Metapopulation-level adaptation of insect host plant preference and extinction-colonization dynamics in heterogeneous landscapes

Species living in highly fragmented landscapes typically occur as metapopulations with frequent turnover of local populations. The turnover rate depends on population sizes and connectivities, but it may also depend on the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host plant species, which show variation in their relative abundances at two spatial scales: locally among individual habitat patches and regionally among networks of patches. Female butterflies in turn exhibit spatial variation in genetically determined host plant preference within and among patch networks. Emigration, immigration and establishment of new populations have all been shown to be strongly influenced by the match between the host plant composition of otherwise suitable habitat patches and the host plant preference of migrating butterflies. The evolutionary consequences of such biased migration and colonization with respect to butterfly phenotypes might differ depending on spatial configuration and plant species composition of the patches in heterogeneous patch networks. Using a spatially realistic individual-based model we show that the model-predicted evolution of host plant preference due to biased migration explains a significant amount of the observed variation in host plant use among metapopulations living in dissimilar networks. This example illustrates how the ecological extinction-colonization dynamics may be linked with the evolutionary dynamics of life history traits in metapopulations.     

The impact of diffusion and stirring on the dynamics of interacting populations

We investigate the combined effects of diffusion and stirring on the dynamics of interacting populations which have spatial structure. Specifically we consider the marine phytoplankton and zooplankton populations, and model them as an excitable medium. The results are applicable to other biological and chemical systems. Under certain conditions the combination of diffusion and stirring is found to enhance the excitability, and hence population growth of the system. Diffusion is found to play an important role: too much and initial perturbations are smoothed away, too little and insufficient mixing takes place before the reaction is over. A key time-scale is the mix-down time, the time it takes for the spatial scale of a population to be reduced to that of a diffusively controlled filament. If the mix-down time is short compared to the reaction time-scale, then excitation of the system is suppressed. For intermediate values of the mix-down time the peak population can attain values many times that of a population without spatial structure. We highlight the importance of the spatial scale of the initial disturbance to the system.     

The impact of climatic factors and host density on the long-term population dynamics of vole helminths

Summary The seasonal and long-term population dynamics of helminths parasitizing voles suggested that density-dependent factors might be important in the population dynamics of common species, whereas density-independent factors predominate in the regulation of the rare species. To test this, we used single and multiple regression to analyse the effects of climatic factors and host density on populations of six species of vole helminths over 12 years. The data do support the idea of a difference between common and rare species of helminths, but they clearly do not support the above hypothesis. The common helminths Heligmosomum mixtum (Nematoda) and Catenotaenia sp. (Cestoda) responded to changes in temperature sum (>5° C days) and precipitation during summer. The combined effect of climatic factors and host density explained most of the variation in the long-term dynamics of these common species. By contrast, the long-term dynamics of the rare helminths Paranoplocephala kalelai (Cestoda), Mastophorus muris, Capillaria sp. and Syphacia petrusewiczi (Nematoda) were explained less well by weather and host density than those of the common ones. Furthermore, the common and rare helminths differed in some ways in their responses to climatic factors.     

A dimensionless number for understanding the evolutionary dynamics of antigenically variable RNA viruses

Antigenically variable RNA viruses are significant contributors to the burden of infectious disease worldwide. One reason for their ubiquity is their ability to escape herd immunity through rapid antigenic evolution and thereby to reinfect previously infected hosts. However, the ways in which these viruses evolve antigenically are highly diverse. Some have only limited diversity in the long-run, with every emergence of a new antigenic variant coupled with a replacement of the older variant. Other viruses rapidly accumulate antigenic diversity over time. Others still exhibit dynamics that can be considered evolutionary intermediates between these two extremes. Here, we present a theoretical framework that aims to understand these differences in evolutionary patterns by considering a virus's epidemiological dynamics in a given host population. Our framework, based on a dimensionless number, probabilistically anticipates patterns of viral antigenic diversification and thereby quantifies a virus's evolutionary potential. It is therefore similar in spirit to the basic reproduction number, the well-known dimensionless number which quantifies a pathogen's reproductive potential. We further outline how our theoretical framework can be applied to empirical viral systems, using influenza A/H3N2 as a case study. We end with predictions of our framework and work that remains to be done to further integrate viral evolutionary dynamics with disease ecology.     

