- Two recent studies looked into the impact of human disturbance on ecological diversity in Amazonia habitats. Another study in the Rupununi region of Guyana found how important maintaining connectivity is to maintaining ecosystem health.
- The first study investigated how forest fragmentation impacts mixed-species flocks of birds. The research found evidence that forest habitat fragmentation in the Amazon has caused mixed-species bird flocks to severely diminish and even disappear.
- A second study evaluated the impact of logging and fire on seed dispersal in tropical forest plots in the eastern Brazilian Amazon. The research team found that Amazon forests which have been heavily logged and burned are populated primarily by tree species with smaller seeds, and smaller fruits.
- The remote Rupununi region provides water connectivity between the ancient Guyana Shield and the Amazon basin. A recent study there identified more than 450 fish species within the Rupununi region. The research illustrated the value of conserving connectivity between diverse habitats.
The Amazon rainforest is falling to pieces — deep cracks intrude ever farther into old growth forest, while devastating wildfires spring up to consume degraded fragments. This shattering, occurring over decades, is the result of an aggressive modern human economic invasion, as the once continuous tropical foliage is supplanted by roads, dams, cattle herds and soy plantations.
With the loss of forest habitat, the rich tapestry of rainforest species too is shredded, as individual species vanish and are replaced by more common, often invasive organisms. The Amazon’s giant anteaters are replaced by rats. The towering, and highly valuable, Brazil nut tree is replaced by weeds. Scarlet Macaws are ousted by vultures.
Whatever our personal value judgments may be around these alterations, human disturbance triggers a panoply of ecological processes. And scientists continue carrying out new studies to illuminate how and why species disappear in the wake of human-caused habitat degradation and destruction — with the hope of slowing or stopping the process.
Fewer flocks flocking together
In the Amazon rainforest, as in forests worldwide, birds of a feather aren’t the only ones who flock together. Dr. Cameron Rutt from Louisiana State University led a study team to see how forest fragmentation impacts mixed-species flocks of birds, who fly together for protection and to take advantage of their various abilities. The research, published in Biological Conservation, found evidence that forest habitat fragmentation in the Amazon has caused mixed-species bird flocks to severely diminish and even disappear.
They developed a unique approach for their work in Amazonas state, Brazil: “Until now, the vast majority of negative effects of habitat fragmentation have been inferred from comparisons of pre-existing fragments with separate control sites,” the researchers wrote. “To directly test the effects of fragmentation on Amazonian mixed-species flocks — a complex and diverse species interaction network — we observed birds before and after re-isolation of three 10-hectare [24.7 acre] fragments at the Biological Dynamics of Forest Fragments Project in central Amazonia.”
Rutt explained that the forest blocks utilized in the research were originally isolated from the primary forest in a large-scale deforestation event in the 1980s. The Amazon rainforest, however, grows back quickly, and the primary forest fragments were soon surrounded by secondary regrowth. For the study, a band of secondary forest was cleared away again, re-isolating the primary forest fragments for a second time, creating a more precise control from which to gather and compare data.
“We quantified species richness and attendance, home range size, proportional use of edge and second growth, and space use for fragment and control flocks before and after re-isolation,” Rutt told Mongabay. “We found that in one of the three [re-isolated] fragments we studied, one of the flocks disappeared entirely within a few years. In the other fragments, where the flocks still existed after re-isolation, there were a few species of obligate flock followers that either vanished or decreased attendance rates.”
Rutt noted that flock space use altered upon re-isolation, as the fragment became “a sort of prison cell where [the birds] were confined by the hard boundaries at the edge of the fragment.” This species decline finding matches up with the groundbreaking island biogeography research by E.O Wilson and Robert H. MacArthur conducted in the Florida Keys in the 1960s, and work carried out in the Amazon by Tom Lovejoy in the 1980s, demonstrating that biodiversity declines in isolated habitats, with greater loss seen in smaller, more insular areas.
Douglas Stotz, a senior conservation ecologist at the Keller Science Action Center at Chicago’s Field Museum, was not involved in the new Amazon study, but reviewed it favorably. He felt the research leveraged long-term ecological research on fragmentation in the central Brazilian Amazon that “would be difficult or impossible to do anywhere else.”
