This blog has been written by Joshua Clark, a volunteer for the Unlocking the Severn project and Severn Rivers Trust. This is a comprehensive study of invasive species, for those of you who are interested in gaining a deeper understanding of the problems they cause.

Invasive Species: What Are They? 

Invasive species, or Invasive Alien Species (IAS), are species introduced by non-natural processes (such as industrial shipping or active introductions by humans) to a new location, that then successfully establish a population and cause detrimental impacts on the local ecosystem. Of the 3224 non-native species (NNS) introduced to the UK, 193 are currently considered IAS by the Joint Nature Conservation Committee (JNCC) [1].  

The initial removal of a species from its native range. With the advent of large scale, global transport by humans, the destination could be almost anywhere.   Collection for the ornamental trade, accidental transport from anything from ship ballast waters to seeds stuck to clothing, deliberate transport for agriculture, etcetera. 
The release or escape of the non-native species into the wild outside of its native range.   Agricultural land repurposing, inability to keep in captivity by ornamental owners, natural life cycle spread of plants, loss of commercial incentive to keep, well-meaning yet ill-advised deliberate releases.  
Escapees integrate into or exploit resources of the novel ecosystem well enough to grow a self-sufficient population that survives without intervention.  Ability to make use of a niche that is not exploited, ability to either successfully apply existing strategies previously used in the native range or rapidly learn new ones. Can increase competition for resources, yet may not yet have outcompeted, or fills a functional niche that has been missing by the absence of another species. 

(Sidenote: Re-introductions of species normally emulate this stage but are kept in check by an ecosystem that evolved alongside the re-introduced native, or by careful management where such natural measures are also absent – an important distinction compared to the non-natives that establish). 

The introduced species has detrimental impacts on the native ecosystem, to the point of causing the local decline of native species, a degraded physical environment that destabilises the landscape.   Application of strategies and adaptations that native organisms are ill-equipped to compete or cope with, such as an alternative hunting strategy, introducing a pathogen they carry that the natives are vulnerable to, or simply a lack of predation that enables exponential population growth.

The invasive may have better capacity to cope with the degraded conditions of a polluted or otherwise disturbed ecosystem. Sometimes the degradation of an ecosystem caused by the species creates more favourable conditions for the invader, at least in the short term.   

Either migration and widening of range through exploration and continued exploitation of resources, which is especially viable in degraded and interconnected habitats, or a secondary displacement by humans.  Overuse of local resources eventually drives the invasives to spread and seek out more bountiful resources. Alternatively, attempts to remove the species which are not thorough enough may give cause for roaming and finding new locations to establish.

Secondary displacement by humans can be caused by any of the same as initial displacement, but also from normally accepted processes such as water abstraction, connecting otherwise isolated regions and aiding the spread.  


Table 1 Some generalised stages of non-native species introduction. The discredited Ten’s Rule stated that 10% of species transported would escape or be introduced into the wild, 10% of those would establish viable populations and 10% of those would cause damage enough to be considered invasive. However the reality is much less tidy, varying based on the stability of the ecosystem, the species involved and many more factors. More recent hypotheses include a 50% and 25% invasion rule for vertebrate and non-vertebrate taxa, respectively, based on more empirical evidence [2]. However, even these are more guidelines than actual rules.  


The progression of an introduced NNS to invasive status is complex but has been summarized in Table 1. Unless a NNS can exploit the novel ecosystem, it will quickly be outcompeted by the native species that have evolved in that environment. However, as ecosystems become more disturbed and introductions become more frequent, the likelihood of a species reaching a stage in a novel ecosystem where it causes disproportionately negative impacts increases [3].   

A slight misnomer?

It should be clarified that the species itself should not be considered ‘bad’ by default – it is purely the damage they cause because of human driven introductions that is a problem. In their native range, there is nothing ‘alien’ about them.  

A complex and ongoing problem

The damage caused by an IAS also varies. Examples include but are not limited to habitat degradation (such as riverbank erosion caused by Himalayan balsam), depopulation of native species through hybridisation (such as the Scottish wildcat with feral cats), or direct competition (such as rhododendron shading out and out-enticing pollinators from native species). 

In 2010, the Centre for Agriculture and Bioscience International (CABI) estimated that direct and indirect costs of IAS at £1.7 billion in the UK in 2010 alone, noting that costs are likely much higher considering the lack of full data and the underfunding of management [4]. These costs are projected to increase, with the global rate of introduction of IAS continuing to rise despite efforts to counter the issue, with the costs of removal becoming exponentially greater the more established a population becomes [3].  

Freshwater System Vulnerability 

Freshwater systems in the UK are especially vulnerable to IAS pressure. First, rivers serve as natural wildlife and dispersion corridors. IAS that thrive in or adjacent to freshwater systems have rapid access to a wide area of resources, providing a higher chance of the population encountering ideal conditions to establish. Seeds can travel far, and populations can be replenished from upstream reproduction, making eradication efforts that much more difficult.

