Out of sight, out of mind?

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The conservation status of the world's freshwater molluscs

By Monika Böhm and Sophie Ledger

If recent conservation news has told us anything, it is that biodiversity decline continues largely unabated. The latest Living Planet Report highlighted how global populations of monitored vertebrates have on average declined by 68% since 1970, with declines particularly pronounced for freshwater vertebrate populations. While much less is known about the population trajectories of invertebrates, a new paper in the journal Hydrobiologia, published last week by ZSL scientists in collaboration with the IUCN SSC Mollusc Specialist Group and a large network of experts, has provided another snapshot on the status of a functionally important invertebrate group: the freshwater molluscs.

Pomacea bridgessi © Emily Watson
Pomacea bridgessi © Emily Watson

Freshwater molluscs – both snails and mussels – play a hugely important role in freshwater ecosystems, in water filtration, nutrient cycling, and food webs. They are excellent environmental monitors, and can tell us a lot about the state of our freshwater systems. However, the new study shows that around one third of species in a randomly sampled set of more than 1,400 species of freshwater molluscs are threatened with extinction. This is primarily due to the impacts of pollution and modification of freshwater systems (damming and water abstraction), while infrastructure development, energy production and mining, invasive species, agriculture and aquaculture, and biological resource use were also frequently listed.

The study is part of a larger project to track trends in extinction risk of entire species groups, the sampled Red List Index (SRLI) project. The SRLI is an adaptation of the International Union for Conservation of Nature’s Red List Index (RLI), a major biodiversity indicator which has been adopted to track our progress towards major biodiversity targets. The RLI featured heavily in the Global Biodiversity Outlook which was published by the Convention on Biological Diversity also last week and shows that globally, none of the 20 Aichi Biodiversity Targets has been achieved.

So why a sampled Red List Index? The IUCN RLI relies on comprehensive assessments and reassessments of all species within a species group to establish extinction risk trends over time. Given that many species groups are highly species-rich, especially within the invertebrates, repeated comprehensive assessments are too time-consuming or outright impossible to carry out. This is why the SRLI employs a random sampling protocol to produce trends over time based on assessments of 900 non-Data Deficient species. For example, globally, there are more than 6,000 valid species of freshwater molluscs. For the SRLI, our project assessed the status of 1,428 species of freshwater molluscs, of which 908 species were found to not be Data Deficient.

Gyraulus sp © Andreas Werth
Gyraulus sp © Andreas Werth

Even this “shortcut” assessment is time-consuming. The initial status assessment for the sample of freshwater molluscs was carried out between 2009 and 2011, with the resulting Red List assessments published on the IUCN Red List of Threatened Species by 2012 and 2013. This has provided an initial status point, or “baseline,” showing that 31% of freshwater molluscs in the sample are threatened with extinction, with risk higher in the gastropods (snails), in river systems and the Nearctic, Palearctic and Australasian realms. We are now embarking on the reassessment of the sample. Once this is concluded in 2022, we will have a trend in extinction risk of freshwater molluscs over time for the first time.

In the meantime, our current study has identified several key issues for the conservation of freshwater molluscs, and it is vital that conservation actions are increased to safeguard freshwater ecosystems and the species that depend on them, especially given the manifold threats impacting these fragile systems. Establishment of protected areas specifically aimed at freshwater conservation, targeted in-situ conservation programmes and clear freshwater policies are needed to safeguard freshwater systems into the future, and directed action to minimise threats, for example through better incorporation of environmental flows in watershed management and environmental impact assessments, are vital to maintain freshwaters for the benefit of all, including freshwater molluscs and humans. In addition, we need continued monitoring of the status of our lakes and rivers. Here in London, ZSL is working with volunteers from grassroots organisations and other stakeholders to health-check our rivers to ensure healthy rivers for both wildlife and humans. 

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