Raptor distributions reveal the conservation consequences of forestry-driven landscape transformation in a grassland biome

The grasslands of South America are among the most threatened ecosystems on the planet, with vast stretches of open land disappearing under the pressure of agriculture and development. In recent decades, a new force has joined the mix: commercial tree plantations. Governments have encouraged the planting of fast-growing trees like eucalyptus to produce timber and pulp, often on land that was previously used for grazing or farming. This creates a complex puzzle for scientists trying to understand how nature responds. When a flat, open landscape is broken up by patches of tall trees, does the variety of life increase because the environment becomes more varied, or does it decrease because the open spaces that many animals need are lost? To answer this, researchers often look at birds of prey, or raptors. These large birds require vast territories to hunt and are highly sensitive to changes in their surroundings, making them excellent indicators of the health of an entire ecosystem. By studying how these birds react to the shifting landscape, scientists can see whether the new mix of trees and fields helps or harms the natural world.

A team of researchers set out to investigate this question in the Mesopotamian Pampas of Argentina, a region where the landscape is rapidly changing. They focused on diurnal raptors, the birds of prey that hunt during the day. The scientists divided the area into seventeen large squares, each covering one hundred square kilometers. Within each square, they drove along unpaved roads, carefully counting every raptor they saw and identifying the species. At the same time, they used satellite images to map exactly how much of each square was covered by eucalyptus plantations, how much was used for cattle, how much was farmland, and how much remained as native grassland or urban area. They were looking for patterns in three different ways of measuring biodiversity. First, they looked at the simple number of different species present. Second, they examined the variety of roles these birds played, such as whether they were large birds that soared on the wind or smaller birds that hunted from perches. Third, they looked at the evolutionary history of the birds, checking if the species present were closely related or if they came from very different branches of the tree of life.

The researchers had two main ideas about what they might find. They wondered if the birds would simply see the new trees as a loss of their open hunting grounds, leading to a steady decline in all types of diversity. Alternatively, they considered that the new mix of trees and fields might create a more complex environment that could support a wider variety of birds, at least for a while. The results, however, told a more nuanced story. The team recorded 153 individual birds representing twelve different species. They found that the sheer amount of tree cover did not directly cause a drop in the number of species, nor did it change the variety of roles or evolutionary history in a simple, straight-line fashion. Instead, the key factor was the complexity of the landscape itself. When the landscape became a highly mixed mosaic of many different land uses without a dominant open space, the number of species and the variety of their roles dropped significantly. The birds that disappeared were the large, specialized hunters that need wide-open spaces to soar and spot their prey. In their place, the landscape began to support smaller, more generalist birds that are comfortable hunting from tree branches or perching in varied environments.

What made this discovery particularly striking was how the different ways of measuring diversity responded to these changes. While the number of species and the variety of their behaviors declined together, the evolutionary history of the group remained surprisingly stable. The birds that left the area were replaced by others that, while different in size and behavior, belonged to the same deep evolutionary families. This suggests that the new landscape acts as a filter that sorts birds based on how they live and hunt, rather than wiping out entire branches of the evolutionary tree. The study also revealed that the specific type of land cover mattered less than the overall arrangement. A landscape that was a chaotic mix of small patches of forest, fields, and grass was harder for the specialized birds to navigate than a landscape that was mostly open, even if that open landscape had some trees in it. The birds that thrived in the forested areas were often generalists, like the great kiskadee or the crested caracara, which can adapt to many situations. In contrast, the birds that vanished were the open-country specialists, such as the burrowing owl, which rely on unbroken grasslands to find their food.

The findings challenge the common assumption that simply planting trees is always a neutral or positive act for biodiversity. In this grassland biome, the expansion of commercial forestry does not necessarily destroy the evolutionary history of the local bird community, but it does reshape who is there and how they live. The loss of large, soaring specialists and their replacement by smaller, perching generalists means that the ecosystem is losing a specific type of ecological function. The study concludes that the real threat to these birds is not just the trees themselves, but the way they break up the open habitat into a fragmented patchwork. To protect these birds, the researchers suggest that conservation efforts should focus on preserving large, continuous areas of native open grassland. Simply limiting the total number of trees planted is not enough; the arrangement of the landscape must be considered to ensure that the open spaces remain connected and usable for the species that depend on them. This approach offers a path forward where economic production and nature conservation might coexist, provided that the unique needs of the grassland ecosystem are respected.