BIOL 4120: Principles of Ecology
Lecture 18: Ecosystem
Ecology
Dafeng Hui
Room: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Ecosystem
Definition: biotic community and abiotic environment functioning as
a system. Includes organism-complex and whole complex of
physical factors.
Ecosystem ecologist:
Forest is a system composed of autographs, heterographs, and
abiotic environment, each component processing and exchanging
energy and matter.
Inputs: exchanges from the surrounding environment into the
ecosystem
Outputs: exchange from inside ecosystem to the surrounding
environment
Closed ecosystem: an ecosystem with no inputs and outputs
Open ecosystem: an ecosystem with inputs and outputs
Ecosystem ecology: exchanges of energy and matter between
ecosystem and environment and among components within the
ecosystem (energy flow and nutrient cycling).
Outline (Chapter 20)
Ecosystem Energetic
20.1 Energy fixed in the process of
photosynthesis is primary production
Flow of energy through a
terrestrial ecosystem starts with
the harnessing of sunlight by
autotrophs.
Rate at which light energy is
converted by photosynthesis to
organic components is referred
to as primary productivity.
Gross primary productivity
(GPP): Total rate of
photosynthesis
Net primary productivity
(NPP): rate of energy as
storage as organic matter after
respiration
NPP=GPP-R
Standing crop biomass: amount of
accumulated organic matter in an
area at a given time
Biomass is expressed as g organic
matter per square meter (g m-2)
Productivity is the rate at which
organic matter is created by
photosynthesis (g m-2 yr-1)
How to measure?
Terrestrial ecosystem:
Change in standing
crop biomass over a
given time interval
(see Hui & Jackson 2006 for
grasslands)
Aquatic ecosystem:
Wrong again?
20.2 Temperature, water, and nutrients
control primary production in terrestrial
ecosystems
Net primary productivity for a variety
of terrestrial ecosystem as a function
of mean annul precipitation (MAP) and
mean annual temperature (MAT) as
well as length of growing season
Deciduous forest in N. America
Warm temperature and
adequate water supply for
transpiration that gives the
highest primary productivity.
(Remember that photosynthesis
and transpiration are coupled
processes)
Global map of primary productivity
Patterns of productivity reflect global patterns of temperature
and precipitation. High NPP in equatorial zone and coastal
region.
Primary production varies with
nutrient availability
Different forest ecosystems
RO, red oak; RP, red pine; SM,
sugar maple, Hem, hemlock;
WP, white pine
20 oak savanna in
Minnesota
20.3 Temperature, light, and nutrients
control primary production in aquatic
ecosystems
Changes in available light,
respiration, NPP with water
depth
Compensation depth:
Depth that available light is
equal to the light
compensation point (NPP=0
or GPP=R)
Effects of nutrient addition on marine
phytoplankton growth rate in 303 experiments
John downing, Iowa State
Conducted 303 experiments
Nitrogen addition stimulated
phytoplankton growth the
greatest, follow closely by Fe,
addition of P showed no effect
Effect of P addition varied among
different ecosystems.
In polluted areas, show negative
effect
Geographic variation in primary
productivity of world’s oceans
1. Great
transport of
nutrient
from bottom
to top
2. Nutrient
from
terrestrial
ecosystems
High productivity is along coastal regions
20.4 Primary
production varies
with time
Park Grass, Rothamsted
Experimental Station in
England
Climatic variation
Disturbance
Stand aging
Aboveground stem
biomass, LAI, and
aboveground NPP for
stands of boreal needleleaf evergreen conifer of
different ages.
20.5 Primary productivity limits secondary
production
Net primary production is the energy available to the heterotrophic
component of the ecosystem
Either herbivores or decomposers eventually consume all plant
productivity, but often it is not all used within the same ecosystem.
Secondary production: net energy of production of secondary
consumers
•Energy stored in plant material, once consumed, passes through the
body as waste products.
•Of the energy assimilated, part is used as heat for metabolism
(respiration)
•Reminder is available for maintenance – capturing or harvesting food
etc, and lost as heat
•Energy left over from maintenance and respiration goes into
production, including growth of new tissues and production of young
Secondary productivity: secondary production per unit of time
Secondary production depends on
primary production for energy
Sam McNaughton
69 studies for terrestrial
ecosystems (from Arctic tundra to
tropical forests)
Similar
relationship
in lake
ecosystems
43
lakes+12
reservoirs
Tropic to
Arctic
Bottom-up
and topdown
control
20.6 Consumers vary in efficiency of
production
Energy use is a complex process. Not all consumers have the same
efficiency
A simple model of energy flow through consumer
I: food ingested by a consumer
A: a portion is assimilated across the gut wall
W: remainder is expelled from the body as waste products
R: of the energy assimilated, part is used for respiration
P: reminder goes to production (new growth and reproduction)
Based on these data, we can calculate:
Assimilation efficiency A/I, ratio of assimilation to ingestion
measure the efficiency with which consumer extracts energy from food
Production efficiency P/A, ratio of production to assimilation
measure the efficiency with which the consumer incorporates
assimilated energy into secondary production.
Secondary
producers are not
necessarily highly
efficient
Assimilation efficiency



