Environmental uncertainty, autocorrelation and the evolution of survival

Survival is a key fitness component and the evolution of age- and stage-specific patterns in survival is a central question in evolutionary biology. In variable environments, favouring chances of survival at the expense of other fitness components could increase fitness by spreading risk across uncertain conditions, especially if environmental conditions improve in the future. Both the magnitude of environmental variation and temporal autocorrelation in the environment might therefore affect the evolution of survival patterns. Despite this, the influence of temporal autocorrelation on the evolution of survival patterns has not been addressed. Here, we use a trade-off structure which reflects the empirically inspired paradigm of acquisition and allocation of resources to investigate how the evolutionarily stable survival probability is shaped in variable, density-dependent environments. We show that temporal autocorrelation is likely to be an important aspect of environmental variability that contributes to shaping age- and stage-specific patterns of survival probabilities in nature.     

Integrating conservation and development: effective trade-offs between biodiversity and cost in the selection of protected areas

Strategies are needed for reconciling competing demands at the regional level when areas are to be selected for protection and there are associated costs, possibly equivalent to forgone development opportunties. As an alternative to the fixed scaling (or weighting) of costs and benefits required by cost-benefit analysis, multi-criteria analyses allow the exploration of alternative weightings and a summary trade-off curve to determine preferred solutions. For alternative sets of areas, total cost could be plotted against total represented biodiversity, but a more consistent approach should look at trade-off space at the level of individual areas. For a given weighting, an area is assigned protection if and only if its contribution to total biodiversity, CB, exceeds its equivalent cost, EC (in biodiversity units). Because CB for a given area depends on which other areas are also protected, it can be more or less than EC. Here we develop an iterative strategy for selecting areas, such that, for a given weighting, an area is in the final protected set if and only if its final CB value is greater than its EC value. Sensitivity analysis is used to identify those areas that: (1) are assigned protection even when low weight is given to biodiversity, or (2) are not assigned protection even when high weight is given to biodiversity. This approach is applicable in principle to any surrogate measure for biodiversity; here examples are presented in which environmental data are summarized as an environmental space.     

Probability of fixation under weak selection: a branching process unifying approach

We link two-allele population models by Haldane and Fisher with Kimura's diffusion approximations of the Wright-Fisher model, by considering continuous-state branching (CB) processes which are either independent (model I) or conditioned to have constant sum (model II). Recent works by the author allow us to further include logistic density-dependence (model III), which is ubiquitous in ecology. In all models, each allele (mutant or resident) is then characterized by a triple demographic trait: intrinsic growth rate r, reproduction variance sigma and competition sensitivity c. Generally, the fixation probability u of the mutant depends on its initial proportion p, the total initial population size z, and the six demographic traits. Under weak selection, we can linearize u in all models thanks to the same master formula u = p + p(1 - p)[g(r)s(r) + g(sigma)s(sigma) + g(c)s(c)] + o(s(r),s(sigma),s(c), where s(r) = r' - r, s(sigma) = sigma-sigma' and s(c) = c - c' are selection coefficients, and g(r), g(sigma), g(c) are invasibility coefficients (' refers to the mutant traits), which are positive and do not depend on p. In particular, increased reproduction variance is always deleterious. We prove that in all three models g(sigma) = 1/sigma and g(r) = z/sigma for small initial population sizes z. In model II, g(r) = z/sigma for all z, and we display invasion isoclines of the 'mean vs variance' type. A slight departure from the isocline is shown to be more beneficial to alleles with low sigma than with high r. In model III, g(c) increases with z like ln(z)/c, and g(r)(z) converges to a finite limit L > K/sigma, where K = r/c is the carrying capacity. For r > 0 the growth invasibility is above z/sigma when z < K, and below z/sigma when z > K, showing that classical models I and II underestimate the fixation probabilities in growing populations, and overestimate them in declining populations.     

Reservoirs and alternate hosts for pathogens of commercially important crustaceans: a review

There is a considerable body of literature describing the causative agents of many diseases of crustaceans. Given that many of these crustaceans support commercially important fisheries, it is somewhat surprising that comparatively little information is available regarding the natural transmission pathways and reservoirs of many of the disease-causing agents. In this paper we review what is known about reservoirs and alternate hosts for several important diseases of commercially important crustaceans and provide recommendations on future areas of research.     

EVOLUTION AND EXTINCTION IN A CHANGING ENVIRONMENT: A QUANTITATIVE-GENETIC ANALYSIS

Because of the ubiquity of genetic variation for quantitative traits, virtually all populations have some capacity to respond evolutionarily to selective challenges. However, natural selection imposes demographic costs on a population, and if these costs are sufficiently large, the likelihood of extinction will be high. We consider how the mean time to extinction depends on selective pressures (rate and stochasticity of environmental change, and strength of selection), population parameters (carrying capacity, and reproductive capacity), and genetics (rate of polygenic mutation). We assume that in a randomly mating, finite population subject to density-dependent population growth, individual fitness is determined by a single quantitative-genetic character under Gaussian stabilizing selection with the optimum phenotype exhibiting directional change, or random fluctuations, or both. The quantitative trait is determined by a finite number of freely recombining, mutationally equivalent, additive loci. The dynamics of evolution and extinction are investigated, assuming that the population is initially under mutation-selection-drift balance. Under this model, in a directionally changing environment, the mean phenotype lags behind the optimum, but on the average evolves parallel to it. The magnitude of the lag determines the vulnerability to extinction. In finite populations, stochastic variation in the genetic variance can be quite pronounced, and bottlenecks in the genetic variance temporarily can impair the population's adaptive capacity enough to cause extinction when it would otherwise be unlikely in an effectively infinite population. We find that maximum sustainable rates of evolution or, equivalently, critical rates of environmental change, may be considerably less than 10% of a phenotypic standard deviation per generation.     

