Ecosystem Production
Objectives

Describe the concept of the ecosystem

Relate the laws of thermodynamics to ecology

Define the types of ecological production
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Discuss how plants allocate net primary production
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Tell how net primary production varies among world ecosystems and why
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Describe secondary production and its allocation

Compare assimilation & production efficiencies of poikilotherms & homeotherms
Ecosystem Concept
 Organisms  physical environment
Organisms Organisms (Intra-specific competition)
Organisms  Organisms (Inter-specific competition)
 Community structure= biota only
 Communities  abiotic = Ecosystem
Ecosystem: biological and physical components of the
environment as a single interactive system
Spatial concept: defined boundaries, often difficult to define
Focus of Ecosystem Ecology
 Ecosystem ecology: focus on the exchange of
energy and matter
Inputs: exchanges from surrounding environment
Outputs: exchanges from inside the ecosystem to
the surrounding environment
– Closed ecosystem: no inputs
– Open ecosystems: with inputs
Three Basic Components

Autotrophs (Producers)
Largely green plants that use energy of sun in photosynthesis to
transform inorganic compounds into organic compounds

Heterotrophs:
Use the organic compounds of producers as a source of food;
eventually break down organic into inorganic
Consumers: feed on living tissue
Decomposers: break down dead material

Abiotic
Soil, sediments, particulate matter, dissolved organic matter, litter
Energy source driven from sunlight
Schematic Diagram
Energy Flow
 Production involves fixation and transfer of
energy from sun
 Energy exists in two forms:
Potential energy (PE): stored energy that is capable
of doing work
Kinetic energy (KE): energy in motion, performing
work at the expense of PE
Work : 1) storage of energy or 2) ordering of matter
First Law of Thermodynamics

Energy is neither created nor destroyed
May change in form, pass from one place to another, or act
upon matter in various ways
However, no gain or loss of energy occurs

Exothermic reaction: energy lost from the system to
surrounding environment:
Wood burning: PE of molecular bonds KE of heat

Endothermic: energy from outside is put into a system
to raise to a higher energy state
Photosynthesis products (sugar) store more energy than the
reactants that combined to form the products
Entropy
 Total energy is maintained in a reaction, but
tends to disperse randomly and in disorder
PE of wood molecules disperse as KE of heat that
disperse and incapable of doing further work
Second Law of Thermodynamics
 When energy is transferred or transformed, part of the
energy is assumes a form that can’t pass on any
further
Entropy increase
– Boiler Coal steam+heat, some heat dispersed to air,
incapable to do work in that system
– Same is true for transfer of energy from one organism to another
in the form of food, some of the energy is lost as heat, unable to
do work, some is stored as tissue, able to to work.
Heat
PE I
KE
PE II
Primary Production

Primary production
Energy accumulated by plants via photosynthesis

Primary productivity
Rate energy is accumulated by plants (kcal/m2 or g/m2)

Gross primary production (GPP)
Total energy assimilated by the plant through photosynthesis

Net primary production (NPP):
Energy remaining after respiration (R), for living processes
NPP stored at organic matter
NPP = GPP – R

Standing Crop biomass (g/m2 or cal/m2)
NPP accumulates over a given time as biomass in a given area
Instantaneous versus NPP is a rate
Energy Allocation

Annual Plants: begin above ground life cycle in spring
Photosynthates to leaves  leaves  photosynthesis
At flowering photosynthates  reproduction

Perennial: maintain vegetative structure over years
Similar allocation to annuals early
Before allocation switch to reproduction, allocation to roots. Roots can be reserves
of food
Reserves  flowering and fruits

Trees and woody shrubs
Early life, leaves >1/2 biomass; later leaves=1-5% biomass
Energy goes toward support and maintenance

Evergreens
Year round photosynthesis in leaves
Don’t draw upon reserves of roots in spring  year round photosynthesis.
Energy Allocation
 Reproduction and vegetative growth
Vegetative growth first, reproduction secondary
 Above-ground vs. below-ground biomass
Low light:
– Allocation of energy to leaves and stems at the expense of roots
– High shoot-to-root ratio
Low water/nutrients:
– Allocation of energy to roots at the expense of leaves and shoots
– Low shoot to root ratio
– Midwest prairies: shoot-to-root ratio of 1:3 due to low moisture
 These are indicators of ecosystem conditions
Climatic Influences
 NPP increases with increasing temperature and
rainfall
Temperature and rainfall influence photosynthesis via the
area of the leaf that can be supported and the duration of the
growing season
PP function of the rate of photosynthesis and total surface
area of the leaf
Terrestrial Ecosystem PP
P= primary production (tn/ha)
B= biomass (tn/ha)
R= PAR solar radiation (kcal/m2/yr)
Nutrient Limitation in Oceans
Vertical separation between zones of PP and
decomposition and nutrient release
 Aquatic ecosystems

Surface = photosynthesis occurs via phytoplankton
Deeper water= nutrients recycle due to death a
N, P, and iron limited in the area of primary productivity
Requires upwelling of nutrient rich water to enhance primary
productivity (Fig 23.6)
PP Varies with Time

Seasonal variations in PP
Wet tropics: little variation
Cold or distinct wet and dry seasons: variation due to
dormancy

Year to year
Climatic: wet and dry years
Herbivory
Fire

Age of ecosystem:
Early stages biomass is in leaf area, later biomass in woody
tissue
GPP goes to maintenance, less towards growth as
ecosystem ages
PP Limits 2° Production
 2° production:
Energy left over from maintenance and respiration
goes into production, including growth of new tissue
and the production of young
 2° production limited by PP
Climatic factors then can control 2° production
Energy Metabolism of Deer
Rainfall versus 2° Production
Consumers Vary in Efficiency of
Production
Not all consumers have the same efficiency of
transforming energy consumed into 2° production
 Homeotherms

High assimilation efficiency: 70 %
but use 98% for metabolism low production efficiency

Poikilotherms
Assimilation efficiency of 30%
79% of their assimilation in metabolism, converting greater
portion of assimilated energy into biomass.
Production Efficiencies
Terms
 PP: Primary Production
 GPP: Gross Primary
Production
 NPP: Net Primary Production
 R: Respiration
 I: Ingestion
 W: Egestion; feces, urine, gas,
etc.
 A: Assimilation as food or
energy absorbed
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Equations
Photosynthetic efficiency
GPP/ solar radiation
Assimilation efficiency, plants
GPP/light absorbed
Respiration
GPP-NPP
Effective PP
NPP/GPP
Assimilation Efficiency, animals
A/I
Ecological growth efficiency
P/I
Production efficiency
P/A
PP, Decomposers, & Climate
 Decomposers limited by amount of food energy
Therefore limited by PP
 Decomposers also strongly influenced by
climate.
Low temps and water limit microbial populations
This then limits decomposition
Decomposition versus Climate