be driven to instability via a Hopf Bifurcation to periodic solutions, as a parameter is varied in parameter space. By considering a special case, an asymptotic perturbation analysis is used to derive the amplitude of the oscillating solutions for the full non-linear system. The present modelling exercise and results show that it is possible to study the population dynamics of disease vectors, and hence oscillatory behaviour as it is often observed in most indirectly transmitted infectious diseases of humans, without recourse to external seasonal forcing.     

樟翠尺蛾种群动态与植物群落结构及气候因子的关系

在昆虫种群的研究中,Ｇｉｅｉｒ和Ｃｌａｒｋ等曾经提出生命系统的基本概念[1,2],认为生命系统由一个对象种群和作用于这个种群的环境所组成。控制昆虫种群增长的有密度制约因子(如食源)和非密度制约因子(如气候环境)。昆虫和植物之间的关系,历来是生态学研究的重要领域。通过长期的定点监测,研究某种昆虫种群动态与各种生态因子之间的关系,不仅可揭示动物和植物及环境之间互相影响的关系,对保护生物学理论的某些方面将有所发展,而且可为林业生产和环保工作提供如何应用生态学原理控制虫害,保护生物多样性和维护生态平衡的理论依据。樟翠尺…