Putting frogs into freshwater food webs

A while ago, I wrote about looking for the end of the food-chain. My co-workers and I had dissected some common species of tadpoles from floodplain wetlands and inspected their gut contents under a microscope. We had found they eat a wide variety of things, from tiny crustaceans and other small insects to bits of aquatic plants and long strands of filamentous algae.

The long, long gut of a marsh frog tadpole.

But I wanted to know, what was the most important thing that they were eating? I’d had a feeling the answer was going to be “even smaller things”, and it turns out I was right!

To answer this part of the question, we did a chemical analysis that compared samples of tadpole muscle tissue with samples of food items such as wetland plants, tree leaves, algae, those teeny tiny crustaceans living in the water, and biofilm. It is a method called stable isotope analysis and it works along the lines that ‘you are what you eat’. And as we report in our new research paper, these results suggested that biofilm was most important. (Twitter summary here). (Full disclaimer, we’re mostly confident that it was biofilm, in some wetlands at some times for a couple of species… check the paper for all the details, DM me for a copy).

And what is biofilm? Biofilm is basically a super nutritious mix of very small things like bacteria, microbes, fungal hyphae, and teeny-tiny invertebrates, that looks like a filmy, fuzzy goo growing on the surface of leaves and branches and anything underwater.

Biofilm – the filmy, fuzzy goop growing on surfaces under water.

Our results were interesting because when we compared what was in the muscles with what we had seen in the gut contents, it looked like tadpole diet is more a case of ‘you are some of what you eat and not necessarily the most obvious or biggest thing’.

The small end of the food web… biofilm supports marsh frog tadpole growth in some Murrumbidgee floodplain wetlands (NSW, Australia)

So, while pieces of plants or long strands of algae were often jam-packed in the tadpoles’ guts taking up a lot of space, they were not contributing much nutritionally. It looks like the really small and least obvious thing growing underwater is a high quality, nutritious snack supporting the growth and health of bigger lifeforms!

Lots of algae, not so much nutrition!

There is definitely more to know before making strategies for wetland management that will support lots of good tasty biofilm and in turn support tadpole growth, but we’re starting to build evidence for what we only suspected before. For example, see Altig et al 2007, Schiesari et al 2009, Schmidt et al 2017. And I definitely suggest doing both gut content dissection and stable isotope analysis, if you want to find the important end of the tadpole food-web.

A glorious Lower Murrumbidgee wetland… good tadpole habitat.

What I saw on my fieldwork. Part 1: shield shrimp!

Often, the weird and wonderful things are the things we see least often. And often the reasons we see them so rarely, are what makes them so weird and wonderful. Take the crazy freshwater crustacean called the shield shrimp in Australia (scientific name Triops australiensis). I’ve been wandering around floodplain wetlands and creeks for a few years now, peering into the water and sweeping nets around looking for tadpoles, and until the last month, I’d only see one of these things.

Snow-plough shield with eyes on top.
Snow-plough shield with eyes on top.

Lotsa legs.
Lotsa legs.

They are weird because they have three eyes – yes really (the middle one tells light from dark). And because they haven’t changed since the early dinosaurs walked the earth. And because it looks like they attached a flattened snow-plough over their head and body, then discovered they couldn’t see, so stuck the eyes on the outside.

And they are wonderful because the eggs can survive the harshest cold and the driest heat and being blown around in the wind for YEARS, before hatching out and finding themselves in temporary pools near the top of Uluru and rocky outcrops at Arapiles, or sandy desert swales and clay-pans, or floodplain creeks in the Murray Darling Basin. Which is where I saw them in September. At a couple of places, I got more shield shrimp in the tadpole nets than tadpoles. Such ungainly looking things, not particularly hydrodynamic-ly designed for an aquatic animal. And at night, I watched them scoot and tumble about, yes definitely not very hydrodynamic.

Shield shrimp swimming around at night (I count five…)

A note on pilgrimages (but mostly about the Lake Eyre Basin).

When I first saw Pelobates fuscus, I was much more excited than I thought I’d be. Yes, this was a delightful if slightly dull-coloured round blob of a European burrowing frog (the ‘common spadefoot’), spotted while traipsing through a wetland in Hungary. But it was more than just a new species I could tick off my global amphibian checklist. This was a ‘pilgrimage’ species. Pelobates fuscus was the subject of several scientific articles I read early in my PhD that were important for learning theories and shaping my understanding of amphibians and floodplain wetlands. I’d read about it, studied it, and now I was looking at it!

My pilgrimage species
My pilgrimage species

I recently got to tick off another item on my pilgrimage list – this time the Lake Eyre Basin (LEB). The LEB looms large in the Australian psyche, but especially so if you are a freshwater junkie like myself, fascinated by how water can move across such a massive, flat, arid landscape and the life it brings with it.

A study in the late 1990s characterised 52 rivers worldwide and found the Cooper Creek and the Diamantina Creek, both in the LEB, to be the most variable for when the water flows, how much comes and how long you have to wait in between flows (i.e. flow timing. magnitude and frequency). They extended the Flood Pulse Concept (Junk 1989), which had only been applied to rivers with predictable regular flows, to include this complexity and so recognise that lots of biological processes, such as waterbird movement and breeding, fish spawning, rely on this natural variability. Again, these were paradigms and theories that made me stop and think, and became central how I understood my own research.

Now, the LEB is not exactly the low-hanging fruit of pilgrimage sites. It’s rather large (one-sixth of Australia’s landmass, or France, Spain and Portugal put together) so what counts as ‘being there’? Could I tick it off my list by just getting my foot over the line of the Basin catchment, or do I need to see Lake? What about putting a toe in the water? Also, it’s a long way away from anything. The sealed roads run out well before you get there. It’s not something you drive past on your way elsewhere, it has to be the destination. And a long weekend from Sydney is unlikely to cut it. Finally, it’s often not there. Well the water isn’t anyway. Like I said it is variable. Subject to continental-sized weather processes as well as high rates of evaporation, on average the lake only fills once in ten years.

Route of our flight from Broken Hill to Birdsville, over Lake Eyre and back again.
Route of our flight from Broken Hill to Birdsville, over Lake Eyre and back again.

Us with our wings about to leave Birdsville.
Us with our wings about to leave Birdsville.

Fortunately, the weather-gods smiled on my pilgrimage to the LEB. In mid-May 2016, my parents and I drove to Broken Hill, and from there took a chartered scenic flight which followed the Cooper Creek north and  stopped at the Birdsville Hotel overnight. The next day we followed the Diamantina Creek and Warburton Channel down stream before and flying across Lake Eyre to William Creek. Due to the rain across the region three days earlier and recent flows down from Queensland, there was water everywhere and the lake was about 60-70% full. A spectacular sight. I can’t adequately describe the experience of following those creeks and flying over the Lake. Really I can’t. The photos don’t do it justice. A worthy pilgrimage site, I was amazed and humbled.

Footnote: huge thanks to Drew from Silver City Air Charter for flying us around and planning the whole trip, highly recommend!

A watery-looking Outback
A watery-looking Outback

Diamantina Creek
Diamantina Creek

Nearly at the Lake water!
Nearly at the Lake water!

Lake Eyre! So much water, so much lake!
Water in Lake Eyre, so much water!

South end of Lake Eyre.
Edge of the Lake.