The role of fauna in seagrass habitats

<p dir="ltr">Animals are both inhabitants and key determinants of ecosystems. The influence of animals on ecosystems may be trophic, or non-trophic, the latter occurring where animals change the availability of resources that shape the ecosystem, or modify environmental conditions. This thesis examined the role that macro- and mega-fauna play in shaping seagrass habitats, and how animal-seagrass interactions are altered under environmental change. Its focus was on non-consumptive positive species interactions, which have received far less attention than consumptive effects. The experiments spanned three species of seagrass in three countries, early to later life history stages of seagrass, and utilized a combination of aquarium and field experiments.</p><p dir="ltr">Infauna may be particularly influential in the early life-history stages of seagrasses, through their bioturbation, which can move particles up and down in the sediment and, resulting modification of sediment conditions. As metabolic processes and animal activity are influenced by temperature, these interactions may be altered under scenarios of climate change. Using aquarium experiments I investigated (1) how different functional groups of infauna interact to shape burial, survival and germination of <i>Zostera marina </i>seeds and (2) how these interactions are influenced by heat-wave conditions. Under present day conditions, cockles had a greater positive influence on seed burial than polychaetes, shrimp or species mixtures. Under the heat-wave scenario, seed burial in the mixed infauna treatment increased to match cockles. Seed burial in turn influenced germination. These results indicate the importance of consideration of infaunal interactions in the establishment of seagrass beds. Despite considerable restoration effort in the Dutch Wadden Sea, <i>Z. marina, </i>success is limited, suggesting a need to provide facilitative infaunal communities.</p><p dir="ltr">In addition to infaunal bioturbators, mega-bioturbators, such as rays, may play an important role in shaping seagrass beds, determining their persistence and lateral expansion. In the Canary Islands, I established mega-bioturbator exclusion ‘stockades’ in middle, edge and surrounding sand of <i>Cymodocea nodosa </i>meadows, to investigate effects of mega-bioturbators on seagrass density and biomass. I hypothesized that in the dense middles of seagrass beds, self-facilitation mechanisms of seagrass would prevent effects of mega-bioturbators (such that effects of exclusion stockades would not be seen). By contrast I expected that in the sandy areas, mega-bioturbators would have a strong negative influence on establishing seagrass, such that their exclusion through stockades would enhance seagrass growth (and hence, lateral extension of the patch). These hypotheses were largely supported by experimental results, with positive effects of mega-bioturbator exclusion on seagrass biomass in the sand but no the middle or edge of seagrass patches. These results support the important role of self-facilitation. They also suggest that mega-bioturbating species play a crucial role in shaping seagrass patch size, and possibly, the shoot densities within seagrass vegetation.</p><p dir="ltr">In addition, animal inhabitants within seagrass may shape community structure by making the environment more or less suitable for occupation of other animals, plants and algae. On the east Australian coast, I documented evidence of a facilitation cascade whereby <i>Zostera muelleri </i>seagrass increases densities and decreases burial of <i>Anadara trapezia </i>cockles, as compared to in sand, which in turn enhances exposed shell substrate for epibionts to colonize. I then examined how this interaction varies across a temperature gradient created by a thermal plume from a power plant. Despite predictions that the facilitation cascade would be strengthened by warming, there was little change across the gradient. Positive effects of warming on epibiont productivity were offset by lower densities and smaller cockles to provide habitat. These results highlight the complexity of ecological interactions in seagrass, and the importance of maintaining these to buffer effects of environmental change.</p><p dir="ltr">Together, the three studies enhance our understanding of how non-trophic plant-animal interactions shape the establishment, growth and persistence of seagrass meadows, and determine their communities. The work also shows the sensitivity of some of these interactions to heatwave anomalies and rising temperature. This raises the possibility that in some instances, seagrass beds will be hit by the double whammy of direct negative temperature impacts and indirect impacts resulting from changes in species interactions. In other instances, these interactions may, however, assist in buffering seagrass from the effects of change. Interspecific relationships can shift from positive to negative, across environmental gradients, spatial scales and life-history stages. Consideration of these interactions will be critical to developing effective conservation and restoration strategies.</p>