Why study demography?

Demography is the study of the birth and death rates of a population. It’s used anytime people are interested in the health of a single population or in comparing multiple populations. Medical researchers study human demography to understand how different countries or areas compare in their birth and death rates, and this information is used to make decisions about how to invest money in health care or to predict how quickly the population is growing. Ecologists study the demography of wild populations mainly to answer two questions: 1) is the population growing, shrinking, or staying stable, and 2) why does the population have the trend that it does? To figure out how the number of animals is changing through time, we could count the animals. But to answer the second question, and understand why the size of the population is changing, we need to study demography.



Number of migratory birds on a 10 ha plot within the Hubbard Brook Experimental Forest, from 1969 to 2005 (from Holmes 2007 Ibis).

Researchers and wildlife managers need to understand why population sizes change in order to effectively do anything about it. For example, deer populations have increased in many parts of the country, leading to problems with overgrazing and deer starvation during harsh winters. One of the main reasons for the population increase has been a decline in the number of natural predators, like wolves and mountain lions, which has enhanced the survival prospects for deer. Managers, therefore, often adjust the number of hunting tags, or permits, in relation to the current deer population, and in this way can maintain deer populations at more natural levels.

So as you can see, a key part of the puzzle is figuring out what factors are causing the population to change in size. A population might be declining if the death rate is too high, if the birth rate is too low, or both. Figuring out which of these is going on is an important first step to designing a management and recovery plan. However, sometimes it’s difficult for researchers to tease apart all the environmental factors that affect a population. It can be especially hard with migratory birds, which often fly thousands of miles between where they breed in the summer and where they spend the winter. To make matters even harder, many birds die during migration, and it’s hard to tell if these deaths are due to bad weather, starvation, or both.

Yet, despite all these difficulties, scientists are learning a lot about how migratory bird populations are limited. So far, most studies have focused on what affects the number of offspring migratory birds raise during the breeding season, which scientists call their reproductive success. These studies have found that the abundance of predators that eat their eggs and young, the amount of food in the environment, and the number, or density, of a bird species breeding in a particular area are some of the most important ecological factors. In this module, we’ll explore how each of these affects the reproductive success of Black-throated Blue Warblers.

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