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.

Sharing love of frogs (even when on holiday…)

When I was first in the Macquarie Marshes, landholders didn’t know there were at least 15 different frog species, and would simply call everything either a green frog or a brown frog. Though after I’d compiled a small identification guide, they’d tell me the proper names of what they’d seen and how many were calling after it rained, and by the time I finished my PhD fieldwork out there, they were all experts! I love telling people about frogs and sharing what I know. Even when on holiday it seems, which is how I ended up doing a guide to the frogs I saw while on holiday in the northern Brazilian Pantanal.

Say cheese!

It’s a small wetland frog research world!

I’m not an expert on Pantanal frogs, but it turned out that finding some experts wasn’t that hard. It seems that while the world is a large place, the ‘world’ I live in is a bit smaller and mostly inhabited by like-minded people. So when I go on holiday and visit wetlands because that is what I like doing, I’m likely to run into other people who like to be around wetlands. This also increases the likelihood that they might be interested in frogs, maybe even researching them. And if the wetland that I’m holidaying at is a natural analogue for the wetlands that I work on here in Australia, then — apparently — there is a good chance that they’ll have read my research papers! Which is how you run into people who know you, even though you’re in a somewhat remote area of a large foreign country far from home, and is how I met my Pantanal frog experts, the awesome Leonardo, Natalia and Marcos.

The Pantanal is the largest continuous floodplain in the world. And like any good wetland system has a lot of frogs, as well as a multitude of other spectacular wildlife such as waterbirds (OK we also have them in Australia, but these are different waterbirds), giant spiny anteaters, capybaras, jaguars, anacondas, and caiman to name a few obvious ones.

Flooded floodplain in the afternoon light.

While I could find plenty of information on these charismatic megafauna, there was a distinct lack of frog knowledge including by the owners and staff of the pousada I stayed at (the wonderful Araras). It seems even though they could hear the frogs and were surrounded by them, they didn’t know any of the species and hadn’t met anyone interested in frogs before. Even just showing them photos of what I’d seen the night before opened up a new world for them, species that were totally unknown yet a very present part of the wildlife of the Pantanal.

Showing the guide how it’s done.

By the end of my stay at Araras, the owners were keen to know more about the frogs and asked me to share what I’d found. Fortunately, I’d kept in touch with Leonardo, Natalia and Marcos and they could identify the 13 different species I’d seen over the three nights. Seeing as a basic identification guide worked last time, when I got home I put one together of the frogs I saw at Araras. The idea was help the owners and staff know more about the frogs around them and share with any future frog-interested guests (check it out here). I also did one for another amazing pousada I stayed at, Park Eco Lodge at Chapada dos Guimaraes (one in English and another in Portuguese). Considering I was there at the start of the dry season and there are many, many more species that I didn’t see in the Pantanal, I think a second edition might require another visit. Who’s coming?

Rhinella major trying to squeeze back under the building.

Known as the Formula 1 frog due to the sound males make, Physalaemus albonotatus.

I feel in love with this one, Boana punctata

Teeny tiny dwarf tree frog Dendropsophus nanus

PS. The Pantanal was the last stop on my tour of different Brazilian ecosystems arranged by Pantanal Explorer and organised for me by the amazing Lara at Eclipse Travel in Sydney. If you want a bang-up Brazilian holiday, email (lara@eclipsetravel.com.au) and tell her I sent you! And if you want do a lot of ticking on your Brazilian bird-list, make sure you request Allan Franco, the best.

Fussy frogs and wet wetlands

Floodplain wetlands in semi-arid and arid regions are important habitats for a variety of wildlife, including frogs. Managing these important habitats requires that we know how wildlife respond to their patterns of natural flows, but surprisingly we don’t have a good understanding of this relationship for many species. While it makes sense that frogs would like inundated wetlands, we don’t actually know if this is the case in many systems, so I set out to determine which species were fond of good flow conditions and which weren’t. I found that while the Macquarie Marshes, a large floodplain wetland with huge conservation significance in inland NSW, supported a diverse range of frogs, not all species responded to flooding in the same way. Knowing this helps us understand which species are more likely to benefit from managed water flows, and which aren’t.

