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 favourable and more helpful, small forest fragments can support far more bird species than would be expected based solely on their size.
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.
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 urbanisation 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 metres. This allowed us to identify how far from the forest the increase in tree-like vegetation makes its greatest contribution to bird species.
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-metre radius — meant fewer local extinctions. The benefit is even greater for forest-dependent birds, which are the most sensitive to forest fragmentation.
In a world where natural habitats continue to shrink, this is a message of hope.
The Conversation