Alternative reproductive tactics (ARTs) are examples of individual heterogeneity in which males adopt one of typically two alternative strategies to mate with females: males are either large, armed fighters or small, benign sneakers. ART expression is often conditionally determined, and variation in the expression of conditional ARTs due to genetic and/or environmental influences can greatly affect population composition and trajectory. For example, ecological feedback mechanisms resulting from strong density‐dependent competition over food have been suggested to explain the observation that the harvesting of scramblers (= sneakers) in closed populations of the bulb mite Rhizoglyphus robini did not result in an increase (expected from quantitative genetics theory) but decrease in fighter expression. Here, we exposed closed bulb mite populations to selective fighter or scrambler harvesting for 5–6 generations under abundant food (to halt ecological feedbacks through density‐dependence) to confirm predictions from quantitative genetics theory. However, we found no evolutionary shift in ART expression; rather, we observed an overcompensatory ecological response, whereby the number of fighters increased when we harvested them. Treatment effects on scrambler numbers could not be tested as there were too few in the experimental populations. Further experiments revealed that starved fighters preferentially killed immature males and immature fighters; possibly to reduce male‐male competition as e.g. immature fighters have not yet developed their lethal weaponry. If this is so, then harvesting adult fighters reduced the killing pressure on immature males in our experiment, which resulted in an overcompensatory number of immature fighters that matured as adults. Our results highlight the complexity of how individual heterogeneity in ARTs affects the ecological and evolutionary processes that determine population fluctuations.
Surprisingly, little is known about how eco-evolutionary feedback loops affect trait dynamics within a single population. Polymorphisms of discrete alternative phenotypes present ideal test beds to investigate this, as the alternative phenotypes typically exhibit contrasting demographic rates mediated through frequency or density dependence, and are thus differentially affected by selection. Alternative reproductive tactics (ARTs), like male fighters and sneakers, are an extreme form of discrete phenotype expression and occur across many taxa. Fighters possess weapons for male-male competition over access to mates, whereas sneakers are defenceless but adopt tactics like female-mimicking. Because fighters in some species mortally injure conspecifics, this raises the question whether fighter expression can feed back to affect population size and structure, thereby altering the selection gradient and evolutionary dynamics of ART expression in an eco-evolutionary feedback loop. Here, we investigated how the eco-evolutionary feedback loop between fighter expression and population size and structure affects the evolution and maintenance of ARTs. We introduced intraspecific killing by fighters in a two-sex, two-ART population model parameterized for the male dimorphic bulb mite (Rhizoglyphus robini) that includes life-history differences between the ARTs and a mating-probability matrix analogous to the classic hawk-dove game. Using adaptive dynamics, we found that the intraspecific killing by fighters can extend the range of life-history parameter values under which ARTs evolve, because fighters that kill other fighters decrease fighter fitness. This effect can be nullified when benefits from killing are incorporated, like increased reproduction through increased energy uptake. The eco-evolutionary feedback effects found here for a dimorphic trait likely also occur in other fitness-related traits, such as behavioural syndromes, parental care and niche construction traits. Current theoretical advances to model eco-evolutionary processes, and empirical steps towards unravelling the underlying drivers, pave the way for understanding how selection affects trait evolution in an eco-evolutionary feedback loop.
Plasticity is the extent to which life history processes such as growth and reproduction depend on the environment. Plasticity in individual growth varies widely between taxa. Nonetheless, little is known about the effect of plasticity in individual growth on the ecological dynamics of populations. In this article, we analyse a physiologically structured population model of a consumer population in which the individual growth rate can be varied between entirely plastic to entirely non-plastic. We derive this population level model from a dynamic energy budget model to ensure an accurate energetic coupling between ingestion, somatic maintenance, growth and reproduction within an individual. We show that the consumer population is either limited by adult fecundity or juvenile survival up to maturation, depending on the level of growth plasticity and the non-plastic individual growth rate. Under these two regimes, we also find two different types of population cycles which again arise due to fluctuation in, respectively, juvenile growth rate or adult fecundity. In the end, our model not only provides insight into the effects of growth plasticity on population dynamics, but also provides a link between the dynamics found in age- and size-structured models.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Growth and growth limitation are important indicators of density dependence and environmental limitation of populations. Estimating individual growth trajectories is therefore an important aspect of understanding and predicting the life history and dynamics of a population. Variation in individual growth trajectories arises due to variation in the environmental factors limiting individual growth. This environmental limitation can vary over time, between cohorts and between individuals within a cohort. For a complete and accurate understanding of individual growth in a population, it is important to include all these sources of variation. So far, statistical models only accounted for a subset of these factors or required an extensive growth history of individuals. Here we present a novel model describing the growth curves of cohorts in a population. This model is derived from a stochastic form of the Von Bertalanffy growth equation describing individual growth. The model is specifically tailored for use on length-at-age data in which the growth trajectory of an individual is unknown and every individual is only measured once. The presented method can also be used if growth limitation differs strongly between age or length classes. We demonstrate the use of the model for length-at-age data of North Sea plaice (Pleuronectes platessa) from the last thirty years. Fitting this model to length-at-age data can provide new insights in the dynamics of the environmental factors limiting individual growth and provides a useful tool for ecological research and management.
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