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Populations How Populations Grow • Since 1930, the world’s human population has nearly tripled. What causes populations to grow? What determines how fast they grow? What factors can slow their growths? • Population – all the individuals of a species that live together in one place at one time. • Every population tends to grow. Limited resources in an environment limit the growth of a population. The statistical study of all populations is called demography. Demographers can predict how the size of a population can change. 3 Key Features Population size – the number of individuals in a population. This can affect the population’s ability to survive. Very small populations are the most likely to become extinct. • Population density – the number of individuals that live in a given area. • Dispersion – the way individuals of the population are arranged in space. A population model is a hypothetical population that attempts to exhibit the key characteristics of a real population. • Growth Rate and Size • A population grows when more individuals are born than die in a given period. So a simple population model describes the rate of population growth as the difference between the birth rate and the death rate. • An exponential growth curve is a curve in which the rate of population growth stays the same, as a result the population size increases steadily. Carrying Capacity- the number of organisms of one species that an environment can support. • When a population is below carrying capacity, the growth rate is rapid. When a population reaches C.C., the death rates begin to rise and birthrates begin to decline. So the rate of growth slows. In real-life, a pop. may actually exceed the C.C. for a short time, but the death rate will be high until the pop. goes back down. Biological Communities How Organisms Interact in Communities • Coevolution – back and forth evolutionary adjustments between interacting members of an ecosystem. – Predators and prey coevolve: predation is the act of one organism feeding on another. Parasitism is when one organism feeds on and usually lives on or in another organism. Parasites do not usually kill their prey (host) bc they depend on it for food and a place to live. – Plants have defenses against herbivores. Virtually all plants contain defensive chemicals called secondary compounds. (sometimes like with mustard plants, backfires…yum! Dijon!) Some animals are made to eat and be fine with these specific plant chemicals. Ex. Some larvae enjoy only a certain cabbage species bc of their chemicals. Symbiotic Relationships and Species • Symbiosis – two or more species live together in a close, long-term association. They can be beneficial, harmful, or neutral. (we already mentioned parasitism or (+,-) • Mutualism – symbiotic relationship in which both participating species benefit (+,+) • Commensalism – symbiotic relationship in which one species benefits and the other is neither harmed nor helped (+,0) Competition Shapes Communities • When two species use the same resource, they participate in a biological interaction called competition. These resources include food, water, space. • The functional role of a particular species in an ecosystem is called its niche. A niche is how an organism live, the “job” it performs within the ecosystem. • The entire range of conditions an organism is potentially able to occupy within an ecosystem is its fundamental niche. • The part of its fundamental niche that a species occupies is called its realized niche. • If two species are competing, the species that uses the resource more efficiently will eventually eliminate the other. This elimination of a competing species is referred to as competitive exclusion. Major Biological Communities • Temperature – most organisms are adapted to live within a particular range of temperatures and will not thrive if temperatures are colder or warmer. • Moisture – all organisms require water. On land, water is sometimes scarce, so patterns of rainfall often determine an area’s life-forms. • A major biological community that occurs over a large area of land is called a biome. • • • • • • • 7 Biomes Tropical rain forests – Rainfall is 80-180 in. per year. Richest biome in terms of number of species (more than 2 million) Deserts – Fewer than 10 in. of rain. Vegetation is characteristically sparse. Savannas – Dry climates with development of grassland. Open landscape with widely spaced trees. (Lions, zebra, etc.) Temperate Deciduous Forests – Relatively mild climates and plentiful rain promote forests. Warm summers, cold winters with rain well distributed throughout the year. (deer, bear, etc.) Temperate Grasslands – Often highly productive when converted to farmland. Also called prairie. (bison, ground hogs, etc.) Taiga – Coniferous forests, with long, cold winters (moose, elk) Tundra – Open, sometimes