You’d think Science, knowing our history of continental land bridges and pre-historic migrants overwhelming natives, would have consensus on how many thousands of years it takes something to dominate its surroundings to become the new “native” – but you’d be wrong …
The latest science involving Didymo rethinks the “invasive” label, as examination of the fossil record of lakes and streams afflicted by the diatom are finding the Didymo has been resident on five of seven continents for many thousands of years.
The Delaware River shows Didymo having been present for tens of thousands of years, rather than recently introduced by fishermen. Dissolved Phosphorus can dip below its normal threshold via numerous temporal phenomena, and with that change in water chemistry, triggers the visual “blooms” that gives the infestation its characteristic unappealing blanket. As quickly as water chemistry is restored, the blooms vanish, explaining one of the great mysteries of Didymo infestation.
Moreover, fossil and historical records place D. geminata on all continents except Africa, Antarctica, and Australia; records place D. geminata in Asia (China, India, Japan, Mongolia, Russia), Europe (Denmark, Finland, France, Germany, Ireland, Italy, Norway, Poland, Portugal, United Kingdom, Sweden), and North America (Canada and the United States), and historical records dating back to the 1960s place D. geminata in South America (Chile; Blanco and Ector 2009, Whitton et al. 2009). The recent blooms of D. geminata are found on each of these continents, where fossil or historical records have been documented, which indicates that attributing all blooms to recent introductions or to range expansion is incorrect.
… and as the last article mentioned, our collective angst in approaching our respective legislatures was a tad premature …
In fact, citing the threat of human-induced translocations of D. geminata or other unwanted organisms, seven US states (Alaska, Maryland, Missouri, Nebraska, Rhode Island, South Dakota, and Vermont), Chile, and New Zealand have passed legislation banning the use of felt soled waders and boots in inland waters (e.g., the 1993 New Zealand Biosecurity Act, Chile’s law no. 20.254, Vermont 2013 Act no. 130 [H.488]). Although such restrictions may reduce introductions of other deleterious aquatic microorganisms, the connection to the spread of Didymo. geminata within its native range seems dubious.
What’s even more interesting is the final definitive science will employ DNA sequencing of the respective colonies to see which continents have unique strains, and which continents may have sourced strains carried by everything from humans to migrating waterfowl.
The assertion that the recent blooms are caused by inad- vertent introductions of D. geminata cells by humans comes from frequent reports of blooms in areas that are used for recreation or monitoring by various agencies (Bothwell et al. 2009). Although Kilroy and Unwin (2011) reported a correlation between the ease of river access and D. geminata blooms in New Zealand, this has not been found in North American studies. In fact, systematic observations at both rivers with frequent human activities and remote rivers not heavily used for recreation or monitoring reveal no association between human activities at a river and blooms in Glacier National Park, in Montana (Schweiger et al. 2011).
Moreover, pathways for introducing D. geminata cells have existed for decades (e.g., felt-soled shoes; the transport of fish, their eggs, and water from areas where D. geminata is determined to be native on the basis of fossil records), making inadvertent introductions by humans difficult to explain, given the recent worldwide synchrony of blooms.
Really good article for the lay person given the science is common sense and easy to follow. I recommend you read it and draw your own conclusions.
As I adore a good conspiracy theory, I find it equally interesting that our fishing media and conservation organizations have published nothing on how scientists are reconsidering earlier theories as more concrete observations accumulate.
I’m sure those that insisted we act responsibly, by first purchasing new wading shoes, donated most generously …