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ECOSYSTEM ECOLOGY
I. Ecosystem = a community of organisms interacting within a
particular physical environment. Stated another way, an ecosystem is a community
plus its abiotic factors, e.g. soil, rain, temperatures, etc.
A. Virtually all energy on earth comes from the sun, via photoautotrophs
(primarily plants), and it is ultimately distributed throughout ecosystems.
B. The autotrophs within the ecosystem are the primary producers,
all other organisms are consumers
1. Consumers which eat plants are called herbivores
2. Consumers which eat animals are called carnivores
3. Organisms such as humans, which eat both plants and animals, are
called omnivores
4. Decomposers, which includes fungi and bacteria,
obtain their energy by breaking down the remains or products of organisms
5. Detritivores are decomposers which eat detritus
- organic wastes and dead organisms
II. Structure of ecosystems
A. Ecosystems are arranged by trophic (feeding) levels
between various producers, the autotrophs, and consumers, the heterotrophs:
1. First trophic level - contains the autotrophs which
build energy containing molecules
a. They also absorb nitrogen, phosphorous, sulfur and other molecules
necessary for life
b. They provide both an energy-fixation base as well
as the nutrient-concentration base for ecosystems
c. Two types of autotrophs:
Photoautotrophs - plants and some Protista
Chemoautotrophs - bacteria
2. Second trophic level - contains the
primary consumers which eat the primary producers
a. These include herbivores, decomposers and detritivores, e.g.
insects, grasshoppers, deer and wildebeest
3. Third trophic level - contains the secondary
consumers, primary carnivores
a. They eat the herbivores, e.g. mice, spiders and many birds
4. Fourth trophic level - contains the tertiary
consumers, secondary carnivores
a. They eat the primary carnivores, e.g. weasel, owl, sharks and wolve.
B. Such linear food chains as described above are probably
rare in nature because the same food source may be part of several interwoven
food chains and many organisms have several food sources
C. Because of this energy flows through ecosystems via food webs,
intricate pathways of energy flow and material cycling
III. Energy flow though ecosystems
A. Gross primary productivity = the rate at which
the primary producers capture and store energy per unit time
1. However, since the primary producers expend energy during respiration
the net primary productivity is considerably lower than the
gross productivity
a. Productivity is usually measured as biomass (dry weight of organic
matter) per unit area per a specified time interval, e.g. kg/m2/yr
B. The trophic structure of an ecosystem is often represented by a ecological
pyramid, with the primary producers at the base and the other levels
above
1. However, because most of the food eaten by organisms is converted to
biomass, or used to maintain metabolic functions, or lost as heat, only
about 10% of the energy makes it to the next level
a. This massive energy loss between trophic levels explains why food
chains can't contain more than a few levels
For example, on the savanna of Africa it takes takes billions of
primary producers (plants) to support millions of wildebeests, which
support a few thousand lions. This also explains why there are so few
large carnivores on earth
C. An energy pyramid is a more useful way to depict an
ecosystem's trophic structure
1. Each block of the pyramid is proportional to the amount of energy it
contains
2. Pyramids may also represent biomass or numbers of individuals
D. The energy pyramid concept helps explain the phenomenon of biological
magnification - the tendency for toxic substances to increase in
concentration at progressively higher levels of the food chain
1. For example, DDT was once a widely used insecticide. However when
washed off croplands into streams and lakes it became concentrated in fish
that were ultimately eaten by birds such as bald eagles. The DDT caused
fragile eggs such that populations of large predator birds rapidly declined.
Since DDT was banned in the US in 1968 bird populations have made dramatic
comebacks
IV. Weather and climate
A. Biome = a large region of land characterized by the
climax vegetation of the ecosystems within its boundaries
1. The distribution and key features of biomes are the outcome of
temperatures, soils and moisture levels (which vary with latitude and
altitude), and evolutionary history
B. Weather = the condition of the atmosphere at any given time
C. Climate = the accumulation of weather events over
a long period of time (temperatures, humidity, wind, cloud cover, rainfall)
1. Climate is dependent upon several factors:
a. Solar radiation
b. The earth's daily rotation
c. The earth's rotation around the sun
d. The distributions of continents and oceans
e. Elevation
D. Heat energy from the sun drives the earth's weather
systems, which ultimately determine the composition of ecosystems
1. Only about half the solar radiation entering the upper atmosphere
makes it through to the earth's surface
a. The ozone layer is responsible for most of this
filtering affect
2. There are different climates on earth because the earth heats unevenly
a. This is because the earth is tilted 23°27' on its
axis
b. Therefore the intensity of solar radiation per unit area is always
less at high latitudes than at the equator
c. Equatorial regions receive about five times more energy than polar
areas
d. The tilting of the earth on its axis also means that at the equator
the days and nights are each 12 hours long, whereas at the poles there are
six month long light and dark periods
e. Therefore low latitudes are warmer than the poles
f. This unequal distribution of heat causes a mass transfer of
heat from the equator to the poles both by air and water
movements
g. It is responsible for the wind and ocean patterns on earth
E. Together, soil and climate affect the growth of primary producers, and
thereby the distributions of ecosystems
F. Temperature and precipitation are the major environmental parameters
which determine the nature of ecosystems
V. Global air and ocean circulation patterns
A. Energy from the sun is more concentrated at the equator
1. Warm equatorial air rises and spreads towards the cooler poles
B. The non equal distribution of land masses creates variations in air
pressure
1. Water resists temperature changes. Therefore land surfaces heat up
faster and cool faster than the ocean surfaces. Air pressure decreases where
air rises (and increases where air sinks). These differences give rise to
winds that disrupt the movement of air from the equator to the poles
C. The spinning of the earth results in the deflection of moving air masses,
this is called the Coriolis effect or force
1. The Coriolis effect deflects moving air or water to the right in the
northern hemisphere and to the left in the southern hemisphere
D. Oceans cover about 71% of the earth's surface
E. As noted above ocean currents are affected by the same factors as air
currents. They are just slower due to the density of water
1. Because water resists temperature changes, the oceans help moderate
climates. For example, compare the climates of Minneapolis, MN with the more
northerly London, England; or the climates of Turlock and San Francisco!
F. Topography, the physical features of a region, also
greatly affect its climate
1. Rain shadows, such as the one created by the Sierra Nevada Mountains,
are created by topographic differences
G. The global circulation patterns of the atmosphere and oceans, coupled
with differences in topography give rise to regional differences in
temperature and precipitation
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