Understanding Populations Sections 4.2 & 4.3- How Populations Change in Size

What is a Population? A population is a reproductive group because organisms usually breed with members of their own kind Example: Daisies in an Iowa field will reproduce with one another but they won’t reproduce with daisies in a Maryland field. Refers to the group in general, as well as to the size of the group

Properties of Populations Described in terms of their density or dispersion Density refers to how crowded a population is Example of density= the number of bass (fish) per cubic meter of water Larger animals tend to have smaller densities

Dispersion describes how organisms are arranged. Random- arranged in NO particular pattern Dispersion describes how organisms are arranged. A population’s dispersion may be uniform, clumped, or random. Uniform- spaced out evenly Clumped- Organisms grouped near resources; most common distribution in nature

How does a population grow? Growth Rate If the growth rate is positive/population grows. If the growth rate is negative/population shrinks. If the growth rate is zero/population stays the same size = - Growth Rate/ Change in size Births Deaths

What else affects population growth? In addition to births and deaths, population growth is affected by immigration and emigration—individuals moving into and out of a population. Migration, seasonal movement into and out of an area, can temporarily affect population size

What can affect the speed of population growth? Populations usually stay about the same size year to year Various environmental factors kill off an individual before reproduction can occur Depends on the Reproductive Potential (R.P.) of the species = maximum number of species produced over a lifetime a) R.P. increases when more offspring are produced at a time b) R.P. increases if the organisms reproduce more often (associated with gestation periods) c) R.P. increases if the organisms reproduce earlier in life. (greatest effect)

Reproductive Potential Examples Hippo- 1 baby/ year (225-250 days per pregnancy) vs. Mouse- 10 babies/litter (19 days per pregnancy) A woman can have more children if she starts getting pregnant at age 22 vs. if she starts at age 32. Bacteria have the greatest reproductive potential.

Exponential Growth Populations sometime undergo exponential growth, which means they grow faster and faster. A larger number of individuals is added to the population in each succeeding time period. Occurs in nature ONLY when populations have plenty of food and space, and have no competition or predators.

Under ideal conditions with unlimited resources, a population will grow exponentially. (ex. Hydrilla)

What Limits Population Growth? RESOURCE AVAILABILITY Carrying Capacity- can the ecosystem support the population COMPETITION

Populations Cannot Grow Infinitely Populations will grow until they reach their carrying capacity. Carrying Capacity is limited by the most severely limited resources Examples of limiting resources include food, sunlight, water, space

How are populations regulated? Density dependent factors Competition, limited resources, predation, disease Density independent factors Natural disasters- tornados, volcanoes, drought, earthquakes