AP Biology
Chapter 54 notes Ecosystems
Ecosystem ecology emphasizes energy flow and chemical
recycling
An ecosystem consists of all the organisms in a community and
all the abiotic factors with which they interact
The laws of physics and chemistry apply, especially energy flow
Energy is conserved but degraded to heat during the ecosystem
processes
Energy and nutrients pass from primary producers (autotrophs)
to primary consumers (herbivores) to secondary consumers
(carnivores). Energy flows through an ecosystem, entering as
light and exiting as heat. Nutrients cycle within the
ecosystem.
Decomposition connects all trophic levels. Detritivores, mainly
bacteria and fungi recycle essential chemical elements by
decomposing organic matter and returning the elements to
inorganic reservoirs ( soil, water, air)
Ecosystem Energy budgets:
The energy produced through photosynthesis is a tiny fraction
of the solar radiation reaching the Earth, but primary
production sets the spending limit for the global energy budget.
Most primary producers use light energy to synthesize energy
rich organic molecules, which can be broken down to generate
ATP. Consumers acquire their organic fuels through the food
web (2nd hand, 3rd hand etc). So, the extent of the
photosynthetic production sets the spending limit for energy
budget of the entire ecosystem
Only a small fraction (1%) of the visible light that does reach the
photosynthetic organisms is converted to chemical energy. But
collectively, the primary producers on Earth create about 170
billion tons of organic material/year.
Gross primary production is the total energy assimilated by an
ecosystem in a given time period. (the amount of light energy
converted to chemical energy by photosynthesis per unit time).
The net primary production, the energy accumulated in the
autotroph biomass, equals the gross primary production minus
the energy used by the primary producers for respiration. Only
net primary production is available to consumers. NPP=GPP-R
Ecologists use net primary production because it’s a key
measurement for the storage of chemical energy that will be
available to consumers in the ecosystem.
Net primary production can be expressed as energy per unit
area per unit time (J/m2/yr) or as biomass (weight) of
vegetation added to the ecosystem per unit area per unit time
(g/m2/yr). Biomass is usually expressed in dry weight because
the water content in plants varies greatly over short periods of
time.
In Marine and freshwater ecosystems:
Light and nutrients limit primary production. Within the photic
zone the factor that most often limits primary production is a
nutrient such as nitrogen or iron. A limiting nutrient is the
element that must be added to in order for production to
increase in a particular area. The nutrients most often limiting
marine production is either nitrogen or phosphorus.
Concentrations of these nutrients are very low in the photic
zone where phytoplankton live. But they are abundant is deep
water where there is not enough light for photosynthesis. In
areas of up-welling (where nutrient rich deep water is
circulated to the surface) the steady supply of nutrients
stimulates growth of phytoplankton populations that form the
basis of marine food webs. Nutrient limitation is also prevalent
in fresh water lakes. The blooms of cyanobacteria
(eutrophication) are caused by the runoff of fertilizers and
sewage from farms and yards
In terrestrial or wetlands ecosystems:
Climate factors such as temperature and moisture affect
primary production on a large geographic scale. More locally,
soil nutrients are often the limiting factor in primary
production. Nitrogen and phosphorus are most often the
limiting nutrients limiting terrestrial and wetland production.
Production efficiency:
The amount of energy available to each trophic level is
determined by the net primary production and the efficiencies
in which food energy is converted to biomass at each link in the
food chain. The amount of chemical energy in consumers’ food
that is converted to their own new biomass during a given time
period is called secondary production of the ecosystem.
Trophic efficiency: is the percentage of energy transferred from
one trophic level to the next – is generally 5-20% depending on
the type of ecosystem. The loss of energy with each transfer in
a food chain can be represented by a pyramid of net
production. (see figure 54.11 pg 1192)
Pyramid of Biomass: each tier represents the standing crop in
one trophic level.
Pyramids of numbers: the progressive loss of energy along the
food chain severely limits the overall biomass of top-level
consumers that an ecosystem can support. This helps explain
why food webs include only about 4-5 trophic levels.
