Biogeography

Bacteria in our guts, insects in a field, trees in a forest and plants borne by air and water around the world are among many examples whose spatial distributions are important and interesting study. Mechanisms of dispersal include making small random moves, being carried along by air of water, and being attracted to certain areas by chemical signals, nutrients, light, etc. Patterns of organisms can be formed by dispersal or aggretation. On the other hand, immobile organisms, like some bacteria on plant roots, can find themselves in habitats where nutrition fluctuates [...]
The patterns of organisms that result from these can be important in how they influence their environment or how they survive adversity. For example, there are foci of infectious diseases in human populations and foci of infestation insects in forests that cause recurring outbreaks.
Biogeography deals with the spatial distribution of organisms. This can be studied by cellular automatas, Markov Chains, random walks (Game of Life) and Reaction-diffusion equations.

From the book:
F.C. Hoppensteadt, C.S. Peskin. Mathematics in Medicine and the Life Sciences. Springer-Verlag. 1992. pp. 83-97