Building a stronger wetland food web

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What do tadpoles eat? It may seem a simple question, but the answer could play an important role in deciding when, and how often, local wetlands receive water.

Research is revealing the important role tadpoles play in the wetland food web.  Environmental water is helping to provide the conditions tadpoles need to flourish and in turn become a source of food for other wetland predators.

The NSW Office of Environment and Heritage manages the delivery of environmental water to support the health of wetlands, creeks and floodplains across the state.  Water managers plan the timing and duration of these flows to meet specific environmental aims such as fish or frog spawning, waterbird breeding, plant regeneration and/or improved water quality.  These decisions are based on a range of scientific data and principles.

So, how does the diet of a tiny tadpole affect the process?

Daphnia carinata, cladocera daphnidae - one of the food sources found in the tadpole's gut. Photo Julli Moreno OEH

Daphnia carinata, cladocera daphnidae – one of the food sources found in the tadpole’s gut. Photo Julli Moreno OEH.

Environmental scientist Dr Joanne Ocock is taking a close look at the food tadpoles eat and the nutrients they absorb from this food as part of a research grant from the Institute of Land, Water and Society, Charles Sturt University.

The aim of her research is to investigate if tadpoles have diet preferences and how they get their energy in floodplain wetlands. This information may then be used to inform planning for future environmental watering events to maximise the tadpole response and strengthen the food chain.

Dr Ocock is using a two part process to determine the foods eaten and the nutrients absorbed by tadpoles.  The content of the tadpole’s gut is viewed under the microscope to identify the materials present. Another technique, stable isotope analysis, is used to measure the chemical make-up of the tadpoles. This is used to determine which foods are absorbed into the tadpole’s body.

The results of the stable isotope analysis are pending, but a close inspection of the tadpole’s gut content is revealing some interesting information.

Early observations suggest that tadpoles are not very selective when it comes to what they eat.  Under the microscope we saw pieces of aquatic vegetation, small invertebrates, seeds and micro-crustaceans.  The marsh frog tadpoles appear to be quite indiscriminate about the foods they eat. They are acting like opportunistic omnivores – if there’s protein about, they’ll eat it. Dr Ocock said.

Two Bridges - one of the sites where tadpole samples were collected. Photo J Ocock OEH

Two Bridges – one of the sites where tadpole samples were collected. Photo J Ocock OEH

Their diet appears to vary from wetland to wetland, depending on the available foods.  It is interesting to note the presence of micro-crustaceans in the tadpole’s diet. Micro-crustaceans are very small animals that live in water and look a bit like shrimp. That’s a favourite food of larval fish and it is thought (according to other scientific studies) that a certain density of micro-crustaceans influences fish numbers within the food web.

“While other studies have focused on the needs of fish within the food chain, this is the first study of its kind in Australia focusing on tadpoles.I believe that tadpoles play a far bigger role in wetland food webs than has been previously thought.  Dr Ocock said.

 The sampling process . . .

Late last year and again in early 2015, tadpole samples were collected from two sites in the mid-Murrumbidgee and four in the lower Murrumbidgee.  Dr Ocock explained the process that was used to collect the samples:

The timing of these field trips coincided with environmental watering events.  Nets were set up and left overnight at sites that were typical of the individual wetlands.  Two species were targeted – the barking marsh frog and spotted marsh frog – because they are known to respond to environmental watering and are the most common and widespread species that occur in wetlands.

I also collected samples of possible food sources from each site – algae, micro-invertebrates, aquatic plants, silt, leaves and so on.  Back at the lab, samples were taken for stable isotope analysis – a process that identifies the ratios of carbon and nitrogen for comparison against samples of potential food sources collected from the site.  That process tells us which of the foods being eaten are actually assimilated into the tadpole’s body.

One of the wetlands where samples were taken Photo J Ocock OEH

Another of the wetlands where samples were collected. Photo J Ocock OEH.

The second part of the process involved viewing the contents of the tadpole’s gut under a microscope.  Results suggest that tadpoles are opportunistic omnivores. Their diets vary from wetland to wetland, according to the available foods.

“The tadpoles in turn become a food source for other animals and contribute to the overall health of the ecosystem. Although early days in this area of research, we’re gaining an insight into how environmental water may be used to help provide ideal conditions for tadpoles which then help to support the broader floodplain wetland food web,” Dr Ocock said.

Joanne Ocock is an environmental scientist with the Water and Wetlands Team, Science Division, NSW Office of Environment and Heritage. Her research into the basal carbon sources used by tadpoles is funded by an early career researcher grant through the Institute of Land, Water and Society, Charles Sturt University.

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