Simulate biological invasions to better understand them
Several definitions exist in the scientific literature to explain precisely what a biological invasion is. For this study, it is defined as a process characterized by the introduction, establishment and then the spread of a species outside its original environment. This process is very difficult to predict, since studies on this subject can only analyze the phenomenon once the invasion has already begun (and therefore only when it has succeeded). In this case, how can we achieve a better understanding of the evolutionary processes at play during an invasion in order to limit its ecological impacts? How to define what allows it to succeed or fail?
A scientific project funded by the National Research Agency has enabled INRAE researchers to simulate biological invasions in the laboratory under controlled conditions, in order to observe their mechanisms and better understand how invasions spread. The model used for this simulation using an experimental approach is the trichogramma, a parasitoid micro-wasp.
Identification of a new mode of expansion
To carry out their experiment, the researchers installed populations of trichogramma in tubes connected to each other by pipes, forming a circuit that can be modified at will and can be enlarged or reduced as they expand. They expected to observe the mode of propagation mostly described to date, which consists, for a few individuals, of migrating to a new area to settle there, reproduce there and form a new subpopulation. Sub-population which in turn will resume the same migration strategy, and so on. It is this dynamic that makes the invasion uncontrollable because it amplifies from very few individuals, and the reduction in genetic diversity accelerates the process by gradually selecting the individuals most capable of disperse. This strategy is said to be “pulled”, because only a few individuals “pull” the expansion.
However, what the researchers observed with the trichogramma is quite different. To colonize a new habitat, Trichogrammas wait until the initial population has reached a certain size. They are indeed sensitive to the density of the population around them, and if there are not enough of them, the individuals do not disperse to colonize new habitats. This strategy is then called “push”, because it is the size of the population that “pushes” the expansion. The invasion is then slower, having to wait each time the installed population grows, and consequently more sensitive to the quality of the environment and the quantities of resources. The genetic diversity also remains stable, unlike that of the more selective “pulled” invasions.
Two different expansion strategies
This discovery has thus made it possible to identify the presence in nature of two distinct expansion strategies, which may differ between species but also vary according to the local ecological context. This will therefore imply a revision of the experimental methodologies but also of the criteria for interpreting the results in future studies on this subject. The additional criterion to be taken into account in particular is to be able to identify, for the species studied, towards which dynamic its invasion strategy tends the most in order to deduce whether it can be predictable or controllable. If its dynamic is rather “pulled”, its speed of expansion will remain stable regardless of its environment. It will therefore be easily predictable regardless of location, but cannot be controlled (i.e. slowed down, slowed down or accelerated). On the other hand, if the invasion dynamic of the species turns out to be rather “pushed”, its speed of expansion may vary from one place to another, depending on the quality of the environment and the availability Resource. It will be much more difficult to predict how quickly it will spread over a given area, but it can be more easily controlled by looking for what environmental factors slow down this invasion or favor it. At the ecological level, species with a strong expansion dynamic will be more controllable, which would make it possible to better prevent future invasions which might ultimately harm the environment.
The data, analyzes and results of this study are also available in OpenAccess on the PeerCommunity platform, in order to guarantee accessibility, transparency as well as the reproducibility of the results and to allow a better advance of knowledge regarding this eco-dynamic. scalable.
REFERENCE
DahirelM., BertinA., HaondM., AngryA., LombaertE., HeelV., FellousS., MailleretL., MalausaT. and VerckenE. (2021), Shifts from pulled to pushed range expansions caused by reduction of landscape connectivity. Oikos130: 708-724. https://doi.org/10.1111/oik.08278
