Aquatic habitat destruction in the Amazon and the unexpected response of fish

When you picture habitat destruction in the Amazon, it is more than likely that you think of scenes of deforestation, agricultural expansion and illegal logging, with statistics such as ‘a football pitch of forest being lost every minute’ (BBC, 2019) springing to mind. While these scenes are both devastating and true, the Amazon is more than just a rainforest, it is also the world's largest drainage basin. It is twice the size of the next largest river basin, Congo, and drains approximately 20% of all Earth's freshwater runoff (Murtoff, 2024). This is not to say that deforestation does not have a profound effect on aquatic diversity, quite the opposite, with one study finding that protecting 13% of Amazon forest in a 18,000 km² study area maintained just 60% of fish diversity (Arantes et al., 2018). However, waterways themselves are also seeing habitat modification at alarming scale in the Amazon.

The Amazon has the highest number of fish and endemic fish species of the world's largest rivers, but just like troubling deforestation statistics in the terrestrial plane, the Amazon is also a global hotspot for dams (Best, 2018). Dams cause a myriad of problems for aquatic organisms, as we have discussed in previous issues (Perry, 2022). These problems include introducing barriers to movement, altering river flow and temperature regimes as well as causing sedimentation and low oxygen. These changes reduce species richness (Franssen & Tobler, 2013) as well as modify (Scharnweber et al., 2024) and simplify tropic relationships, even in the case of small dams (Katano et al., 2006). There are also negative impacts on physiology (i.e., swimming performance (Zhang et al., 2023) and metabolic rate (Parisi et al., 2022)), morphology (Svozil et al., 2020) and spawning (Twardek et al., 2021). Finally, these conditions also drastically reduce the success rate of migration, even when fish passage solutions are installed (Shry et al., 2024).

Despite the devastating impacts of dams on freshwater systems, and an ever-increasing literature base on the topic, new findings continue to reveal unexpected ways that dams affect fish, challenging established narratives. One such dogma is, in areas upstream of dams, where lotic is transformed into lentic, populations respond by becoming more generalist. However, in this issue, Barros et al. (2024) demonstrate that this is not always the case.

Using damming caused by road construction in the Amazon, Barros et al. (2024) were able to show that there was a higher degree of individual specialization of Bryconops giacopinii in dammed streams compared with pristine counterparts. Despite the dammed habitats being less diverse and offering a lower niche width, surprisingly, B. giacopinii individuals consistently used the same food resource, rather than being a generalist and consuming whatever was available. The diet of individuals therefore overlapped less with the rest of the population. Barros et al. (2024) suggest that instead of generalizing to reduce competition by seeking a wider range of food resources, individuals specialized in other, less-consumed items within the population, helping to reduce competition. As there was a lower variety of items to consume within the dammed streams, which were also more predictable (e.g., terrestrial fruits), individuals became specialized to that specific resource.

Furthermore, in the dammed streams, where intraspecific competition was more intense, individuals with narrower niches exhibited poorer body condition than those with broader niches. This suggests that individuals adopting more restricted diets may have lower fitness, and that individual specialisation may not necessarily be beneficial for individuals.

These exciting results show that the response of fish to anthropogenic pressures is not always immediately predictable, with many complex interacting factors at play. Understanding these complexities, with studies like those by Barros et al. (2024), will undoubtably help how we manage and conserve the valuable biodiversity around us.