A Weed Is a Nice Plant at the Wrong Party

How do weeds go wild?  That is a question investigated by Science Now on Feb. 20.¹  A complex relationship between a plant and its microbial partners may keep it in check.  Transplant that species to an unfamiliar territory, and it may go out of control because it no longer has its restraining pathogens, or “natural enemies” (if that metaphor is useful: see 07/03/2003 entry).  Experiments on knapweed have shown two processes at work:

Enemies clearly matter, and that’s especially true in the old country.  When the researchers grew knapweed in French soil, it fared better in soil that had been previously planted with bunchgrass than with knapweed–presumably because the bunchgrass soil had not accumulated knapweed-specific pathogens.  But it appears that enemies aren’t the whole story.  Montana soil showed the opposite pattern: Knapweed planted in soil that had grown knapweed did better there than in once-grassy soil, the team reports in the 19 February issue of Nature.  They think that invasive knapweed has not only escaped its natural pathogens in Montana but is modifying the soil to its own advantage, perhaps by cultivating helpful mycorrhizal fungi.
“This suggests that the contribution of soil organisms in invasiveness is two-fold: [Invasives] escape from the bad guys and [get] help from the good guys,” notes Wim Van der Putten of the Centre for Terrestrial Ecology in Heteren, The Netherlands.

¹Erik Skogstad, “How Weeds Go Wild,” Science Now Feb 20, 2004.

The article starts with the language of warfare, but is it misleading?

It may not make great action footage for nature documentaries, but plants are in constant battle with each other–for space, light, water–and with soil pathogens that threaten to kill or stunt them.  Now it’s becoming clear just how important this subterranean struggle can be.  Plants that escape their natural soil-borne enemies, and strike up alliances with friendly microbes, can become aggressive invaders.

This Malthusian, dog-eat-dog imagery may be opposite the truth (see 07/04/2003 entry.)  If the plants and their soil organisms are in a balance of growth and regulation, that can be a picture a peaceful homeostasis just as much as the regulation that goes on inside a single cell: agonist and antagonist, on-switch and off-switch, accelerator and brake.
    Imagine instead a post-Fall and post-Flood world.  After a worldwide flood and ice age, the ecology was radically changed with continents having drifted apart and land bridges vanished under rising seas.  New groups of organisms, now isolated from one another, settled into new levels of mutual regulation suitable for their climate.  The more isolated the environments became, the more “damage” an invasive species could cause.  Increasing human migrations accelerated the upsets to ecologies that had become established over thousands of years (e.g., the importation of tumbleweeds to the western United States by Russian immigrants, Dutch elm disease, etc.).  An original worldwide balance in nature was replaced by islands, each balanced internally, but out of balance with each other.
    All human investigators have the same data available for study.  Darwinian struggle, Biblical paradise lost – your metaphor will affect how you look at the data, and what questions you will find interesting.