Download Woodland Hills - Science 8 - Lesson 15 Guided Notes Answer Key

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“Let It Burn!”
The forest fire surged ahead as foresters, a professor, and some state officials hold a hurried
conference. The officer in charge trots over to the firefighters. “Get your crews out of here!”
he orders. “We are going to let it burn!”
Only recently, letting a fire burn would have been unthinkable. Conservationists now recognize
that fires are a natural part of the environment, and they are not always bad.
Experts may decide to let the fire recycle the forest. Calcium, potash, phosphates, nitrogen,
trace elements, and other materials left after a forest fire nourish new growth. Many animals
can prosper in the new growth that springs from the ashes. Sometimes, the best thing to do with
a forest fire is to let it burn.
In a minimum of 5 lines, explain whether you agree with the “let it burn” philosophy?
Why or why not?
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UNIT TWO:
ECOLOGY AND EVOLUTION
Change and the Environment
Introduction
Natural disasters and other disturbances change ecosystems, particularly the sizes
of populations.
-After a disturbance, ecosystems go through a series of changes.
Population Changes
To identify environmental changes, scientists might collect evidence about a
population.
-All of the species that inhabit a specific ecosystem.
-The total number of individual organisms of a kind or species in an environment is
the population size.
-The population density is the number of individuals within a given area.
Disturbances such as a hurricane, tornado, flood or fire cause population sizes and densities to
change.
-Change may be immediate due to a disaster destroying resources or killing organisms.
-Change may be slow, and occur after the disaster.
-If nesting sites or food / water sources are destroyed, organisms may slowly move away
or die off, resulting in a decrease in density.
Limiting Factors
Usually population size will change depending on the number of births and the
number of deaths.
-With more births than deaths, the population size will increase.
-With more deaths than births, the population size will decrease.
-Most populations reach a stable point where births equal deaths.
-This equal state is important!
For example: If a population is allowed to keep increasing, organisms will run out of
food and places to live.
-Something in the ecosystem that keeps a population from getting too large is called a
limiting factor.
-Resources such as food, water, climate, shelter, and available space, as well as disease and
predators, are all limiting factors.
Each member of the population must compete for resources.
-Some will get these resources, while others will not.
-Those that get the resources survive; those that do not, move away or die.
-The greater the population density, the greater the effect the limiting factor has on the
population.
In a dense population, diseases spread faster, food does not feed as many members, and there
are fewer places to live.
-Predators thrive on a dense population of prey!
We are not exempt!
-Humans create many limiting factors for nature.
-Cutting down forests to build homes and roads destroys natural habitats.
-Polluting water and air reduces the number of wild organisms that can use it.
-Bringing in species from other parts of the world can change a native ecosystem.
-Populations of native species can decline or disappear.
Every ecosystem has a limited amount of biotic and abiotic factors that can support
life.
-The number of individuals an ecosystem can support is called its carrying capacity.
-A population will usually continue to grow until its carrying capacity is reached.
-Then, the population will stabilize.
Ecological Succession
On the time scale of a human life, some ecosystems may seem stable.
-The appearance of stability is often misleading.
-Ecosystems and communities are always changing.
-Sometimes, an ecosystem changes in response to an abrupt disturbance, such as a severe
storm.
-At other times, change occurs as a more gradual response to natural fluctuations in the
environment.
*Ecosystems are constantly changing in response to natural and human disturbances.
-As an ecosystem changes, older inhabitants gradually die out and new organisms move in,
causing further changes in the community.
-This series of predictable changes that occurs in a community over time is called
ecological succession.
-Sometimes succession results from slow changes in the physical environment; a sudden
natural disturbance from human activities, such as clearing a forest, may also be a cause of
succession.
Primary Succession
On land, succession that occurs on surfaces where no soil exists; after an ecosystem is
completely destroyed.
-Ex: Primary succession occurs on the surfaces formed as volcanic eruptions build new
islands or cover the land with lava rock or volcanic ash.
-Ex: Occurs on bare rock exposed when glaciers melt.
Stages of primary succession after a volcanic eruption:
-When primary succession begins there is no soil, just ash and rock.
-The first species to populate the area are called a pioneer community.
-These can include lichen, fungi and mosses.
The pioneer species on volcanic rocks are often lichens.
-A lichen is made up of a fungus and an alga and can grow on bare rock.
-As lichens grow, they help break up the rocks.
-When they die, the lichens add organic material to help form soil in which plants can
grow.
Soon, animals will begin to move back into the ecosystem.
-Eventually, large trees replace small plants, until no further succession occurs.
-The final stage of succession is called the climax community.
What types of human activities can disturb an ecosystem and cause succession?
-Logging, strip mining, draining a marsh, clearing woodland to grow crops or graze livestock,
removing a beaver dam.
**See diagram on next page!**
Secondary Succession
When a disturbance of some kind changes an existing community without removing the soil,
then secondary succession can follow.
-An ecosystem is disturbed, not destroyed.
-Secondary succession occurs when land cleared and plowed for farming is abandoned.
-It also occurs when wildfires burn woodlands.
Secondary Succession - stages:
-First, fast-growing grasses and non-woody plants appear.
-Then, larger shrubs move in.
-Next, fast-growing trees move in (pines), followed by slower-growing hardwood trees.
-As new plants take root and grow, animals begin to move back into the ecosystem.
-End: Climax community.
**See diagram on next page!**