Green World Hypothesis: herbivores consume a small
percentage of vegetation because predators, disease,
competition, nutrient limitations, and other factors keep their
populations in check. Several factors that keep herbivores in
check: 1. Plants have defenses against herbivores (spines,
noxious chemicals) 2. Essential nutrients, not energy supply
usually limit herbivores 3. Abiotic factors (temp, moisture)
4. Intraspecific competition (territorial behavior) 5. Interspecific
interactions keep herbivore density in check
Biogeochemical cycle:
Atoms present in the complex molecules of an organism when
it dies are returned to the atmosphere, water, or soil by the
action of decomposers. This decomposition replenishes the
pool of inorganic nutrients that plants and other autotrophs
use to build organic matter. These nutrient circuits involve both
biotic and abiotic components.
A General Model of Chemical Cycling:
Gaseous forms of carbon, oxygen, nitrogen, and sulfur in the
atmosphere cycle globally. Other, less mobile nutrients,
including, phosphorus, potassium, and calcium cycle on a more
localized scale, at least over the short term. All elements cycle
between organic and inorganic reservoirs. Ecologists study
chemical cycling by adding tiny amounts of radioactive isotopes
of the elements they want to track or by following the
movement of naturally occurring stable, non-radioactive
isotopes through the various biotic and abiotic components of
an ecosystem.
Biochemical cycles:
Water moves in a global cycle driven by solar energy.
The carbon cycle primarily reflects the reciprocal processes of
photosynthesis and cellular respiration. Nitrogen enters the
ecosystems through atmospheric deposition and nitrogen
fixation by prokaryotes, but most of the nitrogen cycling in
natural ecosystems involves local cycles between organisms
and soil and water. The phosphorus cycle is relatively localized
through the weathering of rocks that adds phosphate to the
soil; with some leaching into the ground water and surface
water.
The decomposition and nutrients:
The rates at which the nutrients cycle in different ecosystems
are extremely variable, mostly as a result of the rates of
decomposition. Temperature and availability of water affect
the rates of decomposition and thus nutrient cycling times. The
rate of decomposition in terrestrial ecosystems increases with
the actual evapotranspiration (annual amount of water
transpired by plants and evaporated from a landscape). Other
factors influencing nutrient cycling, soil chemistry and
frequency of fires.
Human population is disrupting the trophic structure, energy
flow and chemical cycling of ecosystems in most areas of the
world.
Nutrient enrichment:
Example: nutrients in farm soil may runoff into streams and
lakes, resulting in nutrient depletion in one area, excesses in
another, and the disruption of the natural chemical cycles in
both locations.
Nitrogen is the main nutrient lost through agriculture, having a
large impact on the nitrogen cycle. Ex. farmers use fertilizers to
replenish the nitrogen in their fields. Recent studies indicate
that human activities have doubled the amount of fixed
nitrogen available to the primary producers.
The key problem with excess nitrogen seems to be critical load,
which is the amount of nutrient that can be absorbed by the
plants without damaging the ecosystem integrity. Excess
nitrogen eventually leaches into the groundwater or run off
directly into fresh water and marine ecosystems contaminating
water supplies, choking waterways, and killing fish.
Acid Precipitation:
The burning of wood, coal and other fossil fuels releases oxides
of sulfur and nitrogen that react with the water in the
atmosphere, forming sulfuric and nitric acid. The acids fall to
Earth and generally have a pH level less than 5.6. This lowers
the pH levels of aquatic environs and the affects the soil
chemistry of terrestrial ecosystems.
Toxins in the environment: humans release a large variety of
toxic chemicals with little regard to ecological consequences
Biological magnification:
Toxins become increasingly more concentrated in successive
trophic levels of the food web. Top level predators tend to be
the organisms that are most affected.
Atmospheric CO2 has been steadily increasing. The ultimate
effects may include significant warming and other climate
change.
The Ozone layer reduces the penetration of UV radiation
through the atmosphere. Human activities including the
release of chlorine containing pollutants are eroding the ozone
layer.