FERNANDO DE ALMEIDATo eliminate most of the Atlantic rainforest’s toads, frogs and tree-frogs that depend on water to breed, all that is needed is just one hundred meters of barren land separating a stretch of forest from a stream. When they leave the forest in the direction of the streams where they reproduce, these animals have to cross pasture land and plantations and are exposed to being attacked by birds, contamination by agrichemicals and sun that can dry out the moisture in their skin. Many of those that survive the hazards of the journey are unable to make the return journey, because this crossing is even more perilous for their fragile offspring.
A study conducted in São Luís do Paraitinga, in the Paraíba Valley in São Paulo state, and one of the areas where the Atlantic rainforest is quite badly degraded, has shown the extent to which tree felling undertaken to create farms or pastureland is threatening the survival of amphibians, especially when cutting down the forest leaves the land almost bare between the two environments these animals need for feeding and reproducing.
Described in an article in the December 14 edition of Science, this phenomenon may explain the disappearance of most of the amphibians from the Atlantic rainforest and other tropical regions, such as Central America and Australia, where populations of toads, frogs and tree frogs that are essential for balancing these ecosystems are declining. This can also be applied to understanding better what happens to fish in a river when a dam is built and therefore cuts off the route between the environment in which they live and that in which they lay their eggs, suggests the study, which brought together teams from three São Paulo state universities and from the University of Vale do Rio dos Sinos (Unisinos), in the state of Rio Grande do Sul.
In Brazil, the separation between forest and streams is the consequence of a type of occupation that is predominant along a good part of the coastal region where 70% of the population lives. When the country’s colonizers left the coast they almost always settled on the most fertile land, on the plains close to streams. They made clearings to raise cattle and plant tobacco, sugarcane or cotton. While they guaranteed access to water they developed the landscape that is now common throughout Brazil’s Southeast and South: a house at the foot of a hill, surrounded by plantations and a field where cows, some sheep and one or two horses graze. What little has remained of the natural vegetation is often confined to hilltops, far from the streams and rivers that meander through bare valleys.
Though on one hand this type of occupation of the land led to the development of states such as São Paulo, on the other it imposed huge barriers to amphibians and other groups of animals that need both forest and water to live. The authors of this work have called this phenomenon habitat disconnection, because it destroys the link between the water and the land environments that are indispensable for the reproduction of these living beings. Its effect has proven to be much more harmful than two other better known forms of natural vegetation destruction identified in sixties, namely, the reduction of natural environments (loss of habitat) and the isolation of patches of forest (landscape fragmentation).
These three expressions define apparently similar phenomena caused by cutting down an area’s natural vegetation, but they generate very different results. Every time human beings settle in areas of Atlantic rainforest or Cerrado [savannah], for example, they cause what specialists call loss or reduction of habitat, the most obvious effect of which is a reduction in the area of original vegetation. The loss of habitat also alters the soil and rainfall pattern. Right from the start the size of the populations of animals that live in the region start to drop, as their food becomes scarcer.
Deforestation may also leave vegetation confined in smaller blocks, separated from each other by barren areas with little or no connection – this is landscape fragmentation. For animals, the main effects of this are felt over several generations. Species that need large areas to survive – like the muriqui [woolly spider monkey], the largest monkey in the Americas – are unable to cross pasture-land or plantations and remain in a single fragment of forest. “In addition to the immediate impact of reducing the population, there are other long term effects on the animals and plants that survive”, explains environmentalist Paulo Inácio Prado, from the University of São Paulo (USP), one of the authors of the Science study. Over time, animals isolated in patches of forest start mating with close relatives, increasing the risk of genetic diseases.
The situation described in the December article is even more dramatic for 80% of the 483 species of amphibians from the Atlantic rainforest. This is the case of the horned toad (Proceratophrys boiei), whose tadpoles develop in streams until they turn into young adults capable of surviving in the forest. When pasture or plantations replace trees and bushes close to rivers, they increase the difficulties for amphibians and other animals, such as certain species of dragon-fly that need water to reproduce. “Habitat disconnection may drastically reduce a population of amphibians in just one generation”, says, Carlos Guilherme Becker, an environmentalist from Rio Grande do Sul and the main author of the Science article, who carried out this study when doing his Master’ s degree at the State University of Campinas (Unicamp).
