Invasive non-native species week: why it make sense to "Check, Clean and Dry"

To mark Invasive Species Week, I am delighted to welcome my colleague, Paul Walton (Head of Habitats and Species in Scotland) to offer his perspective on the impact of non-native invasive species on native wildlife and what we need to do about it.

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There has been a lot of discussion lately on Invasive Non-native Species (INNS) and their impacts. Whereas their impacts on wildlife globally look to be a huge issue for biodiversity, some commentators have framed the introduction of non-native species as not entirely negative, or, indeed, a positive environmental benefit (for example, see here). The most recent broad scientific analysis of wildlife trends overall, however, identifies the impact of INNS as the second most important driver of global extinctions of plants, amphibians, reptiles, birds and mammals since 1500 - and the most common current threat associated with vertebrate extinctions (see here). The Convention on Biological Diversity now considers the combined effect of climate change and invasive species to be the main driver of biodiversity loss across the planet (see here).

One thing that this debate highlights is that INNS impacts vary enormously between different groups of organisms, and between different environments. For example, much of the discourse claiming that negative impacts of INNS has been overplayed focuses on terrestrial plants in temperate environments.

It is undeniable that, out of the large number of non-native plant species established in the wild in Britain – the exact proportion of plant species is unclear, but it may be well over 30% - only a minority have developed serious invasive impacts on native wildlife.

Now, there is nearly always a lag period between a non-native species being released and establishing in the wild, and invasive impacts becoming evident. With plants, this lag can be extended – sometimes more than a century. So, it's possible that there are potentially invasive plant species out there that have not yet become problematic - but which will in future.

Given that we cannot yet accurately predict which non-native species will become invasive, there is I suspect little room for complacency: the plant species that are invasive include some pretty significant problems. The hybrid Rhododendron x superponticum is responsible for driving a high proportion of our designated Atlantic woodland sites – the Celtic rainforest, so crucial for lichens and bryophytes - into unfavourable condition - something the RSPB and our State of Nature partners Plantlife are beginning to consider in terms of conservation action. It is, none the less, true that many non-native plant species live in terrestrial UK environments with no detectable negative impacts – and many are, of course, beautiful plants in their own right.

It should be remembered, though, that other groups are very different. Among aquatic plants, for example, a much higher proportion of species have become invasive once released into the wild, some having habitat-level impacts that affect whole arrays of species. And for some groups like crayfish, it seems that virtually all species, once moved by people, released into the wild and established, have negative impacts on native species and habitats – sometimes profound ones.

Moreover, there are major differences in the sensitivity of different environments to INNS impacts. It is well known that islands are a case in point. Island endemic species – including that very emblem of extinction, the dodo – have been lost in droves through the introduction by people of non-native vertebrates to islands: rats, cats, snakes, goats and so on. Continental land masses are certainly not immune to INNS impacts, but extinctions to date have weighed strongly towards island species.

Freshwater bodies constitute another environment that is particularly prone to INNS impacts. Those aquatic plants and crayfish can be seriously problematic, as are, in a high proportion of cases, non-native freshwater fish released into new catchments. Aquarium, pond and fish-tank owners should all be aware of this problem: releasing freshwater animals and plants, or allowing them to escape, can have negative consequences for freshwater wildlife.

Recent trends in Europe, however, give added urgency to this issue. Now, everyone who frequents freshwater habitats – anglers, boaters, canoeists, and, yes, birdwatchers in wellies - needs to be aware that they can unwittingly act as vehicles for invasive organisms, moving them into new catchments and helping the spread of INNS.

The reason for the new sense of imperative is a whole suite of freshwater species that is extending across Europe, their spread being assisted by freshwater users moving them between catchments. They include water fleas, mud shrimps, freshwater mussels, snails and fish, and they have one thing in common: they all originate from the same distinct part of the world – the Ponto-Caspian region. This region – comprising the basins of the Black, Caspian and Azov seas - is proving to be a ‘donor hot spot’ for invasive freshwater INNS, with over 100 species known to be spreading out of their native range. The key factor is that these species have, of course, lived together in that region for millennia and have, over that period, have evolved strategies that allow them to coexist, indeed to flourish, when they live together. And in that flourishing, native freshwater life is the loser.

Dikerogammarus or killer shrimp are invasive non-native species that have spread from the Ponto-Caspian Region of Eastern Europe (image courtesy of the Environment Agency)

Now, human activities moving boats and other equipment between freshwater catchments are allowing these species to move across Europe en masse – 23 Ponto-Caspian freshwater species are now well established in the Rhine estuary and Dutch ports. Four of these have recently been found establishing in the UK, so we know that people can unwittingly carry the eggs and larvae of these species across the Channel. Scientific analyses indicate that the risk of invasion by multiple species is high, especially in SE England.

There has been major progress of late, in terms of legislation on INNS – a new EU Regulation aiming to tackle the issue is a significant step forwards, though as ever effective implementation will be the key test. As part of that, EU Member States can develop their own lists of species as priorities for prevention or remedial action, which makes good sense when resources are limited – progress in Scotland in this regard looks positive.

However, the issue is about more than the law. The single best predictor of whether a new species, introduced by people, will establish in the wild is what is termed ‘propagule pressure’. This means, simply, the rate at which eggs, seeds, plant fragments, larvae or adult organisms are actually released. By reducing the number of INNS that are released, slowing down their rate of arrival, the chances of establishment can be reduced and future problems avoided. With the Ponto-Caspian freshwater invasives, this means that by taking simple precautions – cleaning gear, hulls and clothing – when moving between catchments, we can very effectively protect our freshwater wildlife: and with INNS prevention is always far better, and cheaper, than cure.

This is Invasive Species Week. It’s a very timely chance to highlight the importance of simple biosecurity measures like these, and the GB Non-native Species Secretariat is promoting the Check Clean Dry campaign in order to do so. I encourage everyone to visit the website (here), heed the advice, share it, and play a part in protecting the homes of freshwater wildlife.