Species Are Spreading Like Never Before
Plants and animals are expanding their ranges farther and faster these days than ever before in the planet's history. A paper published in the journal Conservation Biology asserts that humans have lately propelled an unprecedented spread of invasive species. Organisms of all kinds have hitched rides to new territory on boats, cars and planes. People have also deliberately introduced exotic plants and animals for food and other products, or to simply enjoy.
Mass invasions of species are certainly not new. They've occurred many times over the last 25 million years, often triggered by a changing climate or the merging or breaking apart of continents. Most studied is the joining of South America with Central America when the Panama isthmus formed three million years ago. The land bridge enabled 37 types of animals to move to a new continent.
But the current rate and scale of organisms moving to new continents, islands and oceans is many magnitudes greater than anything the world has previously experienced.
Evidence for the prehistoric spread of species comes from uncovering ancient pollen and fossils. These have revealed, for instance, how often new species have landed on the Hawaiian Islands. Before humans arrived, Hawaii received 30 new species every million years. Once Polynesians settled there, the rate increased to 20,000 new species per million years. In the last 200 years, the Islands have been gaining new species at a rate of 20 million per million years, or 20 a year.
The accelerated spread of species is occurring not just on land, but also in lakes and oceans. New fish established in the Great Lakes at the rate of 1.7 species a century in prehistoric times. Now it's 20 species a century.
The marine waters of San Francisco Bay used to gain five new types of inhabitants a century. That increased to 170 per century when Europeans began using the area. In the last 30 years, new species moved into San Francisco Bay at the rate of 370 a century.
Besides an accelerated rate of dispersal, other prominent differences exist between the human-driven biological invasions and prehistoric ones. Notably, these days species are travelling between distant continents rather that just adjacent ones.
Europeans colonizing New Zealand and Australia brought with them from home rabbits, deer, sparrows and many other mammals and birds that otherwise could never have gotten to the South Pacific.
Similarly, ballast water discharged from ships has carried many organisms from the Black and Caspian Seas across the Atlantic Ocean and released them into the Great Lakes. The present movement of species is for the first time planet-wide in scope, rather than just a regional event.
Consequently, invasive species are now pervasive. An estimate published ten years ago determined that 3% of the world's uncultivated land area that supports vegetation was dominated by exotic plants.
On continents, 10% of plants, 13% of fish and 3% of birds come from those introduced by people in the last five hundred years. We've altered island biota even more. An average of 24% of plants, 76% of fish and 21% of birds established on islands today are not native species.
The swift spread of invasive species has led to extinctions and a homogenization of plants and animals around the world. The ecological ramifications of multiple biological invasions are rapid and rampant. Several invasive species descending on a region in quick succession can potentially change the local ecology enough to facilitate even more invasions. This hypothetical positive feedback loop is what biologists term an "invasional meltdown."
Anthony Ricciardi. 2007. Are Modern Biological Invasions an Unprecedented Form of Global Change? Conservation Biology. 21(2): 329-336.