same species form an interbreeding unit. Ecologists who study populations determine how
healthy or stable they are and how they interact with the environment, by asking specific
questions, such as, is a certain population stable, growing, or declining, and what factors
affect the stability, growth, or decline of a threatened population?
In determining the health of a population, one must first measure its size or thepopulation
density, the number of individuals per unit area or volume, such as per acre. Population
size or density can also be examined with respect to how individuals are distributed. How
individuals are spaced within a population is referred to asdispersion. Some species may
show a clumped or clustered distribution (Figure23.4) within an area, others may show a
uniform, or evenly spaced (Figure23.5), distribution and still others may show a random,
or unpredictable, distribution.
Figure 23.4: Individuals within this population of the purple loosestrife plant species show
a clumped distribution due to local variation in soils. ( 20 )
Otherfactorsofimportanceinthestudyofpopulationsareageandsexwithinthepopulation.
The proportion of males and females at each age level gives information aboutbirth rate
(number of births per individual within the population per unit time) anddeath rate
(number of deaths per individual within the population per unit time), and this age structure
may give further information about a population’s health. For example, an age structure
with most individuals below reproductive age often indicates a growing population. A stable
population would have roughly equal proportions of the population at each age level, and a
populationwithmoreindividualsatorabovereproductiveagethanyoungmembersdescribes
a declining population.
Another pattern in populations has to do with how they change with time. Survivorship