Parasites detect host spatial pattern and density: a field experimental analysis

Summary A common assumption in mathematical models of parasitism is that the susceptibility to parasitism of an individual host increases both with host density and the degree of host spatial aggregation. To determine whether this assumption is correct in nature, we developed a factorial field experiment with the parasitic marine isopod Hemioniscus balani and its barnacle host Chthamalus dalli. Our factorial design enabled evaluation of the separate effects on parasitism of the two factors (host density and host spatial pattern) and also to assess the host density-spatial pattern interaction effect. Both host density and spatial aggregation were found to lead to increased parasitism, and the interaction effect was nonsignificant. These findings are the first experimental field demonstration that these processes occur in nature, as widely assumed in ecological theory.     

Host density and ectoparasite avoidance in the common lizard (Lacerta vivipara)

Increased transmission of parasites and diseases is generally considered as a major cost of social life. In this study we tested the hypothesis regarding ectoparasites as a cost of living in crowded habitats in the common lizard (Lacerta vivipara). We used two approaches to explore this question. First, we tested if ectoparasite load and prevalence are positively correlated with host density in the field. Second, we experimentally tested if lizards avoid parasitized conspecifics. Contrary to expectation, we found that (1) ectoparasite load is negatively correlated with lizard density; (2) prevalence does not significantly increase with density; (3) unparasitized lizards do not avoid parasitized conspecifics but are attracted by them whatever their parasite load. These findings suggest that ectoparasites cannot be considered as a cost of living at high density in the common lizard, in spite of the potential negative impact mites may have on lizard fitness. Received: 18 August 1996 / Accepted: 7 February 1997     

The role of adaptations in two-strain competition for sylvatic Trypanosoma cruzi transmission

This study presents a continuous-time model for the sylvatic transmission dynamics of two strains of Trypanosoma cruzi enzootic in North America, in order to study the role that adaptations of each strain to distinct modes of transmission (classical stercorarian transmission on the one hand, and vertical and oral transmission on the other) may play in the competition between the two strains. A deterministic model incorporating contact process saturation predicts competitive exclusion, and reproductive numbers for the infection provide a framework for evaluating the competition in terms of adaptive trade-off between distinct transmission modes. Results highlight the importance of oral transmission in mediating the competition between horizontal (stercorarian) and vertical transmission; its presence as a competing contact process advantages vertical transmission even without adaptation to oral transmission, but such adaptation appears necessary to explain the persistence of (vertically-adapted) T. cruzi IV in raccoons and woodrats in the southeastern United States.     

Host selection by the Louse Fly Crataerina pallida, an avian nest ectoparasite of the Common Swift Apus apus

Preferences by parasites for particular hosts may have important implications for the functioning of host–parasite systems, however, this parasitic life-history trait remains little studied. No detrimental effect of Louse Fly Crataerina pallida parasitism has been found on Common Swift Apus apus nestling hosts. Host selection choices may be mediating the effect this parasite has and account for this apparent avirulence. Two aspects of parasite host selection were studied at a breeding colony of Common Swifts during 2008; (1) intra-brood differences in C. pallida parasitism were studied to determine the influence of nestling rank, (2) differences in male and female C. pallida parasitism were investigated, as they may result in varying costs of parasitism to hosts. C. pallida populations were found to preferentially parasitize higher rather than lower ranking nestlings within broods of both two and three chicks. Greater proportions of females were seen upon nestlings than at the nest, and upon higher ranking than lower ranking nestlings within broods. These results indicate that host selection occurs and this may thus account for the lack of parasitic virulence reported within this host–parasite system.     

Factors affecting the divergence of mate recognition systems in the Limnocytherinae (Crustacea,Ostracoda)

Specific Mate Recognition Systems (SMRS) consist of a set of morphological, behavioural and physiological traits which allow mate recognition. The Limnocytherinae, a lineage of non-marine podocopid Ostracoda, have a relatively wide diversity of copulatory modules, a concept largely congruent with the morphological part of the SMRS. The present paper describes the various copulatory modules in some detail and discusses potential mechanisms responsible for the divergence of these modules. Although none of the processes was thus far demonstrated directly, resulting patterns provide indirect evidence that four different mechanisms contribute. Stochastic processes (chance) as well as developmental and other phylogenetic constraints are involved in the initial selection (choice) of modified structures. Subsequent (positive) directional sexual selection on traits of the recognition systems causes radiative speciation within lineages. At all times, natural selection acts on the development of these structures, either stabilising or negative directional. A number of potential tests for these hypotheses are suggested.