In the north of Alagoas, on the border with Pernambuco, the small municipality of São José da Laje is home to a centuries-old sugar and alcohol mill, which over the last few years has been functioning as an important natural laboratory. This is the Serra Grande Mill, which covers an area of 20,000 hectares in which various stretches of the Atlantic rainforest are engulfed by a landscape dominated by sugar cane. On this property the team of ecologist, Marcelo Tabarelli, from the Federal University of Pernambuco (UFPE), found the ideal place to investigate how the transformation of a vast continuous area of native vegetation into isolated stretches of forest – an effect ecologists call fragmentation – affects different species of plants and animals.
In the 9,000 hectares of Atlantic rainforest that have been preserved by the mill, Tabarelli’s team selected 109 stretches of different sizes to analyze – the biggest and best preserved is the Coimbra forest, with its 3,500 continuous hectares of original Atlantic rainforest. Over an eight year period the researchers took note of the biological characteristics of all the trees over 10 centimeters in diameter that they found in four types of environments: 100 meter wide strips on the outermost edge of Coimbra; stretches inside this particular forest; clearings where the native vegetation was recovering; and islands of forest of between 3 and 80 hectares, entirely surrounded by sugar cane. The results, which were published this year in the journal, Biological Conservation, clearly showed that the composition of trees in a stretch of forest surrounded by more forest is very different from the composition of trees in an initially identical fragment, but surrounded by an agricultural crop, like sugar cane.
The ecologists from Pernambuco saw that the number of species of trees on the edges and in the smaller fragments is half that found in inner areas of continuous natural vegetation, like Coimbra. In impoverished forest areas there are four times more species (and more individual ones) of trees that are specialist in colonizing altered areas like clearings or forests undergoing regeneration. These trees are more resistant to the altered conditions of transition zones between the forest and the plantation – the so-called edges -, which generally receive more sunshine and are warmer, less humid and experience stronger winds.
The change in the composition of tree species is not the only alteration observed in these more degraded forest stretches. Ariadna Lopes, a colleague of Tabarelli’s at UFPE, showed that tree communities lose certain reproductive types when forest fragments are isolated from each other. In forest fragments there are no trees pollinated by birds, flies or terrestrial mammals, according to an article published 2007 in PloS One. Only trees that depend on bats and butterflies for pollination survive, but in smaller numbers – around half what was expected. Trees with flowers greater than 2 centimeters are also rare.
These results make it clear that the effects of being on the edge eliminate – at least in part – those trees that need specialist pollination to reproduce. This influence may extend up to 300 meters into the forest. In three fragments only more generalist tree species grow, which according to Ariadna, can be pollinated by various small insects and are more resistant to less ideal living conditions. “This effect makes the smaller fragments behave as if they were entirely on the edge”, says Ariadna.
The reduction in tree species creates a vicious circle: the animal population becomes impoverished, which in turn leads to an even greater reduction in the plant population. Without plants those animals that depend on pollen, nectar, fruit or leaves have no way of surviving in forest fragments where they are isolated, because they are unable to cross the sugar plantation where there is neither food nor shelter. As plants also need animals to reproduce and spread they follow the fate of the fauna by becoming impoverished.
The information collected by the team of zoologist, Rossano Mendes Pontes, also from UFPE, confirms the same tendency among animals that is seen with tree species. The group set traps to capture mammals in the forests of the Serra Grande Mill and saw that none of the fragments houses all the mammalian diversity. Like the plants only more generalist rodents manage to survive in a large number of the smallest islands of forest.
It is not always enough just to have wings for moving between these forest islands. Studies carried out in stretches of Atlantic rainforest in the southeast region of Brazil have shown that birds also suffer from the impact of fragmentation. As part of a Biota/FAPESP program project and collaboration with researchers from the University of Freiberg, in Germany, the team of ecologist, Jean Paul Metzger, from the University of São Paulo, has been investigating the behavior of various bird species in the Morro Grande reserve, on the Ibiúna plain, 40 km from the city of São Paulo.
The researchers found that the blue manikin (Chiroxiphia caudata) and the rufous-breasted leaftosser (Sclerurus scansor), for example, avoid the edges of forests. The article, published this year in Biological Conservation, calculates that round fragments need to be at least 23 hectares in size so that half of its area can maintain the characteristics of the center of the forest that are essential for the survival of these birds. The white-shouldered fire-eye (Pyriglena leucoptera), on the other hand, which prefer the thickest zones of vegetation in the forest thrive in the tangle of bamboo and creepers that are characteristic of the edge.Most of the bird species live in the shelter of the forest. This is what the group from USP showed in a survey that included 62 species of birds and that will be published shortly in Biological Conservation. They saw that the connection between fragments – whether by strips of forest or open field areas up to 30 meters wide separating one stretch of forest from another – is the most important factor for allowing birds to move, with the exception of omnivores and frugivores that depend on larger areas to obtain sufficient food.
For Metzger, there is no magic formula for guaranteeing the ecological functioning of the Atlantic rainforest. Different species have different needs and the current situation has to be dealt with: a highly fragmented forest where there are few extensive stretches of continuous natural vegetation. Given this situation the ideal thing is to preserve some large areas in order to guarantee the survival of animals that are highly sensitive to disturbances, that become intimidated even when they are obliged to cross – even flying – a dirt road and concentrate efforts on maintaining the link between smaller fragments.
Plants would also be grateful. “If we use corridors to unite good size fragments with others, even if they are smaller, the animals are going to cross them, carrying seeds and pollen”, comments Ariadna. Even though being on the edge affects corridors and patches of the forest the researcher explains that the passage of animals would help regenerate the forest and make it less homogenous. This may be the way to recover and maintain the biological diversity that so delighted the Europeans in the 16th century and still has the greatest wealth of Atlantic rainforest.
Small fragments, however, will not solve the problem. Based on the work of his group and of others who are doing similar experiments in the Amazon, Tabarelli sums up his findings: “When the mature forest is extremely fragmented it tends to be transformed into undergrowth and remain as undergrowth, young vegetation formed almost exclusively by colonizing species, with little biological diversity”. According to the ecologist this is the first time that an hypothesis has been formulated that summarizes and integrates the main effects of fragmentation on the forest. The warning is in the comment that the ecologist form UFPE wrote, along with Ariadna and Carlos Peres, from the University of East Anglia, in England, at the invitation of the journal, Biotropica. For Tabarelli, the only way of preserving plant and animal communities with a structure similar to that of a mature forest is to maintain stretches of natural vegetation that cover at least 10,000 hectares and large areas free from edge effects. It may seem like a lot of land but for large predators it is not enough. “Forest fragments of 10,000 hectares are probably insufficient for maintaining viable populations of felines, like the jaguar or the ocelot”, he says. This situation is worrying because to the north of the São Francisco River there are practically no remaining stretches of Atlantic rainforest over 5,000 hectares.
GIRÃO, L.C.; LOPES, A.V.; TABARELLI, M. & BRUNA, E.M. Changes in tree reproductive traits reduce functional diversity in a fragmented Atlantic forest landscape. PLoS One, v. 2(9), p. e908. 2007.
SANTOS, B.A. et al. Drastic erosion in functional attributes of tree assemblage in Atlantic forest fragments in northeastern Brazil. Biological Conservation, v. 141, p. 249-260. 2008.
TABARELLI, M.; LOPES, A.V. & PERES, C.A. Edge-effects drive tropical forest fragments towards an early-successional system. Biotropica. v. 40, n. 7. 2008.