The advent of molecular genetic analysis about 50 years ago is making it possible to determine the relationships between species of plants and animals. As this analytic method becomes more sophisticated and more accessible, we are slowly learning more about how plants and animals have been dispersed around the earth, often great distances, sometimes crossing oceans. Here is an illustration of some of the long-distance dispersal of species across oceans which have been identified (1):
Look closely at the dispersal labeled “b.” This line describes the travels of a genus of plant (Lepidium) in the mustard family (Brassicaceae) which arrived in Australia from California and Africa in two separate dispersal incidents: “This [molecular] pattern is likely explained by two trans-oceanic dispersals of Lepidium from California and Africa to Australia/New Zealand…” (2)
Calibration of the molecular trees indicates the arrival of this plant in Australia/New Zealand in the Pleistocene geologic era, between .3 and 1.3 million years ago. The authors of this study speculate that mucilaginous (sticky surface) seeds were carried by birds: “…sea bird migration pathways between coastal California and Australia/New Zealand and South Africa and Australia/New Zealand are fully compatible with the proposed colonization scenario.” (2)
We know that eucalyptus seeds were brought to California in the mid-19th Century by humans who came by boat. But can we imagine a scenario in which the seeds could have been carried in their protective seed capsules on ocean currents? And is the fact that the seeds were carried in a ship across the ocean really so very different from them being carried on the currents?
Few of these long-distance dispersals have been identified so far, but many more are likely to be identified in the future as this analytic method becomes more widely available.
Next time you hear a nativist say, “It doesn’t belong here” when explaining why a plant or animal must be killed, please think about this example of the natural dispersal of plants and animals across the oceans to new homes. Who is to say that it doesn’t belong here?
- Alan de Queiroz, “The resurrection of oceanic dispersal in historical biogeography,” Trends in Ecology and Evolution, 20 No. 2, February 2005
- Klaus Mummenhoff, et. al., “Molecular evidence for bicontinental hybridogenous genomic constitution in Lepidium sensu strict (Brassicaceae) species from Australia and New Zealand,” American Journal of Botany, February 2004