Diet and cannibalism in plainfin midshipman Porichthys notatus.

The macroscopic and microscopic diversity of potential food items available in the nests of plainfin midshipman Porichthys notatus were quantified and compared with items that were found in the stomach and intestine (digestive tract) of the guarding males. In this species, males occur as one of two possible reproductive morphs: guarder males that care for young and sneaker males that parasitize the courtship and care of guarder males. Although it was predicted that guarder males would have fewer feeding opportunities due to their confinement to the nest, they in fact had more food items in their digestive tracts than did sneaker males and females. Date in the breeding season (a proxy of care duration) and body condition were not correlated with the amount of food consumed by guarder males. The main type of food consumed was P. notatus embryos; 69% of all guarder males sampled had cannibalized offspring. By comparing the diet of both sexes and tactics, this study sheds light on some of the strategies designed to cope with the costs of providing parental care.

Tundra ecosystems observed to be CO2 sources due to differential amplification of the carbon cycle.

Are tundra ecosystems currently a carbon source or sink? What is the future trajectory of tundra carbon fluxes in response to climate change? These questions are of global importance because of the vast quantities of organic carbon stored in permafrost soils. In this meta-analysis, we compile 40 years of CO2 flux observations from 54 studies spanning 32 sites across northern high latitudes. Using time-series analysis, we investigated if seasonal or annual CO2 fluxes have changed over time, and whether spatial differences in mean annual temperature could help explain temporal changes in CO2 flux. Growing season net CO2 uptake has definitely increased since the 1990s; the data also suggest (albeit less definitively) an increase in winter CO2 emissions, especially in the last decade. In spite of the uncertainty in the winter trend, we estimate that tundra sites were annual CO2 sources from the mid-1980s until the 2000s, and data from the last 7 years show that tundra continue to emit CO2 annually. CO2 emissions exceed CO2 uptake across the range of temperatures that occur in the tundra biome. Taken together, these data suggest that despite increases in growing season uptake, tundra ecosystems are currently CO2 sources on an annual basis.

Virulence evolution in a host-parasite system in the absence of viral evolution.

QUESTION: How does virulence evolve in the Drosophila melanogaster/sigma virus (DMelSV) system? ORGANISMS: Drosophila melanogaster (host) and DMelSV (parasite). EMPIRICAL METHODS: Artificial selection on whole-carcass viral titre of infected flies, including two selection regimes (maternal and biparental transmission) and three treatments within each regime (increased titre, decreased titre, and control). The maternal transmission selection regime lasted for six generations, while the biparental transmission selection regime lasted for twelve generations. We further quantified virulence by estimating the fecundity, viability, and development time of infected flies. Finally, we sequenced virus strains at the end of selection. PREDICTIONS AND CONCLUSIONS: Titre is defined here as the number of viral genomes inside a single fly, while virulence is defined as harm to host. We predicted that titre would respond to both increased and decreased selection, that virulence would evolve as a positively correlated response, and that sequence evolution in the viruses would be responsible for these changes. Titre did respond to selection in the biparental regime, although both high and control lines both demonstrated increased titre, while the titre of the low lines did not change. One component of virulence, development time, was positively correlated with titre in the biparental transmission lines (maternal transmission lines were not scored for virulence). However, we detected few (and in some cases, no) genomic changes in the virus, making viral evolution unlikely to be responsible for the response to selection and the association between development time and titre.

Decoupling the effects of logging and hunting on an afrotropical animal community.

