Life History Model Research Articles

IDENTIFYING FITNESS AND OPTIMAL LIFE-HISTORY STRATEGIES FOR AN ASEXUAL FILAMENTOUS FUNGUS

Filamentous fungi are ubiquitous and ecologically important organisms with rich and varied life histories, however, there is no consensus on how to identify or measure their fitness. In the first part of this study we adapt a general epidemiological model to identify the appropriate fitness metric for a saprophytic filamentous fungus. We find that fungal fitness is inversely proportional to the equilibrium density of uncolonized fungal resource patches which, in turn, is a function of the expected spore production of a fungus. In the second part of this study we use a simple life history model of the same fungus within a resource patch to show that a bang-bang resource allocation strategy maximizes the expected spore production, a critical fitness component. Unlike bang-bang strategies identified in other life-history studies, we find that the optimal allocation strategy for saprophytes does not entail the use of all of the resources within a patch.

The logic of skipped spawning in fish

That sexually mature fish skip reproduction, especially in response to poor condition, has been documented in many species. We present results from an energy-allocation life history model that shed light on the underlying logic of skipped spawning, based on the Northeast Arctic stock of Atlantic cod (Gadus morhua). The model predicts that skipped spawning is a regular phenomenon, with up to 30% of the sexually mature biomass skipping spawning. Spawning should be skipped if the expected future gain in reproductive output, discounted by survival, more than balances the expected reproductive success the current year. Skipped spawning was most common (i) among potential second-time spawners and (ii) early in life, (iii) when fishing mortality at the spawning grounds was high, (iv) when fishing mortality at the feeding grounds was low, (v) when natural mortality was low, and (vi) when the energetic and mortality costs associated with migration and spawning were high. Cod skipped spawning more often when food availability was both increased (opportunities for better growth) and decreased (too little energy for gonad development), and this pattern interacted with mortality rate. We conclude that skipped spawning may be more widespread than appreciated and highlight potential consequences for the understanding of stock–recruitment relationships.

Costs of breeding and their effects on the direction of sexual selection

A recent life-history model has challenged the importance of the operational sex ratio and the potential reproductive rates of males and females as the factors most important for the control of sexual selection, arguing that the cost of breeding, interpreted as the probability of dying as a consequence of the current breeding attempt, is the single most important factor that best predicts a mating system. In one species of bushcricket, the mating system can be reversed by resource manipulation. Here, we examine the costs of breeding in this system. Consistent with the model, increased costs of breeding can explain female competition and increased male choosiness under resource limitation. However, this is due to differences in the time required for a breeding attempt, rather than differences in breeding mortality which did not differ between the sexes. In general, males lived longer than females and we discuss the possible reasons behind this pattern of sex-biased non-breeding mortality.

Life-history diversification in the Middle Danubian fish fauna - a conservation perspective

I evaluated the diversity of the life-history attributes of Middle Danubian fish species in relation to a trilateral life-history continuum model. Life-history strategies could be characterized for 70 species. Multivariate analysis identified a gradient in life-history variables from species having larger body size, delayed maturation, longer life span, larger fecundity and no marked parental care to species with the opposite suite of traits. A second gradient further delineated species based predominantly on egg size and the development of parental care. Diversity of life-history strategies tended to increase from headwater to large river habitats. A comparison of life-history attributes between native and exotic fishes showed that exotic species modify the general life-history diversification of the native fauna. Exotic species were rather small, performed relatively well developed parental care and laid a small number of large eggs (i.e. they displayed intermediate-equilibrium strategist features). Temporal changes in the patterns of life-history diversification (i.e. the rarity or disappearance of large bodied, highly fecund species with late maturation (i.e. periodic strategist sturgeons), and the invasion of the intermediate-equilibrium strategist exotic species) could be well associated with the alteration of the Middle Danubian hydrosystem. The present study supports the prediction that life-history variables can be useful in the monitoring and conservation management of running water habitats.

