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Chapter 5 and 6Populations Biology I What Affects Population Size? A population is a group of organisms belonging to a single species that lives in a given area. Populations size is affected by # of births, # of deaths, & # of individuals who enter or leave 5–1 How Populations Grow Characteristics of a Population 1- Geographical Distribution 2- Density 3- Growth Rate 1- Geographical Distribution -area inhabited by a population -could be as small as a few cubic centimeters like a bacteria or as large as millions of kilometers like killer whales 2- Population Density the number of individuals per unit How organisms are dispersed can be important Three patterns of dispersal are random, clumped, and uniform 3- Growth Rate - 3 factors that affect a populations size - number of births - number of deaths - number of individuals entering and leaving a population A population will increase or decrease in size depending on the number added or removed from it A population will grow if: Birth Rate > Death Rate (# of individuals born) (# of individuals who die) A population will stay the same if: A population will decline if: Birth Rate = Death Rate Birth Rate < Death Rate Population Growth 3 2 1 1. 2. 3. 4. 5. 4 5 Initial growth- birth (natality) is high Exponential growth Carrying capacity- steady state Decline- increase in death rate Extinction Exponential Growth If a population has abundant space and food, protection from predators and disease then the organisms in the population will multiply and the population will increase Under ideal conditions with plenty of nutrients, heat, moisture, & light a bacteria can reproduce by splitting in half within 20 minutes producing 2 bacteria In 20 minutes the population is 2 In 40 minutes the population is 4 In 1 hour: the population is 8 In 2 hours: 64 In 3 hours: 512 In one day: 4,720,000,000,000,000,000,000 The pattern of growth is a J-shaped curve and the population is undergoing exponential growth Exponential growth occurs when the individuals in a population reproduce at a constant rate At first, the number of individuals increases slowly, over time the population becomes larger and larger until it approaches an infinitely large size Population size 1 million 500,000 100 One year Under ideal conditions with unlimited resources, a population will grow exponentially Logistic Growth Exponential growth doesn’t continue in natural populations for very long If a new species of organism is introduced into a new environment, at first the population grows slowly, then exponentially, eventually the population growth slows down (the size has not dropped, but the population is growing slower) As resources become less available, the growth of a population slows or stops forming an S-shaped curve- logistic growth Logistic Growth occurs when a population’s growth slows or stops following a period of exponential growth Birth rate decreases and death rate increases Carrying Capacity When growth levels off The population is not growing Carrying capacity- the largest number of individuals that an environment can support When a population overshoots the carrying capacity, then limiting factors may come into effect Population Growth can be Logistic growth Exponential growth characterized by No limits on growth Unlimited resources represented by Constant growth rate J-shaped curve characterized by Limits on growth which cause a Falling growth rate represented by S-shaped curve 5–2 Limits to Growth What can limit growth? Limiting Factors Any factor that causes population growth to decrease Limiting factors, such as availability of food, disease, predators, or lack of space, will cause population growth to slow Under these pressures, the population may stabilize in an S-shaped growth curve Population Carrying capacity 0 J curve S curve Time FOOD PREDATORS SPACE Exponential growth DISEASE Characteristics of Population Growth Density-Dependent Limiting Factors A limiting factor that depends on population size is a Density-dependent limiting factor Density-dependent factors include disease, competition, predators, and parasites. Disease, for example, can spread more quickly in a population with members that live close together. Density dependent factors create an S-shaped curve Organism Interactions Limit Population Size Population sizes are limited not only by abiotic factors, but also are controlled by various interactions among organisms that share a community. Predation affects population size Predation is a density-dependent factor When a predator consumes prey on a large enough scale, it can have a drastic effect on the size of the prey population Populations of predators and their prey are known to experience cycles or changes in their numbers over periods of time Competition within a population Competition is a density-dependent factor When only a few individuals compete for resources, no problem arises When a population increases to the point at which demand for resources exceeds the supply, the population size decreases Competition can also occur between members of different species- this creates evolutionary change The two species competing are under stress to change in way that decrease competition- eventually evolving to occupy separate niches Moose Wolves • The periodic increase in the moose population is quickly followed by a rise in the number of wolves. • This increase in the number of wolves increases the number of moose captured by their predator and the moose population drops. • The wolf population will then decrease after the moose numbers drop because the wolves are less able to find food. • The predator prey cycle can be repeated indefinitely. Parasitism and Disease Parasites can also limit a population’s size Parasites are similar to a predator in that they take nourishment at the expense of their host Density-independent limiting factors and population growth Density-independent factors can affect all populations, regardless of their size Most densityindependent factors are abiotic factors, such as temperature, storms, floods, drought, and major habitat disruption Density-Independent Factors Density-independent factors create a population crash (exponential growth followed by a sudden collapse) Examples: Weather- a drought kills grass Human activity: deforestation- destroys habitat Seasons: a severe winter regulates insect population Population Growth after a Density_____________ Limiting Factor Population Density Time Limitations on Population Growth Reproduction Patterns In nature, animal and plant populations change in size. Biologists that study the factor that determines population growth— Demographers 5–3 Human Population Growth Demography Demography, the study of human population size, density and distribution, movement, and its birth and death rates Demography examines the characteristics of human populations and attempts to explain how those populations will change over time Demographic transition- a change in a population from high birth and death rates to low birth and death rates; Examples- United States and Japan Human Population Growth Industrial Revolution IndustrialBegins Agriculture begins Agriculture begins Revolution begins Bubonic Plowing and Plowing irrigation and plague Bubonic plague irrigation Until about 500 years ago, the population grew slowly. Agriculture, industry, & sanitation made life easier and safer. Human Population We are growing exponentially Due to technological advances: medicines, a dependable food supply, and sanitation The death rate is decreasing and the life expectancy is increasing The human population will eventually reach its carrying capacity Age Structure Population Distribution Per Age Range for Several Countries Stable growth Rapid growth Male Slow growth Reproductive years Age Female Population (percent of total for each country) Population growth depends on how many people of different ages make up a given population Demographers can predict future growth using age-structure diagrams Age-structure diagrams graph the number of people in each age group Age Structure U.S. Population Rwandan Population Ch.6-2: Human Impact on the Environment Air Greenhouse effect • Trapping of heat by gases in the Earth’s atmosphere (carbon dioxide) • This may lead to global warming- increase in the Earth’s temperature from the rapid buildup of carbon dioxide and other greenhouse gases Acid Rain • Burning of fossil fuels release nitrogen and sulfur into the atmosphere which combined with water form nitric and sulfuric acid • Acid Rain kills plants Smog • Mix of chemicals in the air from automobile exhaust • Affects the respiratory system especially those with asthma Human Impact of the Environment Water Excessive nutrients can cause an increase in algae growth and depletion of oxygen for other organisms Over harvesting of fish has reduced the number of fish Human Impact of the Environment Land Erosion • The wearing away of surface soil by water and wind Desertification • The combination of farming, overgrazing, and drought in dry climates which turn productive areas into deserts Deforestation • The loss of forests which leading to severe erosion and changes in soil properties Consequences If humans continue to alter the environment, they will have to deal with the results Examples: deforestation, ozone layer destruction, global warming Humans need to learn to be a part of nature not apart from nature! Ch. 6-3: Biological Diversity Ecosystem diversity- variety of habitats, communities, and ecological processes in the living world Species diversity- the number of different species Genetic diversity- sum total of all the different forms of genetic information carried by all organisms living on Earth today Biodiversity increases as you move toward the equator. Areas around the world differ in biodiversity Biodiversity can bring stability to an ecosystem. Biodiversity is one of the world’s greatest resources Variations within a species also helps protect against disease or parasite Genetically different individuals within a species have a better chance of resisting disease Example: world cheetah population reduced by a single virus Tree plantation has same type of trees (single species) vulnerable to disease, etc..- spread easily Diversity among individuals in a population is important to resistance of population as a whole (genetically different) Species Diversity Insects 54.4% Protists 4.2% Other Animals 19.7% Plants 18% Fungi Bacteria 0.3% 3.4% Loss of Biodiversity Extinction is the disappearance of a species when the last of its members dies Extinction is a natural process and Earth has experienced several mass extinctions during its history A species is considered to be an endangered species when its numbers become so low that extinction is possible Threats to Biodiversity Complex interactions among species make each ecosystem unique Changes to habitats can therefore threaten organisms with extinction Human activity can reduce biodiversity by altering habitats, hunting species to extinction, introducing toxic substances into food webs, and introducing invasive species Habitat loss-One of the biggest reasons for decline in biodiversity Threats to Biodiversity: Habitat fragmentation Habitat fragmentation is the separation of wilderness areas from other wilderness areas Habitat fragmentation creates “islands” of habitat that are isolated from each other Threats to Biodiversity: Demand for wildlife products Hunting has caused many animals to go extinct Humans hunt for food, fur, hides, and body parts In the United States animals are protected from hunting if they are endangered Fish-Eating Birds Threats to Biodiversity: Biological Magnification Increasing concentrations of a harmful substance in organisms at higher trophic levels in a food chain or web Example: DDT Magnification of DDT Concentration 10,000,000 Large Fish 1,000,000 Small Fish 100,000 Zooplankton 10,000 Producers Water 1000 1 Threats to Biodiversity: Invasive Species Invasive species threaten biodiversity Invasive species are organisms that have been introduced into new habitats and often become invasive, reproduce rapidly Invasive species lack densitydependent limiting factors to keep their populations in check and take over areas and out compete existing species Native to South America, nutrias have become pests in coastal areas of se US. These furry rodents eat water plants that protect fragile shorelines from erosion. This destroys the habitats of species native to those ecosystems.