The impact of non-lethal synergists on the population and evolutionary dynamics of host-pathogen interactions

Multiple pathogenic infections can influence disease transmission and virulence, and have important consequences for understanding the community ecology and epidemiology of host-pathogen interactions. Here the population and evolutionary dynamics of a host-pathogen interaction with free-living stages are explored in the presence of a non-lethal synergist that hosts must tolerate. Through the coupled effects on pathogen transmission, host mass gain and allometry it is shown how investing in tolerance to a non-lethal synergist can lead to a broad range of different population dynamics. The effects of the synergist on pathogen fitness are explored through a series of life-history trait trade-offs. Coupling trade-offs between pathogen yield and pathogen speed of kill and the presence of a synergist favour parasites that have faster speeds of kill. This evolutionary change in pathogen characteristics is predicted to lead to stable population dynamics. Evolutionary analysis of tolerance of the synergist (strength of synergy) and lethal pathogen yield show that decreasing tolerance allows alternative pathogen strategies to invade and replace extant strategies. This evolutionary change is likely to destabilise the host-pathogen interaction leading to population cycles. Correlated trait effects between speed of kill and tolerance (strength of synergy) show how these traits can interact to affect the potential for the coexistence of multiple pathogen strategies. Understanding the consequences of these evolutionary relationships is important for the both the evolutionary and population dynamics of host-pathogen interactions.     

Estimating the optimal bottleneck ratio for experimental evolution: the burst-death model

Population bottlenecks are ubiquitous in nature, and are an inherent feature of the experimental protocol for many laboratory selection experiments. These bottlenecks can have profound effects on the rate and trajectory of evolution. In particular, there is a trade-off between sampling the population too frequently and imposing infrequent, but more severe, bottlenecks. In this paper we consider the effects of population bottlenecks, assuming a burst-death model for the life history of the organism under study. This model assumes that generation times are exponentially distributed and that at each generation, individuals in the population have a fixed number of offspring. The model also allows for a constant death rate between bottlenecks. We use this model to estimate the optimal bottleneck ratio, that is, the fraction of the population that should be sampled at each bottleneck in order to maximize the probability that beneficial mutations occur and are not lost. We find that the optimal ratio is roughly constant with respect to many of the model parameters, and that sampling about 20% of the population will maximize the rate of adaptation.     

Life history divergence of sympatric diploid and polyploid populations of brine shrimp Artemia parthenogenetica

In order to study how polyploidy affects life history patterns in animals, we have examined sympatric diploid and polyploid brine shrimp (Artemia parthenogenetica) from China, Italy and Spain under laboratory conditions. At optimal temperature and salinity (25°C and 90 ppt), diploids from the three populations had much higher intrinsic rates of increase, higher fecundity, faster developmental rates, and larger brood sizes than their sympatric polyploids. The Chinese and Italian populations were selected for further analysis to determine the life history responses of diploids and polyploids to temperature and salinity changes. Under intermediate and high salinities, Chinese and Italian polyploids produced most of their offspring as dormant cysts while their sympatric diploids produced most of their offspring as nauplii. This relationship is reversed in the Spanish diploid-polyploid complex. For the Chinese population at 25° C, pentaploid clones had higher developmental rates than diploid clones at 35 ppt; at 90 ppt, diploid clones had higher developmental rates than the pentaploids. Italian diploids and tetraploids had different responses to variation in both temperature (25° C and 31° C) and salinity (30 ppt and 180 ppt). Our results demonstrate that relative fitness of the two cytotypes is a function of environmental conditions and that sympatric diploids and polyploids respond differently to environmental changes. Chinese and Italian polyploids are expected to have lower fitness than their sympatric diploids when the physical environment is not stressful and when intraspecific competition is important. However, polyploids may have advantages over sympatric diploids in stressful habitats or when they encounter short-term lethal temperatures. These results suggest that polyploid Artemia have evolved a suite of life-history characteristics adapting them to environments that contrast to those of their sympatric diploids.     

