There is no doubt that introducing an exotic animal, plant or microorganism will have an impact on the ecosystem around it. But how detrimental is this effect?
Today I’m going to explore a particular invasion and why some scientists believe that a single alien intrusion cannot be the sole perpetrator for major loss and destruction!
An Example of Extreme Extinction:
Brown tree snakes and their invasion of the Island of Guam is one of the most well-known cases of Invasive Alien Species leading to multiple extinctions.
Borga irregularis, natively found along the coast of Australia and Papua New Guinea, was introduced to the island after WW2 when material from New Guinea was returned to the military base on Guam. From 1949 onwards, the invasion began.
But how bad was it really?
As explored by
Rodda and Savidge, the arrival and spread of this alien impacted multiple organisms, including humans. At one-point electrical power outages were experienced every other day due to interference with the electrical infrastructure. You can imagine the hatred felt towards these pesky creatures! Not to mention the fact there have been 10 reported infant deaths due to nighttime attacks of babies in their cots. That’s not all, out of 22 native bird species a whopping 13 went extinct and by 1990 there were only 3 native vertebrate species remaining, as concluded by
Rodda and Fritts.
Why are the Brown Tree Snakes so successful?
Rodda and Fritts explored some reasons why this particular invasion was so damaging;
1. Island Tameness: the native animals already living on the island had evolved without any major exposure to predators. When the snakes arrived, they had no defence mechanism, didn’t know to run away for instance and so they died.
2. Too Many Friends: all of the species that went extinct were in fact native, other cohabiting species of birds and vertebrates had been introduced previously and they had all learnt to live together in harmony. This meant that there was a very high prey population ready for the Brown Tree snakes to feed on when they arrived. It is thought that had the introduced prey not have been there, the snake population would not have survived on only the native species.
However, it’s not all doom and gloom, the island is now relatively free of small pests such as rats so there is always that...
Is it always just the invader’s fault?
Gurevitch and Padilla believe that the evidence and information collected with regards to worldwide extinctions is “anecdotal, speculative and based upon limited observation” and so they decided to explore it further. In 1998
Wilcove et al published a paper that explored the threats on US imperilled species. The main conclusion was that;
“Habitat loss is the single greatest threat to biodiversity followed by the spread of alien species”
More specifically, that habitat loss affects 85% of threatened species and Invasive Alien Species affect 50%.
In 2004,
Gurevitch and Padilla re-evaluated the data obtained in 1998 with the aim of critically analysing the information and being more scientific. From this reassessment they concluded that it was very rare that Invasive Alien Species were the only threat acting upon a species at any one time. These types of threats included pollution, feral pigs and diseases amongst many other things. Interestingly for imperilled plants, there was an average of
2.6 types of threat and for birds
2.8. This then poses the question, are Invasive Alien Species ever the main cause of extinction? Should they be getting all the blame?
As later explored by
Gurevitch and Padilla, in Hawaii they have many areas where feral pigs and goats roam, disturbing the environment by searching for food for example. This commonly leads to a decline in native species but is this reduction due to the invasive alien plants or due to the disturbance caused by the wild animals? Or is it satisfactory to conclude both?
It appears so far that it really depends on the individual species being observed. Sometimes they can cause utter destruction and mayhem for both the natural world and humanity. However, it would seem that in a lot of cases Invasive Alien Species are just adding fuel to the fire, they are another pressure being added to the ecosystem. Even with the Brown Tree Snake, are they even to blame? Or was it militaries fault for transporting them over to an isolated island in the first place?
Check out this blog on biodiversity to see the importance of preventing extinctions
http://goingthewayofthemammoth.blogspot.co.uk!
Head back over next week as I take a look at a common Invasive Alien Species in the UK, causing trouble up and down canal courses and all around wetlands!
