Investigating trophic effects of recolonising generalist predators in complex ecosystems

Abstract

Changes to predator populations can result in ecological changes due to cascading trophic interactions between key players of a food web. Australian (Arctocephalus pusillus doriferus) and long-nosed (Arctocephalus forsteri) fur seal populations are recovering, decades after the cessation of sealing in Australia. What trophic effects may stem from the return of these generalist predators to complex marine ecosystems? This thesis investigated the diets of two fur seal species at their geographic range edge in eastern Australia, using DNA metabarcoding techniques suited to the exploration of ecological interactions, and revealed highly diverse diets in both species. The diets differed seasonally and between both predator species at a new breeding colony, yielding similar patterns to colonies in the centre of their range, in southern Australia. However, the diets of both species converged, containing a high prevalence of coastal prey items, at range-edge, non-breeding sites that harbour younger animals and lower seal population densities. Differences in reef community structure and composition were therefore investigated at recolonizing fur seal aggregation sites compared to reference sites. Schooling fish and browsing herbivores were less common at fur seal aggregation sites. The older aggregation site, with ca. 15 years of use, had a lower total abundance of fish and greater proportional biomass of smaller fish, and a lower abundance of an important herbivore (Centrostephanus rodgersii). Macro-algal cover was also lower at aggregation sites. These results provide some evidence for differences between reef communities adjacent to fur seal aggregation and non-aggregation sites at their recolonization frontier, but they did not correspond to large ecological changes in reef community structure. This thesis provides novel baseline information, early in the recovery of fur seal, to assess potential changes in Australian temperate reefs experiencing the return of predators.