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BIO.9 The student will investigate and understand dynamic equilibria within populations, communities, and ecosystems. Key concepts include a) interactions within and among populations including carrying capacities, limiting factors, and growth curves; A population is made up of individuals of the same species found in the same general location at the same time. Populations of different species interacting in a given habitat constitute a community. An ecosystem consists of a community and the physical aspects of its habitat. Individuals of a population can be randomly dispersed, that is, there is some clumping, and other places where there are few individuals; individuals can be evenly dispersed, where there is almost the same distance between all individuals; or individuals in a population can be clumped, in which individuals are in close proximity to others. These dispersal patterns can be related to resource distribution such as food, water, living space as well as social interactions and mutual defense such as in schooling fish or herd animals. Biological populations grow exponentially, as typified by yeast. Two becomes four, four becomes eight, eight becomes sixteen, etc. Growth is characterized by a doubling time. The shorter the doubling time of the population, the faster the growth rate. Growth of the population will thus start slow and will then increase rapidly in a classic J curve in the absence of limiting factors. All organisms need food, shelter, and water. Whatever factor is in shortest supply will limit the number of individuals in a population. For example, ground finches in the Galapagos are limited by food supply. During the rainy season, reproduction rates are high and food is plentiful. At the end of the rainy season there are many juvenile finches in flocks feasting on the abundant supply of seeds. Six months later at the end of the dry season, or after a period of drought, numbers will be down as the seed supply dwindles and finches are observed only individually or in very small groups. The carrying capacity for a population is the average number of individuals that can be maintained by the resources available. Some populations increase exponentially to their carrying capacity and remain at or near carrying capacity. Others, such as the Galapagos ground finches, fluctuate dramatically about a long term average population, depending on the vagaries of the climate. The deer population in Virginia has exploded in numbers since the colonization of the United States. Removal of natural predators such as wolves and mountain lions, coupled with fire prevention increasing browse and shelter have contributed to the dramatic increase in numbers of deer. The geographic extent of a population can be affected by the abiotic factors of the ecosystem. Temperature range, wind, humidity, and soil type can limit the extent of plants as well as some animals. Since plants provide the basic structure of an ecosystem and are the basis of the food web within an ecosystem, their distribution will necessarily limit the geographic extent and population size of animals within the community that are dependent on those plants. Over time, species within a community co-evolve strategies that contribute to their survival. Predators and their prey must both survive. For example, in the tundra and taiga, the lynx preys upon the arctic hare. When the hare numbers are up, food is abundant and the number of lynx increases. This puts pressure on the hare population and its numbers decrease, thus reducing the food availability for the lynx. Both populations fluctuate around a carrying capacity. However, this is not a complete picture. The favored food of the hare is the arctic willow, a low shrubby plant of the tundra. When it is overgrazed, it produces compounds that are toxic to the hare and hare numbers go down. This means there is less food for the lynx and its numbers go down. The willows recover, and stop producing this secondary compound and numbers of hare and lynx go back up. It’s a complex dance! Other species interactions include parasitism, where one organism actively harms another, such as bacteria like Vibrio cholerae and humans. It is not in the long term interest of a parasite to produce high mortality rates in its hosts. In the case of pathogens with short reproductive periods, evolution will result in lower mortality rates over time. Besides parasitism, other forms of symbiosis include commensalism and mutualism. Everyone who has seen Finding Nemo knows about the commensal relationship between sea anemones and clownfish. In commensalism, one organism benefits (clownfish) and the other is neither harmed nor helped. Of even greater interest is mutualism in which both species benefit. In the Galapagos, crabs feed on the dead skin and mites of marine iguanas and finches eat the ticks from the giant tortoise. Within an ecosystem, each organism has a unique niche, or job. No two species can occupy the same niche. If two species occupy the same niche, one will be driven to extinction, or they will divide the niche up in order to minimize direct competition. For example, all mosquitoes require blood in order to reproduce. When there is more than one mosquito species in an area, they will feed at different times of the night. Both red footed boobies and blue footed boobies occupy the arid and coastal habitats of the Galapagos Islands. They have minimized competition in several ways: the red footed boobies nest in trees, lay one egg and feed further out at sea, preying on very shallow fish. Blue footed boobies nest on the ground, lay more than one egg, and feed near shore, diving deep for fish. Both strategies are successful. b) nutrient cycling with energy flow through ecosystems In most ecosystems the energy source is sunlight. Autotrophs use sunlight and fix carbon from carbon dioxide in photosynthesis. These primary producers are the basis for the food web of an ecosystem. All other organisms in the ecosystem are heterotrophs. Those that feed directly on the primary producers are called herbivores, or primary consumers. On the average, 10% of the energy stored in the lowest