Resilience and stability of Ecklonia radiata kelp forests: the importance of intraspecific facilitation and patch dynamics

Kelp forests dominate coastal environments in temperate and subpolar latitudes around the world and, much like terrestrial forests, create complex habitats that support diverse and productive food webs. Studies of the resilience and stability of habitat-forming ecosystem engineers such as kelp have typically focussed on the role of external factors such as disturbance. Here, I propose that the stability and resilience of these species are also strongly influenced by internal processes. Such that, changes to the environment caused by engineer species positively affect their own demography (e.g. growth, survivorship), resulting in intraspecific facilitation via an 'environment-engineer' feedback. Ecklonia radiata is the most widespread and abundant habitat-forming kelp in Australasia. Unfortunately, this species is threatened by increasing ocean temperatures, overgrazing, and pollution, and is consequently becoming increasingly patchy and sparse in many locations. Existing research has focussed on understanding how external stressors and disturbances influence the population dynamics of E. radiata, however internal drivers such as the environment-engineer feedback are potentially just as important but remain largely unaddressed. My core aims were to determine (i) how engineering of the local environment by E. radiata changes with patch size, and how this influences environment-engineer feedback on the species' demography, and (ii) how the nature of the feedback influences stability and resilience of E. radiata in the face of increasing habitat fragmentation and degradation. Long-term field experiments confirmed that engineering of abiotic conditions within E. radiata habitats is patch size dependent (Chapter 2), and that reductions in patch size disrupt the recruitment of juvenile E. radiata (Chapter 3). These findings directed the construction of an artificial reef system spanning more than a hectare, and onto which over 500 adult E. radiata were transplanted. Using this unique experimental environment, I found that reductions to patch size and adult kelp density impair microscopic and macroscopic juvenile E. radiata due to a breakdown of ecosystem engineering by adult conspecifics, such that demographic collapse occurs in the absence of sufficient adult E. radiata (Chapter 4). Two key outcomes from these field studies were: (1) the provision of suitable habitat and amelioration of physical stressors via ecosystem engineering by adult E. radiata appears critical to juvenile conspecifics and; (2) formation and development of filamentous turf algae ‚Äö- which inhibit kelp recruitment ‚Äö- is a primary cause and effect of the demographic collapse of E. radiata populations. Ecklonia radiata and turf algae habitats exist as alternative stable states, with each state inhibiting the formation of the other. I developed an innovative laboratory experiment to improve understanding of how turf algae may disrupt the recruitment of E. radiata, and found that turf algae create highly modified chemical conditions in the benthic microenvironments with elevated concentrations of oxygen and pH relative to E. radiata assemblages (Chapter 5). Ultimately, this thesis presents results consistent with the hypothesis that positive environment-engineer feedback facilitates the demography of E. radiata. Moreover, the impaired ability of E. radiata to engineer change due to reductions in patch size cause a breakdown in this intraspecific facilitation, leading too reduced habitat stability and resilience. This work contributes to ecological theories of habitat resilience, facilitation, alternative stable states and ecosystem engineers, and provides insights for the future management, conservation and restoration of critically important kelp forest ecosystems.