In tropical forests, hunting nearly always accompanies logging. The entangled nature of these disturbances complicates our ability to resolve applied questions, such as whether secondary and degraded forest can sustain populations of tropical animals. With the expansion of logging in central Africa, conservation depends on knowledge of the individual and combined impacts of logging and hunting on animal populations. Our goals were (1) to decouple the effects of selective logging and hunting on densities of animal guilds, including apes, duikers, monkeys, elephant, pigs, squirrels, and large frugivorous and insectivorous birds and (2) to compare the relative importance of these disturbances to the effects of local-scale variation in forest structure and fruit abundance. In northern Republic of Congo, we surveyed animals along 30 transects positioned in forest disturbed by logging and hunting, logging alone, and neither logging nor hunting. While sampling transects twice per month for two years, we observed 47 179 animals of 19 species and eight guilds in 1154 passages (2861 km). Species densities varied by as much as 480% among forest areas perturbed by logging and/or hunting, demonstrating the strong effects of these disturbances on populations of some species. Densities of animal guilds varied more strongly with disturbance type than with variation in forest structure, canopy cover, and fruit abundance. Independently, logging and hunting decreased density of some guilds and increased density of others: densities varied from 44% lower (pigs) to 90% higher (insectivorous birds) between logged and unlogged forest and from 61% lower (apes) to 77% higher (frugivorous birds) between hunted and unhunted forest. Their combined impacts exacerbated decreases in populations of some guilds (ape, duiker, monkey, and pig), but counteracted one another for others (squirrels, insectivorous and frugivorous birds). Together, logging and hunting shifted the relative abundance of the animal community away from large mammals toward squirrels and birds. Logged forest, even in the absence of hunting, does not maintain similar densities as unlogged forest for most animal guilds. To balance conservation with the need for economic development and wild meat in tropical countries, landscapes should be spatially managed to include protected areas, community hunting zones, and production forest.

Food availability alters the effects of larval temperature on Aedes aegypti growth.

Variation in temperature and food availability in larval habitats can influence the abundance, body size, and vector competence of the mosquito Aedes aegypti. Although increased temperature has energetic costs for growing larvae, how food resources influence the developmental response of this mosquito species to thermal conditions is unknown. We explored how rearing temperature and food affect allometric scaling between wing size and epidermal cell size in Ae. aegypti. Mosquitoes were reared at 22 and 28 degrees C across a gradient of field-collected detritus designed to simulate commonly observed natural larval food resources. Overall, reduced temperature and increased food level increased wing size, but only temperature affected cell size. Females fed the least food had the longest time to maturation, and their increases in wing size induced by cold temperature were associated with larger, rather than more, cells. By contrast, males fed the most food had the shortest time to maturation, and their increases in wing size induced by cold temperature were associated with more, rather than larger, cells. Therefore, food levels can alter the underlying physiological mechanisms generating temperature-size patterns in mosquitoes, suggesting that the control of development is sensitive to the combination of nutrient and thermal conditions, rather than each independently. Conditions prolonging development time may favor increased cell division over growth. We suggest that understanding the effects of climate change on Ae. aegypti vectorial capacity requires an improved knowledge of how water temperature interacts with limited food resources and competition in aquatic container habitats.

A simple model for complex dynamical transitions in epidemics.

Dramatic changes in patterns of epidemics have been observed throughout this century. For childhood infectious diseases such as measles, the major transitions are between regular cycles and irregular, possibly chaotic epidemics, and from regionally synchronized oscillations to complex, spatially incoherent epidemics. A simple model can explain both kinds of transitions as the consequences of changes in birth and vaccination rates. Measles is a natural ecological system that exhibits different dynamical transitions at different times and places, yet all of these transitions can be predicted as bifurcations of a single nonlinear model.

Plants as reef fish: fitting the functional form of seedling recruitment.

The life histories of many species depend first on dispersal to local sites and then on establishment. After dispersal, density-independent and density-dependent mortalities modify propagule supply, determining the number of individuals that establish. Because multiple factors influence recruitment, the dichotomy of propagule versus establishment limitation is best viewed as a continuum along which the strength of propagule or establishment limitation changes with propagule input. To evaluate the relative importance of seed and establishment limitation for plants, we (1) describe the shape of the recruitment function and (2) use limitation and elasticity analyses to quantify the sensitivity of recruitment to perturbations in seed limitation and density-independent and density-dependent mortality. Using 36 seed augmentation studies for 18 species, we tested four recruitment functions against one another. Although the linear model (accounting for seed limitation and density-independent mortality) fitted the largest number of studies, the nonlinear Beverton-Holt model (accounting for density-dependent mortality) performed better at high densities of seed augmentation. For the 18 species, seed limitation constrained population size more than other sources of limitation at ambient conditions. Seedling density reached saturation with increasing seed density in many studies, but at such high densities that seedling density was primarily limited by seed availability rather than microsite availability or density dependence.

