WAMSI 2 KMRP 1.1.2 Key Ecological Processes - Herbivory

Brief description

Herbivory is a key ecological process that sustains food webs, and can regulate the biomass of primary producers in an ecosystem. It has long been hypothesized that rates of herbivory are greatest in the tropics, although strong evidence to support this is limited. The aim of this project was to identify the key species of herbivores, to identify the grazing rates of key herbivores, and in conjunction with project WAMSI 2 KMRP Project 2.2.4 (benthic primary productivity) provide estimates of the proportion of production that is consumed by herbivores. The research on herbivory was focused on the islands and coast of the Bardi Jawi Indigenous Protected Area in the Kimberley (Western Australia), encompassing Jalan (Tallon Island) and Iwany (Sunday Island). Focus of the herbivory study was on one type of habitat (seagrass meadows), and the diet of two species of herbivores (golden-lined rabbitfish and green turtle). Four surveys were conducted between October 2014 and April 2016. At these locations the following measurements or collections were made (not all measurements were made during each survey): (1) Rates of herbivory (three surveys). These data are presented in the report for WAMSI KMRP 2.2.4, here the focus is on assessing rates of herbivory as a proportion of primary production; (2) Collections of golden-lined rabbitfish (Siganus lineatus); and (3) Blood samples from green turtles (Chelonia mydas). Ten green turtles were tagged with satellite tags: 4 in April 2015, and 6 in April 2016. This data record only pertains to data held by CSIRO. For access to all other data generated by collaborative research partners of the KMRP 1.1.2 project refer to the additional metadata field.

Lineage

RUV deployments were completed at Jalan and Ngaloon (two of the sites included in measurements of rates of herbivory) to quantify variation in the composition and relative abundance of potential herbivores. These deployments were made during April 2015. On each of three days, ten remote underwater video cameras (GoPro Hero 4 Silver with a waterproof housing, GoPro Inc, San Mateo, California USA) were deployed in meadows of each of the two main species of seagrass, Thalassia hemprichii and Enhalus acoroides. Each camera filmed for 3 to 4 hours during each deployment. Cameras were placed on steel camera frames; each held two cameras facing in opposite directions. Individual frames were separated by at least 25 m. In the laboratory, 34 minutes from each camera during each deployment were analysed using EventMeasure software (SeaGIS Pty Ltd).

Rates of herbivory: Net rates of herbivory (as a percentage of growth) were calculated from data collected during the companion project WAMSI KMRP Project 2.2.4 (Kendrick et al. 2017). Rates of growth were calculated as mm2 per shoot per day from surveys in which growth was measured using a hole-punch method. Rates of consumption were also calculated as mm2 per shoot per day from tethering experiments.

Rabbitfish: Golden-lined rabbitfish (Siganus lineatus) were collected by spear in October 2014 and April 2015. Ten individuals were collected from Jalan, Laanyi and Ngaloon in each survey. They were weighed (wet weight, in grams) and measured (total length, in mm), and a small piece of dorsal muscle was excised by scalpel. The stomach was removed from individuals taken in October 2014.

Green turtles: Green turtles (Chelonia mydas) were captured during two surveys: April 2015 and April 2015. Turtles were captured using the “rodeo” method, in which individuals are captured in the water by an experienced person jumping from a boat. Upon capture, each individual turtle was weighed and measured (curved carapace length, in mm). Blood was extracted from a vein in the neck using a 22G x 1.5 inch needle, and immediately frozen.

Stomach content analyses: After unfreezing, the stomachs were separated from the rest of the digestive tracts, and rinsed with distilled water. The entire stomach of S. lineatus was used, but only a subsample of the stomachs of C. mydas were used. Their contents were spread on a 13 cm diameter glass Petri dish set over a sheet with 60 randomly-positioned dots. Stomach contents were viewed through a magnifying lamp, and food items were recorded each time they were covering a dot, being identified as close as possible to the species level. All diet sources observed in the stomach were reflected in the study, due to a low diversity of the content which allowed each species to cover at least one dot: 60 dots represented 100 % of the diet's diversity for the last meal of each individual.

Stable isotope ratios: Rabbitfish muscle tissue was thawed, cleaned, dried in an oven at 60°C, and ground into a fine powder using a mixer mill (Retsch MM200, Dusseldorf, Germany). Stable isotope ratios (d13C and d15N) were measured at the West Australian Biogeochemistry Centre using a continuous-flow system consisting of a Delta V Plus mass spectrometer connected with a Thermo Flush elemental analyser. Stable isotope ratios are expressed in ‰ using conventional delta (d) notation d X (‰) = [(Rsample / Rstandard)-1] x 1000; where X is d13C or d15N, and R is the 15N/14N (nitrogen) or 13C/12C (carbon) ratio in the sample and standards (Vienna PDB equivalent for carbon and the IAEA international standard of atmospheric N2 for nitrogen).

Satellite tagging of green turtles: Ten green turtles were tagged with satellite tags: 4 in April 2015, and 6 in April 2016. SPLASH10-F-296A and SPLASH10-F-296C Wildlife Computer Argos transmitter with Fastloc® GPS, temperature and depth recorders were used.