Riverine roadblocks: Vital connections between Amazonian watercourses severed by environmental change

Summary:

  • The Amazon basin has the highest freshwater fish diversity in the world, and provides fish for millions of people along its course
  • The interconnected watercourses of the Amazon are in danger due to dam building and droughts caused by climate change
  • Conservation strategies have been devised based on ‘metapopulation theory’, where connectivity is maintained between the many thousands of watercourses in the Amazon basin
  • These strategies aim to protect fisheries and biodiversity by improving their resilience to change

The Amazon River and its tributaries are home to over 5600 fish species- more than the entire Atlantic Ocean. This plethora of species supports many millions of people living along the banks of the mighty river, who depend on fishing for subsistence and income. The forests, floodplains and lakes of the Amazon are interconnected during half of the year due to seasonal flooding, linking watercourses across a vast area; the river acts as the veins for the ‘lungs of the Earth’. The interconnected nature of these watercourses allows fish to move far and wide across the Amazon basin, causing the seasonal influx of many millions of fish, directly providing food for local people. In addition to this, flooding brings new individuals (and by extension, new genes) into the mix, allowing fish to breed and the next generation to disperse. Indeed, this connectivity is a key part of the life cycles of many economically (and ecologically) important fish species.

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The watercourses of the Amazon are many and linked by seasonal flooding

Unfortunately, these seasonal ‘fish highways’ are at risk. The most imminent threat comes from the building of hydroelectric dams. Dams act as an impenetrable barrier to fish that need to migrate along river systems in order to breed, preventing them from reaching their spawning grounds. The second major threat to the aquatic life of tropical floodplains is climate change, which also acts as a road block for these fish thoroughfares. Climate change extends the dry season, trapping fish in ever-shrinking water bodies, severing the connections between aquatic environments and reducing the influx of food that is usual
ly washed in by floods. These problems will only become more severe in the coming decades, as development encroaches on wilderness, and the climate continues to warm. The reduction in recruitment of young fish due to hampered seasonal flooding will also lead to a reduction in fish stocks, increasing their vulnerability to overexploitation by humans.

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Many species are dependent on Amazonian wetlands, such as the 3m wide Victoria water lily.

A recent study published in Biological Conservation highlights the importance of habitat connectivity to Neotropical fish communities, the threats they face and most importantly suggests conservation strategies aimed at preserving the interconnected nature of Amazonian floodplains. The conservation strategies are based ‘metapopulation theory’- the way that natural populations hedge their bets against environmental change. It posits that in nature organisms exist in metapopulations- small, inter-connected populations that individuals may freely disperse between. Under this pattern, should any natural catastrophe locally wipe out a metapopulation, there will always be a source of new individuals from one of the others able to recolonise. This hugely reduces the likelihood that an entire species will go extinct. Since dam construction is an immediate threat to the connections between metapopulations and to the breeding patterns of many migratory species, Hurd et al. suggest that governments should slow the rate of damn construction on these important watercourses, and where construction must take place that ‘fish ladders’ (artificial steps allowing fish to migrate up and down river) should be built.

Climate change and its effects on the floodplains of the Amazon is an altogether more difficult problem to address. However, the authors promote the establishment of large protected areas in order to encapsulate as much biodiversity as possible, and to facilitate the regulatory functions that intact ecosystems undertake. In addition to this, reducing overexploitation of fish species by banning commercial fishing, only allowing subsistence catches under a quota system would allow fish metapopulations to replenish stocks naturally. These community-based management techniques have been shown to work in the Madre de Dios region of the Peruvian Amazon, and can hopefully be applied across the Amazon basin.

 

References:

Hurd, L.E., Sousa, R.G.C., Siqueira-Souza, F.K., Cooper, G.J., Kahn, J.R., Freitas, C.E.C. (2016) Amazonian floodplain fish communities: Habitat connectivity and conservation in a rapidly deteriorating environment. Biological Conservation 195: 118-127

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