Why are global crayfish populations declining?

Image: Flickr | coniferconifer | Creative Commons

A third of global freshwater crayfish populations are threatened with extinction, according to a newly published report.  A large team of researchers from the UK, Ireland, USA, Mexico, Australia and Austria, led by Nadia Richman at the Zoological Society of London, evaluated the extinction risk of the world’s 590 freshwater crayfish species based on the IUCN Red List categories.

32% of global crayfish species were classified by the team as ‘at risk of extinction’, a figure far higher than for most marine and land-dwelling animals and plants.  This high extinction risk is unlikely to be helped by the fact that only a small proportion of global crayfish populations are covered by existing protected areas for conservation.

Bringing together scattered information on crayfish populations

Distribution of global crayfish populations. Image: Richman et al (2015) Multiple drivers of decline in the global status of freshwater crayfish (Decapoda: Astacidea). Phil. Trans. R. Soc. B

The team undertook the huge task of collecting species-specific data on taxonomy, distribution, population trends, ecology, biology, threats and conservation measures for all 590 global species, using published and unpublished articles, government reports and personal communications up to 2009.

Interestingly, whilst crayfish were found in 60 countries across the world, 98% of species are endemic (i.e. found only in one place) to a single country.  Four described species are now extinct, and 21% of species are ‘data deficient‘, because their populations haven’t been studied by scientists enough to assess their conservation status.

Stressors and threats to crayfish populations

Distribution of threatened crayfish populations in USA, Mexico and Australia. Image: Richman et al (2015) Multiple drivers of decline in the global status of freshwater crayfish (Decapoda: Astacidea). Phil. Trans. R. Soc. B

The team used the ‘standard lexicon’ of biodiversity threats proposed by Nick Salafsky and colleagues in 2008 in Conservation Biology to categorise the threats to global crayfish populations.

Whilst the paper doesn’t go into a great deal of detail about the specific threats, it suggests that in the USA and Mexico, crayfish were largely threatened by the development of urban areas (e.g. modification of watercourses, reductions in natural habitat, increased water temperatures), dam construction (e.g. changing water flows and habitat fragmentation) and water pollution.

On the other hand, in Australia, species were predominantly threatened by the negative impacts of agriculture and logging (e.g. habitat alterations), climate change (e.g. changes to rainfall and temperature) and invasive species.

The impact of invasive species

The impact of invasive species was a major factor in many declining crayfish populations.  In Europe, population declines of between 50% and 80% have been observed in the white-clawed crayfish (Austropotamobius pallipes) and 50% and 70% in the noble crayfish (Astacus Astacus).  These declines were most acute at the northern end of the species’ geographical distributions, where rising temperatures have allowed the larger American signal crayfish (Pacifastacus leniusculus) to move in and outcompete the native species for habitat and food, with a number of negative effects on the wider ecosystem.

The signal crayfish has also brings the infectious and deadly water mould known as ‘crayfish plague’ (Aphanomyces astaci), to which it is immune, but European species are not. In Australia, young crayfish are eaten by invasive predators such as cane toads and feral pigs, species which also damage the crayfishes’ riparian habitat.

Conserving crayfish?

The authors of this study argue that their findings are another indication that not only are freshwater ecosystems under numerous threats, they are also inadequately protected by current conservation schemes.  Richman and colleagues suggest that since there are limited resources available for conservation schemes, it is necessary to prioritise areas for protection – a key motivation for the collation of information on crayfish populations in this study.

A key challenge for conservation is that climate change may mean that crayfish populations need to shift their geographical ‘ranges’ to avoid warming temperatures, requiring a network of potential habitats (rivers, streams and lakes) with connectivity – i.e. those that crayfish can move between – in order to maintain their ecological resilience.  The study suggests that two-thirds of Australian crayfish populations are at risk from climate-related threats, with poor connectivity between new, potential habitats.

Barred Owl (Strix varia) with crayfish at Corkscrew Swamp Sanctuary, Naples, Florida. Image: Matthew Paulson | Flickr | Creative Commons

At present, only a small proportion of crayfish populations are covered by existing protected areas.  But how can this situation be improved?  The authors suggest that freshwater biodiversity is all too often underrepresented in conservation planning schemes because we struggle to put an economic value on it, and conservation funds are more often channelled towards ‘charismatic‘ species with a recognised value.

This ‘if we can better value nature, then we can better protect it‘ argument has become more prevalent in conservation rhetoric in the last decade or so.  But what new economic values of crayfish populations would persuade policy makers to put better protected areas in place for their conservation?  Crayfish are important food sources for larger fish and bird predators, and a central part to cuisine and culture in southern USA states like Missouri and Mississippi.

But their status as rarely seen parts of large and complex freshwater food webs is likely to make any specific economic valuations of their populations tricky.  It’s perhaps helpful to remember that these are beautiful, curious and ecologically important creatures with an intrinsic value in themselves.  But how to recognise these values in conservation planning?

In essence, perhaps this is another example of a key issue facing freshwater conservationists: how do we persuade policy makers that our complex, biodiverse and increasingly threatened rivers and lakes are worth conserving and protecting?

Link to paper in Philosophical Transactions of the Royal Society B