Unit 3: Populations

Population

The individuals that belong to the same species and live in a given area at a particular time

Community

All of the populations of organisms within an given area

Population ecology

The study of factors that cause population to increase or decrease

Population size (N)

The total number of individuals of a particular species in a defined area at a specific time.

Population Density

The number of individuals per unit area at a given time

Population distribution

A description of how individuals are distributed with respect to one another

Sex ratio

The ratio of males to females in a population

Age structure

A description of how many individuals fit into particular age categories in a population

Limiting resource

A resource that a population cannot live without and that occurs in quantities lower than the population would require to increase in size

Density-dependent factor

A factor that influences an individual’s probability of survival and reproduction in a manner that depends on the size of the population.

Carrying capacity (K)

The limit of how many individuals in a population the environment can sustain

Density-independent factor

A factor that has the same effect on an individual’s probability of survival and the amount of reproduction at any population size

Population growth models

Mathematical equations that can be used to predict population size at any moment in time

Population growth rate

The number of offspring an individual can produce in a given time period, minus the deaths of the individual or its offspring during the same period

Intrinsic growth rate r

The maximum potential for growth of a population under ideal conditions with unlimited resources

Exponential growth model (N_t = N₀e^rt)

A growth model that estimates a populations future size (N_t) after a period of time (t), based on the intrinsic growth rate r and the number or reproducing individuals currently in the population (N₀)

J-shaped curve

The curve of the exponential growth model when graphed

Logistic growth model

A growth model that describes a population whose growth is initially exponential, but slows as the population approaches the carrying capacity of the environment

S-shaped curve

The shape of the logistic growth model when graphed

Overshoot

When a population becomes larger than the environment’s carrying capacity

Die-off or Dieback

A rapid decline in a population

K-Selected species

A species with a low intrinsic growth rate that causes the population to increase slowly until it reaches the carrying capacity

r-selected species

A species that has a high intrinsic growth rate, which often leads to population overshoots followed by die-offs

Survivorship curve

A graph that represents the distinct patterns of species survival as a function of age

Type I survivorship curve

A pattern of survival over time in which there is high survival throughout most of the life span, but then individuals start to die in large numbers as they approach old age.

Type II survivorship curve

A pattern of survival over time in which there is a relatively constant decline in survivorship throughout most of the life span

Type III survivorship curve

A pattern of survival over time in which there is low survivorship early in life with few individuals reaching adulthood

Corridor

Strips of natural habitat that connect populations

Metapopulation

A group of spatially distinct populations that are connected by occasional movements of individuals between them

Inbreeding depression

when individuals with similar genotypes produce offspring with impaired abilities to survive and/or reproduce

The theory of demographic transition

The theory that as a country moves from a subsistence economy to industrialization and increased affluence, it undergoes a predictable shift in population growth

Phase I

Slow population growth, because birth rates and death rates are both high, pre-industrialization

Phase II

Rapid population growth, because birth rates remain high while death rates decline

Phase III

Stable population growth, birth rates decline