Visualizing Environmental Science

Population Ecology 161

The growth rate (r) of a local population must take into

account birth rate (b), death rate (d ), immigration (i ),

and emigration (e) (Figure 7.2b). The growth rate equals

(birth rate minus death rate) plus (immigration minus

emigration):

r = (b – d) + (i – e)

Maximum Population Growth

Different species have different biotic potentials (also

called intrinsic rates of increase). Several factors influ-

ence the biotic potential of a species: the age at which

reproduction begins, the fraction

of the life span during which an

individual can reproduce, the

number of reproductive periods

per lifetime, and the number of

offspring produced during each

period of reproduction. These factors, called life history

characteristics, determine whether a particular species has

a large or a small biotic potential.

Generally, larger organisms, such as blue whales and

elephants, have the smallest biotic potentials, whereas

micro organisms have the greatest biotic potentials.

Under ideal conditions (that is, in an environment with

unlimited resources), certain bacteria reproduce by

dividing in half every 30 minutes. At this rate of growth,

a single bacterium increases to a population of more

than 1 million in just 10 hours and exceeds 1 billion in

15 hours. If you plot bacterial population numbers ver-

sus time, the graph takes on the characteristic J shape of

How Do Populations Change in Size?

Populations of organisms, whether sunflowers, eagles,
or humans, change over time. On a global scale, this
change is due to two factors: the rate at which individual
organisms produce offspring (the birth rate) and the
rate at which individual organisms die (the death rate)
(Figure 7.2a). In humans, the birth rate (b) is usually
expressed as the number of births
per 1000 people per year and the
death rate (d) as the number of
deaths per 1000 people per year.
The growth rate (r) of a popula-
tion is the birth rate (b) minus the
death rate (d):

r = b – d

Growth rate is also referred to as natural increase in
human populations.
If more individuals in a population are born than
die, the growth rate is more than zero, and population
size increases. If more individuals in a population die
than are born, the growth rate is less than zero, and pop-
ulation size decreases. If the growth rate is equal to zero,
births and deaths match, and population size is station-
ary, despite continued reproduction and death.
In addition to birth and death rates, dispersal—move-
ment from one region or country to another— affects
local populations. There are two types of dispersal:
immigration (i), in which individuals enter a popula-
tion and increase its size, and emigration (e), in which
individuals leave a population and decrease its size.

growth rate (r)

The rate of change

(increase or decrease)

of a population’s

size, expressed in

percentage per year.

biotic potential

The maximum rate at

which a population

could increase under

ideal conditions.

Global

population Local

population

Increase population:

Births Deaths

Decrease population: Increase population: Decrease population:

Births Deaths

Immigration Emigration

a. On a global scale, the change in a population is
due to the number of births and deaths.

b. In local populations, such as the population of

the United States, the number of births, deaths,

immigrants, and emigrants affects population size.

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