Mark Langdon

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.

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.

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.