What turns an inconspicuous plant into an economically and ecologically damaging alien invader? Or to paraphrase this blog title – why does the Kraken awake? The answer is not simple. Several hypotheses exist, including lack of natural predators in the introduced range (the enemy release or habituation hypotheses).
However alien invaders often take time to become problematic. This lag phase, when plants may be called sleeper weeds, indicates that invaders may be adapting to their new environments. But recent research finds that multiple introductions of invaders during this phase can create a new breed of super weed.
We are seeing an increasing problem with invasive species, due largely to the increased movement of species associated with global trade. Indeed the current epoch has been termed ‘the time of great mixing’
And as with many areas of research, invasion biology has its far share of terminology. A native is a species in its natural home range; an alien species is one found outside of its natural range; a naturalised species is one which has been introduced to a new range, and an invasive species is one which has become problematic (having ecological or agronomic impact) in its new range.
Many species can remain naturalised without becoming invasive and many species introduced to a new range don’t even survive there under natural conditions.
But one of the key areas of interest for invasion biology is what causes the shift from a recently arrived alien species to one which is an aggressive invasive?
When a species is first introduced to a new area, it may occur in limited numbers and will almost certainly lack genetic diversity. Whilst multiple and repeated introductions can increase the likelihood of invasiveness – by simply increasing numbers (known as propagule pressure) – another key factor is the amount of genetic diversity introduced. By combining genetic diversity from a number of sources, new genetic variants and mutations can arise that can give the alien species the potential to adapt and become highly invasive.
Our research, in collaboration with Partners from the Universities of Stellanbosch (South Africa) and British Columbia (Canada), have made some interesting findings and show that human-mediated introductions of plants are more likely than natural processes to introduce material from multiple sources and increase genetic variation in introduced species. In addition, changes to the gene regulation of defence and stress genes appear to play an important role in the evolution of invasiveness in introduced populations. This pattern may be a general trend as it has now been independently observed in three daisy weeds and this is now a leading hypothesis for invasion success.
Recently published work (Dormontt 2014) has highlighted this pattern for the Australian invasive – fireweed, which was first introduced from South Africa to the Hunter Valley in NSW, Australia, in 1918. By analysing genetic variation in contemporary and historical populations (from a plant museum or herbarium), the increase in range and invasiveness of this weed over time since its introduction was directly linked to new genetic variation being introduced from South Africa.
Following this research, specific recognition of the risks of increasing genetic variation in introduced species has been recognised by the Hawke review of the Environment Protection and Biodiversity Conservation or EPBC Act in relation to invasive species in Australia; as follows ‘There is also a need to review the live import list in regards to variants or hybrids. The Invasive Species Council suggested that: The import of genetically distinct varieties of existing permitted species is a major source of pest and weed risk for Australia as new variants may have new features that significantly increase their pest risk or turn existing non‐pest species into invasive risks.’
Following this review and recommended changes to the EPBC Act, the Australian quarantine and importation guidelines for plant and animal material have been changed to limit the introduction of new seed sources of plants that are already listed as introduced, naturalised and invasive in Australia.
These steps should help to limit the introduction of new genetic variation for species in a sleeper or active adaption phase – and should help to keep the Kraken asleep, at least for a little while longer!
Also see the article below by Jill Rowbotham from the Australian
‘Can’t weed them out without entry curbs’
THE raising of the international drawbridge against swine flu has particular relevance to Andy Lowe. The professor of plant conservation biology at the University of Adelaide could do with some drastic action in his own field: weeds.
Of course one person’s weed is another’s gardening experiment, and many plants now identified as weeds have entered Australia at the behest of horticulturalists. Exotic flora has been with us for a long time. The Egyptians were trading adult plants as long ago as 2500BC and during the age of exploration from the 15th century to the 18thcentury, many natural barriers to the movement of flora and fauna collapsed.
Such globalisation is not necessarily a problem. Of 2700 introduced plant species in Australia, only 429 are now regarded as a serious problem, Lowe says.
What concerns him is the realisation that despite a sophisticated understanding of plant biology, Australia may be leaving itself vulnerable to new danger.
“What’s changed is our understanding of what forms a superweed,” Lowe says. “Generally the quarantine service in Australia has concentrated on excluding new species from coming in, but actually when species are already here and recognised as an invasive weed problem, there is no restriction on bringing in new material of the same species because they are already here — it’s too late. There’s nothing more that can be done about it.”
The Australian Quarantine and Inspection Service confirms there is no prohibition on the import of weeds that authorities are no longer trying to eradicate. However, noxious plants such as most blackberry varieties, most scotch broom varieties and fireweed, for example, against which strenuous eradication efforts continue, are all prohibited.
Lowe’s research focuses on the effect on weeds of introduced new material of the same kind. “You can shift those weeds into superweeds,” he says. “What happens is you have the potential for new genetic combinations to arise if you bring in lots of genetic variation. You can get adaptation and evolution even within a short space of time.”
The ballpark figure of costs of control and lost production due to the presence of weeds is $4 billion annually.
Part of Lowe’s research includes the herb fireweed. “It is invasive on the east coast. It was introduced into the Hunter Valley in 1918. It’s a small herbaceous species poisonous to cattle, and in the mid-1980s it appears additional material was introduced from several sources from Africa, where it is native.”
He says it is the introduction of these extra plants by humans that seems to be at the base of the problem, rather than the ability of the original weed to proliferate and colonise habitat.
[Feature image: The Kraken Wakes – a huge legendary sea monster, remade as an alien invader by John Wyndham]
Interview on Spiny rushweed threat, ABC TV Stateline. http://www.abc.net.au/news/video/2010/04/16/2875422.htm
Dormontt EE, Gardner MG, Breed MF, Rodger J, Prentis PJ, Lowe AJ (2014) Genetic bottlenecks in time and space: reconstructing invasions from contemporary and historical collections. PLoS ONE 9:e106874.
Harris AJ, Dormontt EE, Le Roux JJ, Lowe AJ, Leishman M (2012) No consistent association between changes in genetic diversity and adaptive responses in novel ranges for invasive Australian acacias. Evolutionary Ecology 26: 1345-1360.
Prentis PJ, Woolfit M, Thomas-Hall SR, Ortiz-Barrientos D, Pavasovic A, Lowe AJ, Schenk PM (2010) Massively parallel sequencing and analysis of expressed sequence tags in a successful invasive plant. Annals of Botany 106:1009-101.
Prentis PJ, Sigg DP, Raghu S, Dhileepan K, Lowe AJ (2009) Understanding invasion history: genetic structure and diversity of two globally invasive plants and implications for their management. Diversity and Distributions 15: 822-830.
Wilson JRU, Dormontt EE, Prentis PJ, Lowe AJ, Richardson DM (2009) Something in the way you move: dispersal pathways affect invasion success. Trends in Ecology & Evolution 24: 136-144.
Prentis P, Dormontt E, Wilson A, Richardson D, Lowe AJ (2008) Adaptive evolution in invasive species. Trends in Plant Sciences 13: 288-294.
Kang M, Buckley Y, Lowe AJ (2007) Testing the role of genetic factors across multiple independent invasions of the shrub Scotch broom (Cytisus scoparius). Molecular Ecology. 16: 4662-4673.