Guinea-Bissau is a small West African country located on the Atlantic coast, bordered by Senegal to the north and Guinea to the southeast. With a population of approximately 2 million people, it is one of Africa’s smallest nations by land area and population. Guinea-Bissau is not without its charms. The nation boasts an abundance of natural beauty, from the pristine Bijagós Archipelago, a UNESCO Biosphere Reserve teeming with biodiversity, to its dense forests and mangroves that provide sanctuary to a variety of wildlife. Moreover, there is a lot more to be discovered, as this article will show.
The Corubal River is one of the major rivers in Guinea-Bissau, playing a crucial role in the country’s geography and economy. This river, extending about 560 kilometres from the Fouta Djalon highlands in the Republic of Guinea to the Geba estuary near Bissau, encompasses a watershed spanning 24,000 square kilometres. Despite its ecological significance, systematic biodiversity surveys of this region have been sparse—until now. A pioneering study utilising environmental DNA (eDNA) approaches is beginning to shed light on its rich and unique biodiversity.
The Power of eDNA in a Remarkable Study
Environmental DNA refers to genetic material shed by organisms into their surroundings. By collecting and analysing water samples, scientists can detect traces of DNA from a wide range of species without directly observing or capturing them. Traditional biodiversity assessment methods can be cumbersome, costly, and often impractical, especially in remote and diverse ecosystems like the Corubal River basin. For the Corubal River Study, the research team filtered water samples from 11 sites along the Corubal River watershed, employing multiple molecular markers to target various taxonomic groups. A remarkable revelation from the eDNA survey was the vast array of biodiversity hidden within the Corubal River.
A Treasure Trove of Biodiversity
The survey recorded an impressive 2589 amplicon sequence variants (ASVs), offering a glimpse of the river’s true species richness. This number is undoubtedly an underestimation, as the analyses pointed to the need for additional sampling to achieve a more comprehensive survey. Among the identified ASVs, 125 species of aquatic and terrestrial vertebrates were detected. Fish species dominated the findings, accounting for 61.9% of the recorded biodiversity. The abundance of fish in the Corubal River is particularly noteworthy, as it underscores the importance of the river as a vital resource for both local and regional fisheries.
Perhaps the most captivating aspect of this discovery lies in the identification of 11 imperilled species, two of which are Critically Endangered. The Western chimpanzee (Pan troglodytes), one of the two Critically Endangered species, is one of the rarest primates in the world, with population numbers dwindling due to habitat loss and poaching. The other Critically Endangered species, the mussel Pleiodon ovatus, has a restricted range and is highly vulnerable to changes in the aquatic environment.
The eDNA survey also revealed the detection of 21 species new to the country of Guinea-Bissau, which further emphasises the need for ongoing efforts to document the region’s biodiversity. These findings underscore the significance of the Corubal River as an essential hotspot of biodiversity, hosting a unique assemblage of species that should be protected and preserved.
While the Corubal River study showcases the potential of eDNA surveys, it also highlights some challenges that need to be addressed:
1. Reference Libraries: The lack of comprehensive genetic reference libraries for many species in understudied regions limits taxonomic identification. Future efforts should prioritise building these libraries, especially for species of conservation concern.
2. Sampling Effort: Despite the large water volumes filtered and high technical replication, the study suggests that more sampling sites would be needed for a comprehensive representation of biodiversity at the basin scale. Optimising sampling strategies for different ecosystems and taxonomic groups is an essential area for future research.
3. Seasonal Variability: The timing of sampling can affect species detectability. For instance, the study’s dry season sampling may have influenced the low number of amphibians detected.
4. Primer Selection: The choice of genetic markers (primers) can significantly impact species detection. The study found that a general vertebrate primer detected many fish species missed by a fish-specific primer. Developing and testing primers for specific taxonomic groups in different regions will be crucial for improving eDNA survey accuracy.
5. Detection Factors: Some groups, like reptiles, were underrepresented in the eDNA results despite their known presence in the area. Understanding the factors influencing eDNA shedding, persistence, and detection for different taxa is essential for refining the method.
Conservation Significance
The Corubal River’s biodiversity is under threat from human activities, including deforestation, agriculture, and infrastructure development. The eDNA survey has not only unveiled a previously underappreciated biodiversity hotspot in West Africa but also serves as a clarion call for the conservation of this vital aquatic ecosystem. Additionally, The survey’s findings have significant implications for the conservation of the Corubal River basin. They provide a baseline for future monitoring efforts and highlight the potential of eDNA surveys to detect rare and threatened species that might otherwise go unnoticed. The findings presented in this article underscore the critical role that eDNA surveys can play in advancing our understanding of global biodiversity and guiding conservation efforts. With the world facing ever-growing pressures on its natural resources, the insights gained from studies like this one are invaluable, equipping us with the knowledge to act, preserve, and protect the intricate web of life that sustains us all.
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