An international team of researchers involving the University of Bern has investigated the consequences of deforestation and degradation of tropical forests. They were able to show that there are "winner" and "loser" species, whereby the displacement of the "losers" can lead to a decline in the ecological functions of tropical forests.
Tropical forests are the most important reservoir of terrestrial biodiversity. They play a crucial role in the absorption of greenhouse gases and provide important ecosystem services. Nevertheless, they are victims of rapid deforestation and forest fragmentation with a loss of 3 to 6 million hectares per year over the last two decades. A large proportion of today’s tropical forests therefore consist of human-altered landscapes that are subject to local pressures such as logging, hunting and fires. The anthropogenic modification of the landscape can lead to the increase of a few species and the loss of many species.
In a new study, an international team with the participation of Bruno X. Pinho from the Institute of Plant Sciences at the University of Bern now shows that in tropical forest regions with intensive deforestation and forest degradation, fast-growing tree species with small seeds predominate. The change in tree species could have a significant impact on the ecosystem services of these forests - including their important ability to absorb and store carbon.
The tree species referred to as ’winners’ in the study grow quickly, but have a limited lifespan, as their trunks and branches are far less dense than those of the slow-growing tree species they displace. The results were published in the journal Nature Ecology and Evolution.
Over 1,200 tropical tree species studied
For the study, the international research group examined a unique dataset of over 1,200 tropical tree species in more than 270 forest plots in six regions of the Brazilian Amazon and Atlantic rainforest that have been affected by human activities such as deforestation and local disturbances such as logging, hunting and slash-and-burn.
Using various statistical models, the research group analyzed the causal effects of forest loss, fragmentation and local forest degradation on the composition of the forests and determined the characteristics of the so-called ’winner’ and ’loser’ species.
’Our research shows that the tree species that dominate landscapes where forest cover is still high tend to have dense wood and large seeds. These seeds are dispersed by medium to large animals typical of the Brazilian rainforests,’ explained Bruno X. Pinho, first author Pinho carried out much of the research when he was still working at the University of Montpellier.
’In heavily deforested landscapes, however, where the remaining forests are additionally exposed to human pressure, these tree species lose importance in favor of so-called ’opportunistic’ species, which have softer wood and smaller seeds that are eaten by small, mobile birds and bats that are adapted to the disturbance of the forest. These species tend to grow faster and are better able to spread,’ Pinho continues.
Urgent action needed to protect ecosystem functions and flora and fauna
These research findings underline the urgent need to strengthen the protection and restoration of tropical forests in order to preserve these vital ecosystems. ’The severe impacts of forest degradation in some Amazonian regions show the importance of not only tackling deforestation, but also forest disturbances such as selective logging and fires,’ says Jos Barlow of Lancaster University, the
’The functional changes have serious impacts that urgently need to be quantified. They point to a possible deterioration of important ecosystem processes and their services to humans, particularly through changes in biogeochemical cycles - especially the carbon cycle - but also in the interactions between fauna and flora and the regeneration of forests,’ explains Felipe Melo, second author of the study and researcher at the Universidade Federal De Pernambuco in Brazil (now at Nottingham Trent University). Among other things, the researchers show that measures are needed to protect populations of large birds such as toucans and mammals such as spider monkeys, which spread the seeds of ’losing’ slow-growing tree species with large seeds.
Pioneering research and policy implications
’The negative impacts of habitat loss on biodiversity are widely recognized. Less well known and controversial, however, are the independent effects of landscape fragmentation and local degradation. This is mainly because it is difficult to distinguish between causal and non-causal relationships,’ explains David Bauman, research fellow at the French National Research Institute for Sustainable Development (IRD) at the Université de Montpellier and co-author of the study. Some studies report positive, others negative effects of this fragmentation. These often minor effects are only documented in relation to the number of species. However, a small impact on the number of species may conceal the displacement of several species by other species with different ecological strategies, which has a significant impact on the diversity and functioning of these ecosystems.
According to the researchers, understanding these changes and distinguishing between causal and non-causal relationships is crucial for managing fragmented landscapes in a way that preserves these ecosystems and their diversity.
