In the preceding discussion, the factors that limit population size depended on the density of the population. At low density, food and other resources do not limit growth, but as density increases, they exert more and more influence on population growth, spurring competition for available resources. Similarly, at high population density, individuals may be more vulnerable to predation or to infection. Factors such as resource availability and predation are called density-dependent factors.

Not all changes in population growth rates result from population density, however. Density-independent factors influence population size without regard for the population’s density. These factors include events like severe drought or a prolonged cold period, either of which can cause widespread mortality independent of population density. A graph of density-independent effects on populations shows instantaneous, rather than gradual, drops in population size, and these changes can occur at different starting densities.

1010

For example, population growth in Song Sparrows is a function of the number of nesting territories established by males each year, and of their young’s success in those territories. Fig. 46.7 shows that the number of territories generally increases for several years until an unusually dry spring occurs, which greatly decreases the amount of vegetation available to support the insects on which the sparrows feed. The number of territories then plummets, regardless of territory numbers the previous year.