Monday, March 30, 2015

Divergent perspectives on biological invasions: a way forward

50 years have passed since the publishing of the seminal ecological work “The Genetics of Colonizing Species” (GCS) (Baker and Stebbins, 1965). This book covers various topics regarding the introduction of species to different regions, the effects these movements have on the species themselves and sometimes more broadly on the ecosystem into which they are introduced. After 50 years of contribution to ecological discourse, it is worth examining how GCS can help to address some contemporary ecological questions; namely, how can basic science help inform ecological management? And, how much have specific ideas or theories changes in the past 50 years?

To answer the first question, basic science has given us the ability to determine potential invasive qualities in certain species and prevent some introductions. For instance, we know to avoid introducing species that are closely related to species that are already pests or are in some way problematic. Basic science has also helped us develop species distribution maps where an introduced species is and where it could potentially spread to. These maps help us prioritize areas for protection and have helped us prevent the spread of potentially harmful species.

 In regards to the second question, I’m a bit more hesitant to answer. We still don’t have an adequate answer as to why certain species can establish and be very successful, while others don’t. We’ve come up with many hypotheses to try and explain it, such as enemy release (Colautti et al. 2004), novel weapons (Mitchell et al. 2006), empty niche (Elton, 1958), etc. but we still haven’t been able to generalize these ideas. We simply fit these hypotheses in a case by case manner, so really has there been much progress in the past 50 years? To answer this I’ll turn to a current debate where two schools of thought have divergent views on this topic.

On one side, Mark Davis and colleagues (2011) seem to think that we’ve made progress but not enough. With their controversial Nature Comment, they call for the end of invasion biology, stating that the native versus non-native dichotomy within this field is a hindrance to progress as it can promote xenophobia and bias the views of scientists and the public. Davis points out that within this field there is a large emphasis on negative impacts of non-natives, which can take away from the potential positive influences they could have. These researchers are embracing the idea of “novel ecosystems”, which are systems that are rapidly changing as a result of climate change, land use, and increasingly through the introduction of non-natives (Thompson and Davis, 2011).  Moreover, these researchers describe how non-natives have the potential to contribute to conservation goals as they are more likely than native species to persist and provide ecosystem services within these novel ecosystems (Schlaepfer et al. 2011)

Leading the charge from the opposing side is Dan Simberloff ,who believes that within the short span of its existence (about 25-30 years) invasion biology has made significant progress, especially in terms of technological improvements to help prevent or stop the spread of invasive species (Simberloff , 2011; Simberloff et al. 2013). In contrast to Davis and colleagues, Simberloff doesn’t believe that attempting to stop invasions is a lost cause and is able to provide various examples of invasive species that have either been eradicated or brought down to manageable population densities through the continued work of researchers and community efforts (Simberloff and Vitule, 2014). He believes that Davis is downplaying the severity of the impacts non-natives can have, especially when there are no visible effects on the ecosystem (Simberloff and Vitule, 2014). All in all, Simberloff sees great potential in the development of new technologies, but in order to develop them we must have scientists working on these projects, and public support to ensure that there is funding for these projects. 

To learn more about the debate I would highly recommend checking out the webcast of their debate (Conservation Science Webinar - scroll down to Native and Non-native Species: How much attention should managers be paying to origins?). In regards to where I stand with all of this I am unfortunately on the fence. From the GCS I read the chapter titled: “Establishment Aggression, and Cohabitation of weedy species”, authored by John Harper. I very much enjoyed Harper’s chapter as he emphasizes the fact that the introduction of species is non-random. It is the “specialized” species that are capable of moving around the world. Harper points out that these species tend to have particular dispersal or germination traits that are allowing them to establish within new regions. I believe Harper’s views on introduced species mirrors what we currently think. The world is becoming increasingly connected so it is logical that species taking advantage of this connectivity would be the most likely to move around and establish in new areas.

 I think that the “specialized” traits of these species gives us predictable patterns to look for, and that this predictability based on traits plays well into Simberloff’s views of being wary about newly introduced species. An incredible amount of work must be put in to accurately characterize a species, so it’s practically impossible to know everything about a newly introduced species that you’ve just encountered. Therefore, we should be extremely cautious if the introduced species has any “specialized” characteristics. More simply, if the species has any of the 4 risk factors: good dispersal ability, fast reproduction rate, lacks predators or pathogens in the new range, or if it’s primary resource is readily available in the new range, then we should play a more active role in stopping it from establishing (Lerdau and Wickham, 2011).