If you’ve been around frogs for a while, then you’ll know that if it has rained a lot, you’ll see and hear a whole different set of frog species than if it hasn’t. Also if it is still 26⁰C at midnight rather than 12⁰C, again the frogs that you see and hear will be quite different. Figuring out which species you are likely to see in what conditions is important, particularly when you want to determine how to conserve them; just because you didn’t see them, it doesn’t mean they weren’t there, which is especially true for burrowing frogs!

In order to determine how we might be able to manage frog populations by releasing upstream waters (in dams) to replicate natural flows (‘environmental flows’), I needed first to understand how natural floods affect different frog species living in large complex floodplain wetland systems. I wanted to make sure that any managed flows would actually benefit (or not) the frogs that live there. I also needed to know how things like temperature or rainfall or water depth affected how likely I was to see different species.

To do this, I (and my crew of amazing field assistants) spent a lot of afternoons and evenings sloshing through different parts of wetlands and around waterholes in the Macquarie Marshes in NSW. We did this during a large natural flood, and recorded data on weather, vegetation and water as well as all the frogs we came across.

Sloshing around the Southern Lagoons

During four months of surveys at 30 sites in the Macquarie Marshes, we identified 15 frog species, including barking marsh frogs (Limnodynastes fletcheri), wrinkled toadlets (Uperoleia rugosa), desert tree frogs (Litoria rubella) and Sudell’s burrowing frog (Neobatrachus sudellae). On average, we counted nearly 40 individual frogs per site, though sometimes we saw none and once four of us counted nearly 250 in 20 minutes!

Putting all that together, I found that as expected, not all frog species did the same thing at the same time or even liked hanging out in the same places during a flood. However, frogs that had similar features generally shared similar responses. Species that move around on the ground but can’t burrow, such as spotted marsh frogs (Limnodynastes tasmaniensis), were seen in most weather and site conditions, and were more abundant at temporarily flooded wetlands with some aquatic vegetation. Conversely, tree frogs, such as the green tree frog (Litoria caerulea) liked to be around wooded wetlands but needed it to warmer and rainier before they’d be out and about.

Spotted marsh frogs enjoying the wet conditions. Photo: Dave Herasimtschuk.

Green tree frogs doing as they do. Photo: Dave Herasimtschuk.

The remaining species, which had special adaptations allowing them to burrow into the soil, such as the crucifix frog (Notaden bennettii), were rather particular. They were more likely to pop up after some rain the day or night before and they weren’t very keen on the wetlands, preferring ephemeral, rain-fed waterholes.

After unlocking the secret preferences of frogs in a large floodplain wetland during a natural flood, we can now start to get more precise about how environmental water supports frogs. While burrowing frogs might not appreciate flood waters without associated rainfall, we know that ground frogs like the spotted marsh frog do. This means that these frogs are likely to directly respond to and benefit from water releases. And if you’ve got happy frogs, you’ve got a well-functioning wetland!

Thanks to my co-authors, Richard Kingsford (University of New South Wales), Trent Penman (University of Wollongong) and Jodi Rowley (Australian Museum Research Institute). I’d also like to thank landholders and Reserve rangers for permission to access the Macquarie Marshes during this study. Funding and support for the surveys were provided by the NSW Office of Environment and Heritage, the NSW Frog and Tadpole Society, and the Foundation for National Parks and Wildlife Service. For their assistance in the field, I particularly thank Carly Humphries, Jonathon Windsor, Ashley Soltysiak, Sarah Meredith, David Herasimtschuk, Angela Knerl, Diana Grasso, and Bill Koutsamanis.

For the nitty-gritty details, see:
Ocock, J.F., Kingsford, R.T., Penman, T.D. & Rowley, J.J.L. (2016). Amphibian abundance and detection trends during a large flood in a semi-arid floodplain wetland. Herpetological Conservation and Biology 11, 408-425.