Movement of organisms and the dynamics of populations in space

Space does matter at every scale of the hierarchical structure of biology, from nucleic acids to cells, from tissues to organisms, and from populations to the whole biosphere. Spatial patterns that we observe are clearly the result of aggregation phenomena that are constrained by fundamental biological or behavioural mechanisms. The shape of flowers, horns, shells, cones clearly reveals spatial organization, often characterized by a striking and fascinating regularity. Spatial patterning, though less regular and possibly time-varying, is also shown by many populations and ecosystems, e.g. schools of fish, flocks of birds or the tiger bush of many arid regions (see Fig. 1). Waves are also a typical spatial phenomenon that characterizes the functioning of many populations and ecosystems. Of particular importance to the present Anthropocene is the invasion of new species (often alien species that can contribute to the extinction of endemic ones) and the spread of pathogens.

Figure: Aerial view of tiger bush in the W Bénin-Niger National Park. Average distance between two successive gaps in the vegetation is 50 meters. Photo by Nicolas Barbier [CC BY-SA 3.0].

The models illustrated so far are a very approximate description of simple ecological systems, because the real challenges that ecology poses to modelling come substantially from the complex spatio-temporal dynamics of populations, communities and ecosystems. Within this complexity, different scales are clearly seen, often hierarchically organized: for example, an exhaustive explanation of the functioning of a population of small mammals presupposes an understanding of (i) the physiology and behavior of individuals, which occurs on a short time scale (days), (ii) the vital cycle (growth, survival, reproduction), describable on a longer time scale (months), and (iii) the demography of the entire population, occurring on an even longer time scale (years). If we then think that every population is spatially organized and inserted in ecosystems whose essential components range from the microscopic dimensions of bacteria (with the relative space-time scales) to the macroscopic dimensions of large mammals or secular plants, we understand how ecology can be to modellers a challenging field that offers considerable opportunities for the application or development of new techniques. It is the integration of the different scales that poses the greatest challenges compared to mathematical tools that have traditionally been developed to explain relatively homogeneous physical and chemical systems. At the highest level of biological organization, the basic mechanism of spatial dynamics is the ability of organisms to move, leaving their location where they dwell for a time and colonizing new space. Even plants and fungi can actually move via vegetative reproduction (colonization of the surrounding space) or the release of propagules (seeds, spores). Understanding the implications of movement is therefore the fundamental approach to spatial ecology and is thus the subject of this chapter.