POPULATION BIOLOGY CHAPTER 5

What affects population size? Remember, a population is a group of organisms belonging to a single species that lives in a given area. Answer the following questions about what factors can affect population size.

1.Think of a population in your area. How might this population grow in size? 2.How might this same population decrease in size?

How is population size estimated? For stationary organisms: –Quadrant method For moving organisms: –Mark and Recapture For people: –Census

5–1: HOW POPULATIONS GROW A.Characteristics of Populations B.Population Growth C.Exponential Growth D.Logistic Growth

DEFINITIONS Species – groups of organisms that are able to interbreed and produce fertile offspring Population – a group of organisms belonging to the same species and living in certain area

Characteristics of Populations Geographic distribution Population Density - the number of individuals of a population within a given area Growth rate

POPULATION GROWTH Three factors that affect population size: –Number of births –Number of deaths –Number of individuals that leave (emigration) or enter (immigration) a population

EXPONENTIAL GROWTH Exponential Growth – highest rate of reproduction under ideal conditions

LOGISTIC GROWTH Logistic growth – occurs when a population’s growth slows or stops following a period of exponential growth Carrying capacity – greatest number of individuals in a population that a given environment can support

Carrying capacity

QUESTIONS 1.Imagine a small island that has a population of five rabbits. How might each of the following factors affect the rabbit population? a.Climate b.food supply c.predation 2.Now imagine another small island that has a population of 500 rabbits. How would the same factors affect this population? 3.Which of the factors depend on population size? 4.Which factors do not depend on population size?

5–2: LIMITS TO GROWTH A.Limiting Factors B.Density-Dependent Factors 1.Competition 2.Predation 3.Parasitism and Disease C.Density-Independent Factors

LIMITING FACTORS Limiting factors –causes population growth to decrease –List examples

DENSITY-DEPENDENT LIMITING FACTORS Limiting factors that depend on population size –Examples: Competition Disease (contagious) Predation Parasitism

A Density-Dependent Limiting Factor

Competition – competing for resources such as food, space, water, sunlight, mates etc. –Can occur between the same species (intraspecific) or different species (interspecific) –What may happen due to competition? Extinction Adaptation Emigration

–Ways to avoid intraspecific competition (same species): Life cycles (frogs and tadpoles do not compete) Life span (adults die shortly after young are produced) Parental care of young Dominance and Social Hierarchy (chicken’s pecking order)

Role separation (bees and ants) Behavioral and Physiological Changes (killing young, no mating) Emigration Territoriality (birds’ songs, a buck’s scent)

Disease (contagious) –The more crowded an area, the quicker disease spreads Predation – feeding of one organism on another –Predation may be a limiting factor for the prey population while food availability may be a limiting factor for the predator

–predator-prey relationships show cycles of increases and decreases in both populations in response to one another –Predators often kill the old, weak, and young –Predation may keep prey population very near carrying capacity Example: Moose vs. wolf on an island in Lake Superior

Moose Wolves

Parasitism – when organisms live on or in a host; always harmful to host The greater the population density, the greater number of parasites are passed from one organism to another

DENSITY-INDEPENDENT LIMITING FACTORS Limiting factors that are not related to population density –Examples: Temperature (as with ladybugs

How fast are we growing? Until about 500 years ago, the world’s human population remained fairly stable. Then, as advances in medicine, agriculture, and technology occurred, the human population began growing very rapidly. Today, the world’s human population is greater than 6 billion people, and it continues to grow, but at a slower rate.

1.The human population is increasing by about 1.4 percent each year. Assume that the population is 6 billion (6,000,000,000). How large will the population be in one year? 2.If the human population continues to grow at a rate of 1.4 percent per year, the population would double in size (to 12 billion people) in only 51 years! What effect might this increase in population have on the environment and on other people?

5–3: HUMAN POPULATION GROWTH A.Historical Overview B.Patterns of Population Growth 1.The DemographicTransition 2.Age Structure C.Future Population Growth

HUMAN POPULATION World Population as of 5/23/11 (Estimate) 6,920,250,312 World Population as of 2050 (projection) 9,404,251,020 U.S. Population as of 5/23/11 (Estimate) 311,408,847 One birth every seconds One death every second One international migrant (net) every seconds Net gain of one person every seconds U.S. Population for 2050 (projection) 419,854,000 A World Population Clock Source for USA and World Pop data: and

Agriculture begins Plowing and irrigation Bubonic plague Industrial Revolution begins

World Population reached: 1 billion in billion in 1927, (123 years later) 3 billion in 1960, (33 years later) 4 billion in 1974, (14 years later) 5 billion in 1987, (13 years later) 6 billion in 1999, (12 years later) October 12, 1999 was Y6B day (the day the world population reached 6 billion)

PATTERNS OF POPULATION GROWTH Demography – study of human populations Since the 1970’s the worldwide population growth rate is decreasing However, the population size is still increasing (just at a slightly slower rate)

Growth rates higher in developing countries –Birthrates higher –More people at reproductive age Zero population growth is when birthrate = death rate Birthrate, death rate, and age structure affect population growth rate

Demographic transition – as countries modernize, death rates decrease at first without birthrates decreasing. This causes populations to increase rapidly (USA ) Later, birthrates also decrease slowing population growth. Age-structure diagrams – show numbers of people in different age groups

Figure 5-13: Age Distribution U.S. PopulationRwandan Population

RAPID GROWTH 2.1% ANNUALLY

STABLE GROWTH 1.7% ANNUALLY

NEGATIVE GROWTH -0.1% ANNUALLY

MaleFemale Population of the USA by Age and Sex from (in millions)