In general, I’m slightly indecisive about this topic as while I do agree with most of Simberloff’s views I also see the merit with Davis’ idea of “novel ecosystems” since the world is changing so rapidly. With a rapidly changing environment many new questions come to mind, for me the simplest one would be: are species changing (ie adapting and evolving) to better fit the new or “novel” environment?  Using the plant species native to Europe, St. John’s Wort, Maron et al (2004) were able to demonstrate that this species is capable of adapting to a new environment. In the introduced range in North America this invader was becoming better fit to the broad scale abiotic conditions, and was thus experiencing rapid adaptive evolution as this was occurring within the last 150 years. 

With both sides of this debate I find that despite their differing views Davis and Simberloff are still fighting for the same thing: the acquisition of knowledge. Superficially there will always be a debate between the idea of the origin of an introduced species versus the impact of that introduced species. But really underneath it all everyone is still trying to answer the same fundamental questions: How did you get here, and what are you doing?

 So referring back to that second question: how much have specific ideas or theories changed? I don’t think our ideas have really changed, realistically I think we’re still where we started, but we have more information both on the species and the environment, so I do think we’re on our way to a big change. Charles Elton, the founder of this field had written 60 years ago, “we require fundamental knowledge about the balance between populations, and the kind of habitat patterns and interspersion that are likely to promote an even balance and damp down the explosive power of outbreaks and new invasions.” (Elton, 1968)  Invasion biology has only formally been a field of study for about 25-30 years; we’re still at a stage where we’re gathering knowledge. It’s my opinion that its way too early for us to call it quits, but it is the perfect time for us to push the field and make giant leaps. We’re scientists! We can revolutionize this field. We have the technology. We have the curiosity. We can make the world better than it was. More sustainable, more functional, more diverse.


Baker, G.H & Stebbins, J.L. (Eds.) (1965). The Genetics of Colonizing Species. Academic Press Inc

Colautti, R. I., Ricciardi, A., Grigorovich, I. A., & MacIsaac, H. J. (2004). Is invasion success explained by the enemy release hypothesis?. Ecology letters,7(8), 721-733.

Davis, M. A., Chew, M. K., Hobbs, R. J., Lugo, A. E., Ewel, J. J., Vermeij, G. J., ... & Briggs, J. C. (2011). Don't judge species on their origins. Nature,474(7350), 153-154.

Elton, C. S. (1958). The ecology of invasions by animals and plants. University of Chicago Press.

Harper, J. L. 1965. Establishment, Aggression, and Cohabitation of Weedy Species. In H. G. Baker &  J. L. Stebbins (Eds), Genetics of Colonizing Species (pp 245-263). Academic Press Inc.

Lerdau, M., & Wickham, J. D. (2011). Non-natives: four risk factors. Nature,475(7354), 36-37.

Maron, J. L., Vilà, M., Bommarco, R., Elmendorf, S., & Beardsley, P. (2004). Rapid evolution of an invasive plant. Ecological Monographs, 74(2), 261-280.

Mitchell, C. E., Agrawal, A. A., Bever, J. D., Gilbert, G. S., Hufbauer, R. A., Klironomos, J. N., ... & Vázquez, D. P. (2006). Biotic interactions and plant invasions. Ecology Letters, 9(6), 726-740.

Schlaepfer, M. A., Sax, D. F., & Olden, J. D. (2011). The Potential Conservation Value of NonNative Species. Conservation Biology, 25(3), 428-437.

Thompson, K., & Davis, M. A. (2011). Why research on traits of invasive plants tells us very little. Trends in ecology & evolution, 26(4), 155-156.

Simberloff, D. (2011). Non-natives: 141 scientists object. Nature, 475(7354), 36-36.

Simberloff, D., Martin, J. L., Genovesi, P., Maris, V., Wardle, D. A., Aronson, J., ... & Vila, M. (2013). Impacts of biological invasions: what's what and the way forward. Trends in Ecology & Evolution, 28(1), 58-66

Simberloff, D., & Vitule, J. R. (2014). A call for an end to calls for the end of invasion biology. Oikos, 123(4), 408-413.

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