Young Black-throated Blue Warbler that just left its nest (photo by M. Cline).

One of the oldest questions in ecology is: Why don’t population sizes keep on increasing? For example, if an average pair of American Robins can raise four young each year and each bird can breed for three years, then every two birds would produce twelve in their lifetimes! If those reproductive rates continued, pretty soon the world would be teeming with robins. Yet scientists have recognized that those demographic rates don’t continue, and either life-spans get shorter or reproductive rates get lower when population sizes are very large. Populations have been observed to grow quickly when numbers are low and conditions are good, but population growth then slows once there are so many individuals that conditions for each individual become worse. This process is called density-dependence, and it is thought to be a very important process in regulating - or controlling - the size of natural populations. Density dependence stabilizes a population at its carrying capacity, which is the population size that can be stable over time.

Population density is the number of individuals in a given area, so the term density-dependence refers to the fact that the demographic rates vary in relation to density. When density is low, and the population is below its carrying capacity, each individual has plenty of the resources—food, water, nest sites—it needs. Individuals in these good conditions can survive longer and/or breed more successfully, so the population grows. Yet once the population is large, more individuals have to compete for the same amount of resources, so the average share for each one is lower. This leads to lower survival or reproductive success, so the population can stabilize at the carrying capacity. Finally, when there are more individuals than the environment can support, death rates will be high or reproduction will be unsuccessful, causing the population to decline back to the carrying capacity.

In this module we'll explore how predators, food and density influence the reproductive success of Black-throated Blue Warblers. But first we'll learn a thing or two about the biology of the species and the place where they were studied.

Learning module developed by K. Langin, H. Sofaer and S. Sillett for the Hubbard Brook Research Foundation (2009).