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  • pg 1
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
           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

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.
                                               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

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:

         A web page that contrasts the habitats & wildlife of Virginia today with colonial days

         A gateway to Virginia’s natural environment

         Wildlife in Virginia – state programs, wildlife mapping, etc.
         Chesapeake Bay Foundation homepage

         Homepage for the Alliance for Chesapeake Bay

         Environmental Protection Agency homepage for Chesapeake 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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