Tropical forests – anthropogenic interventions are changing tree species diversity

Tropical forests are the most important reservoir of terrestrial biodiversity. They play a crucial role in greenhouse gas sequestration and provide important ecosystem services. Nevertheless, they are victims of rapid deforestation and forest fragmentation, showing a loss of three to six million hectares per year over the last two decades. A major share of today’s tropical forests therefore consists of landscapes modified by human activity which are exposed to local pressure such as logging, hunting and fires.

Now, a new study conducted by an international team in which Dr Bruno X. Pinho of the Institute of Plant Sciences at the University of Bern, Switzerland, demonstrates that fast-growing tree species with small seeds are predominant in tropical forest regions with intensive logging and forest degradation. Changes in tree species could have considerable impacts on the ecosystem services which these forests provide – including their important ability to take up and store carbon.

While the tree species which the study refers to as “winners” grow fast, their lifespan is limited by their trunks and branches being much less dense than those of the slowly growing tree species they are displacing. The results have been published in the specialist journal Nature Ecology and Evolution.

More than 1,200 tropical tree species surveyed
 

For the survey, the international team of researchers examined a unique set of data from more than 1,200 tropical tree species in over 270 woodlands in six regions of the Brazilian Amazon and Atlantic rainforests which had been influenced by human activities such as deforestation and local disturbances like logging, hunting and slash-and-burn clearing.

With the aid of various statistical models, the scientists analysed the causal impacts of forest loss, fragmentation and local forest destruction with regard to the composition of the forests and established the features of the so-called “winner” and “loser” species.

“Our surveys show that those species which dominate landscapes with a still high share of forests tend to have dense wood and large seeds. These seeds are distributed by medium-size to large animals which are typical of the Brazilian rainforests,” explains Pinho, the study’s lead author.

“By contrast, in strongly deforested landscapes, where the remaining forests are additionally exposed to anthropogenic pressure, these tree species lose significance in favour of so-called “opportunistic” species, which have softer wood and smaller seeds that can be eaten by small, mobile birds and bats which have adapted to disturbances of the forests. As a rule, such species are better able to spread,” Pinho further explains.

An urgent need for action to protect ecosystem functions
 

These research results underscore the urgent need to increase the conservation and restoration of tropical forests in order to preserve such vital ecosystems. “The considerable effects which forest degradation has in some Amazon regions show how important it is to not only take measures against forest clearance, but also to combat disturbances of forests such as selective logging and fires,” says Professor Jos Barlow of the UK’s Lancaster University, who conducted the study.

“The functional changes have serious impacts which urgently require quantifying. They point to a possible worsening of important ecosystem processes and their services for humans, especially through changes in the bio-geo-chemical cycles – and here in particular the carbon cycle – as well as interaction between fauna and flora and the regeneration of forests,” explains Felipe Melo, who is the second author of the study and a researcher at Universidade Federal De Pernambuco in Brazil.

One aspect the researchers point to is that measures are needed to protect the populations of large birds such as toucans and mammals like the woolly spider monkey, which spread the seeds of the “losing” slow-growing tree species with large seeds.

Forward-looking research and political implications
 

“The negative impacts of habitat loss on biological diversity are generally recognised. What is less known and is discussed controversially is the independent impacts of landscape fragmentation and local degradation. This is above all because of difficulty in distinguishing causal from non-causal contexts,” explains David Bauman, a senior research fellow at the French National Research Institute for Sustainable Development (IRD) at Université de Montpellier, France, and co-author of the study.

For example, some surveys give accounts of positive impacts of such fragmentation, while others indicate negative impacts. Such frequently minor effects are only recorded with regard to the number of species. However, the displacement of several species through other species with other ecological strategies may be the cause of a minor impact on species numbers, which then has considerable consequences for the diversity and functioning of these ecosystems.

According to the researchers, understanding these changes and distinguishing causal from non-causal contexts are crucial to the management of fragmented landscapes which preserves their ecosystems and their diversity.

(University Bern/wi)

Reference:

Jos Barlow, Bruno X. Pinho et al.: Winner-loser plant trait replacements in human-modified tropical forests. Nature Ecology and Evolution; Dec.’24. DOI: 10.1038/s41559-024-02592-5