Risk Assessment of UK Skylark Populations Using Life-History and Individual-Based Landscape Models

Following a workshop exercise, two models, an individual-based landscape model (IBLM) and a non-spatial life-history model were used to assess the impact of a fictitious insecticide on populations of skylarks in the UK. The chosen population endpoints were abundance, population growth rate, and the chances of population persistence. Both models used the same life-history descriptors and toxicity profiles as the basis for their parameter inputs. The models differed in that exposure was a pre-determined parameter in the life-history model, but an emergent property of the IBLM, and the IBLM required a landscape structure as an input. The model outputs were qualitatively similar between the two models. Under conditions dominated by winter wheat, both models predicted a population decline that was worsened by the use of the insecticide. Under broader habitat conditions, population declines were only predicted for the scenarios where the insecticide was added. Inputs to the models are very different, with the IBLM requiring a large volume of data in order to achieve the flexibility of being able to integrate a range of environmental and behavioural factors. The life-history model has very few explicit data inputs, but some of these relied on extensive prior modelling needing additional data as described in Roelofs et al. (2005, this volume). Both models have strengths and weaknesses; hence the ideal approach is that of combining the use of both simple and comprehensive modeling tools.

Population-level Assessment of Risks of Pesticides to Birds and Mammals in the UK

It is generally acknowledged that population-level assessments provide a better measure of response to toxicants than assessments of individual-level effects. Population-level assessments generally require the use of models to integrate potentially complex data about the effects of toxicants on life-history traits, and to provide a relevant measure of ecological impact. Building on excellent earlier reviews we here briefly outline the modelling options in population-level risk assessment. Modelling is used to calculate population endpoints from available data, which is often about individual life histories, the ways that individuals interact with each other, the environment and other species, and the ways individuals are affected by pesticides. As population endpoints, we recommend the use of population abundance, population growth rate, and the chance of population persistence. We recommend two types of model: simple life-history models distinguishing two life-history stages, juveniles and adults; and spatially-explicit individual-based landscape models. Life-history models are very quick to set up and run, and they provide a great deal of insight. At the other extreme, individual-based landscape models provide the greatest verisimilitude, albeit at the cost of greatly increased complexity. We conclude with a discussion of the implications of the severe problems of parameterising models.

Is Invariance Across Animal Species Just an Illusion?

There is obvious variation in the way different animals live their lives—in their life span, in their age and size at maturity, and in their size as full-grown adults, to name a few attributes. But are there fundamental similarities in the life history strategies that different animals use? Charnov (1) argued that there are: He proposed fundamental similarities—“life history invariants”—to be a major explanatory ingredient of life history evolution. Life history invariants generalize a life history model over species boundaries and over a wide range of animal sizes, leading to an understanding of universal life history strategies. On page 1236 of this issue, Nee et al. (2) call into question the principal method to detect life history invariants. The authors have determined that the approach is misleading, throwing the very existence of the concept into doubt.

The evolution of egg rejection by cuckoo hosts in Australia and Europe

Exploitation of hosts by brood parasitic cuckoos is expected to stimulate a coevolutionary arms race of adaptations and counteradaptations. However, some hosts have not evolved defenses against parasitism. One hypothesis to explain a lack of host defensesisthatthelife-historystrategiesof somehostsreducethecost ofparasitism totheextent thatacceptingparasiticeggsinthe nest is evolutionarily stable. Under this hypothesis, it pays hosts to accept cuckoo eggs if (1) the energetic cost of raising the cuckoo is low, (2) there is time to renest, and (3) clutch size is small. We parasitized the nests of host and nonhost species with nonmimetic model eggs to test whether the evolution of egg recognition by cuckoo hosts could be explained by life-history variables of the host. The most significant factor explaining rates of rejection of model eggs was whether or not a species was a cuckoo host, with hosts rejecting model eggs at a higher rate than nonhosts. Egg-rejection rates were also explained by visibility within the nest and by cuckoo mass. We found little support for the life-history model of egg rejection. Our results suggest that parasitism is always sufficiently costly to select for host defenses and that the evolution of defenses may be limited by proximate constraints such as visibility within the nest. Key words: brood parasitism, coevolution, cowbirds, cuckoos, life-history strategies. [Behav Ecol]

Lack of relationship between simulated fish population responses and their life history traits: inadequate models, incorrect analysis, or site-specific factors?

