# 3.1: Characterizing populations

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One way to describe populations is to look at how many individuals they contain at various times. Or, instead of individuals, it may be more reasonable to consider total biomass— the total weight of all individuals in the population combined. For example, the number of trees may not be as important as their total weight, or the total area of their canopy. Density may also be relevant—how many individuals occupy a unit of area, or the percentage covered by the population in an area. All of these are gross properties of populations that can enter models.

Additional properties that can be taken into account include the age structure—the portions of the population of various ages, and the size structure—the portions of the population of various sizes. Such detail can be important because juveniles or older individuals may not reproduce. Genetic structure can also be important; it is often left out of ecological models, but evolutionary directions can affect the ecology as well.

Another important measure is the rate of change—how fast the population is changing. A population can be constant, increasing, or decreasing, or can fluctuate in complex ways.

Remember that a model is just a simpler view of something complex, and that all scientific models are just approximations of nature. When the full complexity cannot be understood, which is almost always, we can try to construct a simplified model in hopes of finding the essence.