Larger areas contain more species. This is one of the most ironclad laws of ecology, which explains why large natural areas usually receive higher priority in conservation strategies. In fragmented landscapes, this logic has also led small forest fragments to be seen as environments of lower value for biodiversity.
But would it be possible to increase the number of species in a forest fragment without increasing its size?
Our study, published in the journal Proceedings of the National Academy of Sciences, shows that this is possible. When the surrounding landscape is favorable and more helpful, small forest fragments can support far more bird species than would be expected based solely on their size.
Fragmented Forest Landscapes
Forest landscapes modified by human activities are composed of forest fragments of different sizes surrounded by other types of environments. Collectively this is called the matrix. This matrix can be terrestrial, such as cattle pastures, crops and urban areas, or aquatic, such as hydroelectric reservoirs.
In addition to the matrix, the surroundings of the fragments may also include arborescent vegetation (a type of vegetation made up of scattered trees and/or tree-like shrubs and ferns), riparian (riverside) forests, and nearby fragments. Together, the matrix and the different types of vegetation form the surrounding landscape of forest fragments.
The Contribution of the Surrounding Landscape
To understand the role of the surrounding landscape, we compiled data from nearly 2,000 bird species recorded in more than 1,000 tropical and subtropical forest remnants distributed across 50 landscapes in 17 countries in the Americas, Africa and Asia. The study compared forest fragments surrounded by terrestrial matrices modified by cattle ranching, agriculture and urbanization with forest islands formed by hydroelectric reservoirs.
Reservoir islands represent an extreme fragmentation scenario due to the ecological hostility of the matrix. By comparing them with forest fragments surrounded by land-based matrices, we were able to measure how much changing the matrix (from water to land) can increase the number of bird species in forests of the same size.
Using satellite imagery, we also calculated the amount of tree-like vegetation surrounding forest remnants at different distances ranging from 50 to 2,000 meters. This allowed us to identify how far from the forest the increase in tree-like vegetation makes its greatest contribution to bird species.
Improving the Surrounding, Increasing Biodiversity
Fragments surrounded by terrestrial matrices had more species than reservoir islands. This difference in species number increases as fragment area decreases. For example, a one-hectare forest fragment may contain more than twice as many species as islands of the same size.
The amount of tree-like vegetation surrounding forest remnants also matters. In both fragments and islands, more trees in the surrounding landscape – especially within a 300-meter radius – meant fewer local extinctions. The benefit is even greater for forest-dependent birds, which are the most sensitive to forest fragmentation.
How Species Perceive the Environment
Birds living in forest fragments do not necessarily stay within these spaces. The greater the amount of vegetation in the surroundings, the more species can move between fragments and feed on resources available in the matrix, such as insects and flower nectar.
But the ability of birds to fly does not necessarily ensure free movement through the matrix and between neighboring forest areas. Some species adapted to living under the shaded forest environment tend to avoid exposure to open environments. In addition, many species move through the forest interior without the need for long flights. Therefore, even dirt roads can limit bird movement between neighboring fragments.
Bird movement across the landscape is important. If certain species become locally extinct in one forest fragment, they may one day arrive from another fragment. For this recolonisation process to occur, two conditions are necessary: species must be able to cross the matrix, and there must be nearby fragments or vegetation along the way, such as scattered trees and forests along riverbanks that facilitate movement between more distant fragments.
Surrounding Landscape is More Important for Smaller Fragments
In large fragments, species find enough food, shelter, and space to survive. In small fragments, however, space and resources may be insufficient to sustain several bird populations. But when birds can use resources outside the fragment and reach other forest areas, they begin to use an area larger than the fragment’s boundaries. This is why small fragments with a favorable surrounding landscape can support more species.
And most forest fragments are small. Fragmented forest landscapes in tropical and subtropical regions are overwhelmingly composed of small fragments. In the Atlantic forest of South America, 80% of forest fragments are smaller than 5 hectares. This means that biodiversity depends on a favorable surrounding landscape to ensure its survival in the small forests that remain.
Implications for Conservation
Small increases in the number of trees and tree-like vegetation within a 300-meter radius of fragments can significantly reduce species loss. This means that local actions can generate real gains for biodiversity: planting trees, restoring riverside forests, recovering degraded areas, and expanding agroforestry systems – such as coffee and cocoa plantations – can make productive landscapes more favorable to wildlife.
In a world where natural habitats continue to shrink, this is a message of hope.
Protecting forests and other natural habitats is and will continue to be the central strategy for biodiversity conservation. However, our study shows that conservation does not need to stop at forest boundaries. By combining forest protection with improvements to the surrounding landscape, we can increase the conservation value of forest fragments, especially small ones, which are by far the most common in landscapes modified by human activities.
Size is crucial, but it is not everything: what lies outside the forest fragment also determines how many species live inside it.