Chapter 3 Notes: Communities,
Biomes, and Ecosystems
Limiting Factors
Recall from chapter 2, that all ecosystems have
biotic and abiotic factors.
In every ecosystem, there are always certain
factors that are abundant and other factors that
are scarce or hard to come by.
Those factors that are important but scarce or
hard to come by in an ecosystem are referred to
as limiting factors. Especially if those factors
are essential for life.
Limiting Factors
If you are a plant or animal that lives in a desert,
the main limiting factor might be water.
If you are a plant in the rainforest that is
competing for sunlight, sunlight might be the
limiting factor in that ecosystem.
If you are a moose that depends on plants that
are high in nutrients, then forest fires might be
the main limiting factor for a moose, because
forest fires increase the amount of nutrients
available in a boreal forest ecosystem.
Limiting Factors in Ecosystems
Range of Tolerance
All living organisms have a range of tolerance
for certain factors that they can withstand.
A range of tolerance can be described as a
zone within an upper limit and lower limit of
environmental characteristics.
Moose have a range of tolerance for
temperatures down to about -40 and up to
about 60 degrees Fahrenheit.
Above or below these temperatures, moose
may undergo physiological stress.
Range of Tolerance for Organisms
Ecological Succession
As ecosystems mature, eventually they reach a
stage of maturity called a climax community.
Climax communities are not ecologically
productive in terms of biomass and diversity.
Once an ecosystem reaches the stage of climax
community, some type of natural or man-made
disturbance must take place in order to restore the
climax community to a more productive state.
Climax Community
Ecological Succession
Examples of natural disturbances could be
volcanic eruptions, forest fires, glaciation, flooding,
water and wind erosion, storms, hurricanes etc.
Examples of man-made disturbances could be
building roads, agriculture, prescribed forest fires,
logging, clear cutting, development of land etc.
After a disturbance occurs, ecosystems go through
a natural progression or a series of changes called
an ecological succession.
Ecological Succession
One type of ecological succession is called a
primary succession. In primary succession, the
disturbance causes such a severe change that
usually the only thing left is bare rock.
In the case of primary succession, the disturbed
area must reestablish a new layer of soil before
plants can move in and reestablish an area.
Plants and animals that are the first to reestablish
an area after a disturbance are called pioneer
species. lichens are an important pioneer
species in primary succession.
Primary Succession
Ecological Succession
Lichens are important pioneering species,
because they are one of the few examples of
living organisms that can survive on bare rock.
They are also important because they also help
create new soil by producing acids that help
chemically break down the rock into soil, so that
other species can come in and grow after them.
If a disturbance occurs, but the soil in the area is
still intact, then another type of succession
occurs called secondary succession.
Secondary Succession
Ecological Succession
A good example of an event that may cause a secondary
succession is a forest fire. In a forest fire, the trees and
vegetation are removed, but the soil remains intact.
Nutrients from the trees are released back into the soil
once the trees are burned.
Fireweed, alders and willows are all good examples of
pioneer species that help reestablish an area after a
forest fire because they are shade-intolerant.
These pioneer species help the ecosystem by either
nitrogen fixation or they allow shade-tolerant species
like birch and aspen to eventually grow and recolonize.
Ecological Succession
A group of ecosystems in an area with a similar
climate makes up a biome.
The condition of the atmosphere at any given
time or place is the weather.
The average weather conditions in an area
including its temperature and precipitation is
called its climate.
Climates are determined by the distance north or
south of the equator which is called the latitude.
Latitude, Weather, and Climate
Major Biomes of the World
Tundra – low precipitation (15-25 cm), cold
temps (-34-12 C), low growing vegetation,
animals adapted to the cold (caribou, polar
bears, arctic fox, arctic ground squirrels),
permafrost, cold and dark winters.
Boreal forest – moderate prec. (30-84 cm), cold
temps (-54-21 C), coniferous trees dominate,
animals of the boreal forest (moose, lynx, hare,
red squirrel, owls, hawks), short and moist
summers, cold and dry winters.
Boreal Forest (Taiga)
Temperate forest – moderate prec. (75-150
cm), moderate temps (-30-30 C), deciduous
broadleaf trees dominate, animals of the
temperate forest (deer, squirrels, turkeys,
hawks, owls, raccoons etc.), well-defined
seasons, hot summer, cold winter.
Temperate woodland or chaparral –
moderate prec. (38-100 cm), warm temps (1040 C), shrubs dominant vegetation, animals of
the chaparral (coyotes, jackrabbits, lizards,
snakes, insects), summers hot and dry, winters
cool and wet.
Temperate Deciduous
Temperate Woodland or
Temperate grassland – moderate prec. (50-89
cm), moderate temps. (-40-38 C), grasses are
dominant vegetation, animals of the grassland
(gazelles, bison, horses, lions, antelope, deer,
mice, coyotes, fox, wolves, snakes, insects),
summers hot, winters cold, regular fires.
Desert – low prec. (2-26 cm), temps vary, cacti
and other plants adapted to arid conditions
dominate, desert animals (lizards, coyotes,
snakes, jackrabbits, sheep, vultures).
