How do fish invasions change fynbos amphibian communities?

Back in 2017, Naas Terblanche visited me in my office in Stellenbosch. A retired agriculturalist (with an MSc in animal nutrition) turned winemaker, Naas had spent the first 16 years of his retirement in Stanford developing a passion for the frogs that he found there. He explained that he wanted to conduct a scientific study on the different communities of frogs that he found in the area near where he lived, using the skills of sampling and identification that he had developed.

We put together a study that tested the influence of invasive fish on various different habitat types in the region. Spread across two watersheds in the Overstrand, Naas used satellite data to select 200 different freshwater sites and categorise them into different waterbody types. From these we selected 50 that represented a balanced number of each category type.

Naas spent the next two winters visiting each of these 50 sites in turn, identifying the amphibian communities therein. It was long and hard work as he had to visit each site once during each year, and spend the afternoon and evening conducting the surveys so that he saw, captured or heard every species present.

The data showed convincingly that invasive fish were important in determining the composition of amphibian communities. Perhaps unsurprisingly, toads (Raucous toads and Western Leopard toads) that have toxic eggs and larvae are particularly tolerant of invasive fish, while the most intolerant was the plantanna, X. laevis, perhaps because they spend most of their time in the water.

Secondly, the resulting data demonstrated some interesting trends in freshwater habitat types. First, we confirmed that different habitat types contain different amphibian assemblages in the fynbos, with those most valuable being from temporary aquatic habitats. Permanent habitats, such as garden ponds and dams, were not particularly useful for local amphibians, but more for widespread common species. This means that if you want to create a freshwater habitat for conservation purposes in the fynbos, you need to make sure that it is temporary (not permanent), drying out in the summer months and filling in the winter.

Read the paper in full:

Terblanche, N., Measey, J. (2023) The conservation value of freshwater habitats for frog communities of lowland fynbos. PeerJ 11: e15516 https://doi.org/10.7717/peerj.15516

Getting high to work out how low they can go

Graduation time for Dan

Dan van Blerk has been in the MeaseyLab since his Honours project in 2019 (see here), when he conducted a literature review on the impacts of invasive lizards around the world. In late 2019, Dan and I met with Olaf Weyl to plan Dan’s MSc fieldwork over the next two years. With all three of us filled with enthusiasm for the upcoming adventure and what it would achieve, we were about to find out that fate had other plans.

Very sadly, Olaf died later that same year, and subsequently did not get to participate in any of the plans. He is greatly missed, and we continue to remember his initial enthusiasm for the project. Happily, Josie Pegg was soon on hand to take over from the SAIAB side, and in doing so saved the project. However, just as we were about to deploy Dan into the streams of the Western Cape, we were hit by another big setback. The world suddenly shut down, and with it Dan was confined to quarters and we decided that we’d have to conduct at least one of his chapters as a literature review instead of in the field as planned.

During 2021, Dan managed to get two notes published on natural history observations that he’d made during his time in the MeaseyLab (van Blerk 2021 a,b).

Eventually, Dan did get out into the field and surveyed many locations in many streams for ghost frog tadpoles and invasive fish. The work was a real advance for conservation evidence in the impacts of invasive fish on these frogs. Dan presented his work in the Conservation Symposium (see here). We got used to seeing Dan presenting online and in person (see here and here). His findings have already been submitted to Aquatic Invasions, and we hope to announce publication of his chapter soon. Watch this space. Dan defended his MSc work in February this year (see here).

It was great having Dan in the lab, even if he didn’t spend much of the three years physically with us. We wish him all the best in his subsequent career and hope that he’ll keep in touch.

Read more:

van Blerk, D. (2023) The Impacts of Invasive Fish on Ghost Frog tadpoles. MSc thesis, Stellenbosch University. http://hdl.handle.net/10019/32356

Van Blerk, D., Reissig, J., Riley, J.L., Measey, J., Baxter-Gilbert, J. (2021) Observations of infanticide and cannibalism in four species of cordylid lizard (Squamata: Cordylidae) in captivity and the wild. Herpetology Notes 14: 725-729. pdf

Van Blerk, D., Measey, J., Baxter-Gilbert, J.H. (2021) Predation by a Brown Widow Spider, Latrodectus geometricus (Koch, 1841), on a Common Dwarf Gecko, Lygodactylus capensis (Smith, 1849), with a review of the herpetofaunal diet of Latrodectus spp. Herpetology Notes 14: 291-296. pdf

Ghost frogs are under threat from invasive fish - it's official

Dan van Blerk defended his MSc thesis today in what is likely one of the most anticipated events of 2023. Dan has been working for the last 2 years on getting enough data from as many streams as possible to establish whether or not invasive fish (most notably bass and trout) are impacting tadpoles of ghost frogs. The data were unambiguous. Invasive fish are having a major impact on the density of ghost frog tadpoles.

Having this unambiguous data is really good news. It is all very well to speculate that the invasive fish impact tadpoles, and there are a couple of annecdotal studies that show the same. But getting robust data from lots of sites on lots of rivers is no easy job. Dan worked tirelessly, trudging up and down some of the most beautiful scenery in South Africa's fynbos, to collect the necessary data. In truth, Dan greatly enjoyed his time in the field. It did give him a great excuse to play in water and catch fish - two of his most enjoyable pursuits.

Below one of Dan's pics shows the upper portion of a stream beyond which fish cannot reach, and the tadpoles are safe!

His work is critical to conservation managers who require evidence in order to base their conservation management decisions.

We look forward to seeing Dan's work in print shortly!

Read more:

van Blerk, D. (2023) The Impacts of Invasive Fish on Ghost Frog tadpoles. MSc thesis, Stellenbosch University. http://hdl.handle.net/10019/32356

A big step toward conservation of Rough Moss Frogs

Ten years is a relatively short period of monitoring data, yet it can reveal important ecological interactions that can inform conservation managers about on the ground decisions. In this study, published today, we show the clear synergistic relationship between fire and the invasion of pine trees on populations of moss frogs on the Klein Swartberg. 

In the figure below, you can see how the density of calling male Rough Moss Frogs increases in the absence of invasive pine trees, but declines in their presence. In addition, our analyses indicated that this decline was manifest when fire interacted with the invasion.  

We obtained the density estimates of calling frogs in this study from a microphone array using the aSCR methodology (described here). We employed this approach at 12 sites on the Klein Swartberg over 10 years with 35 recordings. This included the initial work undertaken by Debra Stark (see here). Debra's work is published in a book chapter (see here). Currently, the invasion of pines on the Klein Swartberg is incredible, but undergoing control through block burning. 

A picture of Oliver Angus among the pines on the Klein Swartberg in June 2021. 

Further reading:

Angus, O., Turner, A.A. & Measey, J. (in press) In a Rough Spot: Declines in Arthroleptella rugosa calling densities are explained by invasive pine trees. Austral Ecology https://doi.org/10.1111/aec.13273