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'.