Figure 23.5: A population of cacti in the Sonoran Desert generally shows uniform (even)
dispersion due to competition for water. ( 10 )
curves – graphing the population numbers over time - allow us to also study how populations
grow and change, a topic that will be taken into more detail in subsequent lessons.
Births, Deaths, and Migration
Births, deaths and migration all affect population density and growth. The population
growth rate is the rate at which the number of individuals in a population increases. Pop-
ulation growth rate depends on birth rate and on death rate. The growth rate then is
represented by the equation:
growth rate = birth rate – death rate.Accordingtothisequation,ifthebirthrateisgreaterthanthedeathrate,thenthepopulation
grows; if the death rate is greater, then the population declines. If the birth and death rates
are equal, then the population remains stable.
Factors which influence a successful reproduction are age at first reproduction, frequency of
reproduction, the number of offspring, parental care, reproductive lifespan, and death rate
of offspring. In birds,altricial(helpless at birth and requiring much parental care (Figure
23.6)) andprecocial(independent at birth or hatching and requiring little parental care
(Figure23.7)) strategies use different reproductive systems to ensure breeding success.
Migrations and other movements in and out of populations affect population density as
well. Therefore, both birth andimmigration(movement of individuals into a population
from other areas) rates increase the population growth rate, while death andemigration
(movement of individuals out of a population) rates decrease the population rate. The earlier
growth rate equation now looks like this:
growth rate = (birth rate + immigration rate) – (death rate + emigration rate)