Human disturbance affects Amazon plants

Detailed research in the Brazilian Amazon has shown that human disturbance has a significant impact on plants, reducing overall diversity and causing shifts in dispersal mode and seed traits. 

Notably, it increases the proportion of trees with seeds dispersed by animals as opposed to other mechanisms, such as the wind. There are also shifts towards small-seeded species, which are more likely to be dispersed by smaller animals like birds and bats. 

It is not clear, the researchers say, whether the trees can support larger fruit-eating animals that specialise in large-seeded plants and are important for their seed dispersal.

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Fruits on the forest floor. Credit: Marizilda Cruppe, Rede Amazônia Sustentável.

The study, which is published in the Journal of Ecology, brought together researchers from the UK, Brazil, the US and Singapore.

“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,” says lead author Joseph Hawes, from Anglia Ruskin University.

“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, Hawes adds. Forest fires and selective logging disproportionately affect certain tree species, which can influence dispersal patterns. 

Hunting also can reduce seed dispersal by large birds and mammals, leaving smaller animals to disperse smaller seeds.

Hawes adds that as larger-seeded tree species are also often those with higher wood densities, the changes in forest composition could have longer-term implications for both the carbon storage and drought sensitivity of tropical forests. 

The study surveyed 230 forest plots across two regions in the Brazil’s eastern Amazon, recording 26,533 live tree stems from 846 tree species. The plots ranged from undisturbed primary forest, to areas that had been logged, burned, or logged-and-burned. 

Using herbariums and research literature, the researchers compiled information on fruit and seed traits like size, type, shape and dispersal method for each species.

The study focussed on changes in plant communities, rather than changes in animal communities in human-disturbed forests. The researchers warn that isolating these disturbance-specific relationships will likely be difficult due to the multiple drivers of change in human-modified landscapes.

Outside of the seed dispersal method, they did not consider other factors that may influence successful plant recruitment. This was limited by a shortage of information on what constitutes effective seed dispersal by different animal species.

“One of the next steps in understanding the long-term ecological impacts of human disturbances in tropical forests is to build a comprehensive database for plant traits, including measurements such as seed size that were included in our study,” he says. 

“We have contributed our data to the TRY Plant Trait Database, a global research effort to compile and provide free and open access to plant trait data.”

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