RESILIENCE · Understanding the resilience of Amazonian floodplain ecosystems

Understanding the resilience of Amazonian floodplain ecosystems

Context In the Amazon basin, the most severe droughts on record all occurred in the last two decades, each associated with large-scale forest fires. The frequency and severity of drought is predicted to increase further in the future as climate change progresses. Forest fires during drought years, appear to have an especially strong and destructive effect in blackwater floodplain forests. Across the Amazon basin, low-relief alluvial floodplains are covered with dense forests, which are inundated for periods of up to six months each year. These floodplain forests therefore connect the most biodiverse terrestrial and aquatic environments on the planet. They also store vast amounts of carbon in their tree stems and root mats, and play an important role in the regional hydrological cycle. Recent work has shown that blackwater floodplain forests are remarkably susceptible to burn during drought years and can also be particularly slow in recovering after fire. Floodplain forests may exhibit a very low resilience to fire perturbations, and with repeated fires appear to transition to an open, savanna-like state in at least some regions. Societal importance Degradation of Amazonian floodplain forests will have far-reaching effects on several ecosystem functions. A staggering number of freshwater fish species, a biodiversity twice as high as that of the entire great barrier reef, depend on the existence of Amazonian floodplain forests. A transition from tree-dominated to grass-dominated vegetation will likely disrupt several ecosystem regulating functions, including regional rainfall and temperature conditions as well as rates of carbon sequestration and storage. Furthermore, as fish form the main protein source for thousands of people living in the Amazon basin, loss of floodplain forests will ultimately have a significant impact on human welfare in the region. Hypotheses and objectives Previous research suggests that seed dispersal limitations may play a fundamental role in understanding the resilience of blackwater floodplain forests to fire perturbations. Fire destroys the seed bank completely, and the input of new tree seeds during the high-water season might be a crucial bottleneck for the recovery of floodplain forests. In Amazonian floodplain forests, fish play a important role as dispersers of tree seeds during the high-water period. We hypothesize that major disturbances in floodplain forests break-down the interactions between trees and fishes, and that the input of new tree seeds during the high-water season can become a crucial bottleneck for the recovery of floodplain forests after burning. In particular, high tree mortality in burned floodplain forests likely reduces food availability and increases predation risk for fruit-eating fish. As a consequence, seed-dispersing fishes may avoid burned areas, creating a strong seed-dispersal limitation that prevents tree establishment and forest recovery. This project has two main objectives: (1) to evaluate how disturbances in floodplain forests cascade to fish communities, and (2) to evaluate the contribution of seed dispersal limitation (of tree seeds by fish) in explaining arrested succession of burnt Amazonian floodplain forests. Preliminary conclusions Our preliminary analyses indicate that forest fires, and subsequent degradation of Amazonian floodplain forests, result in marked changes in fish communities. Such changes included a change in species composition and diet, as well as a lower fish diversity in burned floodplain forests. These changes likely contribute to the low resilience of these forests to anthropogenic disturbances.

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