Spread of invasive species

The spread of invasive species refers to any species that has been introduced to an environment where it is non-native, and that has since become a nuisance through rapid growth and expansion in that area. These species can alter the morphology of a water body and out-compete natural species, causing significant habitat degradation. In the river environment, many invasive species can cause significant geomorphological alterations (e.g. signal crayfish [Pacifastacus leniusculus] burrowing into banks and undermining them), reduced light penetration (e.g. the development of thick mats of floating pennywort [Hydrcotyle ranunculoides]) and die back of native grass species (due to overshading by Himalayan balsam [Impatiens glandulifera]). In coastal and estuarine environments, invasive plant species can cause over-growing and shading of underlying species (e.g. Sargassum muticum can reduce light, increase sedimentation and reduce ambient nutrient concentrations for native kelp species and eelgrass [Zostera sp.] beds). In both environments, the introduction of invasive animal species can lead to an increase in predation or over-grazing which can result in the exlusion of native faunal species.

Floating pennywort (Hydrocotyle ranunculiodes) Source: GBNNSS

Himalayan Balsam (Impatiens glandulifera) Source: CCW

Removal of in-channel structures 

The removal or modification of the physical barrier created by an in-channel structure can allow the spread of invasive species into an area where this was not previously possible by providing an uninterrupted transport pathway (e.g. for adults, seeds, eggs or larvae). Careful assessment of the risk of spreading invasive species when removing structures is required. A catchment scale approach to managing invasive species should be promoted.