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Amphibian populations in the Atlantic Forest have declined over the last 130 years

The number of individuals of 106 species in the biome fell at some point between the end of the nineteenth century and the year 2020

Rohde's leaf frog (Phyllomedusa rohdei), found in southeastern Brazil

Renato Augusto Martins / Wikimedia Commons

The rapid decline in populations of toads, frogs, and tree frogs—amphibians of the order Anura—is one of the clearest signs of global biodiversity loss. Several studies have highlighted the shrinking of their habitats, environmental pollution, and type B ultraviolet radiation (UVB) as some of the main causes for the reduction in population sizes. A review paper published in the scientific journal Biological Conservation in January suggests that there are other factors contributing to the phenomenon, especially in the Atlantic Forest, such as global warming and the presence of a pathogen discovered in the late 1990s known as amphibian chytrid fungus (Batrachochytrium dendrobatidis).

Data from the last 130 years collected by researchers working with amphibians in the field shows that the number of individuals had fallen in nearly 15% of the approximately 700 known frog species in the Brazilian biome since the end of the nineteenth century. “The Atlantic Forest is home to more declining frog species and populations than anywhere else on the planet—and amphibians are the most endangered group of animals in the world,” says Luís Felipe Toledo, a zoologist from the Biology Institute at the University of Campinas (IB-UNICAMP) and lead author of the study.

The objective of the study, which received funding from FAPESP, was to provide a historical overview of how the decline in amphibians has taken place in the Atlantic Forest since records began to be taken. Only approximately 10% of the biome’s original vegetation remains. The team reviewed the scientific literature, searched for data in museum collections, and interviewed subject matter experts to identify which amphibian populations have declined or disappeared.

The data analyzed cover the states of Espírito Santo, Minas Gerais, Rio de Janeiro, São Paulo, Paraná, Santa Catarina, and Rio Grande do Sul, and range from the end of the nineteenth century to 2020. From this data set, the researchers found that 169 frog populations belonging to 106 species declined at some point in the period in question. The study considered the same species as having more than one population when groups of individuals were separated by a distance of 15 kilometers or more.

One concerning fact was that 128 of the 169 populations that declined at some point were not later rediscovered in the same place or did not recover (never presented the same number of individuals again). This is double the figure suggested in previous studies, say the researchers, and may still be an underestimate. “Many recent declines in amphibian populations have not yet been documented because it takes time to clearly perceive the change,” points out Toledo. “We will only really know which species are disappearing today 10 or 15 years from now.” The frogs most affected by falling populations were groups belonging to the Cycloramphidae, Hylodidae, and Phyllomedusidae families. The study found that a shrinking amphibian population takes an average of 21 years to recover the number of individuals lost.

Reuber Brandão, an ecologist from the University of Brasília (UnB) who did not participate in the study, believes the reason this biodiversity loss is not causing more concern is because Brazil only recently started systematically collecting amphibian samples—other countries have been doing this routinely for much longer. “Much biodiversity was lost before researchers even entered the field looking for amphibians,” says Brandão. “Due to the long history of deforestation in the Atlantic Forest, if there had already been large universities in the country in the early nineteenth century, with many researchers building high-quality biological collections, we would have a very different scenario.”

The review article shows that there was a large decline in amphibian populations in 1979, when records of only 235 toads and frogs were collected in the Atlantic Forest and stored in zoology museums. In comparison, the year with the most samples was 2011, when nearly 4,800 specimens were catalogued.

Other factors can also affect surveys of animal populations and influence perceptions of them, points out biologist Cinthia Brasileiro of the Federal University of São Paulo (UNIFESP). She explains that some species are naturally rare in nature and others are difficult to observe because they camouflage themselves so well or are extremely small. “These characteristics can make them appear to be suffering from species decline, but the research addresses this issue. Using a range of detection methods can also reduce this factor,” says the scientist, who did not participate in the study.

The study does not specifically examine the causes of declining amphibian populations, but it does highlight the impacts of climate change and amphibian chytrid fungus, first described in 1999. The microorganism affects the heartbeat and causes cardiac arrest in infected animals, and can proliferate more quickly when temperatures are warmer. Brandão says the combination of the amphibian chytrid fungus pandemic and extreme weather events is one of the main factors behind the loss of amphibians. According to Brasileiro, the joint impact of these two elements has became even greater since the turn of this century, alongside habitat loss, which is still seen as the biggest cause of amphibian mortality.

For Brandão, the study is important because the methodology could be replicated to analyze amphibian populations in other biomes, such as the Cerrado (wooded savanna) and the Amazon, and because it highlights the important role that museums and scientific collections play in biodiversity conservation. “There are some who think collecting specimens is ethically and morally questionable because they believe the activity is harmful to biodiversity,” explains the UnB researcher. “In reality, it is exactly the opposite: by providing a better understanding of the populations in nature, museum collections are contributing to biodiversity conservation.”

The loss of amphibians may seem like a distant concept to most people, but it has important implications for society. Every species or population that disappears or shrinks makes it more difficult to study animals that could provide substances with medical potential—frog skin often contains elements that can aid drug development. “It also makes the environment more vulnerable. Amphibians eat agricultural pests. Without them, farmers need to use more pesticides, which increases food prices and harms human and environmental health,” stresses the UNICAMP zoologist. “Because they are mostly herbivores, the tadpoles help oxygenate lakes and ponds, preventing eutrophication [excessive plant growth in aquatic environments due to nutrient accumulation], a phenomenon that affects the life of fish and impacts the fishing industry.”

1. Amphibian chytrid fungus in Brazil: From origin to consequences (no. 16/25358-3); Grant Mechanisms Thematic Project; Principal Investigator Luís Felipe Toledo (UNICAMP); Investment R$2,853,660.74.
2. Passive air transport of a pathogen lethal to amphibians in elevated areas: Practical applications for conservation units in the state of São Paulo (no. 19/18335-5); Grant Mechanism Regular Research Grant; Program Biota; Principal Investigator Luís Felipe Toledo (UNICAMP); Investment R$169,301.68.

Scientific article
TOLEDO, L. F. et al. A retrospective overview of amphibian declines in Brazil’s Atlantic Forest. Biological Conservation. vol. 227. jan. 2023.