Introduction to Ecology

Population ecology: the study of populations in relation to their environment * Environmental influences on density and distribution * Age structure * Population size

Population: a group of individuals of a single species living in the same general area
Populations are described by their boundaries and size * Density: the number of individuals per unit area or volume * Population density is determined by immigration and emigration * Dispersion: the pattern of spacing among individuals within the boundaries of the population

Clumped: members of a population are clustered in groups * Most common * Resources tend to be clustered in nature * Social behavior may promote this pattern
Uniform: members of a population are dispersed relatively evenly * Competition may cause this pattern * May also result from social interactions such as territorality
Random: members of a population are distributed without a predictable pattern * Rarest * Resources are rarely randomly spaced * May occur where resources are common and abundant * Absence of strong attractions or repellents.

An organism’s life history comprises the traits that affect its schedule of reproduction and survival * The age at which reproduction begins * How often the organism reproduces * How many offspring are produced during each reproductive cycle * Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

Semelparity: produce all offspring in single reproductive event * Individuals reproduce once and then die
Iteroparity: reproduce in successive years or breeding seasons * Seasonal iteroparity: distinct breeding seasons * Continuous iteroparity: reproduce repeatedly at any time of the year

Reproductive strategy has a strong effect on subsequent age classes of a population
Semelparous organisms have batches of young the same age, called cohorts
Iteroparous organisms have young of different ages
Expect a population increasing in size to have many young and a decreasing population to have few young

Change in population size = births + immigrants entering population - deaths - emigrants leaving population
If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
Exponential population growth: population increase under idealized conditions
Under these conditions, the rate of increase is at its maximum, denoted as rmax

A population grows more slowly as it nears its carrying capacity
A more realistic population model limits growth by incorporating carrying capacity * Carrying capacity(K): the maximum population size the environment can support * Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small
Carrying capacity varies with the abundance of limiting resources
Density-dependent factors: mortality factor that varies with population density * Parasitism, predation, and competition * Predators kill few prey when the prey population is low, more prey when the population is higher * Detected by plotting mortality against population density and finding positive slope
Density-independent factors: mortality factor whose influence is not affected by changes in population size or density * Physical factors – weather, drought, flood, fire
r-selection: density-independent selection, selects for life history traits that maximize reproduction * In density-independent populations, birth rate and death rate do not change with population density
K-selection: density-dependent selection, selects for life history traits that are sensitive to population density * In density-dependent populations, birth rates fall and death rates rise with population density

Density-dependent birth and death rates are an example of negative feedback that regulates population growth * Affected by factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors * Increasing population density intensifies competition for resources and results in a lower birth rate * Accumulation of toxic wastes * As a prey population builds up, predators may feed preferentially on that species * Some populations, intrinsic (physiological) factors regulate population size
Population density can influence the health and survival of organisms * In dense populations, pathogens can spread more rapidly