55.2 How Do Ecologists Study Population Dynamics?
To understand population growth,
ecologists must measure population
processes as well as population traits.
The study of population processes is
known as demography.
Demographic events: births, deaths,
immigration, and emigration.
55.2 How Do Ecologists Study Population Dynamics?
Multiple estimates of population
densities over time can be used to
estimate the rate of population growth
or decline.
• Populations grow when individuals are
added by births or immigration
• Populations decrease by the number
of individuals lost by death and
emigration
55.2 How Do Ecologists Study Population Dynamics?
N1 = N0 + (B – D) + (I – E)
N1 = # of individuals at time 1
N0 = # of individuals at time 0
B = # of births in the time interval
D = # of deaths in the time interval
I = # that immigrated in the time
interval
E = # that emigrated in the time interval
55.2 How Do Ecologists Study Population Dynamics?
A life table keeps track of demographic
events.
A cohort life table tracks a group of
individuals born at the same time
(also called a horizontal life table).
Numbers that are still alive at later
dates (survivorship) are determined.
Mortality is the proportion of each age
class that die before reaching the next
age class.
Table 55.1
55.2 How Do Ecologists Study Population Dynamics?
Age categories in a cohort life table
depend on the organism’s life cycle.
• Age-dependent: track demographic
events as a function of calendar age
• Stage-dependent: track demographic
events by life cycle stages (e.g.,
insect eggs, larvae, pupae, and
adults)
55.2 How Do Ecologists Study Population Dynamics?
Fecundity (mx) = number of female
offspring produced by each female.
• Allows estimates of a population’s
potential for growth.
Fecundity schedules vary among
species because organisms differ in
the number of offspring they can
produce and in the timing of
reproduction.
55.2 How Do Ecologists Study Population Dynamics?
Life tables can be used to predict future
trends.
For cactus finch, mortality rate was
high during the first year, then
dropped.
Mortality rate and fecundity fluctuated
year to year because the birds are
dependent on seed production, which
fluctuates with rainfall.
55.2 How Do Ecologists Study Population Dynamics?
Vertical life tables a population is
sampled at a single point in time,
when a cohort cannot be tracked
through time.
• Can be constructed from a death
assemblage—a collection of bodies or
fossils.

Example: birth and death dates from
tombstones in a cemetery.
55.2 How Do Ecologists Study Population Dynamics?
Survivorship curve: plot of
proportions of individuals surviving
through each life stage (lx).
• Correlated with general patterns of life
history traits, which suggest mortality
factors the population faces.
Survivorship curves fall into three
general patterns:
Figure 55.5 Survivorship Curves (Part 1)
55.2 How Do Ecologists Study Population Dynamics?
• Physiological survivorship curves:

High survivorship through adulthood
(e.g., humans and other large
mammals).

Parental care and low fecundity are
typical.
Figure 55.5 Survivorship Curves (Part 1)
55.2 How Do Ecologists Study Population Dynamics?
• Ecological survivorship curves:

Constant risk of mortality at all ages
(e.g., most birds)
Figure 55.5 Survivorship Curves (Part 2)
55.2 How Do Ecologists Study Population Dynamics?
• Maturational survivorship curves:

Low juvenile survivorship (e.g., most
insects and annual plants)

Produce many offspring, but little or
no parental care
Figure 55.5 Survivorship Curves (Part 2)
55.2 How Do Ecologists Study Population Dynamics?
An organism’s life history strategy
describes how it allocates time and
energy among the various activities
throughout its life.
Life history strategies vary dramatically.
Variations determine how fast
populations can grow.
Environmental conditions can also
influence life history traits.