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Planting in rows

This method can help the movement of small mammals

FABIO COLOMBINIPlants arranged in rows, forming a striped field, are very different from the islands of forest that emerge here and there on the landscape, generally following the hills. It is on such a landscape, in the towns of Guapimirim and Cachoeiras de Macacu in Rio de Janeiro state, that the biologists Prevedello Jaime and Marcus Vinicius Vieira, from the Department of Ecology of the Federal University of Rio de Janeiro (UFRJ), evaluated the movements of small mammals in the midst of manioc plantations. They found that the rows of crops act as corridors that facilitate the movement of animals, as the researchers show in an article in the journal Biological Conservation.

To assess the potential of plantations which connect the remains of forest in the region, the researchers captured marsupials typical of the Mata Atlântica forest – the black-eared opossum, Didelphis aurita, and the gray four-eyed opossum, Philander frenatus – and released them at points that the animals were not familiar with, at least one kilometer away, right in the middle of the manioc plantation, at varying distances from another forest fragment. Each of them carried, strapped to his back, a ball of nylon string which unfolded as the animal moved forward, recording the route. A very effective and simple technique used by biologists, similar to that which allowed Ariadne to get out of the labyrinth in the Greek legend of the Minotaur.

Vieira and Prevedello tested a total of 24 black-eared and 37 four-eyed opossums, and in both species most of the animals tried to find their way home walking through the aisles of rows of manioc instead of crossing them on routes perpendicular to the direction of the rows of crops. “They only left the rows when they were very close to a fragment of forest, at most 50 meters away. Even so, some still chose the paths made by farmers,” says Vieira, who led the study.

In the study area, these marsupials rarely leave the forest, according to the group at UFRJ. More recently, the researchers from Rio de Janeiro city followed some of these animals within the forest fragments and saw that they rarely leave. “Most only venture beyond the area of cover when they discover a loaded fruit tree in the pasture or plantation, close to the fragment of forest,” says Vieira; “in this case, they go to the tree and rush back to the forest, unless there is another fragment nearby: we found that 8% to 10% of the individuals advance to the other fragment.” From these observations, he and Prevedello advocated that unplanned plantations end up causing a gap between the islands of forest that is even more drastic than the inevitable.

Talking to farmers, the environmentalists found that in most cases there is no important reason for the orientation of the rows of manioc, unless they are on a slope. Often they are arranged based along a stream that cuts across the terrain, some other barrier such as a fence, or even on a virtually random basis. “We feel that, in most cases, there would be no resistance to planning the plantations in such a way as to improve the connection between fragments of forest,” Vieira states.

Archives LABVERT / UFRJ Proposition for farmers: joining the patchesArchives LABVERT / UFRJ

According to the professor from UFRJ, until now no one had looked at plantations using this approach. “There are, in international reports, occasional stories of animals following rows of crops, but without this having been tested as we did.” Even if small mammals are reluctant to leave the protection of their forest canopy, Vieira does not think it impossible that the rows might serve as tracks that facilitate migration between one forest fragment and another. “In our study we had no situations where the rows of crops reached as far as the woods,” he says, imagining a situation in which the corridor is presented to the animals unimpeded, right on the edge of their natural habitat.

Vieira has continued his studies to understand the relationship of the inhabitants of the Mata Atlântica forest with the plantations that isolate patches of forest and to assess the impact of simple ideas. “Changing the orientation of the rows is a no cost solution that can have an effect,” he says. Because it is cost free, he believes it is worthwhile even if the effect is modest. After the project of Prevedello Jayme ended, Vieira acquired other students dedicated to studies in the same region. The two mammals in question are the most common among the small mammals in this area of study, but the rows of manioc can also facilitate the movement of other animals such as rodents and lizards.