The replacement of forest with pastures and crops is directly affecting Amazonian fish. In a study published in the journal Neotropical Ichthyology, researchers from Brazil, Colombia and the United States showed that a process similar to that which took place over decades in areas with a long history of deforestation, such as the State of São Paulo, is now being repeated in Rondônia, in the so-called Arch of Deforestation, where the clearing of the forest is recent.
Fish sensitive to changes in the environment are gradually being replaced by a few species that are more resistant to impacts. In addition to the loss of biodiversity, the phenomenon causes a loss of ecological functions performed by the disappearing fish.
“There is a hypothesis within ecology that terrestrial vertebrates would withstand up to 60% of habitat loss before entering the process of population decline and then local extinction. Studying fish from streams, we found that part of the species only supports 10 % of habitat loss and their populations begin to decline in less than ten years after the start of deforestation. Others, however, benefit from losses of more than 70% of habitat”, says Gabriel Brejão, first author of the study, who was conducted during a postdoctoral internship at the Institute of Biosciences, Letters and Exact Sciences of the Universidade Estadual Paulista (Ibilce-Unesp), in São José do Rio Preto.
The results are based on data collected in 75 streams with different degrees of preservation in the Machado River basin, one of the tributaries of the Madeira. To assess the deforestation history of the areas, the researchers consulted satellite images of the region taken between 1984 and 2011.
“Based on historical data, we separate areas into basins that have never undergone change, those that have been deforested for a long time and those that have recently been degraded. We observed that, where deforestation is recent, the rate of species replacement [mais sensÃveis por mais resistentes] was higher than in forested areas and in those with old deforestation”, he explains.
Part of the collection and analysis of the work was carried out by the researcher during his doctorate, at the same institution, with a grant from Fapesp.
The work is one of the results of the project “Fish from upland streams of the Machado River Basin, RO”, funded by FAPESP and coordinated by Lilian Casatti, professor at Ibilce-Unesp.
The investigation was also supported through a project coordinated by Silvio Ferraz, professor at the Luiz de Queiroz School of Agriculture at the University of São Paulo (Esalq-USP) and co-author of the article.
West of São Paulo and West of the Amazon
Casatti says that his group has always worked with fish from streams in the state of São Paulo, which has a history of more than 200 years of intensive land use and replacement of the forest by crops and cattle ranches.
“I wanted to know what the streams would be like in a place that hasn’t changed so much, at least not so long ago. But when we arrived at some points in Rondônia it looked like we hadn’t left the west of São Paulo, such was the silting, the deforestation of the banks, the grass invading the aquatic environment”, recalls Casatti, who coordinated the study.
Streams are especially sensitive to deforestation. Used as a breeding ground and nursery for species that can later migrate to rivers, these water bodies also bring different nutrients from the forest to the rivers. As far as the fish communities that live in them are concerned, a degraded forest has several impacts.
In addition to silting, which is the deposition of soil particles at the bottom of streams, decreasing their depth, the reduction or removal of forest cover also allows the entry of more solar radiation, which increases the growth of aquatic plants undesirable for some species and increases the water temperature.
Fewer fruits, leaves and insects that serve as food for fish are present, in addition to branches and trunks that serve as shelter and even modulate the acidity of the water, another factor that can determine the presence or absence of certain species and functions. ecological they play.
“Losing catfish species that scrape logs that fall into the water, for example, can lead to loss of organic matter processing. Loss of insectivorous fish can increase the number of disease-carrying insects. Carnivorous fish, such as trahira and dourado, exert pressure on more basal species that can reproduce uncontrollably without predators. Habitat quality plays a very important role in maintaining not only species diversity, but ecological functions”, explains Casatti.
“Our results indicate that in areas of more recent deforestation there is a group of species large enough to reverse the loss of functions. This does not mean that what happened in São Paulo will necessarily be repeated in Rondônia. Perhaps this is a sign of that in the initial processes of deforestation there is a ‘buffer’ of diversity that is holding back the loss of functions. We don’t know for how long”, concludes Brejão.