“Habitat disconnection may also affect other groups of animals”, says Becker. Some species of dragon-fly, for example, use the environment in a similar way to toads: they need water to procreate and the forest for food. It is also possible to understand how habitat disconnection is affected by the construction of dams, which hinder the migration of fish for laying their eggs and eliminates lakes or large areas of natural vegetation used by migratory birds for feeding and resting during their long voyages.
Becker identified this ecological phenomenon when he was studying what happened to amphibians, which few people are enthusiastic about investigating. Instead of looking into the swamps and streams of the remainders of the biggest and best preserved parts of the Atlantic rainforest, he joined Prado’s group, which was analyzing how the interaction between the inhabitants of São Luís do Paraitinga and the areas in which they live has modified the region’s biodiversity. There, the little that remains of the Atlantic rainforest is limited to hill tops, separated from rivers by plantations, pasture and populated areas.
An historical survey carried out by Allan Monteiro, from Prado’s team, shows that this pattern of occupation sprang up in the mid-eighteenth century, when the King of Portugal, Dom José I, named Luiz Antônio de Souza Botelho, entitled the Morgado de Mateus, governor of the then province of São Paulo. With the objective of protecting the South of Brazil from the Spanish, the he started creating townships in São Paulo. He divided the east of the state into large areas of uncultivated land and distributed them to whoever had the financial wherewithal to occupy them. “Those sent in by the Portuguese crown started setting up in the valleys where the land was the most fertile and the easiest to occupy . One of the obligations was to make roads, normally along rivers, linking the areas of uncultivated land to the towns”, says Prado. “Even today many minor roads in São Luís do Paraitinga follow the same path as they did at that time.”
Under the guidance of Prado and Carlos Roberto Fonseca, from Unisinos, Becker analyzed what happened to amphibians from the Atlantic rainforest in São Luís do Paraitinga. When he looked for patches of forest in the region he noted that practically none were crossed by a river. After analyzing more than 60 fragments, Becker found only three over 10 hectares in size in which there were streams. Little by little he began to imagine the difficulty that amphibians would have to arrive at streams to reproduce.
To find out whether barren stretches of land really did have a negative effect on the toads, he installed a screen 45 meters long and 1 meter high in the middle of pasture and plantations on hillsides. Under these barriers he buried 60 liter buckets, divided in the middle, which allowed him to distinguish those amphibians that came from the forest to reproduce in the streams from those who were making the journey back home. In March, at the end of the reproduction period, Becker confirmed his suspicions: when there was no vegetation joining the stream to the forest there were fewer amphibians.
Habitat disconnection, however, only affected amphibians that needed to leave the forest to reproduce in the streams and brooks. This effect had no influence on the varieties of toads that live only in the forest and breed in the ground, producing young that are already born to leap around the forest ground. He needed, however, to find out whether the effect of habitat disconnection applied only to the São Luís do Paraitinga region or whether it could be generalized to other areas.
Becker, Prado and Fonseca then joined forces with biologist Célio Haddad, from Paulista State University (Unesp) in Rio Claro; Haddad was coordinating a survey of amphibians in São Paulo, along with Rômulo Batista, at the time a researcher at Unicamp. Together, they calculated loss of habitat, fragmentation and habitat disconnection in other regions in the state. In cross-referencing the information about species diversity with the pattern of deforestation of each of these stretches of Atlantic rainforest, they discovered that the number of species of amphibians was far better explained by habitat disconnection than by the loss or fragmentation of forests, the most widely accepted hypothesis for the decline in amphibian populations.
Besides this new effect, this research gave rise to ideas that may help guide the preservation and recovery of what remains of the Atlantic rainforest and other environments. “By understanding better how human use of land reduces biological biodiversity in these areas, we can think about specific ways to reduce this damage”, says Prado. One of the proposals is to emphasize the recovery of waterside woodland, the vegetation alongside rivers. Although protected since 1965 by the Brazilian Forestry Code, 76% of the waterside forests in São Paulo have been destroyed.
In regions that have not yet suffered the impact of human intervention, the researchers are proposing the creation of biological reserves that include the largest possible number of water resources. Where the environment has already been altered, they are suggesting the restoration of waterside vegetation and the creation of forest corridors that reconnect land and water environments. “Now”, says Prado, “we have more arguments to show land owners and governmental bodies the importance of conserving and also of recuperating this vegetation”.
Biodiversity and social processes in São Luís do Paraitinga, São Paulo (nº 02/08558-6); Modality: Regular Research Aid Line (Biota); Coordinator: Paulo Inácio de Prado – USP; Investment: R$ 121,023.44 (FAPESP)