Relationships between fish population responses to changes in their vital rates and commonly available life history traits would be a powerful screening tool to guide management about species vulnerability, to focus future data collection on species and life stages of concern, and to aid in designing effective habitat enhancements. As an extension of previous analyses by others, I analyzed the responses to changes in fecundity and yearling survival of age-structured matrix and individual-based population models of 17 populations comprising 10 species. Simulations of the matrix models showed that the magnitude of population responses, but not the relative order of species sensitivity, depended on the state (sustainable or undergoing excessive removals) of the population. Matrix and individual-based models predicted population responses that appeared to be unrelated to their species-level life history traits when responses were plotted on a three-end-point life history surface. Density-dependent adult growth was added to the lake trout (Salvelinus namaycush) matrix model, and simulations demonstrated the potential importance to predicted responses of density-dependent processes outside the usual spawner–recruit relationship. Four reasons for the lack of relationship between population responses and life history traits related to inadequate population models, incorrect analysis, inappropriate life history model, and important site-specific factors are discussed.

Integrating the effects of fish exploitation and interspecific competition into current life history theories: an example with lacustrine brook trout (Salvelinus fontinalis) populations

We used data from 17 populations of lacustrine brook trout (Salvelinus fontinalis) of the Canadian Shield, southern Quebec, to test whether early maturity (in males and females) and high reproductive effort (in females) are associated with increased (i) fish exploitation (sportfishing) and (ii) interspecific competition through their effects on growth and survival. The age at maturity of males and females was inversely related to the intensity of both fishing and interspecific competition. Fishing and interspecific competition affect the age at maturity through their effect on adult survival but not on growth, supporting predictions of life history models based on survival. In contrast, we did not find consistent effects of interspecific competition and fishing on the gonadosomatic index of females, which was directly related to survival (in all populations) and to the age at maturity (in exploited populations). These latter results are contrary to the predictions of life history models under the assumption that survival is directly related to growth rate. Our results suggest that reproductive effort and age at maturity are not dependent on growth when survival is independent of growth, as is the case in exploited and sympatric populations experiencing low adult survival but high growth.

Life history consequences of overexploitation to population recovery in Northwest Atlantic cod (Gadus morhua)

Changes to life history traits are often concomitant with prolonged periods of exploitation. In the Northwest Atlantic, 30- to 40-year declines of more than 90% of Atlantic cod (Gadus morhua) have been associated with significant reductions in age and length at maturity, changes most parsimoniously explained as genetic responses to fishing. Increased survival costs of reproduction associated with earlier maturity, resulting in higher natural mortality and shorter life span, negatively affect population growth rate and rate of recovery. Coupled with lower hatching rate among first-time spawners and smaller size at maturity, a modest reduction in age from 6 to 4 years can reduce annual population growth in Atlantic cod by 25%–30%, based on the output of a stochastic, age-structured life history model. Earlier maturity more than doubles the probability of negative population growth every generation. These results underscore the potential for fishing-induced changes to life history traits alone to generate slow or negligible recovery in marine fishes, exacerbating negative impacts on population growth resulting from ecosystem-level alterations to interspecific competition and predation.

Correlates of Species Richness in Mammals: Body Size, Life History, and Ecology

We present the most extensive examination to date of proposed correlates of species richness. We use rigorous phylogenetic comparative techniques, data for 1,692 mammal species in four clades, and multivariate statistics to test four hypotheses about species richness and compare the evidence for each. Overall, we find strong support for the life-history model of diversification. Species richness is significantly correlated with shorter gestation period in the carnivores and large litter size in marsupials. These traits and short interbirth intervals are also associated with species richness in a pooled analysis of all four clades. Additionally, we find some support for the abundance hypotheses in different clades of mammals: abundance correlates positively with species richness in primates but negatively in microchiropterans. Our analyses provide no evidence that mammalian species richness is associated with body size or degree of sexual dimorphism.

