Researchers study the invasive frog’s role in Galapagos food web
by Senckenberg Research Institute and Natural History Museum
OCTOBER 9, 2020
Fowler’s snouted treefrog Scinax quinquefasciatus was introduced on the Galápagos archipelago in the late 1990s. Credit: Senckenberg/Ernst
“I have taken pains to verify this assertion, and have found it true that frogs, toads, and newts are absent from most oceanic islands”—thus states Charles Darwin in his well-known work “On the Origin of Species.” For a long time, this observation by the famous naturalist also held true for the Galápagos Islands, which are inextricably linked to his name. “This only changed with the arrival of Fowler’s snouted treefrog Scinax quinquefasciatus on the archipelago in 1997 or 1998,” explains Dr. habil. Raffael Ernst of the Senckenberg Natural History Collections in Dresden, and he continues, “In our study, we examined the interactions of this newcomer with the local, primarily endemic fauna on Galápagos.”
Ernst and his colleagues were curious to find out what role this 33-to-38-millimeter-long frog plays within the island fauna’s food web. “You might call it a study of ‘eat or get eaten’,” adds Ernst. To this end, the researchers examined the stomachs of 228 frogs they collected during an expedition in 2017. “In total, we were able to identify eleven different groups of invertebrates in the frogs’ stomachs. At 60 percent, butterflies predominated among the prey animals; in addition, we also found remains of cockroaches, arachnids, and grasshoppers,” explains Ernst, and he adds, “The frogs do not show any specific food preference—they simply eat what is most common locally.”
In a second step, the team examined whose menu the frogs might be on, and they hit paydirt with the islands’ endemic diving beetle Thermonectus basillarus galapagoensis. “The beetle larvae also feed on the frogs’ tadpoles. We wanted to know whether this might lead to a natural regulation of the frog’s population,” explains Ernst. To answer this question, the scientists conducted controlled predator-prey experiments. These show that the beetle larvae are usually already ‘sated’ before they have completely consumed the offered tadpoles. “This indicates that the beetles will not be able to sustainably limit the population of this invasive frog species. Therefore, the frog‘s expansion and its impact on the ecosystem should be carefully monitored in the future,” adds Ernst in closing.
Eat and be eaten: trophic interactions of the introduced frog Scinax quinquefasciatus in anthropogenic environments in Galápagos
M. Mar Moretta-Urdiales, Raffael Ernst, José Pontón-Cevallos, Rafael Bermúdez, Heinke Jäger
While the Galápagos Archipelago is known for its endemic flora and fauna, many introduced species have also become naturalised there, especially on the human-inhabited islands. The only amphibian species known to have established on the islands, the Fowler’s snouted treefrog (Scinax quinquefasciatus), is thought to have arrived about two decades ago. Since then, this treefrog has substantially extended its range to the islands of Santa Cruz and Isabela. Our study explores the potential influence of this introduced amphibian on native trophic systems on Santa Cruz and identifies potential antagonists likely to control larval frog populations.
We also identified active predators of S. quinquefasciatus tadpoles: larvae of the endemic diving beetle (Thermonectus basillarus galapagoensis). To determine the efficiency of this predator, we conducted predator-prey experiments in ex situ conditions. Tadpole predation was highest after first exposure to the predator and significantly decreased over time. Our experimental results demonstrate that although T. b. galapagoensis larvae are effective tadpole predators, their feeding saturation rates are likely inadequate for frog population control. Our findings provide the first baseline data necessary to make informed ecological impact assessments and monitoring schemes on Santa Cruz for this introduced treefrog.
Conclusion and future directions
Due to rapid development and the increasing human population, Santa Cruz is prone to invasive species events. Scinax quinquefasciatus is the first successfully invasive amphibian on the island; furthering our understanding of its ecological effect(s) is crucial for management, especially in such a fragile and unique ecosystem. As reproduction for both frogs and beetles in the highlands is apparently restricted to water sources provided in the rainy season and/or anthropogenic structures, we recommend that long- term research be conducted to investigate the frog’s ontogeny, especially in relation to beetle presence/absence.
This diet composition study was limited to higher taxonomic identification levels due to the nature of digested stomach contents (exoskeletons, wings etc.) and economic constraints that prevented us from testing with molecular methods. Further research should address the selection of native, endemic and introduced prey item ratios using DNA-metabarcoding approaches.
Our findings strongly suggest that Scinax quinquefasciatus population growth is likely to remain stable or increase on Santa Cruz. The dietary preferences and predation rates by natural predators on this introduced frog should be taken into account when considering management strategies in the Galápagos Islands.
This project was supported by the Charles Darwin Foundation, Rancho El Manzanillo, the Dean of Research at Escuela Superior Politécnica del Litoral and the Galápagos National Park Directorate (GNPD, permit PC-35-17). This study was financially supported by the Basler Stiftung für Biologische Forschung and Galápagos Conservancy.
We also thank Jacqueline Rodríguez at CDF for her continuous support in laboratory training, the Zoology Museum at the Pontifical Catholic University of Ecuador (QCAZ) for field training and support, the El Manzanillo family for their hospitality and Rebecca M. Brunner for professional edits to the manuscript. R. Ernst would particularly like to thank Marcelo Loyola and Hilla for their support during field work. Thanks to Rolf Sievers for providing the long-term weather data and to Clare Peabody for compiling these. The International Atomic Energy Agency is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. This publication is contribution number 2227 of the Charles Darwin Foundation for the Galápagos Islands.
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