Population Ecology Part III

Life Table (Mortality table)

Life table summarizes survival, mortality, and reproductive patterns of a population.

Age Pyramids

Used to descried the structure of a population

not as good at describing structure of population

Use of life tables and their applications in Ecology

• Population Dynamics and Demography - Age structure Survivorship.

• Conservation Biology - Endangered Species Management, Habitat Restoration 

• Ecosystem Health and stability - Biodiversity Monitoring Invasive Species Control 

• Evolutionary Biology - Life History Evolution 

• Pest Management - Integrated Pest Management Biological Control. 

Types of Life Tables

1) Cohort Life Table

2) Static or time-specific life table

Cohort Life Tables 

Table based on individuals at the same time (and followed throughout the life span) 

l_x meaning

Survival of the life-table variables. Probability of being alive.

used to draw survivorship curves

b_x meaning

Fecundity of age x

m_x meaning 

Proportion of individuals of age x dying by age x+1

s_x meaning

Proportion of individuals of age x surviving to age x+1. 

Static Life Table

A life table constructed by recording the age at death of a large # of individuals.

Snapshot of survival within a population during a short period of time. 

2 ways of constructing life table

• Static Life Tables - Made from mortality data collected from a specified time period. 

• Cohort Life Tables - Follows a group of same-aged individuals from birth (or fertilization) to death.

Problems of cohort life tables

• Organisms have complex life histories.

• Not always possible or feasible to follow a single cohort from birth to death.

Types of Survivorship Curves

1) Type l: Convex Shape (ex; Mammals)

2) Type ll: Linear Shape

• Log-transferred l_x values are often used when presenting survivorship curves

3) Type III: Concave Shape (ex; frogs)

• Will use Log₁₀ transformation notice that calculated values will be negative

m_x meaning 

Number of newborns produced by each age class (Given) 

R or R_x meaning

Reproductive rate of each age class

R₀ meaning (R₀= Sum(l_xm_x)

Net reproductive rate for all years together (whole population) (Average # of offspring produced over lifetime) 

• Assumes: Birth rates/Death rates in population are constant AND have a stable age distribution. 

G meaning

Average generation time: average age of mothers when give birth to their first offspring.

r meaning

‘Intrinsic rate of natural increase’

Also used in population growth models

Population with discrete generations 

Each generation is separate in time, clearly identifiable.

• Changes in #’s are ‘pulses’ or discrete.

ex; Annual plants. 

Population with overlapping generations

Organisms can breed continuously or more than once during year/season.

ex; humans, parasites, bacteria, insects (bug), reproduce continuously. 

r-intrinsic rate of natural increase equation and

r= ln(R₀/G)

G=(sum(l_xm_x)x)/R₀