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.