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Ecosystem Ecology – general points
(1) Biotic community is truly inseparable from the abiotic
environment
(2) Energy flow through communities ultimately stems from
the sunlight that is assimilated by plants (Net Primary
Productivity) and becomes available to the remainder of the
food web
(3) Each step along the way, energy is lost through inefficient
transformation – mediated by biotic factors
(4) That is the biological details (i.e., species identity or
functional group) greatly impacts E-flow through ecosystems
Cont’
(5) Nutrient availability mediates E-flow through ecosystems
because nutrients NOT energy may be most limiting
(6) Nutrient (water, C, N, P) flow through ecosystems is
largely under control of large-scale geological processes
(7) But, the role of individual organisms/species, while often
trivial in terms of global impact, is extremely important in terms
of its impact on the biological world
The Ecosystem Concept – A brief history
Early 1910-1920’s:
Biotic descriptions
of communities
1935
Include abiotic factors
Clements and Gleason
views plant communities
as a superorganism
emphasized individualistic
properties – a fortuitous collection
occurring together in time and space
The Ecosystem Concept – A brief history
1927
Emphasized
energy flow
Charles Elton
characterized feeding relationships among
animals on Bear Island, North Atlantic
coined the terms “food chain” and
“food cycle” to be replaced by food web
Elton’s Pyramids
As one goes up the food chain, one ascends a progression of
sizes as most predators consume smaller prey; larger animals
require more space to find food and hence their numbers go down.
#’s
1
10
100
1000
Biomass
1kg
10
100
1000
Of the total solar radiation, 0 , plants
use some fraction, 1 , herbivores
assimilate less energy, 2 , owing to
plants maintaining themselves before
being eaten and E lost during assimilation
2 = Biological Efficiency
1 of the trophic link (<<1)
1942
Ray Lindeman
visualized a Pyramid of Energy Transformation
– less energy becomes available to higher levels
due to work being done and inefficient transfer.
1953 Eugene Odum – model Energy flow, later adapted for nutrients as well
(1) Energy flows in one direction, absorbed light is lost as heat or transferred
into chemical energy through photosynthesis (Annual Gross Primary
Production) by autotrophic organisms.
(2) Autotrophs spend some energy to respire, other goes into growth, which
becomes available to heterotrophs at the next higher level (Annual Net
Primary Production)
Later, nutrient flows were added:
Unlike energy, nutrients are typically
retained and regenerated in biological
systems. There are some inputs and
outputs (but rather small) and nutrients
pool in some parts of the ecosystem,
but mainly are transferred between
various trophic levels.
• Cycling of nutrients has assumed a near equal status with energy flow.
One reason is that the amounts of the elements and their movement
between ecosystem components provides an index to energy flow, which
may be otherwise difficult to measure.
• Carbon in particular bears a close relationship w/energy because of its
intimate association w/photosynthesis.
• Also, certain nutrients regulate Primary Production
e.g., water in deserts, N & P in oligotrohic lakes
ENERGY FLOW
Photosynthesis:
6CO2 +6H2O  C6H12O6+ 6O2
Or cutting, drying, and weighing
of all plants at the end of the growing
season – Annual Aboveground Net
Productivity (AANP).
Photosynthesis – How efficient?
Light, temp, water, and nutrients all limit photosynthesis.
• Plants are rarely at max (cloud cover, shading)
• Rates saturate at high temp (i.e., diminishing returns)
• Water loss via stomata can shut down PhotoS (Transpiration efficiency ~ 2g/kg water).
For the ecosystem, efficiency is
~ 1-2%. Plants reflect 25-75% of
incoming light and another large
percent is absorbed by other molecules
and lost as heat.
Finally, nutrient availability affects
plant productivity …
C3
C4 CAM
Because ecosystems differ in terms
of plant density, water input and
availability, nutrient availability,
and the uniqueness of individual spp
adapted to particular environments,
Annual Primary Productivity varies
greatly with ecosystems.
Energy transfer between trophic levels is only 5-20% efficient
Ingestion
Egestion
Energy
For Excretion
Detritus (N waste)
feeders
Digestion &
Assimilation
Growth and
Reproduction
ingested E – egested E
= assimilated E
(mainly a function of diet)
Respiration, energy
used to perform work
is lost
assimilated E – respiration
= production
(mainly a function of metabolic rate)
Death
Next level
Ecological Efficiency is the ratio
of biomass at one trophic level that is
incorporated into biomass at the next trophic level.
1-6% active, endotherms
up to 75% for aquatic species
plants: 30-85%
Detritus then, often dominates
food webs – at least in terrestrial
systems.
Herbivory rates
Temp forest: 2-3%
Old fields: 12%
Lakes: 60-99%
Food chain lengths are ultimately limited by ecological efficiencies
Community
NPP
(kcal/m2/yr)
Open ocean
Coastal marine
Temp. grassland
Tropical forest
500
8000
2000
8000
Ingestion
Eff
(kcal/m2/yr)
(%)
0.1
10
1.0
10
25
20
10
5
n
7.1
5.1
4.3
3.2
(5.5)
(6.6)
(4.3)
(4)
Pathways of Elements in the Ecosystem – a generalization
Ecosystem is broken down
into separate compartments (in
a hierarchical manner) or pools
where elements reside.
Movement between pools often
biochemical transformation,
often involving energy.
Photosynthesis adds energy to
carbon, whereas burning fossil
fuels removes energy.
