chapter 54

CHAPTER 54
Ecosystems
Lecture Outline
• Basic structure of ecosystems
• Primary production
• Secondary production & energy transfer
• Cycling of chemical elements in ecosystems
• General model of nutrient cycling
• Nitrogen cycle
• Carbon cycle

Basic structure of ecosystems
• Ecosystem:
consists of populations and communities of organisms and their abiotic environment with which they interact.
• Organisms and their abiotic environment are connected through the movement of energy and matter.
-> the dynamics of an ecosystem involve two processes: energy flow and chemical cycling

-> extreme view:
ecosystems are energy machines and matter processors

All ecosystems share the same basic structure: Fig. 54.1
• Heterotrophs:
make up the trophic levels above the primary producers and depend on their photosynthetic
output.

• Autotrophs:
primary producers, usually photosynthetic (plants or algae).

All ecosystems share the same basic structure:
solar energy is converted into chemical energy by autotroph organisms
= primary producers
(plants & algae)
-> use light energy to synthesize sugars and other organic compounds

solar energy is the initial energy input

tertiary consumers carnivores eating other carnivores

secondary consumers carnivores eating herbivores

primary consumers
herbivores eating primary producers

primary producers

detritivores or decomposers
get energy from non-living organic material

-> link all trophic levels

-> play a central role in material cycling

Flow of energy
and matter in an
ecosystems:

• energy is transferred as organic matter

• but: at each transfer energy is lost as heat

-> all energy is eventually lost from an ecosystem, but matter is recycled

Primary production
= the amount of light energy converted to chemical energy by an ecosystem’s autotrophs
in a given time period
• only ~ 1% of solar energy is captured & converted to chemical energy
• primary production is limited by three primary factors:
- light availability (= energy)
- nutrient availability
-> farmers fertilize plants
- water availability
-> irrigation of crops

• mineral nutrients in the soil can play key roles in limiting primary production in terrestrial systems:
Fig. 54.1

-> primary production can be expressed
as biomass of vegetation added to the ecosystem
per unit area per unit time

• Primary production in freshwater ecosystems:
• depends on light, nutrients and temperature
• sewage and fertilizer pollution adds nutrients to lakes
-> eutrophication
-> shifts lake communities from phytoplankton
to diatoms and green algae =‘green goo’

Secondary production
= amount of chemical energy in consumers’ food that is converted into new biomass during a given time period
• efficiency of energy transfer between trophic levels is usually less than 20%
-> energy loss through:
- respiration
- feces
- heat

• Ecosystem Pyramids:
-> summarize the trophic efficiency
= energy and mass flow at each level in the ecosystem,
expressed as energy (joules) or biomass
-> show loss of energy from a food chain

• Meat versus vegetarian diet:
humans obtain more calories by eating grains directly as primary consumer than by eating other primary consumers
-> implications for human population: worldwide agriculture could feed many more ‘vegetarians’ than ‘meat-eaters’

Green world hypothesis
• herbivores consume relatively little plant biomass
(less than 17 % of total net annual production of biomass)
• herbivores are held in check by a variety of factors:
• Plants have defenses against herbivores
• Nutrients, not energy supply, usually limit herbivores
• Abiotic factors limit herbivores
• Intraspecific competition can limit herbivore numbers
• Interspecific interactions check herbivore densities

Flow of energy
and matter in an
ecosystems:

• energy is transferred as organic matter
• but: at each transfer energy is lost as heat
-> all energy is eventually lost from an ecosystem, but matter is recycled

Nutrient circuits
• Ecosystems receive a constant and inexhaustible influx of solar energy
• But: chemical elements are only available in limited amounts

-> Life on Earth depends on …

• both biological (biotic) and geological (abiotic) processes are involved in nutrient cycling

-> nutrient circuits are therefore called …

Fig. 54.15 General model of nutrient cycling
-> four main reservoirs of elements, defined by their contents and availability of contents to organisms

Nutrient circuits
• Inorganic nutrients (N, P, K, C as CO2, and micronutrients) come originally from air, soil or water, and the weathering of rocks
• They are assimilated into organic form in primary producers first, then move on to consumers, and to deteritivores & decomposers
• Respiration and decomposition returns them into inorganic form in the soil, where they are reassimilated and cycled again

• Exception: ‘long-term storage’ of organic material
• Fossil fuels (oil, coal): sequestered in the geological past
• Fossil fuels are eventually returned to active cycling:
-> slow by erosion or geological upheaval
-> fast by burning them
-> release of CO2 into the atmosphere
-> contributes to global warming

Can we find ways to put some of the released carbon into ‘long-term storage’ ?