Are fluctuating asymmetry studies adequately sampled? Implications of a new model for size distribution.

Previous work on fluctuating asymmetry (FA) has highlighted its controversial relationship with environmental stress and genetic architecture. While size-based measures of FA have been assumed to have half-normal distributions within populations, studies of model developmental mechanisms have suggested other plausible distributions for FA. We investigated the distribution of FA in large empirical data sets of wing shape and wing size asymmetry from three species of insects (cotton aphid Aphis gossipyii Glover, honeybee Apis mellifera, and long-legged fly Chrysosoma crinitus). Regardless of measurement method, FA was best described by a double Pareto-lognormal (DPLN) distribution or one of its limiting functional forms. To investigate convergence of mean sample FA to the population mean at various sample sizes, we sampled repeatedly under a DPLN distribution using parameter values that best fitted our data. Sample variances are much larger, and hence, convergence is slowed considerably with univariate or multivariate size-based measures of FA in contrast to a multivariate shape-based measure of FA. We suggest that much of the past work on FA may be undersampled, and we recommend using multivariate shape-based approaches or collecting larger data sets in future studies. We also discuss the implications of the DPLN distribution for understanding the developmental mechanisms underlying FA.

Chaos and biological complexity in measles dynamics.

Measles epidemiology offers a unique perspective on the construction of models to describe the dynamics of ecological systems. Simple models of measles transmission can generate deterministic chaos by various mechanisms. However, incorporating more biological realism into the model, in the form of age structure and realism in the seasonal forcing function, can suppress complex dynamics. Adding stochastic terms to the models restores complex dynamics, but raises new questions about demographic scale and population structure in these models.

Canonical functions for dispersal-induced synchrony.

Two processes are universally recognized for inducing spatial synchrony in abundance: dispersal and correlated environmental stochasticity. In the present study we seek the expected relationship between synchrony and distance in populations that are synchronized by density-independent dispersal. In the absence of dispersal, synchrony among populations with simple dynamics has been shown to echo the correlation in the environment. We ask what functional form we may expect between synchrony and distance when dispersal is the synchronizing agent. We formulate a continuous-space, continuous-time model that explicitly represents the time evolution of the spatial covariance as a function of spatial distance. Solving this model gives us two simple canonical functions for dispersal-induced covariance in spatially extended populations. If dispersal is rare relative to birth and death, then covariances between nearby points will follow the dispersal distance distribution. At long distances, however, the covariance tails off according to exponential or Bessel functions (depending on whether the population moves in one or two dimensions). If dispersal is common, then the covariances will follow the mixture distribution that is approximately Gaussian around the origin and with an exponential or Bessel tail. The latter mixture results regardless of the original dispersal distance distribution. There are hence two canonical functions for dispersal-induced synchrony

Using Moment Equations to Understand Stochastically Driven Spatial Pattern Formation in Ecological Systems

Spatial patterns in biological populations and the effect of spatial patterns on ecological interactions are central topics in mathematical ecology. Various approaches to modeling have been developed to enable us to understand spatial patterns ranging from plant distributions to plankton aggregation. We present a new approach to modeling spatial interactions by deriving approximations for the time evolution of the moments (mean and spatial covariance) of ensembles of distributions of organisms; the analysis is made possible by "moment closure," neglecting higher-order spatial structure in the population. We use the growth and competition of plants in an explicitly spatial environment as a starting point for exploring the properties of second-order moment equations and comparing them to realizations of spatial stochastic models. We find that for a wide range of effective neighborhood sizes (each plant interacting with several to dozens of neighbors), the mean-covariance model provides a useful and analytically tractable approximation to the stochastic spatial model, and combines useful features of stochastic models and traditional reaction-diffusion-like models. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Spatial heterogeneity, nonlinear dynamics and chaos in infectious diseases.

