Ecological Impacts

The ecological impact of invasive non-native species (INNS) is a major concern which, in past decades, has been over looked, particularly in London. With the projected increase of invasive non-native species and their ability to travel further and faster than ever, we need to be aware of their ecological implications.

Ecosystems are complex and their survival relies on numerous intricate interactions. Understanding the effect that invasive non-native species have on them can be difficult. Generally, a combination of many factors can result in an overall decline in ecosystem health and resilience. Below, we summarise some of the negative effects:

1. Competition for resources

Competition for resources by invasive non-native species is a major factor in the decline of native species. This is because INNS can be more aggressive, have a wider habitat tolerance, high breeding success and fewer natural limiting factors. They survive and breed rapidly. INNS are successful when in direct competition with native species that may have narrower habitat tolerances and a range of diseases and pests that keep them in check.

Competition for resources is not a simple interaction and usually there are additional factors pushing the decline. We see this complicated interaction between the native white-clawed crayfish (Austropotamobius pallipes) and the invasive signal crayfish (Pacifastacus leniusculus). A primary driver behind the decline of the white-clawed crayfish is competition with the introduced signal crayfish for refuges. The population decline is also due to the presence of crayfish plague which is spread by the signal crayfish.

2. Create pressure of native species through direct predation

Direct predation has a dramatic effect on population numbers of native species. This is best seen with species such as the American mink (Mustela vison) which is a highly adaptable, widespread generalist carnivore which predates on water voles (Arvicola terrestris). Water vole populations have undergone a dramatic decline, though other habitat factors and disturbances also play a part in this decline.

3. Introduction and spread of diseases to native species

Humans are not the only animals that allow species to travel throughout the world. Indeed when a new invasive non-native species is introduced it brings along a collection of unique diseases and viruses. One example of this is the squirrel pox virus which was introduced into the UK with the grey squirrel (Sciurus carolinensis). The red squirrel (Sciurus vulgaris) is susceptible to the virus and populations have declined as a result of infection, while the unaffected grey squirrel continues to spread the virus. Another example is the crayfish plague which is spread by the signal crayfish (Pacifastacus leniusculus), and to which the native white clawed crayfish (Austropotamobius pallipes) is highly susceptible.

4. Modification of habitat which is unsuitable for native species

This heading covers two similar concepts but both are subtly different.

Firstly, our general environment has been modified by human activities to the point that it is now more suitable for invasive non-native species. This is mostly due to the high level of disturbance which occurs in London’s habitats and the high number of existing non-native species. More on this can be found within the ‘What and Where’ section, which highlights why urban areas are more vulnerable to invasive non-native species.

Secondly, some invasive non-native species can modify their habitat and prevent native species from using it. Japanese knotweed (Fallopia japonica) is able to produce a monoculture (an area of one singular species) unsuitable for a range of native species. This habitat modification allows the plant to re-shoot in spring as there is little competition in the immediate area for water, nutrients or light.

5. Hybridisation

Some invasive non-native species are able to hybridise with native species. A good example includes the hybridisation of the ruddy duck with the white-headed duck. There is concern that if allowed to continue the native white-headed duck will be threatened. This type of interaction is often hard to manage and define, therefore there is greater concern for the loss of integrity of individual species traits and features.


Many of these factors combine to decrease general ecosystem health, including reduced species diversity and lower resilience to disturbance and other impacts such as habitat fragmentation and climate change.

We need to manage these species and mitigate their effects on a catchment-wide holistic scale rather than looking at any each issue individually.

We are at a point where doing nothing will result in decline in the health of our economy, our health and our environment. LISI has worked to help prioritise species for action, the species of concern list for London can be found on our species of concern page and other information can be found throughout this website.