Research has already shown that invasive species tend to be more tolerant to environmental stress than related non-invasive species. However, a recent study published in Biological Invasions, set out to discover whether this stress tolerance was an inherent trait or whether it was something acquired en route from their natural habitat to the new one.
There’s no doubt that humans aid and abet invasive species. Growing numbers of species conquer new habitats due to human movement. For centuries, human travel has been the source for introducing species to new lands and today, increased global travel and trade means that the cycle is ongoing.
Aquatic organisms in particular travel the world aboard ocean going vessels, and the container shipping industry is continuing to grow, facilitating more travel for these species. The quantity of goods carried by shipping containers has risen from approximately 100 million tonnes in 1980 to about 1.7 billion metric tonnes in 2015. Further, vessels have increased in capacity, between 1980 and 2016, the deadweight tonnage of container ships has grown from about 11 tonnes to around 244 million metric tons. The global shipping of goods helps more and more marine species to reach new ecosystems.
Researchers from the Global Approach by Modular Experiments (GAME) tested this potential for strengthening by placing a variety of mussel species from around the world under thermal stress to mimic the challenges faced by marine invertebrates transported on ship hulls or in ballast water tanks. Many organisms that are transported on ships experience periods of stress, for example when vessels that come from temperate zones travel through tropical seas.
The study used five different mussel species collected from sites in Brazil, Chile, Finland, Germany (Baltic Sea) and Portugal. These five species were exposed to heat challenges that caused 60-85% mortality within 15-28 days. Those that survived were then exposed to a second stress event with their survival compared to the robustness of members of the same species who had not previously been exposed to elevated temperatures.
The results of the study were diverse. While they did not indicate increased resilience in all the species in the study, they did observe that thermal tolerance was significantly enhanced in Semimytilus algosus from Chile and in Mytilus edulis from Germany, and in fact much more so in the latter. This suggests that heat stress can make a population of mussels more robust and therefore potentially more invasive; something that has not previously been considered in bio-invasions research.
Lead researcher for the study, Dr Mark Lenz concluded, “For marine species, human beings and their technology are not only vectors, but they could also be trainers that enhance their invasiveness. This must be taken into account in further investigations.”