Carbon cycle
• The reciprocal processes of photosynthesis and cellular respiration are responsible for the major transformations and movements of carbon:
• atmospheric CO2 naturally present in small amounts:
~ 0.03% of the atmosphere
• CO2 is taken up by plants via photosynthesis
• a fraction is lost back to atmosphere by plant and animal respiration
• organisms die -> decomposers release CO2 by respiration
• some detritus decays slowly or is buried by geological activity (chemical change -> petroleum formation)
-> long-term storage of a small fraction of carbon

Fig. 54.17 The carbon cycle
- natural variations of atmospheric CO2: …
- human impacts on CO2 levels in atmosphere: …

Carbon cycle
= important because of increasing atmospheric CO2 and its role in climate change:
-> atmospheric CO2 concentrations have been continuously increasing since well
before the 1960s

Global warming
• rising levels of atmospheric CO2 may have an impact on Earth’s heat budget
• most light energy hitting the Earth is usually reflected
• CO2 causes the Earth to retain some of the energy that would ordinarily escape the atmosphere
-> …
-> can cause …

Nitrogen cycle
• Nitrogen
• is the main mineral nutrient limiting primary production
• is a player in many forms of pollution
• cycles globally as N2 in the atmosphere

• Nitrogen is abundant in the atmosphere as N2, but N2 is not usable by plants
• usable forms are nitrate NO3- and ammonium NH4+
• N2 is converted to usable forms in several ways:
• by fixation by microorganisms in soil or in the roots of certain plants (mainly legumes = pea family)
• by chemical reactions in the atmosphere -> nitrate and ammonium fall to the soil with precipitation
-> plants take up usable N forms from the soil through their roots, and …

Fig. 54.18 Nitrogen cycle
- only small amounts of N …
- …

• End product of nitrogen fixation by microorganisms is ammonium = ammonification
• Soil bacteria oxidize ammonium -> nitrites NO2- and nitrates NO3- = nitrification
• Some bacteria get oxygen from the nitrate and release N2 back into the atmosphere = denitrification
Example:

Human modification of the nitrogen cycle:
• Nitrogen is added to agricultural
soils in fertilizers as nitrate and
ammonium
-> NO3- and NH4+ runoff causes
water pollution (eutrophication)
• Tillage of crop field accelerates decomposition
-> N is lost to atmosphere
-> lowers availability of soil N
-> more fertilizer needed

-> ability to add N to agricultural systems increases
crop yield , but its consequences must be considered!

Phosphorous cycle
• Phosphorous does not have an atmospheric component
-> …
• losses from soil through leaching into aquatic ecosystems
• gains through weathering of rocks
• P is not as limiting for
plant growth as nitrogen

Fig. 54.19 Phosphorous cycle

Water cycle
C, N, and P occur in several chemical forms during cycling
but: most water cycles in a chemically unaltered form
organisms are mostly made of water, which remains chemically unchanged

-> the water cycle is …

Exception: water is chemically altered during photosynthesis (water --> hydrogen + oxygen)

Fig. 54.16
Water cycle -> driven by high levels of evaporation over oceans

Fig. 54.20 Biological and geological processes in chemical cycling

Cycling of elements depends on:
-- Efficiency of energy capture
-- Production efficiency
-- Rates of decomposition
-- Human activities

Key terms 54
Trophic level
Autotrophs
Primary producers
Heterotrophs
Primary consumers
Secondary consumers
Detritivores
Decomposers
Detritus
Primary production
Biomass
Limiting nutrient
Eutrophication
Secondary production
Production efficiency
Pyramid of production (ecosystem pyramid)
green world hypothesis
biogeochemical cycles
nitrogen fixation
ammonification
nitrification
denitrification
greenhouse effect