Last month, a great headline on BBC caught my eye – The hitchhiking snakes of the Caribbean. The piece lives up to the eye catching title: it is a new and somewhat unexpected twist on an old story. Boa constrictors were most likely introduced to the wilds of Aruba by pet owners releasing snakes they no longer wanted. These snakes rapidly established a breeding population, and their subsequent invasion of the island has been fast and furious, resulting in the demise of tens of thousands of birds and small animals per year. However, given the average speed of a slithering snake, the invasion has been faster than expected. That added speed comes on account of some creative travel arrangements. Boa constrictors like to hitchhike by crawling onto the chassis of a parked car and then hopping off wherever it ends up, meaning they can sometimes wind up many kilometers from where they started. These hitchhiking snakes got me wondering how common hitchhiking by non-humans really is and what impact it has on ecosystems worldwide.
It turns out Aruba’s boa constrictors are a perfect example of one of the hallmark traits of an invasive species: they can take advantage of human movement. A hitchhiking association with humans can significantly contribute to an introduced species becoming invasive and aggressively spreading through new habitats. Quick generation times, generalist habitat requirements, fast growth, tolerance of a wide variety of environmental conditions, and competitive abilities all contribute to the success of invasive species too, but nothing helps the introduction and spread of an invasive species like tagging along with globe-trotting human beings.
In the case of Aruba’s boas, hitchhiking is helping the snakes to colonize and expand their population after they have been intentionally introduced. However, there are also many examples of humans unwittingly transporting hitchhiking species to new locations and thereby inadvertently introducing them to that habitat. For example, zebra mussels originated in the freshwater lakes and rivers of southern Russia, but were first introduced to North America from Europe during the 1980’s when they were unintentionally transported to the St. Lawrence Seaway in the ballast water of boats. From the Great Lakes, they rapidly spread through various waterways by hitchhiking on the hulls of commercial and pleasure boats plying the waters of the canals and rivers criss-crossing the continent. The zebra mussel now causes infrastructure damage in the millions of dollars every year throughout North America. Likewise, the tumble mustard was introduced to North America in ballast or as a contaminant of crop seeds, and rapidly spread along the railway lines through Canada and the United States. A single plant can produce 1.5 million seeds, and grows rapidly, making it a strong competitor against both crops and native plants. In Australia, Bathurst Burr (Xanthium spinosum) was introduced to the country on the tails of horses imported from Chile. This burr plant produces seeds covered in hook-like structures that allow it to stick like Velcro to nearly everything. It’s no small wonder it happened to be in those Chilean horse tails, and by sticking to human clothing, it was readily and quickly spread through the country wherever humans and livestock traveled. The plant is toxic to livestock and damages sheep’s wool, making it a noxious weed. All of these species are good examples of how animal and plant hitchhikers can move along human trade routes, and as such, trade movements are often an important factor in modeling the potential introduction and spread of invasive species.
Non-human hitchhikers aren’t just taking advantage of trade routes either. For example, there is an entire class of invasive and introduced plant species that are referred to as polemochores that have moved around the world in connection with conflicts by hitchhiking on vehicles, aircraft, soldier’s clothing and equipment. The most striking evidence of these hitchikers are on island archipelagos in the South and Central Pacific Ocean, where local floras were irrevocably changed by the intentional and accidental introduction of hundreds of plant species when the previously isolated islands suddenly became populated with troops and war materiel during World War II. In one of the most extreme examples, a pre-war expedition to Johnston Island found only three native plant species, while a post-war expedition found some 24 new arrivals whose establishment and spread was made even easier by the large scale disturbances caused by bomb craters and the building of roads, runways, and supply depots.
There’s no doubt that these non-human hitchhikers are a concern both economically and ecological. The introduction and spread of invasive species can cause damage to human infrastructure, reduce the value of goods, and cause significant changes in native ecosystems. Whether it is snakes under the hood, seeds on your jacket, or seafood hiding out beneath your boat, ecosystems of the world are changing simply because humans move from place to place with speed and frequency that is impossible for most organisms.