Temporal Currency: Life-history strategies of a native marine invertebrate increasingly exposed to urbanisation and invasion
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Biological invasions pose a serious threat to biodiversity world-wide. Through various means, such as competition or predation, invaders can radically change species composition and the functioning of native ecosystems. Even though our understanding of the mechanisms underlying invasion success is improving, there is still a lack of knowledge on the response of native species under pressure from invasion. This study adds to existing knowledge on the responses of a native species to invasion by non-indigenous species. Pyura pachydermatina is a native ascidian in the southeast coast of New Zealand currently under pressure from increased urbanisation and invasion by other ascidian species. The reproductive strategies employed by P. pachydermatina are investigated and the role of these strategies to increase its resistance to invasion are assessed. A population study on the status of P. pachydermatina around the Banks Peninsula was carried out in Camp Bay, Pigeon Bay, and Wainui. Spawning experiments using P. pachydermatina and gonad histology were done regularly during the one year study period to assess its ability to self-fertilise and determine its reproductive period. In addition, predation experiments were carried out to assess the susceptibility of P. pachydermatina early life stages to two amphipod predators. The surveys indicated that the populations of P. pachydermatina in the three sites are different from one another. Wainui has on average the largest individuals of P. pachydermatina and Camp Bay, the smallest. Abundance of P. pachydermatina was highest in Pigeon Bay and lowest in Wainui. The three life stages of Pyura pachydermatina; recruits, juveniles, and adults, were present in all sites at all seasons. The spawning experiments confirmed the species’ ability to self-fertilise and that it has a year-round spawning period. The two amphipod predators, Jassa marmorata and Caprella mutica, were efficient in consuming the egg and larval stages of P. pachydermatina, but did not feed on the settlers. Year-round reproduction and the ability to self-fertilise potentially give P. pachydermatina increased resistance to the effects of urbanisation and invasion. This population study suggested that the species is thriving around the Banks Peninsula. This, combined with previous studies on the non-indigenous ascidian Styela clava that stated the static or declining populations of the potential invaders, gives a positive outlook for the native species for the future. I suggest the use of genetic techniques to assess, in more detail, the population structure and dispersal potential of this native species. I also suggest constant monitoring of native species is required to keep up to date with the current status of the species, which will in turn help management decisions should regional spread of the Lyttelton S. clava invasion occur in the future.