Tortoise Faecal Decomposition Monitoring

As part of the Pre RaCE seasonal monitoring here on Aldabra one of the focusses of our studies is on Soil Ecology. Soils underpin terrestrial ecosystems, and their ecology is key to understanding ecosystem health and the impacts of environmental changes. Soil anchors roots, holds water, stores nutrients and carbon, and is home to numerous macro- and micro-organisms that fix nitrogen, decompose organic matter and are essential to many other species and processes. As part of this monitoring, litter decomposition is being investigated, and one branch of this includes monitoring Aldabra Giant Tortoise Faeces decomposition. The monitoring will track changes in tortoise faecal decomposition rates pre- and post-eradication.

The Aldabra giant tortoise (Aldabrachelys gigantea) is the largest vertebrate on the atoll. It is likely to play key functional roles much like continental herbivores and can be considered an ecosystem engineer. Tortoises are known to modify their habitats through seed dispersal and nutrient cycling via their faeces, which provide soil enrichment and an ideal germination environment for seeds. Therefore it is important to understand decomposition rates of this organic material.This monitoring involves collecting three samples of very fresh tortoise poo, splitting each sample into four replicates to be weighed and dried at different time intervals. For example, for each poo, four replicates each 10g are separated, one of these is placed straight into the oven at 70 degrees Celsius for 48hrs and the other three are placed inside mesh bags which are then deployed back out in the field in a suitable shady location, each replicate is placed at least 50cm away from each other for differing lengths of time (7 days, 30 days and 90 days). This monitoring is to take place once every 6 months (once in the dry season and once in the wet season). This week was the first time this monitoring was being conducted officially for the Pre-Race monitoring. It only takes place on Picard (where the research station is).

On the 8th July, Clarisha and I were tasked with heading out to find three very fresh tortoise poos, from standard mixed scrub habitats, far enough away from the station so that the tortoises were not likely to have been interacting with food thrown outside by people at the station. We headed towards Basin Lebine at 7am and it took us just over an hour to find three fresh faeces samples. We actually collected four just to have a spare. Whenever we found a suitable poo (a pale yellowish brown colour very wet) I took a GPS of the location so we can map where the samples have been taken from. Using gloves we put each poo in a labelled ziplock bag and took it back to the station.

The faeces were then divided into 4 replicates each of exactly 10g. Each sub-sample (apart from one ) was placed in a mesh litterbag. I was in charge of labelling the subsamples with their sample number e.g. TFD-PIC-01 (Tortoise Faecal Decomposition, Picard, Sample 1) and then what time period it would be retrieved at e.g T7, T30 or T90 (days). I put pieces of waterproof paper with the labels inside the bags and also wrote the same information on duct tape in permanent marker and stuck it on the top of the bags as well. Labels of subsamples: T0 (on the day the faeces was first found), T7 (seven days after redeployed on site), T30 (30 days after redeployment), T90 (90 days after redeployment).

After all of the samples were ready I then took the rangers (Angela and Clarisha) and Norisha (Health and Safety Officer) to select a suitable deployment site. I had to choose one site for all three replicates and the 9 total subsamples, located in standard mixed scrub habitat, in a shaded area, and far away enough from station to avoid any type of biases linked to human influence.

Once we selected a site just off of backpath we cleared litterfall and made sure each sub-sample is spaced by at least 50cm apart. I lined the bags so that they were in a grid system with Poo ID along the top (column) (01, 02 and 03) and the rows indicated the time deployed for (T7, T30 & T90) we marked the four outside corners of the plot with red spray paint on rocks and secured the mesh bags with sod staples. I then also tied a short piece of coloured flagging tape to each bag depending on the Poo ID (pink = 01, orange = 02 and blue =03). I placed orange flagging tape in the trees marking the four corners of the plot, and added extra blue flagging tape on the two ‘top’ trees so that if the labels went missing the direction of the columns and rows can still be identified. The next day Rona and William returned to build a tortoise exclusion zone around the plot to attempt to keep any tortoises from trampling the site, so hopefully this works.

T0 was placed straight in the oven whilst we were doing the weighing (wet mass of 10g). I then went into the office on the 10th July to reweigh them and record their dry mass. We will then retrieve T7 next week and have to wait a while to collect T30 and T90. Each time we collect the samples they will be cleaned (debris and plants removed) then dried in the oven at 70˚C for 48h before weighing their dry mass. Once the 90 days is up the decomposition rate will be calculated by comparing the mass remaining over time which gives a decay curve.