Populations Ecology

How Populations Grow CHARACTERISTICS OF POPULATIONS POPULATION GROWTH Four important characteristics: Geographic distribution Density Growth rate Age structure Geographic distribution: describes the area inhabited by a population Example: a few cubic centimeters – a population of bacteria Example: millions of square kilometers occupied by migrating whales in Pacific Ocean Density: number of individuals per unit area – can vary depending on the species and its ecosystem. Example: a saguaro cactus in an Arizona desert has a low density compared to smaller desert plants – what does that mean? POPULATION GROWTH Three factors that affect population sizes: Number of births Number of deaths Number of individuals that enter or leave the population

How Populations Grow If more individuals are born than die in a population, what will the result on population size be? What if birth rate > death rate? Immigration: the movement of individuals INTO an area. Emigration: the movement of individuals OUT OF an area. What are some reasons that individuals might immigrate into a new area and emigrate from an original area? EXPONENTIAL GROWTH Growth of a population occurring when there are unlimited resources and an absence of predation and disease. Example: bacteria reproduce by splitting in half. If doubling time is 20 minutes, after 20 minutes there will be 2 bacteria, 40 minutes 4 bacteria, 60 minutes 8 bacteria, 80 minutes 64 bacteria – in just one day, the bacteria will have grown to 4720000000000000000000 individuals!!  this is exponential growth. Some bacteria have the fastest rates of reproduction of any living organism. Occurs when an individual reproduce at a constant rate.

How Populations Grow LOGISTIC GROWTH Occurs when a population’s growth slows or stops following a period of exponential growth. 1. a few animals are introduced into a new environment 2. animals reproduce and population slowly increases 3. resources are unlimited & population grows exponentially 4. over time, rate of population growth begins to slow down 5. population still grows but at a much slower rate As resources become less available, the growth of a population slows or stops. Population growth may slow when: 1. birthrate decreases 2. deathrate increases 3. or both occur at the same time 4. immigration decreases 5. emigration increases 6. both occur at the same time Carrying capacity: the largest number of individuals that a given environment can support.

Limits to Growth LIMITING FACTORS: a factor that causes population growth to decrease. Can affect the long-term survival of a species. Example: bamboo and panda bears Examples of limiting factors in a population Competition Predation Parasitism & disease Drought & other extreme climates Human disturbance DENSITY-DEPENDENT FACTORS: limiting factors that depend on population size. Affect organisms when populations are large and dense – not when populations are small and scattered. Examples:

Limits to Growth COMPETITION PREDATION PARASITISM & DISEASE When populations become crowded, organisms compete for: Food Water Space Sunlight (plants) Other essentials Can occur between members of the same species as well as members of different species. Examples? PREDATION Predator-prey relationship is one of the best-known mechanisms of population control. PARASITISM & DISEASE Range in size from microscopic bacteria to tapeworms that are 30+cm in length. Do parasites kill their hosts? Why or why not?

Limits to Growth DENSITY-INDEPENDENT FACTORS Affect a population regardless of the population size or density. Examples: Unusual weather Natural disasters Seasonal cycles Some human activities (damming rivers & clear-cutting forests)

Human Population Growth In the US and other developed countries, the current growth rate is low. Some developing countries have human populations that are growing at a rate of 3 people per second! HISTORICAL OVERVIEW For most of human existence, the population grew slowly. Why do you think that is?

Human Population Growth Does human population increase or decrease with time? THOMAS MALTHUS & MALTHUSIAN THEORY: English economist two centuries ago made the observation that human populations were growing rapidly. Believed that populations could not continue growing indefinitely because resources would be used up. Believed that war, famine and disease would limit human population growth. Demography is the scientific study of human populations – examines characteristics of human pop’ns and attempts to explain how they will change over time. Demographic transition is a dramatic change in birth & death rates in a human population – brith/death rates change from high to low. Is complete when birthrate falls to meet death rate. 1. advances in nutrition, sanitation & medicine decreases deathrate 2. birthrate is still high 3. societies modernize – increase education level, raise standard of living decreases birthrate

Human Population Growth