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• W181045627 abstract "Herbivores shape the structure of many marine communities. However, algal-herbivore interactions are complex because algae and herbivores can respond both to each other and their environment. To resist herbivory, higher plants have evolved traits that confer defence and tolerance. Traits of defence reduce the quality of the tissue as a food whereas traits of tolerance reduce the detrimental effects of herbivory on plant fitness. The expression of tolerance and defence both incur costs to plants and tradeoffs can exist between these traits. Additionally, resource conditions, and in particular nutrient supply, can influence the expression of these traits. In contrast to defence, the role of tolerance to herbivory is not well studied in macroalgae but may help maintain the productivity of macroalgae following herbivory. Here I investigate the expression of tolerance in macroalgae and if, similar to plants, it is influenced by the expression of traits associated with defence and resource availability. This thesis uses a subtropical brown alga, Sargassum sp. and a co-occurring herbivorous amphipod, Cymadusa setosa, to investigate macroalgal-herbivore interactions in environments with varying nitrogen supply. Despite their small size (<2 cm), herbivores such as amphipods and isopods (collectively known as mesoherbivores) can significantly affect macroalgal communities and further investigation of these relatively unstudied herbivores will help understand the potential top-down controls on macroalgal communities. Cymadusa setosa uses Sargassum as a habitat and food source. To assess if Sargassum expresses traits of tolerance to herbivory, and how traits associated with defence and nutrient availability influence their expression; the photosynthetic activity, growth patterns and chemical composition of Sargassum were measured in a series of laboratory and field experiments that manipulated herbivory and nitrogen availability. The effects of Sargassum tissue traits on the feeding and habitat preferences of Cymudusa setosa were also determined. In Chapter 2, near infrared reflectance spectrometry (NIRS) is demonstrated to be an effective method of measuring the nitrogen, and carbon content of Sargassum tissue with the benefit of requiring only small sample quantities. It was also effective at quantifying the content of the polyphenolic compounds in brown algae (phlorotannins) that are known to deter herbivores in many systems. The developed NIRS models detected changes in the tissue composition of Sargassum grown over a range of temperature and nitrogen availabilities, demonstrating that environmental factors can have interactive effects on tissue traits. Chapter 3 shows that Sargassum can express traits of tolerance in response to simulated herbivory. Sargassum effectively compensated for loss of tissue in terms of overall growth, yet specific compensatory growth responses were dependent on the type of damage inflicted. Consistent reallocation of resources to secondary apices following the removal of the primary meristematic apex was observed. In contrast to terrestrial plants, no compensatory photosynthetic activity was observed and there was only weak evidence to suggest tradeoffs between traits of tolerance and phlorotannin content. This study suggested reallocation of resources within the thallus is important in regulating compensatory growth in Sargassum. In Chapter 4, nitrogen enrichment did not affect the expression of tolerance to herbivory in Sargassum. The Limiting Resource Model (Wise and Abrahamson 2005), derived for terrestrial plants, successfully predicted these results, demonstrating its potential to predict the effects of resource conditions on the relative expression of tolerance in macroalgae. Mechanical removal of primary apices in Sargassum resulted in enhanced growth of secondary apices. However, Sargassum failed to compensate for amphipod herbivory in the laboratory, suggesting that the severity of herbivory influences the expression of tolerance. Relationships between compensatory growth and phlorotannin content in Sargassum were not strongly influenced by nitrogen availability and were variable, dependent on the growth traits investigated. Biomass traits tended to be positively correlated with phlorotannin content while relative growth traits were negatively or not correlated with phlorotannin content. These observations indicate a need to consider the scale and nature of growth traits used when assessing tradeoffs between defence and tolerance in macroalgae. Chapter 5 demonstrates differences among tissues within an individual Sargassum, and differences among life history stages, influence habitat and feeding preferences of Cymadusa setosa. Amphipods showed feeding preferences for juvenile over mature Sargassum tissue, and both habitat and feeding preferences for apical over basal Sargassum tissue. Preferences were not associated with tissue composition (nitrogen, carbon, phlorotannins) but instead with the physical structure of the tissue. Individual feeding rates were not enhanced on nitrogen enriched Sargassum and thus, amphipods are unlikely to control nutrient induced Sargassum via this pathway. The preferences of amphipods for apical tissue also demonstrate that mesoherbivores can have significant effects on macroalgae whilst only removing small amounts of tissue. In summary, this thesis has generated several contributions to this field of marine ecology. This thesis has developed a powerful and quick method for identifying algal traits and unambiguously demonstrated the ability of Sargassum to tolerate damage. This thesis further found that the ability of Sargassum to tolerate damage is not linked to enhanced photosynthesis, nitrogen availability or tissue phlorotannin content, testing key assumptions underlying studies in both marine and terrestrial systems." @default.
• W181045627 created "2016-06-24" @default.
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• W181045627 date "2010-04-01" @default.
• W181045627 modified "2023-09-27" @default.
• W181045627 title "Macroalgal-herbivore interactions: the roles of algal tolerance, patterns of herbivory and nutrient availability" @default.
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