Mechanisms of invasion: establishment, spread and persistence of introduced seaweed populations

Understanding the mechanisms that facilitate or inhibit invasion of exotic seaweeds is crucial in assessing the threat posed by their incursion and to define control options. In this paper, we consider how life history characteristics of the invading species and properties of the recipient environment influence the likelihood of invasion, giving particular emphasis to how disturbance influences the establishment, spread and persistence of introduced seaweed populations. Very few commonalities in key life history traits emerge since each species possesses a unique set of traits that confers a high capacity for invasiveness. Consequently, for seaweeds at least, predictions of invasibility based on life history characters alone are unlikely to be useful. In contrast, it is clear that disturbance is an important process in the establishment of these invasive species. With the possible exception of Caulerpa taxifolia, disturbance appears to be a critical factor that is either a key requirement (e.g., Codium fragile ssp. tomentosoides, Sargassum muticum and Undaria pinnatifida), or which accelerates (e.g., Fucus serratus) establishment and spread. The role of disturbance in the persistence of the invaders is more complex and depends on the species concerned. In several cases there is substantial evidence for positive feedback mechanisms that enable introduced species to persist in the absence of the disturbance factor that facilitated establishment in the first place. These circumstances define examples of ecological hystereses that pose particular challenges for management and control. The evidence suggests that, in several cases, preventing anthropogenically mediated disturbance to canopies of native seaweeds should be considered as a potential control option to minimise the risk of establishment of exotic species at high densities. However, for these kinds of introduced species, once they are established, control options that primarily target the disturbance are unlikely to represent viable management options.