Transport and Trade of Invasive Alien Species!

Humans have managed to spread to nearly every corner of the globe, but how have they effected the distribution of Invasive Alien Species? 

As the world becomes more accessible for humans, it also does for other organisms. From species hitching a ride, disturbance of environments and the exchange of non-native species between countries, increased trade and transport has led to more invasions.  

At what point did it start to change? 

Philip E Hulme defines 3 key points in time that have caused a notable change to the spread of Invasive Alien Species: the end of the Middle Ages, the start of the Industrial Revolution and the Era of Globalisation. The Era of Globalisation has been marked with both increased income growth and  transport efficiency with both of these factors having an impact on the spread of non-native species. 

Graph showing the relationship between Alien Plant Richness
and Gross Domestic Product 

Increase income growth

Hulme explores the link between Invasive Alien Species richness and the Gross Domestic Product (GDP) of both continents and islands as displayed by the graph to the left. As GDP increases, so does amount of non-native species. The relationship is also stronger for islands, probably because of increased quantities of merchant imports which transport greater amounts of non-native species. 

Increase transport efficiency

The Industrial Revolution and advances in technology have seen a massive increase in the amount of travel that occurs worldwide. Increases in both personal transportation and global trade has had a knock on affect on invasive plants, animals and microorganisms.
Since the 1970's there has been a four fold rise in the volume of global imports that occur and 46% of this involves dry cargo. The dry cargo, mainly transported by containers, pose threat with Invasive Alien Species hitching a ride both inside and outside the containers and ships that transport them.

Also, between 1984 and 2000, 725,000 pests were recorded entering into US ports and 73% of this was via airports rather than seaports.

Disturbance due to transport!

Increasing demand for transport leads to the creation of more roads, railways and a rising use of waterways. As a result habitats get destroyed to make way for this infrastructure and the surrounding areas are continually disturbed and it allows organisms to travel to areas they may have not previously have been able to reach. Hansen and Clevenger compared the number of non-native species occurring in Canada along some railway lines and highways that were next to either grassland or forest. From this study they found:

• Grasslands had the higher frequencies of non-native species and there were more non-native compared to native species for up to 150m from the corridor edge 
• Forested areas had the lower frequencies and more non-native than native species for only up to 10m from the road or railway edge 
• The frequency of Invasive Alien Species decreased with distance from the corridor edge 
• The frequencies of non-native species along highways and railways was very similar 

These results therefore suggest that the transport corridors are having an impact on the distribution of Invasive Alien Species. Grasslands have shown to be more prone to disturbance than forested areas and non-native species are more able to successfully invade and stay there - which coincides with the idea I explored with Himalayan Basalm in my previous post! 

Graph showing the invasion success of
Wild-Caughtand Captive-Bred bird species 

Global exchange of exotic organisms! 

Interestingly, humans bring a lot of non-native species into other countries on purpose through global trade of plants and pets.  For example, exotic animals that are kept as pets and then escape and non-native plants that are introduced into garden centres, have more of an effect than you might expect. 

Carrete and Tella found that it is not the common caged birds that cause issues if they escape but the wild, exotic birds that are brought to countries through the pet market. The graph to the right shows how successful captive bred and wild-caught pet species are in Spain, where 0 = never seen in the wild and 3 =  breeding regularly in the wild. As it shows, captive bred species do not successfully manage to breed and get to level 3 and are therefore not a threat to the environment they enter. 

An example of a wild caught species that has successfully managed to invade places such as the US, Italy, Spain and Czech Republic is the Monk Parakeet, a bird imported from Argentina as shown to the left!

The ideas explored here are all raising questions when it comes to conservation. It is expected that the amount of transportation and disturbance to the environment is only going to increase in the coming years, so how do we make sure that the numbers of native species do not drop too low?

Click the link to find out more http://www.actionbioscience.org/biodiversity/simberloff.html