Unexpected discontinuities in life-history evolution under size-dependent mortality (original) (raw)
Related papers
The importance of growth and mortality costs in the evolution of the optimal life history
Journal of Evolutionary Biology, 2006
A central assumption of life history theory is that the evolution of the component traits is determined in part by trade-offs between these traits. Whereas the existence of such trade-offs has been well demonstrated, the relative importance of these remains unclear. In this paper we use optimality theory to test the hypothesis that the trade-off between present and future fecundity induced by the costs of continued growth is a sufficient explanation for the optimal age at first reproduction, a, and the optimal allocation to reproduction, G, in 38 populations of perch and Arctic char. This hypothesis is rejected for both traits and we conclude that this trade-off, by itself, is an insufficient explanation for the observed values of a and G. Similarly, a fitness function that assumes a mortality cost to reproduction but no growth cost cannot account for the observed values of a. In contrast, under the assumption that fitness is maximized, the observed life histories can be accounted for by the joint action of trade-offs between growth and reproductive allocation and between mortality and reproductive allocation (Individual Juvenile Mortality model). Although the ability of the growth/mortality model to fit the data does not prove that this is the mechanism driving the evolution of the optimal age at first reproduction and allocation to reproduction, the fit does demonstrate that the hypothesis is consistent with the data and hence cannot at this time be rejected. We also examine two simpler versions of this model, one in which adult mortality is a constant proportion of juvenile mortality [Proportional Juvenile Mortality (PJM) model] and one in which the proportionality is constant within but not necessarily between species [Specific Juvenile Mortality (SSJM) model]. We find that the PJM model is unacceptable but that the SSJM model produces fits suggesting that, within the two species studied, juvenile mortality is proportional to adult mortality but the value differs between the two species.
The American naturalist, 2007
The majority of taxa grow significantly during life history, which often leads to individuals of the same species having different ecological roles, depending on their size or life stage. One aspect of life history that changes during ontogeny is mortality. When individual growth and development are resource dependent, changes in mortality can affect the outcome of size-dependent intraspecific resource competition, in turn affecting both life history and population dynamics. We study the outcome of varying size-dependent mortality on two life-history types, one that feeds on the same resource throughout life history and another that can alternatively cannibalize smaller conspecifics. Compensatory responses in the life history dampen the effect of certain types of size-dependent mortality, while other types of mortality lead to dramatic changes in life history and population dynamics, including population (de-)stabilization, and the growth of cannibalistic giants. These responses differ strongly among the two life-history types. Our analysis provides a mechanistic understanding of the population-level effects that come about through the interaction between individual growth and sizedependent mortality, mediated by resource dependence in individual vital rates.
Ecology Letters, 2003
Individual organisms often show pronounced changes in body size throughout life with concomitant changes in ecological performance. We synthesize recent insight into the relationship between size dependence in individual life history and population dynamics. Most studies have focused on size-dependent life-history traits and population sizestructure in the highest trophic level, which generally leads to population cycles with a period equal to the juvenile delay. These cycles are driven by differences in competitiveness of differently sized individuals. In multi-trophic systems, size dependence in life-history traits at lower trophic levels may have consequences for both the dynamics and structure of communities, as size-selective predation may lead to the occurrence of emergent Allee effects and the stabilization of predator-prey cycles. These consequences are linked to that individual development is density dependent. We conjecture that especially this population feedback on individual development may lead to new theoretical insight compared to theory based on unstructured or age-dependent models. Density-dependent individual development may also cause individuals to realize radically different life histories, dependent on the state and dynamics of the population during their life and may therefore have consequences for individual behaviour or the evolution of life-history traits as well.
Growth versus lifespan: perspectives from evolutionary ecology
Experimental Gerontology, 2003
There are many ecological advantages to attaining a large body size as fast as possible (such as reduced risks of being caught by predators or increased reproductive success). However, studies in several taxa indicate that fast growth in itself can have negative as well as positive effects. There appears to be a link between accelerated growth and lifespan: rapid growth early in life is associated with impaired later performance and reduced longevity. In this review we assess the evidence for such within individual trade-offs between growth rate and lifespan, and the potential physiological mechanisms that might underlie them. We discuss the fitness implications of any reduction in lifespan, and point out that certain environmental circumstances may favour a 'grow fast and die young' strategy if this increases overall reproductive success. However, investigation of the intra-specific relationships among growth rate, lifespan and fitness is not straightforward; few studies have controlled for confounding variables such as adult body size or duration of the growth period, and none to date have measured fitness in an appropriate ecological setting. We suggest a number of experimental approaches that might allow the true relationships between growth rate and future performance to be elucidated. q
Evolutionary perturbations of optimal life histories
Evolutionary Ecology, 1995
An optimal age-structured life history is perturbed by increasing the mortality factors specific to an agek. These can be density dependent (DD) or independent (DI), avoidable or unavoidable. The last two refer to whether their effect on any individual depends or not on how much energy it devotes to defence. Agespecific trade-offs between the allocation of energy to defence and fecundity exist: survival probabilities through each agex, P x, are concave decreasing functions of the fecundity per unit size at that age,b x. These are constraints for the optimal life history. The changes induced by perturbation are evaluated by equations that predict whether some extra energy is diverted towards survivorship at the expense of fecundity or vice versa. The model predicts that for DI environments the degree of avoidability of the mortality source perturbed, is a decisive factor for the strategy selected at agek, but not for any other age class. DD environments are more complex since all ages are simultaneously embedded in density effects. The perturbations not only act directly — as in the DI situation — but also indirectly through their effect on equilibrium density,N *. When any kind of mortality source becomes more intense at agek, N * always decreases and all ages react in consequence according to the effect of density on each age-specific trade-off. Either coincidental or opposing reactions can be expected from direct and indirect effects. The resultant strategy for any age would be a matter of magnitude comparisons. Some possible general patterns are discussed.