**Responsible for the
imbalance in the
modern C-cycle
New**
Global Carbon Cycle
3 classes of cycling through
aquatic and terrestrial systems:
(1) Assimilation/dissimilation
reactions via photosynthesis
and respiration
(2) C02 exchange between the
oceans and atmosphere
(3) Sedimentation of carbonates
#’s refers to Billions of
metric tons - 1015 grams
Global Nitrogen Cycle
** in 2000 anthropogenic sources of N exceeded natural sources
at 87 x 106 MT – forecast for 2020: 135 MMT; 2050: 236 MMT
(pesticides - 2000: 3.75 MMT, 2050: 10.1 MMT)
**
Precip
+ 76 kg/ha/yr
E.g., of Local Nitrogen Cycle
Model of N-cycling in a forest
Undisturbed forest
gains 1-3 kg/ha/yr
1200 Kg/ha
Soil
14,000 Kg/ha
Tropical Forest
in Ghana: 1794/4587
Model of N-cycling in a forest
(disturbed Forest)
Precip
+ 76 kg/ha/yr
Fire (wind)
Clear-cut
- 54kg/ha/yr
Large blow-downs
Defoliating insects???
- recovery is quick
- trees remain alive
- no soil erosion
Soil
Insect defoliation and N-cycling in forests
Gypsy moths are an introduced
insect pest that experiences
occasional outbreaks in which
huge swaths of oak forest are
entirely defoliated
Fate of Foliar N (15N-labeled exp)
Normal
Resorption (fall)
Fall leaf drop
Early green leaf drop
Leaching
Insect Biomass
Frass
70%
25
1.4
2
0.7
1
Outbreak
23
17 (new)
29
4
8
23
-47
-8
+28
+2
+7
+22
N has very different fates – Frass/insects biomass
at the expense of resorption
What is the fate of foliar N consumed by insects and
deposited as green fall??
<0.1% Gaseous N
Assimilated: 16%
Frass: 84% N
100% N
Frass: Scatological Research Team – 9% organically extractable
91% salts, such as uric acid
Frass is readily consumed/broken down by microbes,
which immobilizes N in microbial biomass
Oak litter: 81% N recovered, mostly as undecomposed
leaf litter.
Frass: 40% recovered, almost all of it through the soil
But, defoliation disrupts normal N-cycling through
soil-plant systems
Litter N is released from leaf litter slowly via decomposition,
whereas …
Frass N moves quickly into the subsoil where it is retained as
organic matter in mircobial biomass and released more slowly
into the soil pool of N.
As a consequence, short-term effects of Gypsy moth outbreaks
have a small effect on N-cycling through the forest, however,
chronic outbreaks/defoliation can lead to heavy leeching,
depletion of soil N and high concentration of NO3 and NO2
Punch line – species are intimately tied to the cycling and transfer
of Energy and Nutrients through ecosystems.
E.g., #1 – Photosynthetic rates
E.g., #2 – Assimilation rates/ecological efficiencies
E.g., #3 – Impacts of gypsy moths on N-cycling
Will the loss of species richness from communities severely
impact ecosystem processes ?
Will the loss of species richness form communities severely
impact ecosystem processes ?
3 Hypotheses
redundant
Ecosystem
Process/service
e.g., productivity,
pollination
idiosyncratic
Non-substitutable
Species richness
Ecosystem
Process, e.g.,
productivity
Species richness
Non-substitutable implies that every species is unique and its
Effects/impacts cannot be substituted by others.
Ecosystem
Process, e.g.,
productivity
Species richness
Redundancy implies that every species can partially compensate
for each other’s absence, and that there is diminishing returns to
species diversity in terms of their ability to supply ecosystem
services – i.e., not all species are necessary for a fully functional
ecosystem
Ecosystem
Process, e.g.,
productivity
Species richness
Idiosyncratic implies that each unique combination of species
has its own level of ecosystem service provision and is not
predictable a priori
Why should there be a positive correlation be ecosystem
service and species richness?
Ans #1: Sampling artifact. By chance, a larger group of species
is more likely to contain the MOST productive species than a
small group.
Never exhibits over-yielding, that is, no group of species
can ever outperform a monoculture of the best single species
Productivity
group
A, B, C individually
Why should there be a positive correlation be ecosystem
service and species richness?
Ans #2: Niche- complementarity – competition theory:
species will evolve to be distinct and thereby reduce
or eliminate competition.
e.g., root
profiles
Because a group of species can more effectively harvest water,
productivity of the group is enhanced over any single species
C
B
Vs.
A
Exhibits over-yielding
Productivity
group
A, B, C individually
Decomposition of leaf packs in Williamstown, MA
.3
.2
K-value
(decomposition rate) .1
0
-.1
Lonicera maple
beech hemlock
The implication is that a world dominated by Lonicera
releases large amounts of organic matter quickly, but
little remains for the end of the growing season. Therefore,
diversity/richness of stream invertebrates is reduced in
systems with reduced canopy cover
nutrient availability
time
Lonicera maple
beech hemlock
Species diversity/richness and temporal variability
Two lines of thought:
(1) Insurance Hypothesis – rare species are extremely
important under extreme conditions.
(2) Portfolio Effect – More diverse portfolios are less volatile
because, several randomly and independently variable
“items” are less variable than the average item
Services provide by the groups of species include:
stability (the inverse of variability), function during
extreme events (e.g., drought resistance), ability to
resist invasion
Cedar Creek LTER (Univ. MN – David Tilman et al.)
Plant biomass (% cover)
and soil denitrification
rates (less soil nitrate)
increase with species
richness
Plant biomass shows greater
resistance to drought when
species richness is higher
Rare spp response
Loss of species is a concerned because the Earth’s biota
performs many valuable functions and services
Valuing species richness and diversity
Aesthetic
Research and Development
Economical
Service (and $$$)