The dynamics of bacteria-plasmid systems

We introduce a general model for the dynamics of a single plasmid type and a single bacterial cell type, following Stewart and Levin (1977) in subdividing the population into plasmid-bearing and plasmid-free cells. For the particular case of mortality being a linear function of population sizes, we demonstrate the existence of multiple stable states and threshold values, thus illustrating that, in some cases, the establishment of a population of plasmids may depend on introduction of plasmids at sufficiently high levels. We also analyze in detail, for a general monotonically increasing mortality function, the epidemiological case in which plasmids confer a net cost on their hosts, and demonstrate that it is possible for such plasmids to become established. Stewart and Levin previously demonstrated this effect in a more restricted model.     

The force of selection on the human life cycle

In this article, I present evidence for a robust and quite general force of selection on the human life cycle. The force of selection acts in remarkably invariant ways on human life histories, despite a great abundance of demographic diversity. Human life histories are highly structured, with mortality and fertility changing substantially through the life cycle. This structure necessitates the use of structured population models to understand human life history evolution. Using such structured models, I find that the vital rates to which fitness is most sensitive are prereproductive survival probabilities, particularly the survival of children ages 0 to 4 years. The fact that the preponderance of selection falls on transitions related to recruitment combined with the late age at first reproduction characteristic of the human life cycle creates a fitness bottleneck out of recruitment. Because of this, antagonistic pleiotropy with any trait that detracts from the constituent transitions to recruitment is expected. I explore the predictors of variation in the force of selection on early survival. High fertility increases the selective premium placed on early survivorship, whereas high life expectancy at birth decreases it.     

The influence of stage-dependent dispersal on the population dynamics of three amphipod species

In metapopulations, the maintenance of local populations can depend on source–sink dynamics, where populations with positive growth rate seed populations with negative growth rate. The pattern and probability of successful dispersal among habitats can therefore be crucial in determining whether local populations will become rare or increase in abundance. We present here data on the dispersal strategy and population dynamics of three marine amphipods living in pen shells (Atrina rigida) in the Gulf of Mexico. The three amphipod species in this study disperse at different life stages. Neomegamphopus hiatus and Melita nitida disperse as adults, while Bemlos unicornis disperses as juveniles. The two species that disperse as adults have the highest initial population sizes when a new shell becomes available, likely caused by the arriving females releasing their brood into these recently occupied shells. This dispersal pattern results in initially higher population growth, but fewer occupied shells, as noted by their clumped distribution. In contrast, the species that disperses as juveniles accumulates more slowly and more evenly across habitats, eventually dominating the other two in terms of numerical abundance. The metapopulation dynamics of the three species seems to be highly dependent on the life history stage involved in dispersal. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Realistic population dynamics in epidemiological models: the impact of population decline on the dynamics of childhood infectious diseases. Measles in Italy as an example

Most contributions in the field of mathematical modelling of childhood infectious diseases transmission dynamics have focused on stationary or exponentially growing populations. In this paper an epidemiological model with realistic demography is used to investigate the impact of the non-equilibrium conditions typical of the transition to sustained below replacement fertility (BRF) recently observed in a number of western countries, upon the transmission dynamics of measles. The results depend on the manner we model the relation between the (changing) age distribution of the population and contacts. Under some circumstances the transitional ageing phase typical of BRF populations might complexly interact with epidemiological variables leading to (i) a substantial reduction in the amount of vaccination effort required for eliminating the disease; (ii) a significant magnification of the perverse impact of vaccination in terms of the burden of severe age related morbidity.     

