Invasive alien plant species such as Japanese knotweed and Himalayan balsam have a devastating impact. They interrupt natural succession and restrict biodiversity by out-competing native species.
Grows to over 2 metres (7 feet) in height. Roots push through joints in concrete, cavity walls, weaknesses in mortar, between paving slabs or bricks, and can damage drains and sewers.
Effective treatment requires spraying with strong herbicides and burning the weed and roots. This kills the plant but also kills all other plants and insects in the vicinity, with knock-on effects on pollinators such as honey bees.
The common red squirrel in the UK was virtually wiped out by the introduction of the more aggressive, disease-carrying American grey squirrel. The release was unplanned; grey squirrels were introduced from North America in 1876 as ornamental species for stately homes.
By 1930, damage was recognised and it became illegal to release grey squirrels. Grey squirrels live in higher densities and cause considerable damage to nesting birds and trees. In urban environments they raid bird feeders, bins, and damage roofs.
There is now a call for identification of brownfield sites that require conservation rather than development. Research is carried out by pressure groups and charities such as Buglife, the Wildlife Trust, and the RSPB.
Several sites have been protected. Canvey Wick, Essex was the first brownfield site designated as a Site of Special Scientific Importance (SSSI). It was a landfill and oil refinery site that closed in the 1980s and lay derelict; in 2005 it was designated as an SSSI and transformed into a nature reserve. The area has more species per square metre than any other site in the UK and has been called 'England's Brownfield Rainforest’.
Local communities are claiming areas of urban wasteland as valuable ecological assets for leisure and recreation, education, and urban farms.
Key principle: Conservation of brownfield sites protects biodiversity while redevelopment conserves greenfield sites. The tension between these goals requires careful policy and community engagement.
| Pathway | Outcome | Ecological impact |
|---|---|---|
| Redevelopment | New buildings, infrastructure | Ecosystem destroyed |
| Invasive species | Native species outcompeted | Succession interrupted |
| Conservation | SSSI, nature reserve | Biodiversity protected |
Regreening vs conservation: Regreening may convert wasteland to agriculture or amenity grassland; conservation preserves the existing wasteland ecosystem and its succession stages.
Plant vs animal invasives: Plant invasives (Japanese knotweed) disrupt succession directly; animal invasives (grey squirrel) impact fauna and tree health. Both reduce biodiversity.
Use located examples: Canvey Wick (93 hectares, Essex, first brownfield SSSI, landfill/oil refinery closed 1980s) demonstrates conservation success and high biodiversity.
Balance the trade-offs: Discuss both redevelopment benefits (greenfield conservation, urban sprawl reduction) and costs (ecosystem destruction). Examiners expect evaluation, not one-sided description.
Invasive species case studies: Japanese knotweed (physical damage, herbicide treatment) and grey squirrel (red squirrel decline, urban damage) are core examples.
Link to pressure groups: Buglife, Wildlife Trust, RSPB for conservation advocacy and research.
Issues and management link to Factors Affecting Development (how succession creates value that is then threatened) and Urban Wasteland Ecosystems (characteristics that make sites worthy of conservation).
The tension between brownfield redevelopment and conservation reflects broader debates about sustainable development and biodiversity offsetting.