trophic level (primary producers) is converted to biochemical energy in the second trophic level (herbivores). Secondary consumers, carnivores, feed on the herbivores, storing 10% of that energy in the third trophic level. Top predators, the fourth trophic level, store 10% of the energy available in the third level, and so on. Moving up through the trophic levels, numbers of individuals decreases, the size of individuals increases, and total biomass decreases as a general rule. As a consequence of this, toxins such as DDT that are concentrated in the flesh of herbivores as they graze on contaminated plants become even more concentrated in higher and higher trophic levels. Thus the worst environmental effects of DDT were present in raptors at the top of the food chain. carnivores herbivores primary producers Many organisms feed at different levels. An omnivore such as a coyote will eat plants, insects, small rodents as well as carnivores such as snakes or birds. Energy and nutrients can follow a complex path through a food web. It is not a linear food chain. At the very top of the food web are decomposers such as bacteria and fungi that recycle nutrients to the soil. In an ecosystem, matter is recycled, but energy cannot be recycled. The entire system depends on a continuous supply of energy from sunlight. Water, carbon, nitrogen, oxygen and phosphorous are cycled through the biotic components of the ecosystem; they enter through photosynthesis and respiration and are returned to the abiotic environment through decomposition. But nothing happens without energy. You can examine specific nutrient cycles in your textbook. c) succession patterns in ecosystems; If a region is devoid of life, it will rapidly be colonized by organisms from other places. If there is no preexisting soil, that is, if the area has never contained life, the process is called primary succession. New volcanic islands or lava flows, land that has been under glaciers will undergo primary succession. Areas that have had life stripped off by agriculture or by forest fires will undergo secondary succession, as there is intact soil, often containing the seeds of plants to speed the process. Pioneer species are those that can exist under extremely harsh conditions. They need little by way of soil or water. They are autotrophs and have a high reproductive rate and efficient dispersal mechanisms. They will modify the environment in such a way that other species can now successfully colonize the region. As each species succeeds earlier species, the community gradually changes, eventually reaching a stable climax community. Please view the power point on ecological succession. d) the effects of natural events and human activities on ecosystems; Natural events can destroy an ecosystem in a region. Hurricanes can have dramatic effects in coastal areas; lightening induced forest fires and ice damage from storms can create a mosaic of habitats within an ecosystem. A mosaic of habitats can often be highly beneficial to the diversity of life present in a geographic region and different stages of succession can be present at the same time. To a limited extent, man can have a beneficial effect with proper forestry techniques. Prairie in many locations is maintained as a subclimax community by fire. Fires are naturally set by lightning, however, the plains Indians often set fires to drive buffalo herds toward waiting hunters. Unfortunately, man has shown a geologically unprecedented ability to modify the environment. Stone Age man was a super predator. Extinctions of large Pleistocene animals can be documented to follow the dispersal of man into the western hemisphere during the ice ages. In the last few centuries, agricultural practices of poor soil conservation have led to desertification and the destruction of ecosystems. Harvesting and burning the rainforest for subsistence agriculture is presently altering a key ecosystem and reducing global diversity. Man induced global warming is causing the loss of coral reefs as well as Arctic and Antarctic ecosystems dependent on the existence of sea ice. We have over fished the sea; cod fisheries have totally collapses and show no signs of recovery. We have transported alien species into new environments (both intentionally and by accident) where they out compete and drive endemic faunas and floras to extinction. Starlings were released in Central Park by a well meaning Shakespeare fan; they are presently out competing many North American song birds. Kudzu and multiflora rose have dramatically altered the habitat in the Southeastern United States; water hyacinths clog southeastern waterways and change the fluvial ecosystem. Putting dams on rivers have caused warm muddy rivers such as the Colorado to run clear and cold. Water diversion of the Colorado has resulted in the total destruction of the delta ecosystem of the Colorado in the Gulf of California. It is not possible to give a complete list of man’s impact on the biosphere. Experts estimate that we are causing the extinction of three species of organisms every hour. e) and analysis of the flora, fauna, and microorganisms of Virginia ecosystems including Chesapeake Bay and its tributaries. Please examine these websites for information regarding Virginia ecosystems: http://www.virginiaplaces.org/natural/index.html A web page that contrasts the habitats & wildlife of Virginia today with colonial days http://www.vanaturally.com/ A gateway to Virginia’s natural environment http://www.dgif.state.va.us/ Wildlife in Virginia – state programs, wildlife mapping, etc. http://www.cbf.org/site/PageServer?pagename=homev3 Chesapeake Bay Foundation homepage http://www.acb-online.org/ Homepage for the Alliance for Chesapeake Bay http://www.epa.gov/region3/chesapeake/ Environmental Protection Agency homepage for Chesapeake Bay http://www.deq.state.va.us/bay Virginia Department of Environmental Quality Chesapeake Bay After reviewing material in your textbook, go to the file labeled BIO.9 Review Response and open it in Word. Type your answers below each question and make them a distinctive readable color or font. E-mail this file as an attachment.
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