What happens on a daily basis now, used to occur just twice in millions of years – for this one horn snail at least.
Snail’s airlift chance before age of globalisation: mistaking heron’s leg for reed?
An international team of scientists led by the Smithsonian Tropical Research Institute in Panama have investigated the historical dispersal of a specific type of marine horn snail – and discovered it crossed the American land bridge separating the Atlantic and Pacific Ocean twice over the course of more than three million years, when North and South America (re)connected.
When the land connection formed it separated the population of (the predecessor of) Cerithideopsis snails, after which Cerithideopsis californica and Cerithideopsis pliculosa formed on different sides of the Isthmus. Both snails now live on either side, because -genetic evidence shows- first a Pacific population crossed over to the Atlantic about 750,000 years ago – and then the reverse migration took place some 72,000 years BP.
The snails were probably carried over Mexico in the feathers or on the legs of marine or marshland birds, the researchers write in their publication in Proceedings of the Royal Society B. Because it is unlikely a viable population could arise from a single cross-over, such events are really quite rare in Earth’s history.
The age of air travel and cargo ships
That however has changed. Nowadays thousands of airplanes connect the land masses of our continents each day. Apart from humans and other intended cargo, they can bring along anything from a bit of moss in someone’s shoe soles to a spider in a suitcase, a virus or any other small organism.
But that’s just on land. Meanwhile similar numbers of cargo ships travel the world’s oceans. That has happened for centuries now, but practice has changed – and total traffic intensified. Many ships take in ballast water to balance their weight, releasing it whenever they get new cargo onboard or for other reasons have to lose weight again.
If you were a fish, when would you rather have lived?
For fish for instance being swallowed by such Jonah’s whales is an a lot more comfortable migratory option for inter-ocean hitchhiking than to have to cling on to some heron’s toenail for hours or days on end – considering their fear of heights.
[From the Carribean to the Pacific and from the Mediterranean to the Indian Ocean fish these days of course have a third option: swimming – through Panama and Suez Canal.]
For ecology however this means the difference between an occasional helping hand to optimise biodiversity by transporting genes from the one coastline to the other – and clear-cut damage, by randomly introducing invasive species, which may not fit local food chains and thereby undermine ecosystem health. Rule of thumb: too much of anything, etc.
On the menu: 21st century ocean soup
The biodiversity damage of continuous inter-ocean and intercontinental traveling is however the one ecological problem that really does not seem to offer any solution route. Because there is no stopping globalisation, is there?
We may have to wait until the process evens out. Throw in all of Earth’s species, stir it for a century or so – and see what sort of soup we end up with. Don’t expect much, from a culinary perspective. Ask any cook. Soup is never good for diversity.
© Rolf Schuttenhelm | www.bitsofscience.org