boggy biome. Water is usually unavailable bc it’s frozen. (foxes, lemmings, caribou) Human Impact on the Environment Global Change • Things you do can ultimately influence every ecosystem on Earth. • Coal that we burn for electricity is rich in sulfur. Scientists have discovered that the sulfur introduced into the atmosphere by smokestacks combines with water vapor in the air to produce sulfuric acid. Rain and snow carry the sulfuric acid back to the earth’s surface. This acidified precipitation is called acid rain. • In 1985, someone noticed that the ozone levels (protect us from the sun) were 30% less than they were 10 yrs ago. ??? This means more U.V. rays hitting Earth causing skin cancer, and cataracts. In fact, the # of cases of melanoma (bad bad skin cancer) has doubled since 1980. What’s Destroying the Ozone? • The major cause of ozone destruction is a class of chemicals called chlorofluorocarbons (CFCs). • Throughout the world, CFCs are commonly used as coolant in refrigerators and air conditioners, as aerosol propellants in spray cans, and as foaming agents in the production of plastic-foam cups. Global Warming •When fossil fuels are burned, carbon atoms of the fuel molecules combine with oxygen atoms from air, yielding carbon dioxide (CO2) •The chemical bonds in CO2 absorb solar energy, trapping heat within the atmosphere. The warming of the atmosphere that results from greenhouse gases is known as the greenhouse effect. Ecosystem Damage • In many countries, modern agriculture introduces large amounts of chemicals into the global ecosystem. These chemicals include pesticides, herbicides, and fertilizers. Some industrialized countries (like the U.S.) try to monitor/eliminate these chemicals. Unfortunately, most toxic chems are still available. • Ex. DDT was a pesticide that was taken up by the roots of plants. They are then stored by the fatty acids of animals that ate the plants. As this toxin passes up the food chain, it accumulates (high concentration). This process is called biological magnification. DDT was hurting the bald eagle pops by making the egg shells weak (no new offspring). Loss of Resources • The U.S. is one of the most productive agricultural countries on Earth, largely bc of its fertile soils. These rich topsoils have accumulated over thousands of years and cannot be replaced and it’s being lost at a rate of several centimeters each decade. Turning over soil to eliminate weeds, overgrazing and practicing poor land management all allow wind and rain to remove the topsoil. • We also cannot replace ground water. Much of our drinking water is stored in aquifers (porous rock reservoirs). Much of it is wasted by washing cars, watering lawns, and leaky faucets and toilets. Not to mention all the nonpoint source pollution (changing the oil in your car and not properly disposing of it, pouring out paint thinner, etc. • And our population is still growing. Will we choke ourselves out????!!! Real Pop.s and Growth Patterns • Density-independent factors – environmental conditions (weather, climate, etc.) Ex. Mosquito pops grow in the summer bc it’s warm but decrease in the winter. • R-strategists – grow exponentially when environmental conditions allow them to reproduce. Temporarily large populations. They reproduce early in life and have many offspring that need little or no parent care. (Type III survivorship) • K-strategists – grow slowly. Pop density is usually near the c.c. These organisms have a long life span, few young that are slow maturing and reproduce late in life. (Type I survivorship) 16.2, How Populations Evolve • After Mendel’s pea plants, biologists wondered if dominant alleles spontaneously replaced recessive alleles within populations. • Using algebra and a simple application of probability, they showed that the frequency of alleles in a population and the ratio of heterozygous to homozygous individuals do not change unless the pop is acted on by things that favor the allele. • Ex. If a dominant allele is lethal, it will not become more common just bc it is dominant. It will actually become more rare as the dominant individuals die. • Hardy-Weinberg principle – the frequencies of alleles in a population do not change unless the evolutionary forces act on the population. Other Issues… • Mutation – mutation rates are very slow. They are rare and usually result in death. • Gene flow – the movement of alleles into or out of a pop. (from immigrants adding new alleles, and emigrants taking alleles away.) • Nonrandom mating – some individuals prefer to mate with others nearby or that are their own phenotype. Inbreeding is a type of nonrandom mating that causes a lower frequency • Genetic drift – in small populations, the frequency of an allele can be greatly changed by a chance event. (ex. Fire killing most of the pop.) • Natural selection – causes deviations by directly changing the frequency of alleles.