N Nature of the Physical Environment Determines What Can Live There

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

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