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A significant amount of focus has been directed towards how climate change can lead to biodiversity decline. Now, MIT scholars have demonstrated the opposite is also valid: Reductions in biodiversity may threaten one of Earth’s most effective mechanisms for combating climate change.
In a publication featured in PNAS, the researchers revealed that after deforestation, naturally-regenerating tropical forests, with robust populations of seed-dispersing creatures, can sequester up to four times more carbon than similar forests with a lower number of seed-dispersers.
As tropical forests represent the Earth’s largest land-based carbon reservoir, these findings enhance our comprehension of a powerful ally in the fight against climate change.
“The outcomes highlight the significance of animals in sustaining thriving, carbon-rich tropical forests,” states Evan Fricke, a research scientist in the MIT Department of Civil and Environmental Engineering and the primary author of the study. “When populations of seed-dispersing animals diminish, we risk undermining the climate-mitigating capability of tropical forests.”
Fricke’s co-authors include César Terrer, the Tianfu Career Development Associate Professor at MIT; Charles Harvey, an MIT professor of civil and environmental engineering; and Susan Cook-Patton of The Nature Conservancy.
The research integrates a broad spectrum of data on animal biodiversity, movement, and seed dispersal across thousands of species, alongside carbon accumulation statistics from numerous tropical forest locations.
The researchers assert that the results provide the clearest evidence thus far that seed-dispersing animals are crucial to forests’ capability to sequester carbon, and that the findings emphasize the necessity to acknowledge biodiversity loss and climate change as intertwined elements of a fragile ecosystem rather than distinct issues in isolation.
“It’s been apparent that climate change endangers biodiversity, and now this research reveals how losses in biodiversity can intensify climate change,” Fricke remarks. “Grasping that these issues are interconnected helps us see the relationships between these challenges, and how we can confront them. These are issues we must address concurrently, and the role of animals in tropical forest carbon shows that there are mutual benefits to supporting biodiversity while combating climate change.”
Connecting the dots
The next time you watch a clip of a monkey or bird enjoying a fruit, think about the fact that these creatures are playing a vital role in their ecosystems. Studies have indicated that by digesting seeds and later defecating in different locations, animals aid in the germination, growth, and long-term survival of plants.
Fricke has been researching animals involved in seed dispersal for nearly 15 years. His previous studies have indicated that without animal-driven seed dispersal, trees have lower survival rates and struggle more with adapting to environmental changes.
“We are now contemplating more about the roles animals might occupy in influencing the climate through seed dispersal,” Fricke states. “We know that in tropical forests, where over three-quarters of trees depend on animals for seed dispersal, the decline of these animals could impact not only forest biodiversity but also their recovery from deforestation. Moreover, we recognize that globally, animal populations are in decline.”
Regenerating forests is frequently mentioned as a method to mitigate climate change impacts, yet the effect of biodiversity on forests’ carbon absorption capacity hasn’t been fully assessed, particularly at larger scales.
For this research, the team merged data from thousands of distinct studies and utilized new methods for quantifying diverse but interconnected ecological processes. After analyzing information from over 17,000 vegetation plots, the researchers chose to concentrate on tropical regions, investigating data regarding the habitats of seed-dispersing animals, the quantity of seeds each animal disperses, and their influence on germination.
Subsequently, the team integrated data revealing how human activities affect the presence and movement of various seed-dispersing animals. They discovered, for instance, that animals exhibit reduced movement in areas with a larger human impact.
By synthesizing all that data, the researchers developed an index of seed-dispersal disruption that illustrated a connection between human activities and declines in animal seed dispersal. They then examined the correlation between this index and records of carbon accumulation in naturally regenerating tropical forests over time, accounting for variables like drought conditions, fire prevalence, and the presence of grazing livestock.
“It was a substantial endeavor to consolidate data from thousands of field studies into a map showing seed dispersal disruption,” Fricke comments. “Yet it enables us to shift from merely identifying the presence of animals to truly quantifying the ecological roles those animals fulfill and comprehending how human pressures influence them.”
The researchers acknowledged that the quality of animal biodiversity data could be enhanced and introduces uncertainty into their conclusions. They also noted that other factors, like pollination, seed predation, and competition, influence seed dispersal and can restrict forest regrowth. Nevertheless, the findings were consistent with recent assessments.
“What is particularly novel about this study is that we’re quantifying these effects,” Fricke remarks. “Finding that seed dispersal disruption accounts for a fourfold variance in carbon absorption across the numerous tropical regrowth sites analyzed indicates that seed dispersers are a key factor in tropical forest carbon dynamics.”
Measuring lost carbon
In forests identified as potential regrowth locations, the researchers discovered that declines in seed dispersal were associated with reductions in carbon absorption each year averaging 1.8 metric tons per hectare, equating to a 57 percent decrease in regrowth.
The researchers maintain that the findings indicate that natural regrowth efforts will be more effective in landscapes where seed-dispersing animals have experienced less disruption, including areas that were recently deforested, are adjacent to high-integrity forests, or exhibit higher tree coverage.
“In discussions regarding planting trees versus permitting trees to regrow naturally, regrowth is essentially cost-free, while planting trees incurs expenses and tends to result in less diverse forests,” Terrer states. “With these results, now we can ascertain where natural regrowth can occur effectively due to animals facilitating seed dispersal, and we can also identify areas where, due to the impact on animals, natural regeneration is unlikely, necessitating the active planting of trees.”
To aid seed-dispersing animals, the researchers advocate for initiatives that protect or enhance their habitats while alleviating pressures on species, ranging from wildlife corridors to regulations limiting wildlife trade. Restoring the ecological roles of seed dispersers can also be achieved by reintroducing seed-dispersing species to areas where they have vanished or planting particular trees that attract these animals.
The findings may also enhance the accuracy in modeling the climate impact of naturally regenerating forests.
“Neglecting the influence of seed-dispersal disruption may overstate natural regrowth potential in certain areas while underestimating it in others,” the authors note.
The researchers believe the findings pave the way for new explorations within the field.
“Forests provide a significant climate benefit by sequestering roughly a third of all human carbon emissions,” Terrer comments. “Tropical forests are undoubtedly the most critical global carbon sink, yet their ability to sequester carbon has been diminishing in recent decades. We will next investigate how much of that decline can be attributed to increasing extreme droughts or fires compared to declines in animal seed dispersal.”
Ultimately, the researchers aspire that this study enhances our understanding of the intricate ecological processes that govern our planet.
“When we lose our animal populations, we lose the ecological foundation that sustains the health and resilience of our tropical forests,” Fricke concludes.
The research was supported by the MIT Climate and Sustainability Consortium, the Government of Portugal, and the Bezos Earth Fund.
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