There is currently considerable interest in the role of nonlinear phenomena in the population dynamics of infectious diseases. Childhood diseases such as measles are particularly well documented dynamically, and have recently been the subject of analyses (of both models and notification data) to establish whether the pattern of epidemics is chaotic. Though the spatial dynamics of measles have also been extensively studied, spatial and nonlinear dynamics have only recently been brought together. The present review concentrates mainly on describing this synthesis. We begin with a general review of the nonlinear dynamics of measles models, in a spatially homogeneous environment. Simple compartmental models (specifically the SEIR model) can behave chaotically, under the influence of strong seasonal 'forcing' of infection rate associated with patterns of schooling. However, adding observed heterogeneities such as age structure can simplify the deterministic dynamics back to limit cycles. By contrast all current strongly seasonally forced stochastic models show large amplitude irregular fluctuations, with many more 'fadeouts' of infection that is observed in real communities of similar size. This indicates that (social and/or geographical) spatial heterogeneity is needed in the models. We review the exploration of this problem with nonlinear spatiotemporal models. The few studies to date indicate that spatial heterogeneity can help to increase the realism of models. However, a review of nonlinear analyses of spatially subdivided measles data show that more refinements of the models (particularly in representing the impact of human demographic changes on infection dynamics) are required. We conclude with a discussion of the implication of these results for the dynamics of infectious diseases in general and, in particular, the possibilities of cross fertilization between human disease epidemiology and the study of plant and animal diseases.

Chaos and complexity in measles models: a comparative numerical study.

Recurrent epidemics of measles in developed countries offer a proving ground for current theories of complicated dynamics in ecological and epidemiological systems. This paper contrasts the basic forced SEIR model for measles with a variety of more complicated and realistic models, showing that variations in seasonal forcing and age-structured mixing patterns can generate a wide range of global dynamics. The well-known chaotic dynamics of the forced SEIR model appear to be absent from more realistic models, suppressed by the buffering effect of a low-risk group of pre-school children. These results, and the variety of measles dynamics seen in real populations with different demographic and geographic patterns, point out the need for age- and spatially-structured measles models and suggest caution in the construction of models for complicated systems.

Analytic models for the patchy spread of plant disease.

Plant epidemiologists have long been concerned with the patchy nature of plant disease epidemics. This paper presents a new analytical model for patchy plant epidemics (and patchy dynamics in general), using a second-order approximation to capture the spatial dynamics in terms of the densities and spatial covariances of healthy and infected hosts. Using these spatial moment equations helps us to explain the dynamic growth of patchiness during the early phase of the epidemic, and how the patchiness feeds back on the growth rate of the epidemic. Both underlying heterogeneity in the host spatial arrangement and dynamically generated heterogeneity in the spatial arrangement of infected plants initially accelerate but later decelerate the epidemic.

Space, persistence and dynamics of measles epidemics.

This paper explores the relations between persistence and dynamics in measles epidemics. Most current models, including the stochastic seasonally forced and age-structured models examined here, fail to capture simultaneously the observed dynamics and persistence characteristics of epidemics in large urban populations before vaccination. Summary measures of persistence and trienniality allow us to compare epidemics in England, New York and Copenhagen with results of non-spatial and spatial stochastic models. Spatial (metapopulation) structure allow persistence and triennial dynamics to coexist i this class of models. The spatial dynamics of measles, for which detailed spatiotemporal data are available, may serve as a useful test of ideas applicable to other epidemiological and ecological systems with an important spatial component.

Impact of vaccination on the spatial correlation and persistence of measles dynamics.

The onset of measles vaccination in England and Wales in 1968 coincided with a marked drop in the temporal correlation of epidemic patterns between major cities. We analyze a variety of hypotheses for the mechanisms driving this change. Straightforward stochastic models suggest that the interaction between a lowered susceptible population (and hence increased demographic noise) and nonlinear dynamics is sufficient to cause the observed drop in correlation. The decorrelation of epidemics could potentially lessen the chance of global extinction and so inhibit attempts at measles eradication.