Theoretical and empirical investigations on population stability and dispersal evolution using laboratory populations of Drosophila melanogaster

Abstract

Global climate is changing rapidly and is accompanied by large-scale fragmentation and destruction of habitats. These environmental adversities not only affecting the life-history and behaviour of organisms but also compromise the stability of local population dynamics. Although a number of control methods for stabilizing the dynamics had been proposed, there were no comparisons of the relative efficiencies of these methods. More critically, majority of these methods had not yet been validated empirically. Therefore I undertook theoretical study that compared six such control methods in terms of attaining a given level of stability over four different life history/environment combinations. This analysis showed that although no single method was unambiguously superior to others, the methods that involved culling reduce the risk of extinction more efficiently, whereas methods that involve only restocking are better in reducing temporal fluctuations in population sizes. Subsequently, using a series of experiments, I validated the efficiency of four well-known control methods in enhancing the stability of widely fluctuating, extinction-prone populations of Drosophila melanogaster. These experiment showed that methods which incorporate both culling and restocking, can simultaneously achieve multiple kinds of stability and therefore are strong candidates for field applications. I also showed that the choice of methods under a given condition would depend critically upon what kind of stability needs to be attained. Non-Drosophila specific, biologically realistic simulations suggested the generalizability of these results over a wide range of taxa.

Subsequently, in order to obtain mechanistic insights about the determinants of dynamics and stability, I built an individual-based, stage-structured model of Drosophila dynamics. This model included parameters that are common to the life-history of several holometabolus insects. I calibrated this model using data from laboratory populations of Drosophila melanogaster. The calibrated model could capture various aspects of Drosophila dynamics under four different nutritional regimes. Further simulations showed that unequal sex-ratio and sex-specific culling are greatly influenced by fecundity, whereas complex interaction between juvenile nutritional levels and adult fecundity determines the efficiency of Sterile Insect Technique, a widely-used pest control method.

In the context of global climate change and habitat degradation, dispersal is one of the traits that is likely to be crucial for the survival of many populations. Consequently, understanding the causes and consequences of evolution of dispersal has been a major area of interest in ecology and evolutionary biology. I report the results of an artificial selection experiment for increased dispersal using four large (N~2500) outbred populations of D. melanogaster. Dispersal propensity, ability and kernel evolved rapidly in the selected populations. The differences persisted even in the absence of proximate drivers for dispersal. Interestingly, the selected lines did not incur any major life-history costs, but behavioral traits like activity, exploration and aggression were increased. I also investigated the metabolomic changes that took place in these lines to accommodate the excess energy demand of active dispersal and, as a correlated response, led to the observed behavioral changes.

Finally, I summarized the main results of these studies, discussed their potential implications and mentioned possible avenues for further work.