Spread by introduced macroalgae and their interactions with native herbivores

Introduced macroalgae have become abundant and widespread in the world's oceans and there is a perception that they may produce serious impacts. Despite this, few exotic algae have been studied in detail, and much remains to be known about the mechanisms of invasion and impacts of even those species that have received the most attention. This thesis examines range expansion by introduced seaweeds and their interactions with native herbivores, a component of the invaded community most likely to directly affect the spread of these seaweeds and to experience the direct and indirect effects of their presence. In the first quantitative examination and comparison of the spatial and temporal dynamics of multiple seaweed invasions (22 separate events) at a regional scale, I show that introduced macroalgae spread quickly (kilometers to hundreds of kilometer per year), in many cases after an initial lag when expansion was much slower. Although rapid, the rates are also quite variable, emphasizing the likely role of multiple interacting factors in determining where propagules are delivered, and whether they become established. I then use a series of field-based studies to examine the potential of green sea urchins to influence the distribution and abundance of the introduced alga Codium fragile. My results suggest that although urchins have the potential to exert strong control over populations of C. fragile, the outcome of interactions between the two species is likely to depend on their biotic and abiotic context. Next, I examine the consequences of consumption of C. fragile for urchins. I showed that, although urchins can feed on the introduced alga at a high rate, they are unable to compensate for its low nutritional value and suffer reduced growth and gonad production compared to urchins feeding on native kelps. Finally, I examine the effect that algal secondary metabolites, algal physiogamy, and dietary history have on urchin feeding behaviour. I identify a chemical anti-grazing defense produced by C. fragile that may contribute to the indifference that sea urchins and other generalist grazers normally exhibit towards the alga. I then show that the shape of C. fragile makes it easier for urchins to eat than the native kelps that they normally prefer, and that this preference can be reversed under certain conditions.