Next, with much of our rivers under constant pressure from factors including sewage releases and agricultural run-off, many of our native freshwater ecosystems and species are in decline or vulnerable. This reduces competition for IAS during their acclimatisation and establishment phase should they be able to tolerate those conditions.

Human abstraction and release of freshwater, such as the use of inter-basin water transfers to resupply depleted water stores, also allows species to rapidly translocate between isolated waterbodies while also altering conditions outside of favourable natural parameters for natives – a factor likely to be exacerbated by the climate change crisis [5].  

Policy & Legislation Rundown 

In the UK, the major legislation pertaining to invasive species is Section 14 of the Wildlife and Countryside Act 1981 (as amended). The Act is effectively a compilation of legislation pertaining to human interaction with wildlife in the UK, with associated penalties for violating certain clauses. 

Section 14 specifically provides clauses ruling that spread of a non-native species into the wild in the UK is an offence – the Schedule 9 List categorizes species of particular concern. There are notable exclusions, such as the industry-driven exception of the mass releases of common pheasant and red-legged partridge for the purpose of shooting. It also includes clauses prohibiting their sale or transport at any life stage, the powers to specify specific non-native species as illegal to possess and the groundwork for governmental institutions to provide code of practice pertaining to individual species. Organisations such as NGOs and ecological consultancies must apply for licences for the permission to handle, eradicate and dispose of species on the list.  

More recently, the Invasive Alien Species (Enforcement and Permitting Order) 2019 was passed to take over from the previous list of species of European Concern (some 66 species of international concern requiring likewise international methods to control).  

The last major update to the Schedule 9 list was the adoption of the new indicator using data from the GB Non-native Species Information Portal (itself prioritising expert opinion over purely quantitative data, recognising a paucity of data available for non-native species), raising the list from 49 species to 179.  Since 2017, the list has been updated annually based on expert input received and considered by the JNCC. Methods used to determine IAS spread include use of the National Biodiversity Atlas for creating maps of sightings and hence current potential range [1].  

Adjacent legislation can further compound IAS pressures. In recent weeks, the JNCC consultation on the quinquennial review of the Schedule 5 and 8 lists (which protect against the killing, disturbance and sale of certain animals and the picking and sale of certain plants respectively) reveals plans to strip protections from any species that does not hold the IUCN Red List status of “Critically Endangered” or “Endangered” [6]. Aside from the notion that a species does not deserve protection unless it has already reached a point teetering on the edge of no return, this also ignores the potential damage cause by the release of captive wild animals with invasive pathogens, such as the Bsal fungus that has already been shown elsewhere to cause huge declines in amphibian populations, from the wildlife trade [7].  

IAS Introduction Drivers – Agricultural Escapees: Invasive Crayfish Species

Figure 1 Ventral photos of the native, white-clawed crayfish and the invasive signal crayfish. Left image and left specimen in the right image displays the physical characteristics of the white-clawed crayfish, with pale white to brown colours along the underside, and chelipeds (commonly referred to as claws) that are quite small and narrow relative to body size compared to the large, bright red chelipeds of the signal crayfish visible on the specimen on the right of the image on the right. Photo credit: Rachel Davies of Severn River Trust.

There are currently seven recognised species of crayfish present in the UK, of which only the white-clawed crayfish is native. The primary driver of the white-claw’s decline has been a plague that the IAS American signal crayfish carries which the white-claw has not evolved an immune response for. To find out more about the species and risks they pose, please read the Severn River Trust article by Jodie Richford [8].

Most of the introduced species of crayfish in the UK escaped from aquaculture farms after an initiative in the 1970s to create an industry to export farmed crayfish to European countries, with the signal crayfish being the most common. The industry was not as successful as hoped, leading to a collapse. With little incentive to keep them, preventing their escape became a low priority, and with their ability to roam overland from their ponds into natural watercourses and combination of adaptable traits, the species quickly spread. Note that this lack of prevention of escape into the wild pre-dated the Wildlife and Countryside Act of 1981, so such early releases were unpunishable. Prohibition of keeping crayfish did not occur until 1996, and, by then, signal crayfish had already caused widespread decimation [9].

IAS Impact – Indiscriminate Invertebrates: Demon (Dikerogammarus haemobaphes) and Killer (Dikerogammarus villosus) Shrimp

Both demon and killer shrimp are present in UK rivers (first recorded in 2012 and 2010 respectively), with demon shrimp more widespread (figure 3). While the killer shrimp is better studied and recognised from invasions across Europe, both species have been shown to have convergent impacts on ecosystems they are introduced to, resulting as part of their traits that make them successful invaders.

