Growing up, we’re always told to learn from our mistakes. When we trip over our shoelaces, we learn how to tie them so that the laces are out of the way when we walk and we don’t have to worry about future embarrassment. Every time we fell off our bike when learning to ride, we made a mental note to try not to repeat the movement that caused the imbalance. Even as adults, we learn from rejection—whether for a job, a date, or even publication in a journal—and we try to improve for the next time. This concept of using past experience to do better in the future is inherent in our lives, whether or not we realize it. However, in the world of science where failure is not sexy, those “mess-ups” often get swept under the rug.
In a recent article I read, researchers Rafael Zenni and Martin Nuñez at University of Tennessee sought to lift up the rug and talk about the “elephant in the room” of invasion biology: failed invasions. In their 2013 Oikos article, they argue that considering reasons for failed invasion is just as important—if not more important—for understanding the reasons for an invasive species’ success across different environments. In invasion biology, as well as science, negative or nonsignificant results and failed invasions are often deemed less important than studies showing significant results and drastic effects of invasive species, which are a hot topic in conservation biology. However, the same hypotheses that invasion biologists have posed for invasion success—propagule pressure, predator release, genetic variability or plasticity, novel weapons—can also serve as the reasons for the downfall of a non-native species. In their review, they gathered (with some serious effort I might add) 76 examples where there was intraspecific variation in invasion success outside the species’ native range. In other words, rather than comparing the differences between native and non-native populations, they were interested in what made populations of a species successful and non-successful in areas outside their native range. This is a less commonly used comparison, but may be better for ruling out certain invasion hypotheses, and determining the relative importance of others (Blackburn et al., 2011).
Of the studies they collected, only one third of them even attempted to provide reasoning for failed invasions. The main hypotheses Ruiz and Nuñez focused on were propagule pressure, abiotic resistance, biotic resistance, mutualist release (in contrast to predator release), and genetic constraints (inability to be plastic across environments) and they suggest how including information from failed invasions may change our current theories about these hypotheses.
Reason #1: Propagule Pressure
Having enough propagules during the initial stages of invasion is undoubtedly important for establishing a population in the initial phases of invasion. Even then, some small populations still fail despite being able to naturalize to an environment for reasons unrelated to initial propagule pressure. They recommend using propagule pressure as a null hypothesis for invasion studies, then moving onto other hypotheses if high density of propagules cannot explain the success or failure of an invader.
Reason #2: Abiotic resistance
Like propagule pressure, having just the right environment to start a non-native population may be a first step to success because abiotic factors are acting on individuals before they hit reproductive maturity (Moyle and Light 1996, Castro et al. 2002). So in a sense, this is the “make it or break it” stage for the recruits, and they could still fail despite swarming in large numbers.
Reason #3: Biotic Resistance
Even past the recruitment stage, life doesn’t get any easier on an invader, whether or not the environment is “just right”. Having to compete with other species for space and resources, the invader will either have to outperform the native species, or exploit their weaknesses. In a similar sense, an environment under constant disturbance provides more opportunities for an invader to jump in than one that rarely changes. But if the invader is not able to exploit those opportunities, understanding why can be useful for management and eradication efforts.
Reason #4: Genetic Constraints
Though the role of genetic variability or adaptability could be very important for understanding the success or failure of an invader, none of the studies on failed invasions included comparison of genetic characteristics or tests of potential hypotheses such as the bottleneck effect. For all we know, species could be experiencing bottleneck effect in different environments throughout their non-native range leading to their demise. But without comparing genetic constraints across environments outside their native range, we’ll never know how important of a component that may be to their success.
Reason #5: Lack of mutualists
This hypothesis serves as a sort of contrast to the ‘enemy release’ hypothesis by Keane and Crawley (2002), and reminds me a lot of my lab mate’s work. She works with an invasive legume that may rely heavily on a rhizobial mutualism to successfully invade different habitats with varying degrees of precipitation. In other words, this mutualist may be a crutch upon which the success of the species depends. This just goes to show that having friends around in a new place is just as important (if not more important) than not having enemies from home.
One thing that stood out to me in this paper was their argument for comparing not just between native and non-native populations, but also between non-native populations across environments or communities that did or did not succeed. I’ve only ever come across studies comparing traits between the native and non-native parts of a species range, and those have been very helpful for understanding whether there’s carryover or changes in species traits that allow them to become widespread. However, the idea of understanding what does and doesn’t work across places away from home makes just as much sense in ruling out potential mechanisms for success and understanding the relative importance of others. To understand this better, I like to think of it as going to a bar you’ve never tried before in the hopes of meeting new people:
In your normal bar, you’re the regular—you’ve made a lot of friends, know the bartenders, and know who to avoid. But you’re getting bored. You decide to try something new, and go outside your comfort zone to a bar you’ve not been to before with the intention of making new friends. Though some tactics used in your normal bar may also work well for “breaking the ice” with a stranger in the new bar, other normally successful strategies may be met with awkward glances or Irish exits from your converser. Nothin’ wrong with that, you learn from it and move on.
Now, let’s say your signature story about “that one time at band camp” was a huge success in this bar and have gained the courage to walk down the street to another bar to try your hand at chatting up more people. At the second new bar, you tell your story, regaling your experiences in great detail, but just get crickets from everyone. Despite the social environment in both new bars being the same, you didn’t succeed in making friends in both, but ignoring the fact that you failed in one of the bars doesn’t help anyone. To help yourself in future conversations, it’s just as helpful to understand the things that work well between normal and new bars as it is to know what will and will not make you a social butterfly across all new bars you try. The same goes for failed invasions, knowing what made them fail is just as important as knowing what makes non-native species succeed.
The bottom line: though including data on failed invasions may change what we think we know about invasions and predicting potential invasive ranges of species, it is important to consider them to help support or deny existing hypotheses about reasons for invasion success. Furthermore, Diez et al. (2009) points out that if failures are ignored, the probability of success would have been overestimated for most invasive species. So I ask the following question: If we’ve always been taught to learn from our mistakes, what’s stopping studies of widespread invasive species from doing the same?
Citations:
Blackburn, T. M. et al. 2011. A proposed unified framework for biological invasions. Trends Ecol. Evol. 26: 333–339.
Castro, J. et al. 2002. Mechanisms blocking Pinus sylvestris colonization of mediterranean, mountain meadows. J. Veg. Sci. 13: 725–731.
Diez, J. M. et al. 2009. Learning from failures: testing broad taxonomic hypotheses about plant naturalization. Ecol. Lett. 12: 1174–1183.
Keane, R. M. and Crawley, M. J. 2002. Exotic plant invasions and the enemy release hypothesis. Trends Ecol. Evol. 17: 164–170.
Moyle, P. B. and Light, T. 1996. Fish invasions in California: do abiotic factors determine success? Ecology 77: 1666–1670.
Zenni, R. D. and Nuñez, M. A. (2013), The elephant in the room: the role of failed invasions in understanding invasion biology. Oikos, 122: 801–815.
