Tackling invasive species

What are invasive species?

Invasive species are plants or animals that invade habitats where they are not naturally found, competing with native species and often thriving to the point of dominance. These species can be either native or non-native to the ecosystem, as long as they exhibit invasive traits.

However, invasive species are most commonly associated with non-native species introduced into new environments. These species, known as Invasive Non-Native Species (INNS), include examples like the Grey Squirrel and Himalayan Balsam. Native to other parts of the world, these species have been introduced to new ecosystems through human interference, disrupting the natural balance.

Native species often lack the ability to compete with these non-native invaders, leading to their decline. For instance, the native Red Squirrel is threatened by the non-native Grey Squirrel.

Himalayan Balsam in Langdale Valley 

Managing invasive species, especially invasive non-native species (INNS), is crucial as they directly threaten biodiversity. A good case study in the Langdale Valley is the Himalayan Balsam, a familiar plant known for its purple/pink flowers resembling a police helmet, and its ability to grow up to 2 meters tall. It typically thrives on wet, disturbed ground, especially riverbanks.

As an annual plant, Himalayan Balsam grows and dies within one season. Its survival strategy involves growing rapidly and producing as many seeds as possible. When its seed heads are ready, they 'pop' and disperse thousands of seeds, outcompeting surrounding plants by choking them of light and creating a monoculture. Studies have shown that Himalayan Balsam decreases the species diversity of ground insects.

In Langdale, there is a wet woodland designated as a Site of Special Scientific Interest (SSSI) due to its population of the rare native Touch-me-not Balsam. This plant is the food source for the rare Netted Carpet Moth and is directly threatened by its invasive cousin. Many ranger hours are dedicated each year to removing Himalayan Balsam from this SSSI woodland. 

Why do we manage bracken?

Bracken is an example of a native invasive species. Originally a typical woodland plant, it has become invasive in other habitats due to excessive human disturbance, such as woodland clearance. Bracken is now commonly seen on fell sides, forming extensive beds that cover vast areas. However, this is not its natural state; in theory, woodlands would have stood where bracken is now found.

In its natural woodland habitat, bracken is relatively sparse and grows alongside other typical woodland floor plants. It thrives in canopy gaps and is light-loving. The extensive clearance of woodlands has created new habitats for bracken, allowing it to outcompete other plants due to its rapid growth and extensive root system. Additionally, bracken is toxic to grazing animals, so it is often left unchecked. This is why we see such extensive bracken on our fells.