Download Demography, life tables and survivorship curves

Demography, life tables and survivorship curves
Population demography
 Populations can be described by vital statistics or demographics
 Size
 Change in size over time (growth)
 Age Structure
 Density
 Distribution
 Rates of dispersal between populations
 Sex ratio
Population size
 Number of individuals that contribute to a population’s gene pool
 Can be determined by Capture-Recapture Method for mobile species
Capture-recapture method
 Sample small number and extrapolate
 Capture, mark, and release individuals
 Return later and capture second sample
 Count the number of marked individuals and use this to estimate total
population
Capture-recapture equation
Marked individuals
Marked Individuals
in sampling 2
=
in sampling 1
Total capture
Total population size
in sampling 2
Factors that increase populations
 Natality - Production of new individuals - (births, germination, etc.)
 Fertility - Measure of actual number of offspring produced
 Immigration - Organisms introduced into new ecosystems
Factors that decrease populations
 Mortality - Death Rate
 Survivorship - Percentage of cohort surviving to a certain age
 Life expectancy - Probable number of years of survival for an individual of a
given age
 Life span - Longest period of life reached by a given type of organism
 Emigration - Movement of individuals out of a population
Life history patterns
 Patterns of timing of reproduction and survivorship vary by species
 Summarized in survivorship curves and life tables
Life table
 Follows the fate of a given cohort of individuals
 A cohort is a group of individuals born in the same designated time interval
 Tracks age-specific patterns
 Population is divided into age categories
 Birth rates and mortality risks are calculated for each age category
Survivorship curve
 Can be created from life tables
 Follows survival of a given cohort of individuals over time
 Graphs survival per 1000 individuals versus age on a logarithmic scale
Survivorship curves
Survivorship curves
 Log(lx)
 Shows risk of mortality
Survivorship curve for Poa annua
Survivorship curves
 Four general patterns:
 Full physiological life span
 Probability of death unrelated to age
 Mortality peaks both early and late in life
 Mortality peaks early in life
Survivorship curves
Crowding
 Annual plant Erophila verna
 Varied planting density
 Found shift in survivorship curves
 Intraspecific competition
 Density can affect all BIDE factors
Life expectancy