The Role of Intraspecific Variation, Species Interactions, and Eco-evolutionary Feedbacks in Population Persistence
Creators: name: Nabutanyi, Peter
Publisher: name: Universität Bielefeld
Year: 2022
Subjects: subject: 570

description: Theoretical and empirical research have shown that ecological and evolutionary processes can occur on similar timescales with each influencing the dynamics of one another through eco-evolutionary feedbacks. Eco-evolutionary feedbacks have been investigated with respect to population and species persistence, especially in small populations. However, the role of some evolutionary processes, such as balancing selection mechanisms, in the persistence of small populations remains unclear. Furthermore, recent studies have shown that intraspecific trait variation (ITV) in interacting species also influences population and species persistence. However, the effects of coevolution and incorporating ITV in two or more interacting species remain primarily speculative. This dissertation aims to develop quantitative eco-evolutionary models and use them to evaluate the role of intraspecific variation and eco-evolutionary feedbacks in small populations. My dissertation consists of four major studies, addressing one main question each: How do eco-evolutionary feedbacks between population size and genetic diversity under balancing selection lead to population extinction? How does the interaction of various genetic problems in small populations influence their extinction risk and minimum viable population size? How do joint ITV and covariance affect the persistence and the ecological dynamics of interacting species? How do ITV and coevolution affect the minimum viable population (MVP) sizes of interacting species? In brief, my dissertation consists of a total of six chapters. In Chapter 1, I discuss the general role of intraspecific variation, eco-evolutionary feedbacks, and species interactions in population persistence. In Chapter 2, we develop quantitative stochastic and deterministic eco-evolutionary models to show how the loss of genetic diversity from loci under balancing selection can lead to eco-evolutionary extinction vortices. We find that the per-capita rate of population decline and the per-locus rate of loss of genetic diversity increases with decreasing population size and genetic diversity. We also tested the so-called early-warning indicators on our simulated data and found them of limited use. Generally, the signals produced from populations with just a decline in population size without causing extinction could not easily be distinguished from those with a decrease that led to eventual population extinction. For Chapter 3, we expanded the eco-evolutionary models in Chapter 2 to allow the interaction of two or more genetic mechanisms. The mechanisms included three different forms of balancing selection and deleterious mutations that captured inbreeding depression and mutation accumulation. We estimated the smallest population size that allows persistence on a long timescale as a measure of extinction risk. As number number of loci under selection increased, minimum viable population (MVP) size rapidly increased until a critical number of loci, where MVP sizes exhibited an asymptotic increase (to infinity). Thus, the asymptotic increase indicated that no finite population size was sufficient to keep the population at equilibrium. Contrary to verbal speculations, the interaction of the different genetic mechanisms did not necessarily increase MVP size when the number of loci under selection was constant. In Chapter 4 we explore the role of joint ITV, i.e., ITV in two interacting species, in species coexistence via nonlinear averaging. We first employ numerical integration techniques and Taylor approximations to evaluate interaction parameters over a trait distribution. Then, the interaction parameters from the different techniques where used in common Lotka Volterra interaction models to evaluate species dynamics. We find that joint ITV and covariance can significantly alter the interaction parameters. Similar to genetic variation, substantial effects of ITV on interaction parameters affected the persistence of species. The last study in Chapter 5 developed a stochastic coevolutionary individual-based model that incorporated intraspecific variation in either two, one, or none of the interacting species. At least one of the coevolving species was subjected to a bottleneck. We calculated the survival probabilities of each species at the end of the simulation time and estimated MVP sizes required for the survival of either one or both species. We generally find that changes in the population size of one species significantly affected the survival and MVP size of both species. We also generally found that coevolution can significantly facilitate but also derail species survival after the bottleneck. Lastly, Chapter 6 gives a brief discussion of the main findings from the four studies. To conclude, our models did not focus on any particular biological system and, therefore, our results are general to conservation management. In addition, the results are either completely new, confirm observations from earlier studies, or clarify some of the speculations in the literature. The major conclusions from each study are: (1) Loss of genetic diversity from loci under balancing selection can cause eco-evolutionary extinction vortices and early-warning indicators may not detect such vortices. (2) Extinction risk increases with an increasing number of loci contributing to fitness reduction. Moreover, the interaction of the different genetic problems does not necessarily increase extinction risk for a fixed number of loci under selection. (3) The joint ITV and covariance need to be accounted for in modeling interacting species for better species dynamics predictions. and (4) ITV and coevolution can facilitate but also hinder species persistence after a bottleneck in interacting species.