Both exhibit intraguild predation (IGP) behaviour, where they kill potential competitors from other species regardless of whether they will subsequently eat them. Although with their omnivore diet they will eat and attack indiscriminately, placing additional pressure on juvenile or larval stages of native fish and invertebrates [10]. Killer shrimp have also been shown to actively eject native amphipods from shelter rapidly upon introduction, forcing them into the water column and increasing their vulnerability to predation [11]. These strategies trend towards homogenization of species present, with the cannibalistic capacity acting as a buffer from classic predator-prey population dynamics.

One aspect of interest is that these IAS are less efficient at leaf-shredding than the native species they typically devastate, while also reducing the efficiency of shredder survivors as they prioritise avoidance. This reduction of litter processing changes the benthic habitat and nutrient availability for the detritovore trophic system [12].

Figure 2 National Biodiversity Atlas (NBA) maps of recorded demon shrimp (left) and killer shrimp (right) distribution, retrieved on 14/07/2021. Note how killer shrimp populations are restricted to fewer, discrete sites whereas demon shrimp have managed to invade along watercourses around England.  

IAS Management – Natural Control: Himalayan Balsam (Impatiens glandulifera)

Himalayan Balsam was first introduced to the UK in the 19th century as part of the ornamental trade, propagating across the country over time. The impacts of Himalayan balsam are extensive: they have explosive seed pods and dense, rapid growth that shades out shorter native plants, creating monocultures. When they die back in autumn they leave soils exposed as hydrological pressures increase. Please read the ‘Focus On…Himalayan Balsam’ article by Jodie Richford for more information.

Figure 3 Photo of the mature stage of Himalayan balsam. The flowers are highly attractive to pollinators, often to the exclusion of native species. Stock photo credit to Shutterstock.

One category of management is the nature-based solution (NBS) ‘natural control’, whereby a species that would normally control an invasive species in its native range is introduced into the invaded ecosystem. Himalayan Balsam in the UK is being infected with Himalayan balsam rust fungus in controlled releases. The fungus co-evolved with the plant, with each of its five life stages all affecting Himalayan balsam. In short, the fungus feeds on the living stem tissue, warping and stunting the plant, and reduces the amount of leaf area capable of photosynthesis as the spores develop in the leaves [13]. Releases in the UK began in 2015, with releases at 47 sites as of January 2020.

Since the project began, at least three biotypes of Himalayan balsam have been identified in the UK, indicating that the plant has been introduced at least three times from different locales within its native range. As biotypes of the plant are resistant to strains that do not originate from the same area, for each biotype a matching strain has had to be molecularly identified from the native range. This method has increased successful rust infection rates and the natural control is showing promise for the future control of Himalayan Balsam [14]. Labour-intense management of the plant continue for now however and will likely still be required should a full eradication be attempted.

Figure 4 Photo of Himalayan balsam cut back as part of manual conservation efforts. The plants are left in situ to dry out and die before being disposed of, to minimise the chance of spreading during disposal. Photo credit: Rachel Davies of Severn Rivers Trust.



1] Harrower, C.A., Rorke, S.L., Roy, H.E. 2020. JNCC. UK Biodiversity Indicators 2020 B6 Pressure from invasive species technical background document. [Online]. Available from:

2] CAB International. 2018. Eds Jeschke, M.J. and Pyšek, P. . Invasion Biology: Hypotheses and Evidence. 124-131.

3] Seebens, H., Blackburn, T., Dyer, E. et al. 2017. No saturation in the accumulation of alien species worldwide. Nat Communications 8, 14435

4] CAB International. Williams F. et al. The Economic Cost of Invasive Non-Native Species on Great Britain. Available from:,93,196

5] Gallardo, B. and Aldridge, D.C. 2018. Inter-basin water transfers and the expansion of aquatic invasive species. Water Research 143 (1), pp. 282-291.

6] 2021. Quinquennial Review 7. [Online]. Available from: Accessed on 01/07/2021.

7] Cumbria Amphibian and Reptile Group. 2020. Biosecurity web page. [Online]. Available from:

8] Severn Rivers Trust. Rochford, J. 2021. All about crayfish. [Online]. Available from:

9] The Fish Site. 2008. Non-native crayfish in the UK. [Online]. Available from:

10] Bacela-Spychalska, K. and Van Der Velde, G. 2012. There is more than one ‘killer shrimp’: trophic positions and predatory abilities of invasive amphipods of Ponto-Caspian origin. Freshwater Biology. 58 (1) pp. 730-741.

11] de Gelder, S. et al. 2016. Competition for shelter sites: Testing a possible mechanism for gammarid species displacements. Basic and Applied Ecology. 17 (5) pp. 455-462

12] MacNeil, C. et al. 2010. Direct and indirect effects of species displacements: an invading freshwater amphipod can disrupt leaf-litter processing and shredder efficiency. Journal of the North American Benthological Society. 30 (1) pp. 38-48

13] CAB International. 2018. The proposed solution. [Online]. Available from:

14] CAB International. 2020. Progress with Weed Biocontrol Projects. [Online]. Available from:



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