Deforestation is starting to affect fish diversity in Amazon streams

Species sensitive to habitat change are gradually being replaced by more resistant species, according to a study conducted in Brazil. The trend is leading to a loss of the ecological functions performed by the vanishing species.

By André Julião  |  Agência FAPESP – Replacement of forest with cattle pasture and crops is directly affecting fish in the Amazon region. In a study published in the journal Neotropical Ichthyology, researchers affiliated with institutions in Brazil, Colombia and the United States show that a similar process to that seen for decades in areas with a long history of deforestation, such as São Paulo state, is now occurring in a swathe of Rondônia, North Brazil, known as the “deforestation arc”, where logging is recent.

Fish species that are sensitive to environmental change are declining, while relatively few species capable of withstanding its impact survive. Besides the loss of biodiversity, the trend is leading to a loss of the ecological functions performed by the vanishing species.

“There’s a hypothesis in ecology that terrestrial vertebrates can withstand the loss of 60% of their habitat before their population starts to decline and they eventually become locally extinct. Our research on fish species in upland streams showed that some species can withstand only 10% habitat loss and that their populations start to decline less than ten years after the onset of deforestation, whereas others benefit from more than 70% habitat loss,” said Gabriel Brejão, first author of the article. The study was conducted while he was a postdoctoral intern at São Paulo State University’s Institute of Biosciences, Letters and Exact Sciences (IBILCE-UNESP) in São José do Rio Preto, Brazil.

The results were based on data collected from 75 streams with varying degrees of conservation in the Machado River basin. The Machado is one of the tributaries of the Madeira River. To assess the history of deforestation in the areas, the researchers analyzed satellite images produced between 1984 and 2011.

“We used historical data as a basis for separating the areas into basins that had never undergone change, had been undergoing deforestation for a long time, and had recently been subject to degradation,” Brejão said. “We found that sensitive species were replaced by resistant species at a higher rate in areas where deforestation was recent than in forested areas and historically deforested areas.”

He conducted part of the data collection and analysis for his doctoral research at the same institution with a scholarship from FAPESP.

The study is one of the outcomes of the project “Terra firme stream fish in the Machado River basin, Rondônia”, funded by FAPESP and with Lilian Casatti, a professor at IBILCE-UNESP, as principal investigator.

It was also supported via a project led by Silvio Ferraz, a professor at the University of São Paulo’s Luiz de Queiroz College of Agriculture (ESALQ-USP) and penultimate author of the article.

Western São Paulo and western Amazonia

Casatti and her group have long studied stream fish in São Paulo state, which has had 200 years of intense land use and replacement of forests by cattle pasture and crops. “I wanted to know what streams were like in a region that has not changed so much, or at least where land-use change is more recent,” Casatti said. “However, we found that some parts of Rondônia closely resembled western São Paulo in terms of stream silting, destruction of riparian forest and grasses invading the aquatic environment.”

Streams are particularly sensitive to deforestation. Serving as breeding grounds for species that may then migrate to rivers, these water bodies also carry nutrients from the forest into the rivers. A degraded forest has various impacts on the fish communities that live in streams.

Besides silting (deposition of sediment on the stream bed, reducing water depth), removal or reduction of forest cover leads to increased solar radiation, which fuels the growth of aquatic plants that are harmful to some fish species and drives up water temperatures. 

Declining amounts of the fruit, leaves and insects on which fish feed, and fewer submerged branches and tree trunks to serve as shelter or modulate water acidity, are also factors that can determine the presence or absence of certain species and the ecological functions they perform.

“The loss of suckermouth armored catfish species that scrape organic material from the surface of logs that fall into the water, for example, reduces the amount of organic material processed in streams,” Casatti said. “The loss of insect-eating species can lead to an increase in insects that transmit disease. The loss of carnivorous species such as trahira [Hoplias spp.] or dourado [Salminus brasiliensis] leads to an increase of pressure from more basal species that can reproduce far more prolifically without predators. Habitat quality plays a very important role in maintaining both species diversity and ecological functions.”

“Our findings show that in recently deforested areas there’s a sufficient number of species to reverse function loss,” Brejão said. “This suggests that what happened in São Paulo won’t necessarily happen in Rondônia. It may be a sign that a ‘safety cushion’ exists in initial deforestation processes, so that function loss hasn’t yet begun. We don’t know how long this can last.”

The article “Taxonomic and functional turnover of Amazonian stream fish assemblages is determined by deforestation history and environmental variables at multiple scales” is at: www.scielo.br/j/ni/a/zq5j99bzX9JDDyxwZxyGXDG/.