Mark Langdon

Sea urchins can have a significant influence upon the ecological structure of coral reefs through both bioerosion of substrata and by affecting competition for space. Loss of reef structure can limit space for algal and coral recruitment which further alters the balance between reef growth and reef destruction. Urchins are important grazers in many marine systems and can cause major ecosystem changes when their numbers reach high levels (generally after a decline in the numbers of their fish predators). However, the relative importance of the role of urchins in influencing the composition and structure of coral reef habitats has rarely been explored. This thesis investigated the habitat preferences, distribution, grazing, bioerosion, and behaviour of the grazing urchin Echinometra mathaei at Ningaloo Marine Park (NMP), Western Australia. Coral reef habitats of the NMP were characterised using field surveys and validations of broad-scale hyper-spectral benthic habitat maps; the effects of habitat type and different closure regimes (e.g. Sanctuary zones) on urchin distribution and abundance were then examined and compared. This thesis represents the first study to quantify the grazing and consequent bioerosion rates of E. mathaei at Ningaloo Reef and the first to study their animistic behaviour and diurnal movement patterns. Data were collected from over 100 sites within the Marine Park, focussing on near shore, lagoonal and back reef areas within Sanctuary zones and adjacent Recreation zones. Data analyses indicated that the distribution of urchins was variable and appears not to be affected by the management zones of the park (i.e. no significant evidence has been found of indirect effects from fishing of known urchin predators). However, habitat type had a major influence on urchin distribution; urchin abundances were higher on nearshore intertidal and sub-tidal reef platforms, lagoonal patch reefs and shallow backreef platforms than in other habitats. Data analysis showed strong positive correlations between urchin densities and habitats that contained turf algae, and a combination of limestone pavement and turf algae. Grazing and bioerosion studies demonstrated that although E. mathaei grazing plays an important ecological role, concomitant bioerosion may play a more central role in influencing the structure of coral reef communities than grazing at the NMP. Urchin, morphometrics and gut content analyses from different habitats in four regions of the NMP indicated higher mean urchin densities, size and subsequent bioerosion rates in southern regions than in the north of the park. Bioerosion rates from Ningaloo Reef (1.0 - 4.5 kg m-2 year-1 of CaCO3) were found to be comparable to degraded (overfished) reef systems in other parts of the world, but without accurate estimates of CaCO3 accretion rates it is difficult to determine the degree to which bioerosion is affecting reef growth at the NMP or if it is any more or less significant than in other parts of the world. Results from this study suggest that habitats at Ningaloo with high E. mathaei densities are more likely to be niche habitats that co-exist with other coral reef habitats as part of a healthy ecosystem. Video footage of diurnal movement revealed that E. mathaei did not leave their burrows to graze but were systematically “gardening’ turf within longitudinal burrows at night and sheltering from predators during the day. Observations of animistic behaviour experiments showed that they would also defend their burrows when threatened by intruding conspecifics but the majority of interactions would result in urchins coexisting in the same longitudinal burrow. This type of territorial grazing behaviour within long, tube-like burrows has been documented for other urchin species (e.g. the northern Atlantic echinoid, E. lucunter) but never for E. mathaei. Defence of (and sharing of) longitudinal burrows may also be associated with other predation avoidance behaviour.

Herbivore grazing is a well-documented cause of habitat decline in terrestrial systems, but marine examples from seagrass meadows are rare. Here we present evidence that isolated urchin grazing events have caused further localized losses to seagrass meadows already degraded by eutrophication or other anthropogenic disturbances. By 1992 a substantial scar in Posidonia meadows at Luscombe Bay in Cockburn Sound, Western Australia, had been caused by grazing urchins. When seagrass transplants were placed at the site more than a decade later most were grazed and did not survive. GIS analyses on imagery from 1985 to 2004 indicated that rapid seagrass meadow decline coincided with the presence of an unusually large aggregation of the grazing urchin Heliocidaris erythrogramma. Evidence of some seagrass recovery after 1993 was also apparent after the manual removal of the urchins in late 1992. Restoration efforts in seagrass meadows should consider the potential for grazing damage, as is commonplace in terrestrial systems.

Sea urchins can have a significant influence upon the ecological structure of coral reefs through bioerosion of substrata and also by affecting competition for space. They are important grazers in many marine systems and can cause major ecosystem changes when their populations reach high levels (generally after a decline in the numbers of their fish predators). However, the relative importance of the role of sea urchins in influencing the composition and structure of coral reef habitats has rarely been explored. This study is linked to the CSIRO Wealth from Oceans Ningaloo Collaboration Cluster, Component 1: Habitat Mapping and Biodiversity. It has so far examined coral reef habitats and macroinvertebrate (particularly urchins) distribution and abundance within Ningaloo Marine Park. Field sampling has been undertaken at over 100 sites within the Park, focussing on near shore, lagoonal and back reef areas within Sanctuary zones and adjacent Recreation zones. Data analyses so far indicate that the distribution of urchins is not affected by the management zones of the park (i.e. no significant evidence has been found of indirect effects from fishing of urchin predators). However, habitat type has a major influence on urchin distribution, e.g. urchin populations were higher on nearshore intertidal and sub-tidal reef platforms, lagoonal patch reefs and shallow backreef platforms than other habitats. In coral reefs in other parts of the world, unusually high urchin populations can indicate overfishing. So far, this study has found no indication of fishing pressure indirectly affecting urchin densities, which suggests that the current zoning may be effective. However, further analyses of the data, particularly those from the nearshore sanctuary areas where shore based fishing activities are allowed, are yet to be completed. If fishing pressure is having an effect, it is likely to be seen in those areas where urchin abundance is predicted to be highest.