Grassland or Prairie
Tropical savanna – moderate prec. (50-130
cm), high temps (20-30 C), grasses and
scattered trees, savanna animals (lions, hyenas,
cheetahs, elephants, giraffes, zebras etc.), hot
and rainy summers, cool and dry winters.
Tropical seasonal forest – high prec. (more
than 200 cm per year), high temps (20-25 C),
forest vegetation, tropical seasonal forest
animals (elephants, tigers, monkeys etc.),
deciduous trees drop their leaves in the dry
season, rainfall is seasonal.
Tropical Savanna
Tropical Seasonal
(Deciduous) Forest
Tropical rain forest – extremely high prec. (2001000 cm per year), high temps. (24-27 C), large
diversity of plant life, tropical rain forest animals
(chimpanzees, Bengal tigers, orangutans, toucans,
sloths, cobras, jaguars, insects), high humidity, no
seasons, hot and wet all the time.
Mountains – mountainous terrain varies with
elevation, changes in elevation resemble changes
in latitude from an ecological standpoint.
Polar – cold all year, covered in ice, no vegetation,
animals highly dependant on the sea for food.
(polar bears, penguins, krill, seals)
Tropical Rainforest
Mountain Biomes
Aquatic Ecosystems
A large portion of the earth is water. It should be
no surprise that many ecosystems in the world
are aquatic ecosystems.
Approximately 97.5% of the water in the world is
saltwater (oceans).
Roughly 2.5% of the water is freshwater.
How much of the 2.5% freshwater is found in...
Lakes and rivers?
Distribution of water graph
River and Stream Ecosystems
As streams travel from their headwaters to their
destination (ocean, stream confluence, lake
etc.) the character of the stream changes
creating different habitats.
Stream headwaters have a steep gradient, low
volume (discharge), waters are cold and rich in
oxygen but poor in nutrients. Stoneflies,
caddisflies, mayflies, rainbow trout, grayling.
Streams closer to their destination have a
shallow gradient, high volume, warm waters
that are low in oxygen but rich in nutrients.
River Continuum Concept
Lake Ecosystems
Lakes are either Eutrophic (shallow, warm, more
sunlight, nutrient rich, more vegetation)
or Oligotrophic (deep, cold, less sunlight, nutrient
poor, less vegetation).
Most of the life in a lake is found close to the
shoreline (littoral zone). More light means more
plants. More plants means more food.
The open water area in the middle of the lake is
called the limnetic zone. The food chain in the
limnetic zone is dependent on algae and plankton.
Lake Ecology
Lake Ecology
Lake Zones
Lake Zones
Lake Ecosystems
The bottom of a lake is the benthic zone. The
zone in a lake below light is the profundal zone,
which is generally the most unproductive.
No sunlight means no plants. No plants means
no food. Not much life on the bottom except
mostly decomposers and detritivores.
Seasonal changes in the water temperature and
water chemistry cause changes in lake ecology.
4 degrees Celsius (39-40 F) is when water is
most dense. This causes thermal stratification.
Thermal Stratification in Lakes
During the summer the top layer of the lake (the
epilimnion) warms up faster than the bottom.
The bottom of the lake stays at 4 Celsius because
the water at 4 Celsius has the highest density.
As autumn approaches the epilimnion cools off
and eventually reaches 4 Celsius.
When this occurs the density throughout the lake
is the same and wind causes the water throughout
the lake to mix nutrients. Dissolved oxygen is
replaced and carbon dioxide is removed.
Thermal Stratification in Lakes
As winter approaches, the temperature of the
epilimnion lowers until it reaches 0 Celsius.
At 0 Celsius the top of the lake freezes and the
bottom of the lake (the hypolimnion) stays at a
constant temperature of 4 Celsius.
As spring approaches, the ice melts and begins
warming until it reaches 4 Celsius.
Once it reaches 4 Celsius, once again the wind
causes the lake to mix and turnover.
The name of these critical time periods are called
spring and fall turnover.
Thermal Stratification
Spring and Fall Turnover is important to lake
ecosystems, because during summer and winter
the amount of carbon dioxide increases, the pH
lowers (becomes acidic), and the amount of
dissolved oxygen decreases.
Spring and Fall Turnover helps to replenish the
lake ecosystem with new nutrients and dissolved
oxygen, and it helps to eliminate the amount of
carbon dioxide that has accumulated by cellular
respiration of animals.
It also helps regulate the pH levels of the lake.
Thermal Stratification
Other Important Aquatic Ecosystems
Wetlands – marshes, swamps, and bogs.
Estuaries – Freshwater and saltwater mix.
Intertidal zones – Land meets the ocean.
Open Ocean (Pelagic) – similar in many ways
to lake ecology only bigger. There are different
zones of the open ocean like the photic zone,
aphotic zone, benthic zone and abyssal zone.
Coastal Ocean and Coral Reefs – the most
diverse ecosystem of the ocean.
Marine Ecosystems
Marine Ecosystems
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