This post is a guest contribution by Kathryn Turner, a PhD student at the University of British Columbia, who studies the evolution of invasive thistles. Kathryn writes about her scientific interests at the slyly named site Alien Plantation and tweets under the handle @KTInvasion.
Invasive species are a big problem. A real big problem. In the US alone, invasive species cost nearly $120 billion in damages per year (Pimentel 2005). 42% of species on the Threatened and Endangered list are there primarily because of invasive species.
Which leaves us with two questions. First, most obviously, how is it that a species is able to come into a new environment that it is not adapted to, surrounded by new environmental conditions and foreign biological interactions, and thrive? Thrive so exaggeratedly, that it can out-compete and displace species which have been there for millennia, adapting precisely to those environmental conditions and biological interactions? How can an individual survive to propagate a population? How can any species accomplish this? Second, less obviously: why can’t more species do it? Humans transport animals and seeds (and spores and larvae, etc, etc) around all the time, but only 10% establish self-sustaining populations, and only 1% spread to new habitats, becoming potentially invasive; this is known as the ‘tens rule’ (Williamson 1993) – a funny ‘rule of thumb’ for which I could never quite figure out the math.
A partial answer to the first question, you probably already guessed: We’re the problem. We’ve turned the planet into a biological melting-pot. If we can transport one individual, it’s very likely we will transport more than one. Though the colonization event may result in a genetic bottleneck (potentially limiting population growth and persistence through inbreeding depression, and decreasing standing genetic variation (Sakai et al. 2001)), it isn’t as common as you might think, and it doesn’t seem to do the erstwhile invader much harm. Dlugosch and Parker (2008) found that losses of genetic variation are a common feature of introductions (overall loss of 15% of allelic variation in introduced populations), but significant amounts of genetic variation can be maintained via multiple introductions. Additionally, we tend to introduce species to places that are already ripe for invasion – places which we have disturbed, by living there, building roads, cultivating crops or livestock, or even climate change. By disturbing these habitats, by changing the adaptive landscape, we knock the native species off their game, and decrease the home field advantage.
So then, why can’t more species do this? Why aren’t we completely surrounded by invaders? What is it that allows a species to be invasive? This is a bit tricky. It may be a semantic argument – after all, what do you call an invasive? Arbitrary debates rage over how long a species has to exist in an area before it’s considered native, or how far away it has to originate for it to be considered non-native, or how to tell the difference between something which is a non-problematic exotic species, and one which is biding its time until invasion. If you think about it on geological time scales, aren’t most species really invasive? Perhaps, as Davis et al (2011) argued recently, species shouldn’t be judged on their origins. And yet origin, again and again, seems to matter. Knapp and Kuhn (2012) demonstrate, that at least for the flora of Germany, invasive species have a different suite of functional traits than the natives, different even that wide-spread, very successful natives. And in eastern North American forests, invaders get the advantage over natives by growing a full month longer into the autumn (Fridley 2012). Since Baker in the 1960s, it has been every invasion biologist’s favorite pastime to make lists of the traits which distinguish invaders (Baker 1965).
And yet no list ever fully satisfies. In the end, each invasion may be largely idiosyncratic. Invasions represent repeated evolutionary experiments, in the briefest of time scales. For those species which are capable of spreading, there is often a lag phase between establishment and spread. This lag time may allow for evolutionary change, such as adaptation to the new habitat (Sakai et al. 2001). During the lag phase, multiple introductions can increase genetic diversity within the invading population and enable the generation of superior new genotypes through recombination and hybridization (Schierenbeck and Ellstrand, 2008). At any stage, there is ample opportunity for genetic change to occur, through selection or drift. An understanding of the ecological or genetic differences between invasive and native source populations is key to understanding why some species merely show up to the party, and others take it over.
Baker, H.G and Stebbins, G.L. (1965). The genetics of colonizing species: proceedings. Academic Press.
Chown, S., Huiskes, A., Gremmen, N., Lee, J., Terauds, A., Crosbie, K., Frenot, Y., Hughes, K., Imura, S., Kiefer, K., Lebouvier, M., Raymond, B., Tsujimoto, M., Ware, C., Van de Vijver, B., & Bergstrom, D. (2012). Continent-wide risk assessment for the establishment of nonindigenous species in Antarctica Proceedings of the National Academy of Sciences, 109 (13), 4938-4943 DOI: 10.1073/pnas.1119787109
Davis MA, Chew MK, Hobbs RJ, Lugo AE, Ewel JJ, Vermeij GJ, Brown JH, Rosenzweig ML, Gardener MR, Carroll SP, Thompson K, Pickett ST, Stromberg JC, Del Tredici P, Suding KN, Ehrenfeld JG, Grime JP, Mascaro J, & Briggs JC (2011). Don’t judge species on their origins. Nature, 474 (7350), 153-4 PMID: 21654782
DLUGOSCH, K., & PARKER, I. (2008). Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions Molecular Ecology, 17 (1), 431-449 DOI: 10.1111/j.1365-294X.2007.03538.x
Fridley, J. (2012). Extended leaf phenology and the autumn niche in deciduous forest invasions Nature, 485 (7398), 359-362 DOI: 10.1038/nature11056
Knapp, S., & Kühn, I. (2012). Origin matters: widely distributed native and non-native species benefit from different functional traits
Ecology Letters DOI: 10.1111/j.1461-0248.2012.01787.x
Pimentel, D., Zuniga, R., & Morrison, D. (2005). Update on the environmental and economic costs associated with alien-invasive species in the United States Ecological Economics, 52 (3), 273-288 DOI: 10.1016/j.ecolecon.2004.10.002
Sakai, A., Allendorf, F., Holt, J., Lodge, D., Molofsky, J., With, K., Baughman, S., Cabin, R., Cohen, J., Ellstrand, N., McCauley, D., O’Neil, P., Parker, I., Thompson, J., & Weller, S. (2001). THE POPULATION BIOLOGY OF INVASIVE SPECIES Annual Review of Ecology and Systematics, 32 (1), 305-332 DOI: 10.1146/annurev.ecolsys.32.081501.114037
Schierenbeck, K., & Ellstrand, N. (2008). Hybridization and the evolution of invasiveness in plants and other organisms Biological Invasions, 11 (5), 1093-1105 DOI: 10.1007/s10530-008-9388-x
Williamson, M. (1999). Invasions Ecography, 22 (1), 5-12 DOI: 10.1111/j.1600-0587.1999.tb00449.x