A life history model for the San Francisco Estuary population of the Chinese mitten crab, Eriocheir sinensis (Decapoda: Grapsoidea)

First discovered in San Francisco Bay in 1992, the Chinese mitten crab,Eriocheir sinensis, has become established over hundreds of km2 of the San Francisco Estuary. Ecological and economic impacts of this invasive species motivated our search for a greater understanding of the crab’s life history as an important step in better management and control. Data for this life history model comes from the authors’ research and scientific literature. Juvenile crabs migrate from the Estuary into fresh water where they develop into adults. Environmental signals may stimulate gonad development that is followed by a downstream migration beginning at the end of summer. Mating occurs after the crabs reach saline water. Embryos are carried until hatching, and the larvae undergo five zoeal stages before settlement. Our model projects rates of development at various temperatures and growth increments, supports a minimum of 2 years in low salinity or freshwater habitat, and predicts that most California mitten crabs are at least 3 years old before becoming sexually mature. Environmental factors strongly influence the timing and duration of the crab’s life stages, and are discussed in the context of a gradient of development times for worldwide populations of this important invasive species.

A life history model of somatic damage associated with resource acquisition: damage protection or prevention?

A resource acquisition–allocation model is developed to examine the trade-off between reproduction and somatic protection. Unlike previous studies, resource intake is not assumed to be constrained: instead, resource intake is free to vary, with increased intake being associated with an increased risk of somatic damage. This gives rise to an optimal resource intake as well as an optimal allocation strategy. This paper studies the relative importance of acquisition and allocation strategies in regulating acquisition-related mortality. Under the optimal allocation strategy mortality rate increases with age, in accordance with the disposable soma theory of aging. Contrary to the usual interpretation of the disposable soma theory, this increase in mortality can arise from an increase in the resource acquisition effort rather than a decrease in the resources allocated to protection. At early ages resource acquisition is found to be the primary path for regulating life history costs, whilst allocating resources to protection becomes more important later in life. Models for targeted and non-targeted damage repair are considered and the robustness of our results to the structure and parameterization of the model is discussed. The results from our models are discussed in light of published data. Resource acquisition is shown to be a potentially important mechanism for controlling somatic damage which deserves further study.

Increase of inbreeding by stocking on wild population assessed by using individual-based life history model

The genetic impact of stocking on natural populations was assessed by using individual-based life history models. Models were constructed that included density-dependent mortality at the early life stage, natural mortality, fishing mortality, and the number of released fish. These variables were varied by using random numbers. The focus was on the genetic impact of the number of released fish, the number of parent fish producing seedlings that were released, the sex ratio of the parent fish, fishing mortality, and the methods to select the parents of the seedlings. When brood stock size in the hatchery was five males and five females, the coefficient of inbreeding was about 27-fold as high as that in the case of 50 males and 50 females at the 50th generation. The coefficient increased about fivefold at the 50th generation, when the sex ratio of the parents changed from 50 males and 50 females to 10 males and 90 females. When parents were taken from wild populations at random, extreme fishing mortality reduced genetic diversity.

Marking Salamander Egg Masses with Visible Fluorescent Elastomer: Retention Time and Effect on Embryonic Development

Although marking methods are available for identifying larval and adult amphibians, techniques for egg masses have not been tested. Individual egg masses can be difficult to relocate and monitor because they are similar in appearance and often found within communal aggregations. We evaluated a method of marking spotted salamander (Ambystoma maculatum) egg masses with visible fluorescent elastomer (VIE) and provided the first assessment of an identification technique for this life stage. Laboratory and field experiments tested for effects of VIE on the development of embryos, retention in the jelly matrix and the practicality of this technique in the field. We found no effect of VIE on survival or hatching of embryos, nor on body size and developmental stage of hatchlings compared to unmarked controls. In the laboratory, marks remained intact and identifiable within the jelly matrix until hatching was complete (>35 d). In the field, marks in the outer jelly matrix remained intact for 83% of developmental days; marks in the inner matrix remained intact longer for 97% of developmental days. We found this method efficient for large samples and were able to locate VIE-marked egg masses at depths typical of oviposition sites (0.05–0.45 m). We suggest that the VIE technique has broad applications and would be suitable for other species with similar egg jellies. When combined with methods for larvae and adults, this technique may facilitate identification for multiple life stages and improve tests of individual-based allocation, population and life history models.

