Population Ecology and the Effects of Density

Population: a group of individuals of the same species living in an area * Population ecology: analyzes the factors that affect population size and how and why it changes over time
Density: # individuals / area * Can be determined by: * Counting the number of individuals in the population (rarely done) * Sampling techniques (count small areas, average the areas, and then use the averages to estimate total population size)
Dispersion: pattern of spacing between individuals, how the population is spread out * Clumped: individuals gather in patches * Uniform: evenly spaced individuals in a population * Can be due to territoriality * Random: unpredictable spacing; not common
The size of a population is not static, affected by: * Births/deaths * Immigration/emigration * Demography: the study of the vital statistics of populations and how they change over time * Life table: an age-specific summary of the survival pattern of a population * Represented by a survivorship curve

Type I curve: low death rate during early/middle life and high death rate later in life
Type II curve: constant death rate over the lifespan of the organism
Type III curve: high death rate early in life and lower death rate for those that survive early life

Exponential growth model: a population living under ideal conditions (ie easy access to food, abundant food, free to reproduce, etc) * Population grows rapidly * A population growing exponentially grows at a constant rate * J shaped curve
Logistic growth model: a population's per capita growth rate decreases as population size approaches a maximum imposed by limited resources, the carrying capacity( K). * The density of individuals exceeds the system’s resource availability

Populations are influenced by natural selection and environmental factors * Life history: the traits that affect an organism’s schedule of reproduction and survival * Three variables affect life history * When reproduction begins * How often the organism can reproduce * The number of offspring produced per reproductive episode
K-selection (density-dependent selection): selection for life history traits that are sensitive to population density * Seen in high density populations that are close to carrying capacity (K)
R-selection (density independent selection): selection for life history traits that maximize reproductive success * Seen in low density populations with little competition
Density-dependent regulation: as a population increases, factors can slow or stop growth by decreasing birth rate and increasing death rate * Competition, predation, toxic wastes, territoriality, disease, intrinsic factors (ie reproduction rates)
Density-independent regulation: factors that exert their influence on population size, but the birth/death rate of a population does not change * Weather, climate, natural disasters