Preserving tropical rain forests is complex. Important economic plans in Brazil called for clearing forests in order to use the land for large cattle ranches. Yet the Brazilian government wanted to preserve as much land as necessary to sustain plant and animal species. In 1976 a study was developed to determine how big a tract of rain forest must be to maintain its biological integrity. For the study, biologists cut down Brazilian rain forest to isolate tracts of various sizes: five of 1 hectare, five of 10 hectares, one of 100, two of 1,000, and one of 10,000. They identified all species in each tract before and after it was isolated. They wanted to observe which species leave or die out, and in what order, so that they could discover the minimum critical size that a species requires for its survival and how species depend on one another to survive. The project's early findings were suggestive. In rain forests, a disturbance of one species can interfere with the well-being of many others. For example, ant-following birds depend entirely on the forays of army ants to flush out other insects such as grass-hoppers and crickets. In 10-hectare plots the colonies of army ants, which in their periodic movements range over as much as 30 hectares, could not survive; deprived of the flushed insects, the birds also vanished.