Population Ecology
Chapter 56
Environments vary
 Nature of the physical environment determines what can live there
 Temperature
 Water
 Sunlight
 Soil
Response to environmental variation
 Maintain steady-state internal environment
 Homeostasis
 Organisms have physiological, behavioral, or morphological
mechanisms to maintain homeostasis
 Change with the environment
 Conformers
Physiological mechanisms
 Acclimate to environment by making physiological adjustments
Morphological mechanisms
Behavioral mechanisms
Adaptation to environmental variation
 The ability to alter morphology, physiology and behavior is a result of natural
selection
 Natural selection can also influence more “set” features in an organism
 e.g., Allen’s Rule
What is population ecology?
 A population is a group of individuals of one species living in the same area
and interbreeding
 Ecology is the study of organisms and their interactions with the environment
and each other
 Population ecology is therefore the study of how an organism’s response to
the environment affects the properties of populations
 Emphasis on population growth and demography
Characteristics of population ecology
 Population range - area throughout which a population occurs
 Pattern of spacing of individuals
 How population changes in size through time
Population range
 No population occurs in all habitats throughout the world
 Each population of organisms has factors that restrict its geographic location
or population range
 Ranges can be narrow or broad
Endemic species
Devil’s hole pupfish
Populations
 Organisms must be adapted for the environment in which they occur
 Each population has its own requirements
 Predators, competitors or parasites may prevent a population from occupying
an area
 Ranges undergo expansion and contraction
The human effect
 Humans alter the environment and some species have altered their range to
areas not previously occupied
Dispersal
 Natural
 Human-aided
 Invasive species
Dispersal mechanisms
Dispersal mechanisms
Dispersal mechanisms
Population distribution
 Describes how individuals within a population are distributed geographically
Random distribution
 No strong interaction among individuals
 Uniform environment
Uniform distribution
 Territorial behaviors
 Competition for resources
 Allelopathy
Clumped distribution
 Response to uneven environment
 Common in nature
 Social interactions
 Greater density in interior of range
Metapopulations
 A population that consists of several subpopulations linked together by
immigration and emigration
 Metapopulations are found where patches of suitable habitat are separated by
patches of unsuitable habitat
 Source-sink dynamics – A type of metapopulation structure whereby some
habitats have higher quality than others
Vernal Ponds
Populations
 Source-sink metapopulations: some areas are suitable for long-term habitat
others are not
 Extinctions in some areas due to unfavorable conditions
Source-sink metapopulations
 Habitat patches vary in quality
Metapopulations
 Metapopulations can have two implications for the range of a species
 Continuous colonization of empty patches
 Prevent long-term extinction
Population Ecology
Demography
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
 Fecundity (birth rate) - Production of new individuals
 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 and mortality rates 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 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
Life expectancy