From pushed to pulled invasion dynamics: causes, consequences and applications for biocontrol – PushToiDeLa

We formulate theoretical predictions on the dynamical properties of populations in expansion, with the help of individual-based modelling. This approach allows us to investigate whether classical predictions made on infinite populations in continuous space can generalize to more realistic ecological contexts where space is discrete, heterogeneous, individuals are in limited number and stochastic effects are strong.

We also test these predictions using experimental invasions in laboratory microcosms. Most empirical data on invasions suffer from major biases related to the detection of low-density populations and to the over-representation of successful invasive populations. The use of laboratory microcosms is an efficient strategy to circumvent these methodological shortcomings. We chose as our main biological model parasitoid wasps of the genus Trichogramma, whose small size and short development time make excellent models to simulate invasions in laboratory microcosms. Our privileged access to a unique collection of more than 200 diverse Trichogramma strains will allow us to investigate the impact of different life-history traits on expansion dynamics.

In complement to these more fundamental approaches, we also take interest in how classical biological control provides empirical insight into the ecology of introduced populations in real-life ecosystems. In this project, we analyze datasets on the establishment and expansion of natural enemies populations introduced into an agro-ecological landscape to control the populations of invasive pests.

At this stage of the project, we have found that:
(i) Pushed and pulled waves differ not only in the dynamics of neutral genetic diversity, but in a large set of other indicators, many of which are based on demographic characteristics only. This important result is a first validation of the idea that a set of empirical indicators can help determine the type of an expansion wave.
(ii) Our experimental design managed to generate either pushed or pulled waves and that, according to theory, pushed waves retained their neutral genetic diversity longer than pulled waves
(iii) The dynamics of introduced populations in the field were characterized by a shift in demographic regime when the number of established populations increased, and secondary colonizations showed an acceleration of growth related to the continuous propagule pressure from already established sources nearby.

We successfully performed the first empirical work on the evolutionary dynamics of pushed expansion fronts. We demonstrated that (i) theoretical predictions obtained in continuous contexts could generalize into discrete, stochastic environments ; (ii) the genetic and velocity aspects of the pushed/pulled distinction, which have been assumed to be tightly linked can be decoupled when (weakly) pushed expansions are caused solely by reduced connectivity ; (iii) reducing connectivity may in some cases limit the loss of genetic diversity while not impeding spread rates.

This work has been the subject of two oral presentations at conferences (national and international), one manuscript has been written and is available as a preprint (https://www.biorxiv.org/content/10.1101/2020.05.13.092775v2) , two other manuscripts are currently under writing.

In colonization processes, propagation waves may be « pulled » when a few individuals on the front drive expansion, or « pushed » when all individuals from the core are involved. Although these concepts directly relate to the ecological properties of populations in a novel environment, so far they have not been applied to invasion biology or the use of beneficial organisms for pest control. We aim at demonstrating that the nature of the wave may affect key components of the expansion dynamics of an introduced population, with potential important consequences on its spatial spread and ultimately its invasion success. Our approach is built on a close dialogue between modelling and experiments, with inputs from PDE modelling, stochastic simulations, population dynamics, invasion biology, ecology, statistics and biological control. Experiments will be run in laboratory microcosms using Trichogramma as model invaders, and in the field, using the planned introduction of a novel biological control agent as a special case of a biological invasion. Our results will allow us to estimate the relevance of the concepts of pulled and pushed waves in ecology and to propose a unifying framework for the study of expansion dynamics. On a more applied perspective, a major challenge in crop protection is to predict, anticipate and manage the progression of invasion fronts, either to prevent the spread of invasive pests or to enhance the dispersal of biological control agents. In this context, our results will also contribute to improve the predictions of colonization dynamics of invasive pests and their introduced natural enemies, and to design innovative levers for population management in agricultural landscapes.