Vary widely among animal groups
Endotherms are much more efficient
than ectotherms
Carnivorous animals (even
ectothermic ones) have high
assimilation efficiency than
herbivores (meat vs veg)
Production
efficiency varies
mainly according
to taxonomic
class
20.7 Ecosystem have two major food chains
Food chain and food web review
Food chain is a flow of energy
Feeding relationships within a food chain are defined
in terms of trophic or consumer level
1st level: Autotrophs or primary producer
2nd level: herbivores (1st level consumers)
Higher level: carnivores (2nd level consumers)
Some consumers occupy more than one trophic level
Within any
ecosystem,
there are two
major food
chains
Difference
1. Source of
energy for
herbivores
2. Energy flow
direction
3.
interconnected
20.8 Energy flow through tropic levels can
be quantified
Energy flow within a
single trophic
compartment
Consumption
efficiency:
In/Pn-1
20.8 Consumption
efficiency determines the
pathway of energy flow
through the ecosystem
In terrestrial ecosystems and
shallow water ecosystems, with
their high standing biomass and
relative low harvest of primary
production by herbivores, the
detrital food chain is dominate.
In deep water aquatic
ecosystem, with low biomass,
rapid turnover and high rate of
harvest, grazing chain may be
dominate.
20.9 Energy decreases in each successive
trophic level
Energy pyramid
End


Normally ecosystems have two major
food chains
Terrestial grazing chain not very
important
• Only 2.6% of primary production


Insects very important
Detrital chain is very important
• 35% of primary production



Food chains are interconnected
Energy flows through trophic levels
Energy decreases with each tropgic
level


Assimilation efficiencies vary widely
among endotherms and ectotherms
Pattern of flow varies
20.1 Laws of thermodynamics
govern energy flow
Energy
Potential energy: stored energy; capable of and available for performing work
Kinetic energy: energy in motion. It performs work at the expense of potential energy
Work: at least two kinds: the storage of energy and the arranging or ordering of matter
First law of thermodynamics:
Energy is neither created nor destroyed.
Exothermic and Endothermic
Second law of thermodynamics
When energy is transferred or transformed, part of the energy assumes a form that cannot pass on any
further
Entropy increases
As energy is transferred from one organism to another in the form of food, a portion is stored as energy
in living tissue, whereas a large part of that energy is dissipated as heat.
Biomass and productivity
Trophic Dynamic View of
Ecosystems

Lindeman (1942) concluded the ecosystem
concept is fundamental to the study of energy
transfer within an ecosystem.
• Suggested grouping organisms within an ecosystem into
trophic levels.

Each feeds on level immediately below.
• As energy is transferred from one trophic level to another,
energy is degraded.
• As energy is transferred from one trophic level to
another, energy is degraded:



Limited assimilation
Consumer respiration
Heat production
• Energy quality decreases with each successive trophic level.
 Pyramid-shaped energy distribution.