Why the bad press?

More often than not invasive species are portrayed in a negative light; this is fuelled by both the research of ecological conservationists and by personal experiences of recreational gardeners. I have also put a large emphasis on the need to remove invasive species and prevent the introduction of future invasives throughout my blog. Before I progress any further, I feel that it is necessary to present the basis for this school of thought by outlining some of the key ways invasive species can have a negative impact on the environment.

Predation

Prior to the introduction of an invasive species, a given ecosystem is likely to be at an equilibrium state with an established food web. Invasions have the potential to introduce a new predator to the ecosystem, which will consequently alter the population dynamics. In particular this could have extreme consequences if the invasive species preys on a keystone species. Predation may also result in the loss of endemic species. When the Brown Tree Snake was introduced to Guam in the late 1940s it had a devastating effect on the island’s bird populations. The snake was associated with the extinction of at least seven avian species from the island, some of which were endemic, and was also linked to a decline of many other bird species (Pimm,1987).

Competition

Water, nutrients, minerals and light are just a few of the resources that are often essential for life. What’s more is that these are often limited, and despite differences in the relative requirement of resources between species, there is often interspecific competition to acquire them. It may not come as a surprise to you that invasive species are often very strong competitors. For example the Himalayan Balsam (mentioned here), has the ability to outcompete a variety of plant species that may also be present along the canal edge, excluding other organisms and decreasing species richness (Hulme and Bremner, 2006)

Perhaps a less obvious way that invasive species can drive biodiversity decline is via competition for reproductive mates, leading to hybridisation (Mooney and Cleland, 2001). During the 1950s the Ruddy duck was introduced to the UK from North America. This species can sexually reproduce with the native White-headed duck population. Unfortunately, the reproductive behaviour of White-headed duck females indicates a sexual preference for male Ruddy ducks, resulting in an increased incidence of hybridisation and a decline in the native endangered population (Green and Hughes, 1996). Consequently wide scale culling programmes have been implemented across Europe to reduce Ruddy duck populations (NNSS).

Disease

As invasive species move around the globe, they may also act as disease vectors, transporting potential harmful pathogens to new geographic locations. If animals, plants and humans are not equipped with the appropriate immune response, then transported diseases may have severely detrimental effects. There is increasing research into whether the presence of certain invasive species can have a negative effect on human health as result of associated pathogens (e.g. Allan et al., 2010). 

I hope that these examples help to illustrate just some of the reasons why the Millennium Ecosystem Assessment highlighted invasive species as one of the major drivers of biodiversity decline. Furthermore, not only do invasive species have an ecological impact, but they also have an economic impact. Surprisingly it is estimated that as much as $137 billion per year is spent controlling and compensating for the effects of invasive species populations across the USA (Pimentel et al., 2000). 

I will leave you with this video, explaining that as a consequence of the negative effect on biodiversity and human health, the EU is currently proposing to implement a ban on the future intorduction of invasive species...

Until next time!

Elena