Allee effects, host tree density and the establishment of invasive bark beetles

Abstract

The aim of this thesis was to understand the role of Allee effects and dispersal during the establishment and spread phases of bark beetle invasion events, a topic currently central to invasion ecology. Allee effects are defined as a positive relationship between population size and fitness. It has long been proposed that establishment probability and spread potential is dependent on propagule pressure, but few empirical studies have directly tested this hypothesis. I investigated the establishment potential and dispersal capabilities of bark beetles across large geographical scales in New Zealand and the United States of America. My overarching hypothesis was that bark beetle species in two different feeding guilds (saprophytic and secondary) would have different Allee thresholds, affecting their establishment and spread success. In New Zealand, I found that two saprophytic bark beetles (Hylurgus ligniperda and Hylastes ater) were capable of dispersing at least 25 km from the nearest host Pinus patch through a fragmented and geographically complex landscape. The observations help to explain the widespread distribution of these insects throughout both the North and South Islands. Point introduction experiments of H. ligniperda in open (field) and closed (tent) environments revealed component Allee effects during both the colonization and larval development stages, leading to a demographic Allee effect. However, even the smallest releases (N = 10 H. ligniperda) sometimes resulted in colonization, indicating the high invasion potential of this insect species. In the USA, both regional host density and an estimated Allee threshold helped explain the spatial distribution of a secondary bark beetle, Ips pini, through a heavily fragmented agricultural landscape. A large emergence pulse of I. pini in spring lifted disjunct populations over the estimated Allee threshold, apparently allowing the beetle to persist in the landscape at intermediate host densities. The colonization probability of I. pini was also positively related to the number of beetles released (a component Allee effect), but a strong negative density-dependent interaction during larval development offset this so there was no demographic Allee effect. This work provides baseline data for theoretical ecologists modelling Allee effects, conservation ecologists conducting re-introductions, invasion ecologists concerned with future introductions of non-native species and policy makers targeting higher risk bark beetle species.