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Biomedical subjects

P A Abrams

Publications and source records attributed to P A Abrams.

7 recordsLinked to original sources
PubMed2004

Interpreting the von Bertalanffy model of somatic growth in fishes: the cost of reproduction.

We develop a model for somatic growth in fishes that explicitly allows for the energy demand imposed by reproduction. We show that the von Bertalanffy (VB) equation provides a good description of somatic growth after maturity, but not before. We show that the parameters of the VB equation are simple functions of age at maturity and reproductive investment. We use this model to show how the energy demands for both growth and reproduction trade off to determine optimal life-history traits. Assuming that both age at maturity and reproductive investment adapt to variations in adult mortality to maximize lifetime offspring production, our model predicts that: (i) the optimal age of maturity is inversely related to adult mortality rate; (ii) the optimal reproductive effort is approximately equal to adult mortality rate. These predictions are consistent with observed variations in the life-history traits of a large sample of iteroparous freshwater fishes.

Age Factors↗
PubMed2004

The interaction between reproductive lifespan and protandry in seasonal breeders.

The timing and duration of reproductive activities are highly variable both at the individual and population level. Understanding how this variation evolved by natural selection is fundamental to understanding many important aspects of an organism's life history, ecology and behaviour. Here, we combine game theoretic principles governing reproductive timing and the evolutionary theory of senescence to study the interaction between protandry (the earlier arrival or emergence of males to breeding areas than females) and senescence in seasonal breeders. Our general model applies to males who are seeking to mate as frequently as possible over a relatively short period, and so is relevant to many organisms including annual insects and semelparous vertebrates. The model predicts that protandry and maximum reproductive lifespans should increase in environments characterized by high survival and by a low competitive cost of maintaining the somatic machinery necessary for survival. In relatively short seasons under these same conditions, seasonal declines in the reproductive lifespans of males of equivalent quality will be evolutionarily stable. However, over a broad range of potential values for daily survival and maintenance cost, reproductive lifespan is expected to be relatively short and constant throughout a large fraction of the season. We applied the model to sockeye (or kokanee) salmon Oncorhynchus nerka and show that pronounced seasonal declines in reproductive lifespan, a distinctive feature of semelparous Oncorhynchus spp., is likely part of a male mating strategy to maximize mating opportunities.

Aging↗
PubMed1999

Adaptive host preference and the dynamics of host-parasitoid interactions.

Models of two independent host populations and a common parasitoid are investigated. The hosts have density-dependent population growth and only interact indirectly by their effects on parasitoid behavior and population dynamics. The parasitoid is assumed to experience a trade-off in its ability to exploit the two hosts. Three alternative types of parasitoid are investigated: (i) fixed generalists whose consumption rates are those that maximize fitness; (ii) "ideal free" parasitoids, which modify their behavior to maximize their rate of finding unparasitized hosts within a generation; and (iii) "evolving" parasitoids, whose capture rates change between generations based on quantitative genetic determination of the relative attack rates on the two hosts. The primary questions addressed are: (1) Do the different types of adaptive processes stabilize or destabilize the population dynamics? (2) Do the adaptive processes tend to equalize or to magnify differences in host densities? The models show that adaptive behavior and evolution frequently destabilize population dynamics and frequently increase the average difference between host densities.

Adaptation, Biological↗
PubMed1994

Evolutionarily stable growth rates in size-structured populations under size-related competition.

The competitive interactions between individuals in size-structured populations usually change as a function of the individuals' sizes. A general model of a density-dependent size-structured population is used to investigate the size-specific birth and death rates that result when growth rates can be adjusted adaptively. If there is no cost associated with faster growth, the evolutionarily stable growth rates result in an ideal free distribution of individuals among size classes, provided that competition within size classes is stronger than competition between size classes. When the population is stationary, this ideal free distribution is characterized by identical ratios of expected number of offspring per unit time to probability of death per unit time for all size classes with growth rates less than the physiologically maximum level. If more rapid growth reduces birth rate or increases death rate, the size-specific ratios of births to mortality increase with the organism's size. If the population is growing in a density independent manner, but there is a cost to growth, there should be an increase with size in the ratio of reproductive output to the quantity (population growth rate minus survival probability). Available evidence about size-specific birth and death rates in some size-structured populations is discussed.

Animals↗
PubMed1986

Character displacement and niche shift analyzed using consumer-resource models of competition.

This paper analyzes the adaptive responses to competition (both character displacement and niche shift) in a two consumer-two resource model. The model includes density dependence that is unrelated to the resources that are explicit in the model. This could be due to another resource dimension, parasites, or interference competition. Competitors adapt by changing their relative consumption rate constants on the two resource types. This model can result in mutually divergent, parallel, or mutually convergent displacement of competitors. Parallel displacement may entail net divergence, net convergence, or no net change. Parallel change with net convergence is most likely when the competitors have similar constraints on the possible values of consumption rate constants, unequal allopatric abundances, and significant intraspecific density dependence. Numerical calculations of displacements are presented for several models and the effect of a number of different possible alterations of the model are discussed. The evolution of resource handling and processing efficiency, and displacement in the presence of additional selective pressures on the character are considered in detail. The results have implications for questions about maximization of population size, the relationship of character displacement and the competition coefficient, and "null" models in the study of competition. Differences between this and previous theoretical works are discussed. It is argued that conditions allowing parallel or convergent displacement are not biologically unlikely, and possible examples are discussed. Data on resource partitioning seem to be more consistent with the results reached here than with previous theory.

Animals↗