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.