New research finds tropical forest disturbance goes beyond species loss and includes a shift towards smaller seeds and an increase in the proportion of trees dispersed by animals, impacting how the ecosystem functions.

The findings are published in the British Ecological Society journal, Journal of Ecology.

The study looked at areas in the Brazilian Amazon with varying levels of disturbance from activities like logging or burning. The researchers found that not only did human disturbance reduce overall tree diversity, it increased the proportion of trees with seeds dispersed by animals as opposed to other mechanisms like wind.

Disturbance also led to a significant shift towards small-seeded species, which are more likely to be dispersed by smaller animals like birds and bats. It is not clear if these trees can support larger fruit-eating animals that specialise in large-seeded plants and are important for their seed dispersal.

The researchers observed similar effects in secondary (re-grown) forests recovering from clear felling. Older secondary forest had functionally similar plants to the most heavily disturbed primary forest.

Dr Joseph Hawes, lead author of the study of the Museu Paraense Emílio Goeldi and Anglia Ruskin University, and now at the Norwegian University of Life Sciences, said: “Previous studies in disturbed tropical forests have often found plant communities are more likely to rely on seeds dispersed by wind and other abiotic mechanisms, rather than fruit-eating animals. In contrast, our study found that disturbance led to tree communities in which a greater proportion of species and individuals rely on animal dispersal.”

There are likely multiple reasons for this shift. Forest fires and selective logging disproportionately affect certain tree species, which can influence dispersal patterns. Hunting can also reduce seed dispersal by large birds and mammals, leaving smaller animals to disperse smaller seeds.

On the implications of a shift towards smaller-seeded tree species, Dr Hawes added: “Smaller-seeded tree species are becoming more prevalent in forests heavily disturbed by human activity. As larger-seeded tree species are also often those with higher wood densities, these changes in forest composition could have longer-term implications for both the carbon storage and drought sensitivity of tropical forests.”

Professor Jos Barlow, co-author from Lancaster University, the Federal University of Lavras in Brazil, and who began this work in 2010 through the Sustainable Amazon Network (Rede Amazônia Sustentável), said: “This highlights the especially important role played by large-bodied fruit eating animals in the Amazon and helps to underline the need to avoid the loss of these animals and to help encourage their recovery in human-modified forests.”

Dr Ima Vieira, co-author from the Museu Paraense Emílio Goeldi, said: “Most forest restoration focusses on the vegetation, but we also need to consider fauna in restoration projects because of their important mutualistic interactions with plants. Our study provides further evidence that fauna are key to restoring biodiversity-rich ecosystems in the Amazon.”


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