If you’ve been following (listening to?) our tweets ((I never thought I’d ever write that, but time and the social web wait for no-one.)) you might have seen this enigmatic little gem:
Population genetic diversity influences colonization success. K. M. CRAWFORD. Molecular Ecology: http://goo.gl/JFHh
And, possibly, ignored it. It must have registered at some level though, because when I saw Inbreeding bad for invasives too at C.J.A. Bradshaw’s blog Conservationbytes.com I thought, “Hmmn, I wonder if that’s the thing we tweeted”. ((Which gives me the opportunity to remind you that we don’t tweet everything we write about here, nor do we write here about everything we tweet.)) And it was. Bradshaw does a great job of explaining how it is that reduced genetic diversity contributes not only to a population’s risk of extinction, but also to its ability to invade new habitats.
Crawford & Whitney measured greater population-level seedling emergence rates, biomass production, flowering duration and reproduction in high-diversity populations compared to lower-diversity ones. Maintain a high genetic diversity and your invasive species has a much higher potential to colonise a novel environment and spread throughout it.
Of course, this is related to propagule pressure because the more individuals that invade/are introduced the more times, the higher the likelihood that different genomes will be introduced as well.
So far the experimental evidence comes only from Arabidopsis thaliana, the white rat of the plant biologist. But I’d be willing to bet that if you could measure such a thing as invasiveness and persistence for crop varieties, where people, rather than nature, determine how many propagules survive and spread, the ones that are both widespread and long-lasting are also the ones with the most genetic diversity.
I wonder where the notoriously genetically undiverse Japanese knotweed (just one clone in the UK) fits into the picture? Seems to be doing quite well hereabouts.
How does Japanese knotweed reproduce and spread? Is it seeds (possibly apomictic) or vegetative bits spread by eager road-diggers and the like?
Definitely vegetatively, although inter-species hybrids occur with Russian vine sometimes. They’re are not as invasive as the knotweed parent. I would guess that most invasion foci originate as rhizomatous material dumped or transported during earth moving activities, although stems (as well as rhizomes) can and do root and establish new colonies when washed downstream in riparian habitats. The mass of the propagules must be a key factor in their success rather than their number.
I suppose that the kind of disturbed habitats in which Japanese knotweed grows and establishes tend to be occupied by a limited group of ruderal species and maybe the relative simplicity of these communities (missing functional groups, trophic levels, suites of predators and herbivores) makes invasion success more likely, despite knotweed’s lack of genetic diversity. I think that has been linked to the relative invasibility of different Pacific island groups, where the number of indigenous species was correlated with the invasion success of adventives (Daehler 2006). Dunno. Water hyacinth is another invasive plant with low genetic variability, in China at least (Li et al 2006), yet it seems to be doing very well. Is the aquatic environment relatively homogeneous and therefore the few existing genotypes are just very well adapted?
I don’t doubt that Crawford is right though – just trying to work out in my mind why vegetatively propagated weeds with limited genetic diversity are sometimes amongst the most invasive. I’ll shut up now.