When it comes to spreading their seeds, many trees in the rainforest rely on animals, clinging to their fur or hitching a ride within their digestive tract. As the seeds are spread around, the plants' prospects for survival and germination are increased.

Researchers from UConn and the San Diego Zoo Institute for Conservation and Research examined how the overhunting of seed dispersing animals is changing tree communities in Western Amazonia, such as those in Manu National Park [Credit: Varun Swamy]

But in many tropical forests, over-hunting is diminishing the populations of those animals, and, as a result, changing the make-up of the forests themselves.

A new study of the Amazon rainforest by researchers at UConn and the San Diego Zoo Institute for Conservation and Research, published in the Journal of Ecology, examines what happens to plants if their seed dispersers are no longer present. They found that theoretical models predicting a dire impact on plant communities and huge decreases in the amount of carbon stored in tropical forests are not supported by the facts. Instead, the effects on the ecosystem are less straightforward and less immediately devastating.

"Yes, there is a negative effect, but there isn't 100 percent mortality," says Robert Bagchi, assistant professor of ecology and evolutionary biology at UConn. "The story is more complex and much more subtle."

Whereas the models used in the previous studies did not use actual data on items such as mortality, survival, growth, and spatial distribution, Bagchi and his fellow researchers explored the question in greater detail, using a statistical technique they recently developed with extensive data collected on tree communities in the 80,000 km2 Madre de Dios river basin, located in the southeastern corner of Peru's Amazon rainforest.

In Western Amazonia, as many as two-thirds of all tree species rely on native, fruit-eating mammals such as spider monkeys and tapirs, or birds like guans, trumpeters and toucans, who are able to travel fairly large distances and carry any ingested seeds far from their parent trees.

Dispersal is advantageous for seeds because spreading out will give seedlings an edge over specialized natural predators who might otherwise wipe out aggregations of undispersed plants.

"The idea is that the seeds escape," says Bagchi. "A lot of pathogens and insects are quite specific about which plants they will eat, and if there is no dispersal and their desired plants are densely aggregated, those plants will be clobbered."

In tropical rain forests, as many as two-thirds of all tree species rely on native, fruit-eating mammals such as capuchin monkeys who are able to travel fairly large distances and carry any ingested seeds far from their parent trees. What happens to the forests when these seed dispersing animals are over hunted? [Credit: Varun Swamy]

In addition, the tree species dispersed by these animals also store the most carbon.

Unfortunately, the large-bodied animals and birds are the favorite quarry of hunters for bush meat.

The researchers examined tree communities in the tropical rain forests of Western Amazonia, in terms of forest spatial organization and carbon storage capacity. They did find that tree communities in hunted forests appear to be undergoing a reorganization, where saplings of species that rely on large hunted animals for dispersal are now growing closer to each other and forming denser clumps in hunted forests.

But the long-term implications for biodiversity and the biomass of forests are not yet clear. And the expectation that without their dispersers, seeds of these plant species will land in the "kill zone" of insects and diseases under their parents and be replaced by other species that store less carbon, culminating in huge decreases in the amount of carbon stored in tropical forests, has not materialized.

A number of factors could be contributing to the reason that previous theories are not proving true, Bagchi says.

Smaller seed dispersers that often increase when their larger competitors are hunted out may be compensating. Additionally, the trees analyzed in the study were already at least 10-15 years old, so follow-up studies will instead focus on the early lives of these trees, starting at the germination stage.

Questions the researchers hope to pursue include, What are the survival rates of undispersed seeds in hunted forests? Is limited dispersal by smaller animals enough to ensure a seed's survival? How do these stages fit together -- does high survival at a later stage compensate for low survival of undispersed seeds?

"We can't simplify the process to just a linear one," says Bagchi. "We need data following the whole process, from seed dispersal to trees growing into adults."

Bagchi also cautions that although these findings are somewhat hopeful in light of previous modeling studies, tropical forests in South America, Asia, and Africa are becoming ever more stripped of their diversity of flora and fauna, fundamentally changing the structure of these complex systems.

Source: University of Connecticut [February 15, 2018]

Tropical forests span a huge area, harbor a wide diversity of species, and are important to water and nutrient cycling on a planet scale. But in ancient Amazonia, over 500 years ago, clearing tropical forests was a way of survival to provide land for families to farm and villages to prosper. Researchers at the University of New Hampshire used high-tech tools to more precisely view where these cleared sites were and how much lasting impact they had on the rainforest in the Amazon Basin in South America.

Map developed using satellite imagery and spatial modeling showing estimated past human impact  on the Amazonian forest [Credit: Michael Palace/UNH]

"One of the key mysteries in this area of the world is that no one knows how many people lived in Amazonia before European contact," said Michael Palace, an associate professor in the Earth Sciences Department and Earth Systems Research Center at UNH. "Once the Europeans arrived, indigenous populations were devastated due to disease, slavery and displacement so it's often hard to determine lasting impact. It is important to understand the resilience or fragility of these forests to past human disturbance, which allows for appropriate planning on the use of natural resources."

One of the few indicators of human settlements are the terra preta, or Amazonian Dark Earths (ADEs). These are black, human-made soils that are enriched by materials left behind from past societies, pottery remnants, charcoal and other artifacts frequently associated with human origin. There is very little rock and metal in that part of the world (a former ocean bottom), and wood, bone and other organic materials decay quickly in the humid tropics.

In their study, recently published in the journal Ecosphere, the researchers used imagery from NASA's terra satellite, MODIS, and geospatial modeling to predict the ADE probability across six million square kilometers of the Amazonia. They found that biomass (primarily weight of trees in forests), tree height, and tree cover were all lower at ADE sites than adjacent random non-ADE locations. The ADE sites were also more susceptible to drought.

Indigenous people most likely used slash-and-burn techniques to clear the forests so the ADE sites may have been pre-selected due to the propensity for drought or intensity of dry seasons. Researchers also observed differences in spectral properties (reflective light) between ADE and non-ADE sites that are likely driven by forest structure and tree species, indicating that remnant forests are still showing impacts from past human settlement patterns.

Researchers further explain that the high spatial heterogeneity in ADE sites across the Amazon suggests that pre-Columbian occupation by indigenous people was complex and varied substantially across this ecologically diverse region. But they speculate that because of their proximity to the rivers, ADEs may have become areas of interest for other groups to settle and reoccupy, further impacting the forests and vegetation in the ADE areas.

Source: University of New Hampshire [January 18, 2018]