Although populations are made up of individuals, populations possess additional properties that individuals lack. Among these properties are birth rate, death rate, age structure and genetic fitness. Such properties apply only to the group level.
Population density refers to the population size relative to some unit of space.
It is generally expressed in terms such as trees per acre, Vinegar Weeds per square meter, coyotes per square kilometer, etc. Crude density includes all the land within the organism's range whereas Ecological density includes only that portion of land that can actually be colonized by the species. The crude density of the Checkered Fritillary in Butte County is very low but where it is found in may be very abundant. This variation in density is important to both the species in question and its predators. With its relatively high ecological density, the forager merely has to know its specific habitat.