The loss of the mixed flocks, especially fruit-eating birds, could have an important long-term impact on the forest structure, and Stotz believes that his team’s results could be used to help conservationists create more effective tropical bird preserves.
“The time period of this study was reasonably short,” covering only three to four years. “If anything, it understates what might happen across time in an isolated fragment, further emphasizing the need for mitigation strategies if we intend for small preserves to have value of bird conservation,” in Amazonia and possibly beyond, Stotz said.
Shifts in seed and fruit dispersion
Multiple studies have found forest loss and degradation over time to correlate with species loss and biodiversity reduction. But these effects are complex, with more happening than is easily observed, as the activities and health of individual species impact the activities and health of other species, subtly altering the existing ecological web through many interactions between all manner of plants and animals.
A recent study that looked at such interspecies interactions, published in the Journal of Ecology, evaluated the impact of human disturbances, such as logging and fire, on seed dispersal in tropical forest plots in the eastern Brazilian Amazon.
“We aimed to collect data on a gradient of human-impacted forests, going from forests that have been slightly disturbed by fire or logging, to forests that have been heavily disturbed by repeat burnings and intense logging,” lead author Joseph Hawes told Mongabay.
The study looked at small forest plots in Pará state, Brazil, where all the tree species were identified, measured and registered. THEN, knowing what trees were present in the various plots, researchers looked deeper to see how an important ecological process — seed dispersal networks — were impacted by varying degrees of human disturbance.
Pattern shifts weren’t immediately obvious. “When you walk into a disturbed forest it might appear as if it hadn’t been impacted to the untrained eye, but there are actually many subtle effects that are going on underneath the surface,” Hawes said.
The research team found that Amazon forests which have been heavily logged and burned are populated primarily by tree species with smaller seeds, and smaller fruits. These fruits’ characteristics provided an advantage to smaller animals, who prefer smaller fruits and disperse smaller seeds, while also inhibiting larger species, such as primates, who consume large fruits and disperse larger seeds.
“Disturbance reduced tree diversity and increased the proportion of lower wood density and small‐seeded tree species in study plots,” the study authors concluded. Interestingly, “Older secondary forests had functionally similar plant communities to the most heavily disturbed primary forests.”
The study did not try to separate the effects of fire and logging, because both impacts were occurring simultaneously in the same human-disrupted forests and ecosystems. Hawes did discover how logging helps lead to the loss of large-fruit trees in Amazonia. Loggers typically select hardwood trees (which also happen to produce larger fruits) for harvesting, because such trees produce heftier timber profits.
Fire, the other disturbance looked at, is rarely natural in Amazonia, which is very wet, and where fires are typically set by people to enhance agricultural activity. As a result, tropical rainforest fires do not burn hot, like the raging blazes seen in the planet’s temperate zone, but are low-level blazes that smolder and spread primarily through the understory.
No matter the disturbance, smaller seeds replaced bigger ones. “There are still a large number of fruiting trees in disturbed forests, but they tend to be trees with smaller seeds, that are more likely to be dispersed by small animals like bats rather than larger bodied animals such as monkeys,” Hawes observed.
He concludes that. even while a heavily disturbed forest may remain standing and appear healthy, it still loses biodiversity and resilience. “We’ve lost a web of complex interactions that are necessary for a healthy ecosystem for years to come,” Hawes concluded.
Hans ter Steege, professor of tropical tree diversity at Utrecht University, a researcher at the Amazon Tree Diversity Network (ATDN), and not involved in the recent study, told Mongabay that previous research had produced models pointing toward the importance of animals in forest recovery; this latest study supports that modeling with field data.
Ter Steege pointed to other studies that show forest biomass and richness can be recovered relatively quickly, but that original species composition can take a much longer time to be restored. “Large animals are fundamentally important to help the succession from just gaining biomass, to [regaining] a composition that resembles undisturbed forest.”
What can we do? Conserving connectivity
Lesley de Souza, a conservation ecologist at Chicago’s Field Museum, has conducted field studies for the last 15 years in South America, primarily in the Amazon basin. Her latest work, published in Frontiers in Forests and Global Change, took her to the Rupununi region of central Guyana, where she surveyed fish species.