On the role of body size for life-history evolution
Ecological Entomology, 1997
1. Body size is a central element in current theories of life-history evolution. Models for optimal age at maturity are based on the assumptions that there is a trade-off between development time and adult size and that larger size provides a reproductive advantage.
Life history evolution: successes, limitations, and prospects
Naturwissenschaften, 2000
Life history theory tries to explain how evolution designs organisms to achieve reproductive success. The design is a solution to an ecological problem posed by the environment and subject to constraints intrinsic to the organism. Work on life histories has expanded the role of phenotypes in evolutionary theory, extending the range of predictions from genetic patterns to whole-organism traits directly connected to fitness. Among the questions answered are the following: Why are organisms small or large? Why do they mature early or late? Why do they have few or many offspring? Why do they have a short or a long life? Why must they grow old and die? The classical approach to life histories was optimization; it has had some convincing empirical success. Recently non-equilibrium approaches involving frequency-dependence, density-dependence, evolutionary game theory, adaptive dynamics, and explicit population dynamics have supplanted optimization as the preferred approach. They have not yet had as much empirical success, but there are logical reasons to prefer them, and they may soon extend the impact of life history theory into population dynamics and interspecific interactions in coevolving communities.
Alternative Models for the Evolution of Offspring Size
The American Naturalist, 1989
Eleven years ago, I proposed a simple model for the evolution of offspring size, pointing out a general correlation between parental care and relatively large offspring size consistent with that model (Shine 1978). My basic premise came from Williams (1966): natural selection should adjust the life history such that organisms spend little time in life-history stages characterized by high mortality. Hence, optimal egg sizes should be determined by the relative survival rates of eggs and free-living juveniles. If the egg stage is a "safe harbor" (as in species with parental care), whereas juvenile life is hazardous, selection should favor an increase in egg size and thus a decrease in the duration of the high-risk juvenile phase. This safe-harbor hypothesis for the evolution of offspring size has recently been criticized on several grounds by Nussbaum (1985, 1987), Sargent et at. (1987), and Nussbaum and Schultz (1989). Important alternative ideas on the topic have also been developed by Ito (1980) and Ito and Iwasa (1981). Analysis ofthese contributions suggests at least four alternative explanations for the observed correlation between parental care and offspring size: (1) parental care favored the evolution of larger eggs (Shine 1978); (2) larger eggs favored the evolution of par~ntal care (Nussbaum 1985, 1987); (3) parental care and offspring size coevolved, with an increase in one favoring an increase in the other (Nussbaum and Schultz 1989); or (4) some third factor simultaneously selected for parental care and larger egg size. Although I maintain that several criticisms of the safe-harbor hypothesis are in error, I also believe that the general correlation between parental care and large offspring size may be due to a combination of processes rather than the single process I envisaged in my 1978 paper. This note outlines my objections to recent criticisms and proposes a method for testing these competing explanations. CRITICISMS AND ALTERNATIVES The mathematical formulations of Sargent et at. (1987) and Nussbaum and Schultz (1989) are more sophisticated than my own 1978 version, but they unequivocally support my basic argument that the relative survival of embryos and free-living juveniles should be crucial determinants of optimal offspring size. These authors also extend my model to cover varying intensity and/or effec
Fitness can be profoundly influenced by the age at first reproduction (AFR), but to date the AFR–fitness relationship only has been investigated intraspecifically. Here, we investigated the relationship between AFR and average lifetime reproductive success (LRS) across 34 bird species. We assessed differences in the deviation of the Optimal AFR (i.e., the species-specific AFR associated with the highest LRS) from the age at sexual maturity, considering potential effects of life history as well as social and ecological factors. Most individuals adopted the species-specific Optimal AFR and both the mean and Optimal AFR of species correlated positively with life span. Interspecific deviations of the Optimal AFR were associated with indices reflecting a change in LRS or survival as a function of AFR: a delayed AFR was beneficial in species where early AFR was associated with a decrease in subsequent survival or reproductive output. Overall, our results suggest that a delayed onset of reproduction beyond maturity is an optimal strategy explained by a long life span and costs of early reproduction. By providing the first empirical confirmations of key predictions of life-history theory across species, this study contributes to a better understanding of life-history evolution.