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

Legal Aliens (Part 2)

Organisms are often restricted to their native location due to physical barriers, therefore humans are largely responsible for the spread of invasives around the world. However, despite putting the blame on humans in one of my earlier blog posts, it is important to realise that naturally occurring global change events can also facilitate the introduction of invasive species. If barriers to invasion are removed as a consequence of natural global and environmental change, I wonder if it could be argued that these species are ‘legal’ aliens? Hence the Sting inspired title of this blog post. Let's take a look at a specific example...

The ocean dominates the globe and accounts for approximately 70% of the total surface area, creating a huge barrier for organisms in their quest to invade new locations. To argue that the ocean should be fully removed to enable species to spread naturally is as ridiculous as it sounds! However, the Earth is a dynamic system and you may be surprised to hear that the movement of  tectonic plates can result in the removal of the ocean barrier at specific locations around the world. Such an event occurred approximately 3 million years ago, when movement of the Earth’s tectonic plates joined North America and South America by a thin strip of land, referred to as the Isthmus of Panama. 

Letter A points to the Isthmus of Panama (Google Maps)

The formation of this land bridge between the two continents is a historically and geographically important event, and lead to the Great American Interchange (GAI) (Wallace, 1876). Prior to the formation of the Isthmus of Panama, numerous invasive species had already become well established in North America. This could not be said for South America however, where few invasive species had been introduced, meaning that there was a high proportion of endemics present (Marshall, 1988).

During the GAI, evolutionary distinct aquatic and terrestrial organisms were able to expand their range, resulting in reciprocal invasions between North and South America. Marshall (1988) studied the movement of land mammals throughout that GAI and calculated that in total 38 genera walked across the Isthmus of Panama from South America to North America, and 47 genera walked from North America to South America. This exchange is thought to be in equilibrium when the size of the source fauna present on each continent before the GAI is taken into consideration (Marshall, 1988).

Species: Cuvieronius. Moved from North America to South America during the GAI
Photo Credit: Sergiodlarosa

A consequence of the GAI was that a large proportion of South American endemic species were exposed to high levels of competition and predation for the first time. It is interesting to note that since North American genera reached South America 3 million years ago, there has been further diversification, and today approximately half the genera present in South America have evolved from North American species. This did not occur in North America (Marshall, 1988).

This example shows that invasives can have a dramatic effect on the species composition of non-native ecosystems as a consequence of natural global change. Additionally this interchange could not have been prevented, therefore my personal opinion is that despite the impact that natural invaders have, if they are introduced as a consequence of past or future global change then they should be considered 'legal'.

Introduction

Hi my name is Elena and I’m currently studying for a Masters in Climate Change at UCL. As someone new to the blogging world, the understanding I have is that you should blog about something you’re passionate about. So here’s why I've chosen my topic..

If somebody asked me to imagine an idyllic landscape, there is no doubt that what would immediately spring to my mind is the typical British countryside. Perhaps this is because I have so far been unadventurous in my travels and aside from growing up in London, the only other place I have lived for a prolonged period of time is Yorkshire. However, wherever your dream destination, I am almost positive that the environmental conditions present would allow for a range of insects, animals, plants and trees to survive. Furthermore, depending on the location you choose, the organisms present will differ dramatically. In so many words I have just described the term global biodiversity (Convention on Biological Diversity).

You may be interested to hear that species usually thought to be typical of a given location are not always native. Personally I find it remarkable that organisms are able to survive and reproduce outside their natural range, establishing new populations in new locations. These are referred to as alien species (IUCN, 2000).

It is inevitable that you will have encountered multiple alien species throughout your lifetime, here are just some of the UK culprits…

Photos taken from: NNSS

So what’s the big deal?

As an undergraduate I studied Biology, so it is not news to me that biodiversity is declining at an alarming rate. Importantly, invasive alien species can have a hugely negative impact on biodiversity (IUCN, 2000). These organisms have the potential to dramatically change the environment that you know and love. 

I aim to uncover how these species arrive in new locations across the globe. In doing so I hope to gain a greater understanding of the role that humans have in facilitating this movement. I want to understand how detrimental invasive alien species can truly be to biodiversity. The plants and animals pictured above contribute to the ecology of Britain, could there be any advantages of invasive alien species in a time when global biodiversity is declining so dramatically?

"Invasive alien species" is quite a mouth full however, so throughout my blog I will refer to such species as 'Invasives'!