Propagule pressure rather than population growth determines colonisation ability: a case study using two phytophagous mite species differing in their invasive potential.
Colonisation, i.e. the establishment of dispersed individuals in an unoccupied area, is a fundamental biological process that drives the distribution and range dynamics of organisms. It is crucial to understand the basic mechanisms of colonisation, especially in invasive species. In this study, we investigated the importance of dispersal success (the proportion of settlers and founders) and intrinsic population growth rate for colonisation ability in phytophagous mites that spread both passively and actively. We performed laboratory experiments using two eriophyid mite species: Aceria tosichella [wheat curl mite (WCM)] and Abacarus hystrix [cereal rust mite (CRM)]. These are obligate herbivores of economic importance because they feed on cereal plants, including wheat. To test the colonisation ability of WCM and CRM on wheat when dispersed actively by walking and passively when blown by air currents, we estimated the number of individuals that established new populations after dispersal and calculated the intrinsic population growth rate (r) for both species. WCM had a higher colonisation ability on wheat than CRM. This resulted from the higher dispersal success of WCM; however, the two species did not differ in r. This pattern was consistent for both dispersal modes. Moreover, both species had higher colonisation success when dispersing actively, albeit with a very limited spatial range. These results underline the role of successful dispersal in the colonisation ability of species. The knowledge of the colonisation process of WCM and CRM is important for understanding their invasiveness and for predicting their potential geographical distributions.