Abstract
The role of extrinsic mortality in shaping life histories is poorly understood. However, substantial evidence suggests that extrinsic mortality interacts with density-dependence in crucial ways. We develop a model combining Evolutionarily Stable Strategies with a projection matrix that allows resource allocation to growth, tissue repairs, and reproduction. Our model examines three cases, with density-dependence acting on: (i) mortality, (ii) fecundity, and (iii) production rate. We demonstrate that density-independent extrinsic mortality influences the rate of aging, age at maturity, growth rate, and adult size provided that density-dependence acts on fertility or juvenile mortality. However, density-independent extrinsic mortality has no effect on these life history traits when density-dependence acts on survival. We show that extrinsic mortality interacts with density-dependence via a compensation mechanism: the higher the extrinsic mortality the lower the strength of density-dependence. However, this compensation fully offsets the effect of extrinsic mortality only if density-dependence acts on survival independently of age. Both the age-pattern and the type of density-dependence are crucial for shaping life history traits.
Highlights
Extrinsic mortality is risk of death that is independent of an organism’s strategy of survival and reproduction [1]
When DD acts uniformly on fertility, the results that increased extrinsic mortality c leads to decreases in the age at maturity and the fraction of resources allocated to repairs holds across all sets of parameter values analyzed (Fig 2A and 2D; see Fig 1AD in S2 File for the effect of repair efficiency)
The three cases we examine are: (I) DD acts on survival with strength that decreases with age; (II) DD acts on survival with strength increasing with age; and (III) DD acts on production rate with strength that declines with age
Summary
Extrinsic mortality is risk of death that is independent of an organism’s strategy of survival and reproduction [1]. While extrinsic mortality is used in many models and invoked as a key factor affecting life history patterns in many organisms, its role in nature is poorly understood. Some see it as a central driver of life history evolution, while others think it may play no role at all. We show that the type of density-dependence, what it acts on, has significant implications for the effect of extrinsic mortality on the life history
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