E. Pujol-Buxó, C. García-Guerrero and G. A. Llorente
Have you ever asked yourself, as a biologist, when should a species be considered “invasive”? You may even have discussed it with your colleagues. And when is a species “allochtonous”? Seems easy to define from a geographic point of view, but… what does an evolutionary point of view have to say about it? When does a species behave as allochtonous? Will an introduced population of a species ecologically act as allochtonous if it is introduced very near its natural range? Going further… what can we expect when species are introduced within the same ecoregion that comprises its native populations? This is what I asked myself – and even more – before planning the study “Alien vs. Predators: Effective induced defenses of an invasive frog in response to native predators” published in Journal of Zoology. If we choose my favorite study system, the introduced populations of the Mediterranean painted frog (Discoglossus pictus), in a practical sense the question is more simple: we have had Discoglossid frogs, Libellulid dragonflies and Notonectid backswimmers all around the Mediterranean basin for millions of years… so, focusing on their predator-prey relationships, will they ever function as unknown species if we are translocating these species inside this ecoregion? The fact is that although Discoglossus pictus has been introduced from another continent (Africa), its invasive (European) range is included in the same Mediterranean ecoregion. This means that both areas share several – most, in fact – genera and species of aquatic predators. So I thought I could plan some experiments with this array of species to shed some light onto this complex set of questions.
The first step was to see if in a natural environment – in order to control the presence of predators we had to mimic the “natural environment” with a complex mesocosm set – inducible defenses of the introduced frog in the presence of native predators would be clearly detectable. In other words, we wanted to know if the phenotypic plasticity of the introduced frog could be something similar to a native-native relationship with the predators. However, even when putting a lot of effort and motivation into a project, there can also be unwanted incidents: the project was in near termination when an unwanted and uncontrolled extra set of predators entered the mesocosms… ‘but how?’ you could ask. If you are reaching the article for the first time through this blog post, you will now have the privilege of laughing at what went wrong. How did some predators sneak into some mesocosms, forcing us to discard them? The story is that, for the clearing and maintenance of the experimental area, our university uses a small sheep herd. Somehow the sheep decided – and managed – to find their way into the enclosed area of active experimentation, and they also discovered that by destroying some nets they could access the mesocosms’ water and drink it. It must have been nice for them during the hot spring, but for us the result was close to a total disaster after three months of hard work – and not counting in previous planning. Luckily enough, the herd was somehow “scientifically grateful” and left untouched enough mesocosms of each treatment to make us believe that it was worth keeping up the experiment using this subset. Finally, we saw that continuing the experiment had been the right choice when the results confirmed our suspicions: the frogs clearly reacted to the native predators. And even more: the results were so clear that the lack of some mesocoms would not be a problem at all. Nice! Now, that motivated me enough to move to the second round of experiments in lab to answer the next question.
Once the results of the previous experiment were clear, the next question we wanted to answer was: are these inducible morphologies and behaviour really effective in reducing predation risk of the introduced frogs? Or it is just some spurious outcome? For this we had to plan some predation trials, and – believe me – it is really a fun work to do. We reared induced and non-induced tadpoles, predators, and prepared some water tanks with plastic plants and natural stones: after some weeks, we were ready for enjoying the work in this new experiment. I have to admit I was eager to see in front of my eyes – and I mean, the action, not the plot of data – how the native predators tried to feast on the introduced tadpoles and how the induced individuals managed to avoid it. Would this be the outcome? Or would the results in this second experiment contradict the first, showing us that what we thought as “inducible defenses” were in fact a non-adaptive change in morphology and behavior? More importantly for our motivation, this time nothing could go wrong: sheep are not allowed in the lab. The results were clear, easy to interpret, and did not contradict the first experiment. And yes, making preliminary tests with Notonecta backswimmers allowed us to enjoy seeing them in action. Backswimmers are indeed spectacular predators when they become hungry: we could not keep them together with the Libellulid darter nymphs because they ate some of them as well. So, morphological results were similar, and induced tadpoles really survived better the predation trials than non-induced. Summing up, we detected marked reactions of antipredator phenotypic plasticity in Discoglossus pictus tadpoles and these reactions seemed clearly effective in reducing their mortality and injury rates against the predators.
More experiments would be needed to confirm this, but we believe that the introduced frog benefits from a previous experience of these – or very similar – predator species. In conclusion, even though native and invasive ranges of Discoglossus pictus are in different continents, the similarity of predator communities of both areas may make this fact unimportant in terms of predator-prey interactions during its larval phase. Going back where we started, the introduced frog is not behaving like an introduced species in this case. We hope that the readers find the study interesting and inspiring for further work on the topic.