Relating results of chronic toxicity responses to population‐level effects: Modeling effects on wild chinook salmon populations

Standard toxicity tests assess the physiological responses of individual organisms to exposure to toxic substances under controlled conditions. Time and space restrictions often prohibit the assessment of population-level responses to a toxic substance. Compounds affecting various toxicity endpoints, such as growth, fecundity, behavior, or immune function, alter different demographic traits and produce different impacts on the population. Chronic effects of immune suppression, reproductive impairment, and growth reduction were examined using life history models for Chinook salmon (Oncorhynchus tshawytscha). Modeled immune suppression acted through reductions in age-specific survival, with first- and second-year survival producing the greatest changes in the population growth rate (lamda). A 10% reduction in various reproductive parameters all produced a similar lamda, but different sensitivity and stable age distributions. Growth reduction models incorporated effects to both survival and reproduction and produced additive effects. Overall, model output indicated that for Chinook salmon, alteration of first-year survival has the greatest relative impact on lamda. Results support the importance of linking toxicity endpoints to the demographic traits that they influence and help generate toxicity tests that are more relevant for the species. Life history modeling provides a useful tool to develop testable hypotheses regarding specific and comparative population-level impacts.

THEORY OF COMPLEX LIFE CYCLES: APPLICATION IN PLETHODONTID SALAMANDERS

Abstract Experimental studies of complex life cycles in plethodontid salamanders are reviewed and evaluated in the context of seven theoretical life-history models that focus on amphibians having biphasic life cycles. Such life cycles are found in the single species of the subfamily Hemidactylinae and in some members of the subfamily Spelerpinae and genus Desmognathus. In general, data on plethodontids provide only limited support for the predictions of the models. Given that the models have been formulated mainly for pond-breeding amphibians, several investigators have suggested that ancestral adaptation to streams in biphasic plethodontids have prescribed different evolutionary trajectories in the metamorphic response of these salamanders, in comparison with pond-breeding frogs and salamanders. Larval and metamorphic characters tabulated for biphasic plethodontids show that there is considerable inter- and intraspecific variation in larval periods and sizes at metamorphosis. In all lineages, in most spe...

Survival during Early Life Stages of the Desert Tortoise (Gopherus agassizii) in the South-Central Mojave Desert

Early age classes of the Desert Tortoise (Gopherus agassizii) are particularly vulnerable to predation by several mammal and bird species. We studied tortoise survival at the Sand Hill Training Area of the Marine Corps Air Ground Combat Center, Twentynine Palms, California, from 1998 to 1999. We radiographed and thread-spooled 25 females to determine reproductive and nesting ecology. Predators consumed 11 of 42 monitored nests during the first 70 days of incubation. Ninety-one of 132 eggs in nests fenced after 70 days developed into healthy neonates. We recorded 0.84 and 0.91 neonate survival probability during dispersal in 1998 and 1999, respectively. Applying neonatal survival to egg success, we predict that 40% of individuals survived from oviposition to hibernation. Our findings support other chelonian life-history models indicating relatively high mortality early in life. Our data also suggest that neonatal Desert Tortoises are less susceptible to predation than was previously thought, perhaps because of their cryptic coloration and secretive habits. The Common Raven (Corvus corax) was not found to be a source of neonate mortality during this study.

A Perron Theorem for positive componentwise bilinear maps

It is proved that a Perron type theorem holds for positive maps with bilinear components whose defining matrices satisfy a maximality assumption with respect to certain entry ratios. The result is applied to a life history model which includes sexual reproduction.