BIOL 4120: Principles of Ecology
Lecture 18: Ecosystem
Ecology (Energy in the
Ecosystem)
Dafeng Hui
Office: 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.
Forest Ecosystem
Forest is a system composed of autotrophy, heterotrophy, 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 22)
18.1 Ecosystem function obeys thermodynamic
principles
18.2 Primary production provides energy to the
ecosystem
18.3 Many factors influence primary production
18.4 Primary production varies among ecosystems
18.5 Only 5%– 20% of assimilated energy passes
between trophic levels
18.6 Energy moves through ecosystems at different
rates
18.7 Ecosystem energetics summarizes the movement
of energy populations
18.1 Ecosystem function obeys
thermodynamic principles
History of Ecosystem Ecology
Alfred J. Lotka, 1925
Energy transformation and
thermodynamic principles
Raymond Lindeman, 1942
Pyramid of energy (left)
Eugene Odum, University of
Georgia, 1953
Fundamentals of Ecology
E. P. Odum developed a “ universal” model of energy flow through
ecosystems. The energy ingested by organisms at each trophic
level is reduced by respiration and excretion, so that less energy is
available for consumption by the next trophic level.
Laws of thermodynamics govern
energy flow
First law of thermodynamics:
Energy is neither created nor destroyed.
Second law of thermodynamics
When energy is transferred or transformed, part of
the energy assumes a form that cannot pass on any
further.
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.
18.2 Primary production provides
energy to the ecosystem
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
Productivity is the rate at which
organic matter is created by
photosynthesis (g m-2 yr-1)
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)
How to measure?
Terrestrial ecosystem:
1. Flux based
Measure photosynthesis (equipment: LiCor,
Eddy flux method)
net photosynthesis
2. Biomass based estimation
Change in standing crop biomass (SCB) over
a given time interval
NPP=delta SCB +loss of biomass due to death of plant + loss due to
consumption. (see Hui & Jackson 2006 for grasslands)
18.4 Primary production varies
among ecosystems
Patterns of productivity reflect global patterns of temperature
and precipitation. High NPP in equatorial zone and coastal
region.
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
Recap: Energy in ecosystems
1st and 2nd law of thermodynamics
Primary production provides energy to the ecosystem
Many factors influence primary production
Primary production varies among ecosystems
18.5 Only 5%– 20% of assimilated energy
passes between trophic levels
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 consumers
• Energy stored in plant material, once consumed, some passes
through the body as waste products.
• Of the energy assimilated, part is used as heat for metabolism
(respiration) and 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
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, convert nutrient to
body biomass (digestion, absorption)
E: remainder is expelled from the body as waste products (egested
energy). animal excrete small portion as nitrogen-containing
compounds (as ammonia, urea, uric acid) (excreted energy)
R: of the energy assimilated, part is used for respiration (respired
energy)
P: remainder 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
Secondary consumers: 60-90%
Production efficiency P/A,
ratio of production to assimilation
measure the efficiency with which the consumer
incorporates assimilated energy into secondary
production.
Homoeothermic: low, 1 % (birds) -6% (small
mammals)
Poikilotherimic: high, as much as 75%.
Production
efficiency varies
mainly according
to taxonomic
class
Endotherms have
low P/A
Invertebrates
have high P/A
Vertebrates:
ectotherms have
intermediate
Energy flow through trophic levels can be
quantified
Energy flow within a
single trophic
compartment
Consumption
efficiency:
In/Pn-1
Ecological
efficiency (food
chain efficiency):
Pn-1/Pn
14/200=7%
18.6 Ecosystems have two major
food chains
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: omnivores.
Within any
ecosystem,
there are two
major food
chains
Difference
1. Source of
energy for
herbivores
2. Energy flow
direction
3.
interconnected
18.7 Energy decreases in each successive
trophic level
Energy pyramid
M. Imhoff and L. Bounoua (NASA’s Goddard Space Flight Center) used
satellite-derived data to estimate the human appropriation of
terrestrial NPP (HANPP)
Mean annual HANPP = 24.2 Pg (1 Pg = 1015 g) = 20 percent of
terrestrial annual NPP
HANPPWestern Europe/south central Asia = 70 percent
HANPPSouth America = 6 percent
18.8 Energy move through different
ecosystems at different rates
Ecological efficiency: determine how much energy assimilated by
plants reach high level of tropic levels.
Another feature of energy transfer is the rate of energy transfer (how
fast or how long energy stays in one tropic level)
Residence time (years):
energy stored in biomass (g m-2)
=--------------------------------------------------net productivity (g m-2 yr-1)
Also called biomass accumulation ratio
Tropic: =42 kg m-2/ 1.8 kg m-2 yr-1 = 23 yrs
Residence time for litter pools
Residence time (years):
litter accumulation (g m-2)
=--------------------------------------------------rate of litter fall (g m-2 yr-1)
Forests:
Tropic: 1-2 yrs
Temperate (southeastern US): 4-16 yrs
Mountain and boreal forests: more than 100 yrs
Net Ecosystem Productivity
NEP: a measure of net carbon accumulation
NEP = NPP – Soil heterotrophic respiration
= GPP – Plant Respiration – Soil heterotrophic respiration
= GPP – Aboveground Plant Respiration – Soil Respiration
NEP: 1%– 2% of the total gross primary production (~2 billion tons)
Fossil fuel burning: 8 billion tons of carbon
The End
Relationship of Secondary
production and primary production
Secondary production depends on
primary production for energy
Sam McNaughton (Syracuse Uni.)
69 studies for terrestrial
ecosystems (from Arctic tundra to
tropical forests)
Similar
relationship
in lake
ecosystems
43
lakes+12
reservoirs
Tropic to
Arctic
Energy flow through trophic levels can be
quantified
Energy flow within a
single trophic
compartment
Consumption
efficiency:
In/Pn-1
Ecological
efficiency (food
chain efficiency):
Pn-1/Pn
14/200=7%