The remote Rupununi is covered by expansive savanna, wetlands, and forests, a mystical-seeming landscape that helped inspire Pixar’s animated film Up. The vast area provides water connectivity between the ancient Guyana Shield and the Amazon basin —the locations for two of the most biodiverse, carbon rich, and intact forests in the world.
De Souza explained that this hydrological connection forms during the wet season, when river and lake waters rise, and ceases in the dry season when water levels decline. “It’s like a heartbeat, up and down. The waterways fill up, the connection is formed; the water drops, and the connection is cut,” de Souza said. “This process drives diversity because it creates a huge variety of habitats and aquatic systems that are changing all year long,”
The Rupununi research team uncovered incredible aquatic biodiversity. They identified more than 450 fish species within an area smaller than the U.S. state of Connecticut; that’s more fish species than are found in the entire Mississippi River basin, which stretches from the river’s delta in the Caribbean Sea to the Rocky Mountains.
In the biodiversity-rich Rupununi region, ecosystems converge. “There’s no way to separate what’s happening underwater from the top of the forest canopy. The macaws perched on top of the giant trees are dependent on the fish swimming in the river, and vice versa. The river brings nutrients into the soils that are absorbed through the roots and permeate the entire landscape. On the other hand, the fish feed upon the seeds and fruits that fall from the trees,” explained de Souza.
Two main goals of the study were to highlight the importance of the Rupununi terrestrial and aquatic corridor, and explain why it is necessary to maintain the vital water connection between a diverse mosaic of varied habitats. Additionally, the research found a new hydrological corridor dubbed the Sand Creek portal, running between the Takutu and Rupununi rivers.
“The Rupununi Portal is a unique biogeographic region, [representing] one of the only seasonal connections between highly diverse rivers. We have shown that complex habitats that mix forest and savanna are the most biodiverse areas, and that the Rupununi Savanna serves as a corridor for some species and a barrier to others. This complexity of ecosystems leads to high diversity that is deserving of protection,” the study found.
Like tropical ecosystems everywhere, the Rupununi faces threats from large-scale agribusiness, mining and logging. De Souza said foreign companies, many coming from across the Brazilian border, are trying to buy up land leases in the savanna and convert these diverse ecosystems to monocultures of soy and rice. Other companies want to extract gold. All of these activities would violently disrupt the complex ecological interactions unfolding within the region’s watersheds.
The Rupununi is Inhabited by around 7,000 indigenous Makushi and Wapishana, who depend on the rivers, wetlands, forests, and savannas for their sustainable lifestyles. De Souza said her study purposely focused on fish because they’re a particularly important source of protein and food security for indigenous communities.
Working in cooperation with the locals, de Souza has presented the Guyana government with a plan to create a protected conservation area for the region in order to maintain healthy ecosystems and bountiful biodiversity.
“There are over 20 indigenous communities in the area. They all depend on this landscape’s natural resources to hunt and fish. Healthy rivers and forests are crucial to their persistence and livelihoods,” de Souza concluded. “For me, the most important piece is people. We need to ask the locals what their aspirations are for the region. We need to ask: what do you want your future to be?”
Banner Image caption: The Long-winged Antwren (Myrmotherula longipennis) is part of the Amazon mixed-species flocks observed in the study. Image courtesy of Cameron Rutt.
Hawes, J.E., Vieira, I.C.G., Magnago, L.F.S., Berenguer, E, Ferreira, J, Aragão. L.E.O.C., Cardoso A., Lees, A.C., Lennox, G.D., Tobias J.A., Waldron, A., Barlow, J. A large‐scale assessment of plant dispersal mode and seed traits across human‐modified Amazonian forests. Journal of Ecology, 2020.
Rutt C.L., Mokross, K., Kaller, M.D., Stouffer, P.C. Experimental forest fragmentation alters Amazonian mixed-species flocks. Biological Conservation, Volume 242, February 2020.
De Souza, L.S., Armbruster, J.W., Willink, P.W. Connectivity of Neotropical River Basins in the Central Guiana Shield Based on Fish Distributions. Frontiers in Forests and Global Change, Volume 3, 2020.
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