title: Alternative stable states, demography and intraspecific facilitation in the intertidal ecosystem engineer Hormosira banksia
Creators: name: Lewis, RD
Publisher: name: University of Tasmania
Year: 2020

description: In the intertidal zones of southern Australia and New Zealand, the fucoid brown macroalgae Hormosira banksii is often the dominant canopy-forming species. H. banksii is commonly found in association with an understory of red coralline turf algae. H. banksii is well recognised as an important habitat-forming or ‘ecosystem engineering’ species for its ability to add structure and complexity to its habitat. This important engineering species ameliorates harsh environmental variables which are common in intertidal areas such as wave activity and factors caused by prolonged exposure which include extreme temperatures and desiccation. Canopy forming species such as H. banksii add diversity to intertidal communities and make the environment more favourable for other species by providing complex structures which not only ameliorate the effects of environmental variables, but also provide benefits such as a refuge from predation, substrate for attachment and food source. In intertidal habitats disturbance is common, H. banksii populations are impacted by a range of natural and anthropogenic disturbances, these can include; trampling by foot traffic in populated areas, storm water and sewage effluent discharge, extreme temperatures, desiccation and storms increasing the intensity of wave activity. Threats to this important ecosystem engineer are becoming more common with the increasing human populations in coastal areas and climate change increasing the intensity and frequency of extreme weather events. Several studies have focussed on the associated assemblages, community structure and effects of disturbance events on H. banksii, however few have focussed on the effects of these disturbance events on canopy density and recruitment. The aims of this thesis were to determine; (i) how different gradients of disturbance intensity affected the recovery and recruitment of Hormosira banksii and its associated understory of coralline turf algae, (ii) how the survivorship of H. banksii was influenced by, and the effects of adult density on recruitment and post-recruitment mortality, and; (iii) to explore how ecosystem engineering species facilitate their own recruitment and survivorship through positive feedbacks in marine environments. In chapter 2, I manipulated the density of H. banksii canopy and coralline turf understory in a field experiment on the rocky shores of northern Tasmania. Percent cover of both canopy and understory were manipulated to eight different levels, the recovery of these plots was then monitored over the following two years. The aim of this experiment was to determine whether a threshold of disturbance exists at which the recovery of H. banksii was inhibited, or an alternate stable state dominated by coralline turf would competitively exclude H. banksii as previous studies have shown. I found that even where total removal of canopy or understory occurred, although slowly, the percentage of both cover types increased over following 24 months to densities near what was observed prior to manipulation. No change in species composition was observed regardless of the intensity of the disturbance. It could be assumed that factors which have caused alternate stable states between H. banksii and coralline turf and other types of fucoid algae in other studies, were largely absent from this system. In chapter 3, I followed the recruitment, growth and mortality of individual H. banksii plants at all growth stages over 18 months, these subplots were located within the plots that were manipulated in chapter 2. This aimed to determine the influence of the ‘adult’ population on the recruitment, growth and post-recruitment survivorship of H. banksii. We found that recruitment and post-recruitment survivorship were density-independent and within this system, adult populations appeared to have little influence on recruitment. Although H. banksii recruitment occurs all year round, we found evidence of a seasonal increase in recruitment over summer and that survivorship of recruits and adult plants was high overall. Chapter 4 reviewed the literature surrounding intraspecific facilitation in marine ecosystem engineers incorporated a meta-analysis of studies which experimentally addressed this phenomenon. We examined the overall effects of these species on their own demographic traits, whether the effects differed between intertidal and subtidal habitats, functional groups and between different types of manipulative studies. We found that overall, published works on marine ecosystem engineers mostly revealed they facilitate their own growth, recruitment and survivorship. This effect was consistent across functional and habitat groups and demographic traits. Overall, this study indicates that on the northern Tasmanian rocky shore, the population of Hormosira banksii appears to be resilient and stable, from our experiments it does not seem to be impacted by the same stressors which are causing localised decline in canopy forming fucoid algae worldwide. It examines the effects of different gradients of disturbance on H. banksii, documents, its recovery and did not find evidence of alternative stable states within this system. This study provides an insight into the demography of the ecosystem engineer Hormosira banksii and is one of the first to follow individual plants over a long term period and observe small scale recruitment, growth and mortality.