Surveys were undertaken of key invertebrate and plant groups in lagoon areas throughout the Ningaloo Marine Park, focussing on identifying and quantifying soft corals, sponges, echinoderms (urchins and sea cucumbers) and seagrasses; this information was linked to the underlying habitat structure in the Ningaloo Reef lagoons, used in a parallel habitat mapping project based on hyperspectral aerial photography. The biodiversity surveys conducted throughout the Ningaloo Reef system have shown that there are clear differences in the lagoon systems in different parts of the Marine Park, with clear biogeographic separation of sponges, soft corals and seagrasses. This has important implications for the distribution and abundance of many animals, including some of commercial importance. The northern section of the Marine Park (north of Point Cloates) is more strongly tropical than the southern section, which has many temperate species present. There are also several locations within the Ningaloo Reef system that are quite unique and don’t necessarily match the surrounding lagoon environment. Examples include Coral Bay, Bateman Bay and the Point Cloates region. These observations will be important in the management of the Ningaloo Marine Park; the northern and southern sections of the Marine Park may need to be managed differently. Surveying biological groups over the whole Marine Park is difficult and time consuming; further sampling in more locations and at different times of year are required to build on the findings of our study. There are clear seasonal differences in primary productivity in the lagoons, which likely drive significant community-wide changes throughout the year. These seasonal variations will also affect the validity of the habitat maps, which were based on imagery collected at only one time of year. Seasonal sampling will improve the reliability of the habitat maps and also give a better understanding of how the Ningaloo system operates.

SSurveys were undertaken of key invertebrate and plant groups in lagoon areas throughout the Ningaloo Marine Park, focussing on identifying and quantifying soft corals, sponges, echinoderms (urchins and sea cucumbers) and seagrasses; this information was linked to the underlying habitat structure in the Ningaloo Reef lagoons, used in a parallel habitat mapping project based on hyperspectral aerial photography. The biodiversity surveys conducted throughout the Ningaloo Reef system have shown that there are clear differences in the lagoon systems in different parts of the Marine Park, with clear biogeographic separation of sponges, soft corals and seagrasses. This has important implications for the distribution and abundance of many animals, including some of commercial importance. The northern section of the Marine Park (north of Point Cloates) is more strongly tropical than the southern section, which has many temperate species present. There are also several locations within the Ningaloo Reef system that are quite unique and don’t necessarily match the surrounding lagoon environment. Examples include Coral Bay, Bateman Bay and the Point Cloates region. These observations will be important in the management of the Ningaloo Marine Park; the northern and southern sections of the Marine Park may need to be managed differently. Surveying biological groups over the whole Marine Park is difficult and time consuming; further sampling in more locations and at different times of year are required to build on the findings of our study. There are clear seasonal differences in primary productivity in the lagoons, which likely drive significant community-wide changes throughout the year. These seasonal variations will also affect the validity of the habitat maps, which were based on imagery collected at only one time of year. Seasonal sampling will improve the reliability of the habitat maps and also give a better understanding of how the Ningaloo system operates.

As part of the CSIRO Wealth from Oceans Ningaloo Collaborative Cluster programme currently underway in Western Australia, this study aims to examine lagoonal habitats and biodiversity within Ningaloo Reef. Key habitat types were identified using information from hyperspectral remote sensing and were used to develop a stratified sampling approach. Two focal areas were selected, based on sanctuary zones within Ningaloo Marine Park: Osprey Bay and Coral Bay in the north and south respectively. A nested quadrat sampling regime was used to attempt to link field-collected data with remotely-sensed data, collected at different scales. Preliminary results confirm that northern sections of Ningaloo Reef differ greatly from the south, with a greater diversity of habitats present in the broader lagoons in the south. Greater areas of coral are found close inshore and across the entire reef at the southern location, compared with the northern section, which has a broad expanse of sand and limestone pavement before grading to corals further offshore (back-reef and reef-crest). These differences in habitat may have implications for the overall biodiversity of the two locations and more broadly along the reef.

The overall objective of this research project is to add to the general understanding of coral reef ecology and more specifically, advance the existing knowledge of the role of sea urchins in coral reef ecology at Ningaloo Marine Park. This study will examine marine grazers (particularly sea urchins), investigating their habitats, home range, reproduction, distribution, larval recruitment and settlement, and trophic relationships at Ningaloo Marine Park. The indirect effects of different closure regimes (e.g. Marine Protected Areas (MPA’s) such as sanctuary zones) on urchin ecology within Ningaloo Marine Park will be examined at length, both temporally and spatially over the next two to three years and will provide important new information which will aid in the formulation of future management strategies for the conservation and stewardship of Ningaloo Marine Park.