Allee effects and the persistence of small populations

Abstract

The intent of this thesis was two-fold. Firstly, to examine the role of the Allee effect in the establishment success of insect pest populations during the early phases of invasion. Secondly, to examine how spatial management at the point of introduction could render populations more vulnerable to the Allee effect and facilitate their eradication. Both aims are important, as despite strong theoretical findings that a demographic Allee effect can prevent small founder population establishment, empirical evidence is limited. Additionally, social pressure is demanding alternative ‘greener’ tools to achieve eradication of new incursions. The Allee effect is defined as a positive relationship between population size and fitness and is an important factor that underpins the widespread effect of propagule pressure on establishment success. By using a weed biological control agent, Neolema ogloblini, as a model insect pest species, I investigated the impact of propagule size on establishment success and found evidence that cautiously supports the existence of a strong demographic Allee effect governing the establishment of this species. Further studies suggested that the demographic Allee effect is driven by at least one potential component Allee effect; that is, mate-finding failure. Heavy predation on the immature stages by generalist predators was found during predator exclusion trials, but further studies are needed to demonstrate whether it constitutes a predator-driven component Allee effect. Increased aggregation of individuals allows necessary interactions (mating, predator dilution, etc.) and minimizes the negative impacts of the Allee effect. My study indicated that adults of N. ogloblini potentially utilize feeding-induced plant volatiles as a cue to find their host, thereby increasing dispersal success among host patches and aggregation on host plant patches. As the Allee effect plays an important role in the dynamics of newly established and low-density populations by driving small populations to extinction, it is critical for influencing outcomes of eradication efforts. The Allee effect, together with other processes occurring at low population sizes (e.g., demographic stochasticity), eliminate the need for removal of all individuals in a population during management actions. Results of my study indicated that limited resources at the point of introduction motivated dispersal to adjacent host patches. A higher number of adults successfully located and settled on host patches that were closer to the point of introduction (higher connectivity level) than sites where adjacent patches were further away (medium connectivity or low connectivity). Subsequent host removal of patches at the point of introduction induced additional dispersal at high connectivity treatment sites only, significantly reducing meta-population size. Meta-population size correlated significantly positively with probability of survival and per capita population growth rate. Therefore, management actions that reduces landscape connectivity (limiting and isolating host patches), and consequently population size, could subject remaining populations to the Allee effect and achieve eradication.