Populations have many Characteristics Populations go through 3 Stages Scientists study how populations or organisms change as they interact with each other & the environment POPULATION DYNAMICS = study of why populations change & what causes them to change. 4 Characteristics define a Population: Population size; population density; population spacing & age structure Scientists can predict a Population Change

Growth, Stability & Decline All populations go through growth, stability & decline. CARRYING CAPACITY = maximum number of individuals an ecosystem can support(with its available resources); if a habitat has enough resources, the population grows fast, then it stabilizes and population stays about the same for a while. This stability stage is usually the longest time period. Often, it is followed by a sharp decrease in population size. During growth phase it can occur very quickly (as in bacteria reproducing, or gradually as in a population of seals).

Darwin’s Observations of Population Growth Charles Darwin was a naturalist scientist who wrote On the Origin of Species based on his observations in the Galapagos Islands. He noted the following about population growth: All populations are able to grow rapidly Populations tend to remain constant in size There are limits to natural resources Within a given population there is genetic variation He knew that if there were no limitations to growth, populations would grow rapidly. But also realized, there are natural limits to growth in the real world. If birth rate doesn’t match the death rate, a population can decline until it becomes extinct.

Population Size Population size= number of individuals in a population at a given time. (varies from habitat to habitat, & even within a habitat). In our area, mosquito population size changes during different times of the year. Many deserts experience blooming & growing during our spring, but by summer (with less water & higher temperatures) there is a rapid decline in plant population size.

Population Density Population density= measure of the number of individuals in a certain space at a particular time. Related to population size; if size increases and they all stay in same area, density increases too. Some species live in high density (bees) while others live in low density populations (blue herons). RANGE = the distribution of a population across a large geographic area. The population density within that range may vary (more grasshoppers in middle of prairie than its edge). Habitats located in the middle of a population range usually have a greater density than its edges (more resources)

Population Spacing Population spacing= the pattern in which the individuals of a population are found. 3 main types: clumped, uniform and random. Clumped spacing: individuals form small groups within a habitat (elephants do this due to social nature, salamanders due to habitat-moist rotten logs found in 1 area) Uniformly spaced: individuals living at an equal distance from each other; plants often due this due to competing resources (if too close, some die out) Random spacing: dandelions/weeds grow where ever.

Age Structure Scientists divide a population into 3 groups based on age: Postreproductive: organisms can no longer reproduce Reproductive: organisms capable of reproduction Prereproductive: organisms not yet able to reproduce The age structure of a population affects how much it can grow. On graph below, postreproductive age range is over 45, while prereproductive range is 0-14

Predicting Population Change Scientists use size, density, spacing & age structure to describe a population & predict how it may change over time. A population may change due to its surroundings(frogs living in pond that gets saltier will probably experience a population decrease). Scientists can also predict change by looking at the distribution of a population. (New York City released a population of 60 starlings in 1890; they now number over 200 million spread across the U.S., Canada & Mexico. The population rose as they moved into new habitats with available resources.