Describing Populations Lecture 4.2

Environmental Science Lesson 4.2: Describing Populations 1. 2. 3. 4. OBJECTIVES: Explain the usefulness of tracking population size. Define population density. Describe the 3 ways populations can be distributed. Explain what age structure diagrams tell you about a population. POPULATION SIZE ➢ Population size describes the number of individual organisms present in a given population at a given time. ○ Steady or increasing population is often a sign of a healthy population. ○ Rapidly declining population can mean extinction is coming. POPULATION SIZE ➢ Decline of the passenger pigeon ○ The passenger pigeon was once the most abundant bird in North America ○ It nested and bred in the forests of the upper midwest and southern Canada, but began being hunted when forests were cut down ○ The last passenger pigeon on Earth died in Ohio’s Cincinnati Zoo in 1914. POPULATION SIZE ➢ Determining population size ○ Population size is estimated using sampling techniques ○ Ecologists count the number in a smaller sample area, then use that info to estimate the number of individuals in the larger overall area ○ Example: If there’s 100 oak trees in one sq km of a large forest, it’s reasonable to estimate that there’s about 1000 oak trees in 10 sq km of the same forest ○ Sometimes you can only estimate population size by finding signs of an organism rather than the organism itself (tracks, droppings). POPULATION DENSITY ➢ Population density: the number of individuals within a population per unit area ○ Example:1500 golden toads were counted within 4 sq km (988 acres). Population density is 1500 toads/4 sq km, or 375 toads/sq km POPULATION DENSITY ➢ Different densities ○ High population density: ■ Can make it easier for organisms to group together and find mates ■ Can cause competition for resources ■ Overcrowded organisms may become more vulnerable to predators ■ Close contact can increase transmission of disease ○ Low population density: ■ Organisms benefit from more space and resources ■ May be more difficult to find mates and companions POPULATION DENSITY ➢ Example: When many of the “splash zone” habitats of the Harlequin frog dried up, the frogs had to gather in the few splash zones that remained. This resulted in overcrowding, which caused the spread of disease, predator attack, and parasitic flies. One very small population is believed to remain. POPULATION DISTRIBUTION ➢ Population distribution (or population dispersion) describes how organisms are arranged within an area. ➢ Three distributions types: 1. Random distribution: individual organisms are arranged within a space in no particular pattern ■ Can occur when needed resources are found throughout an area. POPULATION DISTRIBUTION ➢ Three distributions types: 2. Uniform distribution: individual organisms are evenly spaced throughout an area ■ Can occur when individuals hold territories or compete for space POPULATION DISTRIBUTION ➢ Three distributions types: 3. Clumped distribution: individual organisms arrange themselves according to the availability of the resources they need AGE STRUCTURE & SEX RATIOS ➢ Age structure (or age distribution): describes the relative numbers of organisms of each age within a population ➢ Age structure diagram (or age pyramid): visual tools scientists use to show the age structure of populations ○ Width of each horizontal bar represents the relative size of each age group AGE STRUCTURE & SEX RATIOS ➢ Age structure diagram (or age pyramid) (continued): ○ Population with even age distribution is likely to remain stable (births keep pace with deaths). ○ “Bottom heavy” populations are capable of rapid growth-- mostly reproductive or pre-reproductive individuals ○ Populations that are wider at the top are likely to decline over time. AGE STRUCTURE & SEX RATIOS ➢ Sex ratios: proportion of males to females ➢ Unbalanced sex ratio: too many males or females ➢ 50:50 sex ratio is often ideal