University Of Tasmania
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Dynamic resilience and stability of Ecklonia radiata : the importance of density-dependent ecosystem engineering feedbacks

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posted on 2023-05-28, 09:01 authored by Masayuki TatsumiMasayuki Tatsumi
Kelps are the dominant marine ecosystem engineers of temperate subtidal rocky reefs around the globe. Kelp forests create a complex habitat structure, modify abiotic factors and support diverse and highly productive systems. However, kelps face several threats and degradation of kelp forests has been reported in a number of places including south-eastern Australia. To date, most studies of threats to kelps focus on how external factors contribute to the degradation but there are also important internal drivers that will also influence kelp stability and resilience. Because ecosystem engineers modify the abiotic and biotic environment, the changes they make may positively feedback to affect their own demography. Consequently, a decline in adult density may lead to a decline in engineering capacity, a change in the modified conditions and reduction in, or loss, of positive effects. Ecklonia radiata is the most widespread habitat-forming kelp in Australia, however, range of stressors, such as ocean warming, more severe and frequent storms, overgrazing and pollution have been impacting its populations causing localised declines in population density. Although studies have been conducted to understand the implications of these changes, how a decline in adult density will affect the demography (reproduction, recruitment, post-recruitment growth and survivorship) of E. radiata are poorly understood. The central aims of this thesis were to determine; (i) how a decline in the density of Ecklonia affects engineering of critical abiotic factors and the link between those abiotic changes and both the understory community and Ecklonia reproduction, recruitment and post-recruitment growth and survivorship, (ii) whether abiotic factors modified by E. radiata were important mechanisms affecting the early post-recruitment survivorship and growth of E. radiata sporophytes, and; (iii) how density-dependent effects on reproductive output affected recruitment of both gametophytes and sporophytes. Chapter 2 presents a study where I manipulated the density of adult E. radiata in a field experiment for 24 months to four different levels (zero, low, medium and high) and measured changes to abiotic factors, associated understory algae and E. radiata recruit demography. These manipulations revealed density-dependent engineering of some, but not all abiotic factors. Most notably, light increased as E. radiata density decreased; scour increased at low and medium E. radiata densities compared to high; while sediment accumulation increased in the absence of E. radiata compared to any treatments with E. radiata. Despite some density-dependent changes in abiotic factors, the understory algal community and demography of E. radiata recruits did not always reflect these changes due to large variation in these metrics within the density treatments which highlighted the variable nature of these processes acting on small-spatial scales on natural reefs. In chapter 3, I manipulated three modified abiotic factors (light, scour and water flow) in the field to identity their role as mechanisms affecting the survivorship and growth of early sporophyte recruits. Overall, this experiment revealed low light / low scour (ambient flow) as important for very small / early sporophytes, but the response of slightly larger sporophytes was more complex. By week 6 in the absence of scour, light and water flow interacted and the highest survivorship occurred with ambient flow/low light and low flow/ambient light. Importantly, these larger sporophytes respond positively and grow much faster under higher light. Chapter 4 tested density-dependent issues related to reproduction. Specifically, how reproductive capacity of E. radiata in the field (the amount of zoospores released per individual) was affected by adult E. radiata density and how recruitment of microscopic stages (gametophytes and sporophytes) was influenced by zoospore density in two different seasons. Zoospore released per individual did not vary greatly in relation to adult density indicating that the total amount of zoospores produced in a E. radiata forest is likely to increase as the number of reproductive adults increases. Gametophyte and sporophyte recruitment were dependent on the density zoospores, with clear optimal ranges (90-355 mm\\(^{-2}\\) in spring and 21-261 mm\\(^{-2}\\) in winter) and threshold densities (< 6.5 mm\\(^{-2}\\) in spring < 0.5 mm\\(^{-2}\\) in winter) of zoospores for successful recruitment. Overall, this study indicates that some abiotic factors (light, scour and sediment accumulation) change in response to a declining density of Ecklonia and that low scour and combinations of light and water flow are important to post-recruitment survivorship and growth. However, the evidence for the break-down of a positive demographic feedback with a decline in E. radiata density was not strong. Nonetheless this study did indicate that a high density of reproductive E. radiata sporophytes is likely to result in a high density of zoospores and subsequently higher recruitment of microscopic sporophytes beneath the canopy. The sub-canopy conditions beneath a high-density E. radiata forest (low light, reduced scour and understory algal abundance) are also likely to result in high survivorship of microscopic sporophytes. Once light increases due the creation of localised gaps in the canopy, these recruits grow fast allowing the canopy to reform highlighting a key component to the resilience of E. radiata forests.


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