The impact of homologous recombination on the generation of diversity in bacteria

The imprint of demographic and selective processes on bacterial population structure needs to be evaluated as deviation from the expectations of an appropriate null neutral model. We explore the impact of varying the population mutation and recombination rates theta and rho on ideal populations, using a recently developed model of neutral drift at multiple loci. This model may be fitted to experimental data to provide estimates of these parameters, and we do so for seven bacterial species (Neisseria meningitidis, Streptococcus pneumoniae, Streptococcus pyogenes, Staphylococcus aureus, Helicobacter pylori, Burkholderia pseudomallei and Bacillus cereus), illustrating that bacterial species vary extensively in these fundamental parameters. Historically, the influence of recombination has often been estimated through its influence on the Index of Association I(A). We show that this may be relatively insensitive to changes in either mutation or recombination rates. It is known that biased sampling can lead to artificially high estimates of I(A). We therefore provide a method of precisely separating the effects of such bias and true linkage between alleles. We also demonstrate that by fitting the neutral model to experimental data, more informative and precise estimates of the relative roles of recombination and mutation may be obtained.     

Spatio-temporal heterogeneity in coral recruitment around Moorea, French Polynesia: Implications for population maintenance

The spatio-temporal variability of scleractinian coral recruitment was investigated from December 2000 to December 2003 around Moorea Island (French Polynesia). Nine stations, each with 20 terracotta tiles, were placed on the outer reef slope, at 3 sites (Vaipahu, Tiahura, Haapiti), and at 3 depths (6, 12 and 18 m). The relative contribution of the different families of recruits (Pocilloporidae: 60.4%, Poritidae: 18.8%, Acroporidae: 11.2%) and the very low recruitment rates (maximum: 35 recruits per tile, higher mean densities of 10.8 recruits per tile, average 40.77 recruits m^− 2 year^− 1) recorded at Moorea are similar to recruitment patterns recorded on sub-tropical reefs. Over the duration of the study period, we detected a marked seasonal variability in recruitment rates, with the peak recruitment for all families recorded in December-March periods, corresponding to periods of warmest SSTs. However, recruits of Acroporidae were also relatively abundant in the September-December period in some years, which coincides with the known spawning periods of some Acropora species. Total recruitment rate decreased after the first year of the survey, and was probably the result of the bleaching event that occurred in early 2002, which may have reduced fecundity of some coral populations. A lower proportion of recruits were found on the upper surface of the tiles (14.5%), compared to the lower surfaces (57.1%) and sides (28.4%), which is likely the result of intense grazing by herbivorous fish and urchins. We detected a patchy distribution at the station scale, and a significant variation in recruitment patterns among depths and sites. Pocilloporidae recruited more abundantly at 6 and 18 m, whereas Poritidae were generally more abundant at 12 m depth. In contrast, Acroporidae showed no clear depth pattern during the study period. Recruitment was lowest at the most exposed site (Haapiti), especially for Acroporidae, and probably reflects lower settlement rates and/or higher early post-settlement mortality caused by frequent high swells and their associated strong currents. These distinctive characteristics in recruitment patterns of the 3 dominant families of recruits underline the important role of life history strategies in understanding the spatial patterns, dynamics and maintenance processes of coral populations. The relatively low coral recruitment rates recorded from this study indicate that recovery from severe or frequent perturbations will be slow.     

Ostertagia ostertagi: protective immunity in calves. The development in calves of a protective immunity to infection with Ostertagia ostertagi

Resistance to the establishment of Ostertagia ostertagi was tested in calves which had been infected daily with 1000 larvae for 30, 80, 150, and 250 days. The proportion of a challenge dose of 30,000 larvae which became established decreased steadily and by Day 250 was only $\text{1}\text{20}$ of the proportion that became established in susceptible control animals. Neither the age of the host nor the age of the larvae used affected the proportion that became established. It is shown that the increasing resistance to the establishment of worms, and an average life span of 28 days, of worms which developed beyond the early fourth stage, adequately account for the course of infections in calves infected daily. Evidence is presented which suggests that the host's previous experience of infection increases the proportion of the worms that are inhibited in their development.     

