Emerging predictable features of replicated biological invasion fronts

Abstract

Biological dispersal shapes species' distribution and affects their coexistence. The spread of organisms governs the dynamics of invasive species, the spread of pathogens, and the shifts in species ranges due to climate or environmental change. Despite its relevance for fundamental ecological processes, however, replicated experimentation on biological dispersal is lacking, and current assessments point at inherent limitations to predictability, even in the simplest ecological settings. In contrast, we show, by replicated experimentation on the spread of the ciliate Tetrahymena sp. in linear landscapes, that information on local unconstrained movement and reproduction allows us to predict reliably the existence and speed of traveling waves of invasion at the macroscopic scale. Furthermore, a theoretical approach introducing demographic stochasticity in the Fisher-Kolmogorov framework of reaction-diffusion processes captures the observed fluctuations in range expansions. Therefore, predictability of the key features of biological dispersal overcomes the inherent biological stochasticity. Our results establish a causal link from the short-term individual level to the long-term, broad-scale population patterns and may be generalized, possibly providing a general predictive framework for biological invasions in natural environments.hat is the source of variance in the spread rates of biological invasions? The search for processes that affect biological dispersal and sources of variability observed in ecological range expansions is fundamental to the study of invasive species dynamics (1-10), shifts in species ranges due to climate or environmental change (11-13), and, in general, the spatial distribution of species (3,(14)(15)(16). Dispersal is the key agent that brings favorable genotypes or highly competitive species into new ranges much faster than any other ecological or evolutionary process (1,17). Understanding the potential and realized dispersal is thus key to ecology in general (18). When organisms' spread occurs on the timescale of multiple generations, it is the byproduct of processes that take place at finer spatial and temporal scales that are the local movement and reproduction of individuals (5, 10). The main difficulty in causally understanding dispersal is thus to upscale processes that happen at the shortterm individual level to long-term and broad-scale population patterns (5,(18)(19)(20). Furthermore, the large fluctuations observed in range expansions have been claimed to reflect an intrinsic lack of predictability of the phenomenon (21). Whether the variability observed in nature or in experimental ensembles might be accounted for by systematic differences between landscapes or by demographic stochasticity affecting basic vital rates of the organisms involved is an open research question (10,18,21,22).Modeling of biological dispersal established the theoretical framework of reaction-diffusion processes (1-3, 23-25), which now finds common application in dispersal ecology (5,14,22,(26)(27)(28)(2...

Generalized receptor law governs phototaxis in the phytoplankton Euglena gracilis

“…The linear landscapes (Fig. 1) used in the experiments were channels drilled on a Plexiglas sheet (61). A second Plexiglas sheet was used as a cover, and a gasket prevented water spillage.…”

“…Density estimates were obtained by placing the linear landscape under the objective of a stereomicroscope (Olympus SZX16), taking pictures (with the camera Olympus DC72), and counting individuals through image analysis as in ref. 61. Stationary density profiles were measured after 210 min from the introduction of cells in the landscape.…”

Range expansions transition from pulled to pushed waves as growth becomes more cooperative in an experimental microbial population

“…Melbourne and Hastings (31) have carried out a very detailed comparison between theory and experiment for a laboratory population of flour beetles and showed that the unavoidable heterogeneity of the founding organisms leads to large variation in the rate of spread between replica populations. At the microscopic scale, Wakita et al (32) tested the expected relation between the rate of spread and nutrient availability in Escherichia coli, and Giometto et al (33) used the theory of pulled waves to describe the expansion of tetrahymena in linear channels. All of these studies, however, focused only on the rate of invasion and did not test theoretical predictions for the shape of the invading fronts.…”

“…However, such experiments can guide our thinking, show which theoretical predictions may be observable in nature (34,35), and help develop new models (36). For example, range expansions of microbial populations have revealed the dependence of the invasion velocity on the supply of resources (37) and demographic stochasticity (33). Experiments with microbes have also demonstrated the strong effect of range expansion on competition (38)(39)(40)(41)(42)(43)(44) and neutral evolution via the founder effect or gene surfing (45,46).…”

BEMOVI, software for extracting behavior and morphology from videos, illustrated with analyses of microbes

“…Several studies show, however, that 2D tracking successfully predicts spread rates even in higher dimensional systems (Giometto et al. 2014; Pennekamp 2014). Complex environments with many physical obstacles (e.g., debris particles in our medium) prevent reliable tracking in three dimensions, because individuals may be frequently invisible to one of the three required cameras.…”