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Wealth in danger

Shortage of large animals may reduce the diversity of species in regenerating forests

Eduardo CesarToucan: one of the best dipersers of fruits and seedsEduardo Cesar

Abandoned pastures and farmlands located in the Mar mountain range (Serra do Mar), provided that they are close to the Atlantic Rain Forest, may once again be covered by hill forest. But the diversity of tree species will depend on the action of large vertebrates, like mammals and birds, that ensure the survival of the plants by spreading fruits and seeds over new territories. The problem is that both the birds and the mammals that are essential to maintaining the forest are coming under pressure from hunting, and their habitats are becoming smaller all the time – indirectly, the forest is also losing out.

“With the reduction in the population of large fruit eaters, the bits of forest that cover the mountainous regions tend to disappear, or at least to lose their diversity”, explains ecologist Marcelo Tabarelli, from the Federal University of Pernambuco (UFPE). When this relationship of dependence of the forest on animals is broken, there may be a loss of up to 50% of the diversity of trees in the regenerated forest, according to a work that he carried out with primatologist Carlos Peres, from the University of East Anglia [RJS29], England. The forest has some 150 species per hectare (10,000 square meters) – apparently a high diversity, but one that is, for example, lower than the 300 species per hectare shown in the south of Bahia.

Published in April in Biological Conservation, the study by Tabarelli and Peres reconstitutes the process of regenerating forests from the action of birds and mammals – known as dipersers [RJS30], in view of their role in dispersing seeds. They point out that the size of the seeds is decisive for the survival of the trees: the larger ones are spread by large animals, that are easy to hunt and hence rarer. It is, for example, bats and small fruit eating birds, still common in the forests, that spread the seeds of the embauba (Cecropia glaziovi), one of the first species to appear in the forest, with a slender trunk and leaves in the shape of an open hand. Then there is the Hymenaea ssp, typical of mature forests, a species that may be jeopardized because the agouti (Dasyprocta ssp) that disseminates its seeds is becoming rarer all the time.

The conclusions are based on observations carried out in six areas of the Atlantic Rain Forest under regeneration – land with secondary growth- in three states of the southeast: Rio de Janeiro (Macaé de Cima), São Paulo (Cubatão, Intervales, Iporanga) and Paraná (Morretes and Santa Virgína). They are remains of forests at an altitude of up to 1,100 meters, surrounded by cattle farms or agriculture, and an age that varies from 5 to 120 years. The most recent are to be found in Intervales and Iporanga, while the oldest are in Cubatão, Macaé, Morretes and Santa Virgínia. The relationship is clear: the greater the age of the forest, the greater the dependence on animals to disperse the seeds. In a forest that has been regenerating for only five years, 52.9% of tree species depend on birds and mammals for their seeds and fruits to be scattered. In a mature forest, though, this percentage goes up to 98.7%.

Two years ago, in an article published in Nature, Tabarelli had the relationship between the size of the fruit or seed and the animals that eat them. According to him, in the Atlantic Rain Forest above the São Francisco River, 31.6% of the species of trees that need fruit eaters for dispersion depend on birds with beaks that open more than 15 millimeters. This time, the work done with Peres associates the size of the dispersed seeds and fruits with the age of the forest.

The small ones, less than 0.6 centimeters in length – like miconia[RJS33] (Miconia ssp) seeds-, are prevalent in all the areas studied. The bigger ones, over 1.6 centimeters long, produced, for example, by palm trees -, account for less than 25% of the total found. “In the mature forests, the small seeds are diminishing, while in the course of the regeneration process the medium sized ones, from 0.6 to 1.5 centimeters, are increasing”, explains Tabarelli. This means that the pioneer plants are replaced by species from mature forests, which are dispersed by medium and large sized fruit-eating animals.

Monkeys and birds
“The greater the diversity of fruit eating birds and animals, the greater will probably be the wealth of trees in the regenerated forest”, says he. Amongst the chief fruit-eating mammals – called as such when over half the diet of a species consists of fruit – there are three species of monkeys: one sort of howler monkey (Alouatta fusca) and two of woolly spider monkeys, Brachyteles arachnoides, found in São Paulo, Paraná and Rio de Janeiro, and Brachyteles hypoxanthus, which occurs in Minas Gerais and Espírito Santo. Amongst the best birds for disseminating fruits and seeds in the Atlantic Rain Forest are the toucans (Ramphastos vitellinus) and the aracaris (Pteroglossus and Baillonius), the dusky-legged guan (Penelope obscura) and the black-fronted piping-guan (Pipile jacutinga).

The spider monkeys, the largest primates of the America, which weigh up to 15 kilos when adult, stand out as the most versatile dipersers: they feed on the fruit of no less than 14 species of the main families of trees – the myrtaceae and the lauraceae – found in the forests of the Mar mountain range. The myrtaceae are a group of trees that includes the pitanga trees (Eugenia florida), wood guavas (Myrcia glabra), guabirobas (Campomanesia guabiroba) and the Blepharocalyx salicifolius, while the lauraceae include the Cryptocarya mandioccana, the Nectandra grandiflora and sassafrass (Ocotea pretiosa).

On the basis of this information, Tabarelli and Peres reached a conclusion as to how the forest regenerates. In the first stage, the trees depend of direct sunlight for germination and growth – they cannot tolerate shade. The most common pioneer is the embauba, but others taking part in the initial recuperation of the forest are Policourea marcgravi, a madder, and Rapanea umbellata, of the Myrcinaceae family, besides representatives of the Melastomataceae and the Flacourtiaceae families, all from the secondary Atlantic Rain Forest, now regenerated.

The pioneers are trees fated to death in the process of regeneration of the Atlantic Rain Forest. They are trees with a short life cycle, from 25 to 50 years, and are between 15 and 25 meters high – as they grow quickly, they protect their successors, the definitive trees, which accept the shade. “The trunk of a pioneer hardly ever exceeds 30 centimeters in diameter”, says Tabarelli. But the species from the mature forest have a longer life cycle, more than 50 years. They have slow growth and are taller, reaching from 20 to 35 meters, with a trunk that may attain over 1 meter in diameter. When regeneration begins, there are three or four species of mature forest trees per hectare. When it ends, and then forest is practically mature, this number rises to 150 to 200 species per hectare.

To allow the flow of seed dispersers, the regenerating area should not be at a distance of more than 50 meters from a remnant of the Atlantic Rain Forest. “If the distance is greater, many mammals will not cross the area in the open”, says Tabarelli. But the breaking up of the natural landscape, with its obvious risks, does not occur only in the Atlantic Rain Forest, but all over the country. “Small pieces support small populations of animals and are more accessible to hunters”, is the ecologist’s comment. Without urgent measures for conservation, this breaking up tends to increase, and if the current power game prevails, the Atlantic Rain Forest tends to turn itself into a set of archipelagos with thousands of little islands of forest, in which the majority of mature forest trees will be replaced by shrubs and by a few pioneer trees.