This is not an easy question to answer. Almost every species on the planet relies directly or indirectly on water to survive, but that doesn’t make them freshwater species.
So what is the definition a freshwater species? Iain Gunn, from the Centre for Ecology & Hydrology Edinburgh, helps to explain:
“A freshwater species is defined as any taxa that is associated with, or occurs within and by fresh waters, i.e. ditches, lakes, ponds, pools, riparian zones, rivers, streams and wetlands etc.”
For many species groups, classifying them as freshwater is reasonably straightforward (e.g. fish and amphibians are never found far from water in the UK). Yet other groups may prove more difficult. For example, you wouldn’t be surprised to see dippers (left), coots, or swans listed as freshwater species, yet house martins (right) or swallows wouldn’t necessarily live in within freshwaters, but rely on them for their food source (flying insects).
Freshwater ecosystems are losing species at 5 times the rate of terrestrial ecosystems, so knowing what species rely on freshwater habitats and may be affected by habitat loss is incredibly important. To aid with this, a checklist of UK species has recently been published, lead by Iain Gunn.
The checklist was compiled, primarily, under the auspices of the NERC Hydroscape project. The aim for compiling the checklist was to allow querying of freshwater species data in the Biological Records Centre (BRC) but to also to query freshwater species data from the BRC via the UK Lakes Portal, as well as to update the freshwater species list supplied to the UK Species Inventory (UKSI) partners, such as Recorder 6, National Biodiversity Network (NBN) Atlas and iRecord.
This checklist updates Whitton et al. (1998), who estimated that 5000 freshwater species occurred in the UK, though this was acknowledged to be an underestimate. Gunn et al. (2018) estimate that 8367 taxa have now been identified. However, this is also an underestimate as the list doesn’t currently include algae (apart from stoneworts) or micro-organisms such as bacteria, fungi and viruses.
The following eight major groups were identified as being associated with fresh waters in the UK: algae, amphibians, birds, fish, invertebrates, macrophytes, mammals and reptiles. Of which the biggest grouping are invertebrates with 5,810 listed taxa (broken down into 28 discrete groups), followed by macrophytes with 2,216 listed taxa (9 discrete groups).
The checklist can be cited as:
Gunn, I.D.M.; Carvalho, L.; Davies, C.E.; Edwards, F.K.; Furse, M.T.; Maitland, P.S.; Raper, C.; Siriwardena, G.M.; Winfield, I.J. (2018). UK Checklist of freshwater species. NERC Environmental Information Data Centre. https://doi.org/10.5285/57653719-434b-4b11-9f0d-3bd76054d8bd
Feature headline image: Martin Krzywinski/EMBO (https://www.wired.com/2013/11/wired-data-life-martin-krzywinski/).