Probing of mortality rate by staying alive: The growth‐reproduction trade‐off in a spatially heterogeneous environment

Abstract

In many annual plants, mollusks, crustaceans and ectothermic vertebrates, growth accompanies reproduction. The growth curves of these organisms often exhibit a complex shape, with episodic cessations or accelerations of growth occurring long after maturation. The mixed allocation to growth and reproduction has poorly understood adaptive consequences, and the life‐history theory does not explain if complex growth in short‐lived organisms can be adaptive. We model the trade‐off between growth and reproduction in a short‐lived organism evolving in a metapopulation. Individuals occupy risky or safe sites throughout their lives, but are uncertain regarding the risk of death. Modelled organisms are allowed to grow and produce offspring at specified time points (moults), although we also consider scenarios that approximate continuous growth and reproduction. Certain combinations of risky to safe sites select for strategies with mixed allocation to growth and reproduction that bet‐hedge offspring production in safe and risky sites. Our model shows that spatially heterogeneous environments select for mixed allocation only if safe sites do not become the prevailing source of recruits, for example, when risky sites are frequent. In certain conditions, growth curves are multi‐phasic, with allocation to growth that stops, remains constant or accelerates during adult life. The resulting complex growth curves are more likely to evolve in short‐lived organisms that moult several times per adult life. Our work shows that spatial heterogeneity can select for growth that accompanies reproduction and provides insights into the adaptive significance of complex growth curves. Short‐lived crustaceans are particularly predisposed to exhibit complex growth patterns as an adaptive response to spatially heterogeneous environments. Our results suggest that standard statistical growth models assuming adult growth rate to only decelerate over life are not well suited to approximate growth curves of short‐lived crustaceans. A free Plain Language Summary can be found within the Supporting Information of this article.

Spatial heterogeneity in the environment can favour the evolution of bet‐hedging in resource allocation to growth and reproduction

“…However, growth also continues after maturation in many short‐lived (e.g. annual) organisms that produce dormant offspring, for example annual plants and cladocerans (Ejsmond, Kozłowski, & Ejsmond, ). The production of dormant offspring spreads the risk for temporal environmental uncertainty in these species, so it seems unlikely that simultaneous allocation to growth and reproduction would have evolved in relation to temporal environmental variation.…”

“…To address the evolution of growth that accompanies reproduction in short‐lived organisms, Ejsmond et al () developed an elegant life‐history model – published in this issue of Functional Ecology – where mortality risk varies spatially among habitat patches and the modelled organism is unable to infer the ambient mortality risk. Their analysis is coarse‐grained, with each individual experiencing a constant mortality risk during its lifetime because they stay in the same habitat patch for their whole lives.…”

“…Ejsmond et al () conducted their study using a metapopulation of short‐lived organisms as a model system, but I believe that their predictions are more general. Their analysis is coarse‐grained, with each individual experiencing only one environmental state during its lifetime.…”

“…Therefore, it seems likely that the prediction for the evolution of simultaneous allocation of resources to growth and reproduction would hold in any coarse‐grained environment whenever there is a high probability that a newborn individual ends up in a dangerous environment and has no possibility to escape the high mortality risk in space or time during its lifetime. This prediction should be most applicable to short‐lived organisms (Ejsmond et al, ). The take‐home message from Ejsmond et al () is that spatial heterogeneity can favour the evolution of such bet‐hedging strategies that traditionally were thought to evolve only in response to temporal variation.…”

“…This prediction should be most applicable to short‐lived organisms (Ejsmond et al, ). The take‐home message from Ejsmond et al () is that spatial heterogeneity can favour the evolution of such bet‐hedging strategies that traditionally were thought to evolve only in response to temporal variation.…”