Individual-based models (IBMs) describe individual organisms as autonomous, unique entities. Some IBMs deal with quite simple individuals, which are characterized by just their position. Other IBMs include the individual's adaptive decisions of what to do next, so that individuals are characterized by a set of state variables, for example, age, sex, size, social rank, energy reserves, memory, etc. But why should ecological models be based on the representation of individuals in the first place? Models always have to simplify, so why do we not ignore individuals, their variability, and their behavior, and rather consider average individuals as in classical theoretical population ecology?
There are three main reasons why it can be necessary to represent individuals in ecological models:
1. Individual variability. Individuals usually are different, even if they are of the same age. If resources like food or space are scarce, individuals that are larger, stronger, or have more experience than others may have a competitive advantage. Moreover, during their life cycle, individuals not only change in size but often also in food requirements, behavior, trophic interaction, etc.
2. Local interactions. Most mathematical population models assume global interactions, that is, all individuals interact with all other individuals, but real interactions are local, which can be important. For example, the global density of individuals may be low enough to provide, on average, enough food or space for each individual, but local density may in some places be much higher than global density.
3. Adaptive behavior. Individuals seek to maximize fitness, that is, survive and produce as many offspring as possible that reproduce themselves. To achieve this aim, they adapt their behavior to the current state of themselves and their biotic and abiotic environment (Figure 1). Behavior not only includes movement, foraging, mating, etc., but also growth, development, and life history. Adaptive behavior is very likely to affect or even determine system-level properties of populations, communities, and ecosystems. For example, herbivores like elks change their foraging behavior in the presence of predators, for example, wolves, by selecting other types of habitat. This in turn affects the structure and dynamics of the vegetation and of the entire herbivore community, etc.
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