The impact of nonlinear functional responses on the long-term evolution of food web structure

We investigate the long-term web structure emerging in evolutionary food web models when different types of functional responses are used. We find that large and complex webs with several trophic layers arise only if the population dynamics is such that it allows predators to focus on their best prey species. This can be achieved using modified Lotka-Volterra or Holling/Beddington functional responses with effective couplings that depend on the predator's efficiency at exploiting the prey, or a ratio-dependent functional response with adaptive foraging. In contrast, if standard Lotka-Volterra or Holling/Beddington functional responses are used, long-term evolution generates webs with almost all species being basal, and with additionally many links between these species. Interestingly, in all cases studied, a large proportion of weak links result naturally from the evolution of the food webs.     

The impact of diapause on the evolution of other life history traits in flesh flies

Summary Flesh flies (Sarcophagidae) collected in Costa Rica and Panama lack the pupal diapause that is characteristic of flesh flies from the temperate zone and tropical Africa. The absence of a diapause capacity in the neotropical species correlates with several other life history traits: in most species the post feeding wandering phase of the third larval instar is longer and duration is more variable, adult life is longer, clutch size is smaller, and more clutches are produced. Among species that have the capacity for diapause, risk is invested primarily in the diapausing stage and other life stages are brief. Though diapausing species are short-lived, they produce as many or more progeny than nondiapausing species by increasing clutch size. The slower and more variable developmental rate and increased adult longevity desynchronizes development and permits the nondiapausing species to spread an environmental risk over different stages of the life cycle, thus offering an alternative to diapause. Other traits such as body size, developmental velocity, thermal constant thresholds, thermal constants, age at first reproduction, and the interval between clutches do not appear related to the capacity for diapause.     

The adaptive dynamics of the evolution of host resistance to indirectly transmitted microparasites

We use adaptive dynamics and pairwise invadability plots to examine the evolutionary dynamics of host resistance to microparasitic infection transmitted indirectly via free stages. We investigate trade-offs between pathogen transmission rate and intrinsic growth rate. Adaptive dynamics distinguishes various evolutionary outcomes associated with repellors, attractors or branching points. We find criteria corresponding to these and demonstrate that a major factor deciding the evolutionary outcome is whether trade-offs are acceleratingly or deceleratingly costly. We compare and contrast two models and show how the differences between them lead to different evolutionary outcomes.     

A note on the dynamics of montane virginPicea yezoensis forest in the Greater Khingan Mts,Northeastern China

The vertical growth ofLarix gmelini in the north of the Greater Khingan Mts., north-eastern China is very fast during the first 50 yr. ThoughPinus sylvestris var.mongolica shows a higher growth rate, it will be replaced by the larch population.Picea jezoensis is the most tolerant tree species and will replace all other trees.The fire cycle is 110–120 yr. Most of the montane areas from 800 to 1000 m a.s.l. are not occupied by spruce forest.There is a sapling bank with large numbers of saplings at ages below 20 yr in the understory in the spruce forest. The sapling bank is the key to maintain the steady state of the spruce forest. The persistent sapling bank in the life history of spruce governs the dynamics of the spruce forest controlled by the intensity and frequency of disturbances.The author is H. Q. Wu     

Rapidly converging numerical algorithms for models of population dynamics

We propose algorithms for the approximation of the age distributions of populations modeled by the McKendrick-von Foerster and the Gurtin-MacCamy systems both in one- and two-sex versions. For the one-sex model methods of second and fourth order are proposed. For the two-sex model a second order method is described. In each case the convergence is demonstrated. Several numerical examples are given.