Download Lecture 8 - Susan Schwinning

ECOSYSTEM ECOLOGY
… the integrated study of biotic and abiotic components of ecosystems
and their interactions.
To achieve this integration: follow the path of matter and energy.
Divides ecosystems into stores and fluxes.
A focus on material exchanges, instead of numbers
of individuals:
death
renewable
resources
reproduction
a population
C in soil
C (e.g. in
forage)
C (e.g. in
grazers)
C in atmosphere
N (e.g. in
forage)
N (e.g. in
grazers)
N in soil
N in atmosphere
Matter fluxes through a typical primary producer:
Absorbs light
O2 release
CO2 uptake
water vapor
release
More live
biomass
Soil nutrient
uptake:
N,P,S,K,…
Water uptake
litter
root exudates
(complex sugars,
allelochemicals?,
leached N
Matter fluxes through a typical primary consumer:
O2 of air intake
C, N, H2O, etc.
in grass
C, N, H2O, etc.
in urine
C, N, H2O, etc.
in dung
C, N, H2O, etc.
in milk
CO2 of air
expelled
Methane, CO2
C, N, H2O, etc.
in the dead
cow
C, N, H2O, etc.
in a calf
TASKS OF ECOSYSTEM MODELS
Identify energetic constraints on material exchanges (by quantifying
primary productivity)
Close material budgets
Represent all major pools of materials in an ecosystem, exchange of
materials between them, and linkages that may exist between the flow
and storage of different materials (= ecosystem stoichiometry)
Population models:
•
Good at capturing constraints that act at the level of the
individual (e.g. reproductive problems associated with low
population numbers, chance events, genetic change,
behavior) .
•
Bad at capturing environmental constraints (available energy,
mass balance, climate impacts).
•
Risk of violating environmental constraints by an abstraction
too far removed from biochemical mechanisms.
Ecosystem models:
•
Good at characterizing the physical/biochemical constraints on
growth.
•
Bad at representing complexities based on the interaction of
individuals.
•
Risk of oversimplifying population processes by reducing
dynamics to the exchange of materials.
THE NATURE OF THE ENERGETIC CONSTRAINT
IN ECOSYSTEMS
H2O
H2O
C,N,P
etc.
C,N,P
Coupled energy and water balance for
terrestrial surfaces:
Surface Energy Balance Equation:
Rnet
H
LE
G
S
Water Balance Equation:
P
S
E
R
Rnet  H  LE  G  S
Net radiation: the difference between incomingand outgoing
radiation.
Sensible (convective) heat flux: energy exchange between the
surface and the atmosphere through temperature change of air.
Latent heat flux (evapotranspiration): energy exchange between
the surface and the atmosphere through phase change of water
(evaporation, condensation, melting, sublimation, etc).
Ground heat flux: energy exchange between the surface and the
ground.
Stored heat in vegetation
P  S  E  R
Precipitation inputs
Changes in ecosystem water storage
Evapotranspiration
Runoff
Energy flow through living organisms are
coupled to the flow of carbon:
Net Primary Production (NPP)
Net carbon gain in biomass
(= total carbon absorbed by plants (GPP) – carbon released
by plant respiration Rp)
The rate of
carbon fixation
per unit area:
Gross Primary
Production
(GPP)
The rate of
plant
respiration per
unit area (Rp)
Net Primary
Production
into the trophic
web
Globally, NPP is primarily controlled by precipitation and
temperature:
Net Ecosystem Exchange (NEE)
= Carbon absorbed or released by the entire ecosystem
(GPP – ecosystem respiration)
The rate of
carbon/energy
fixation: Gross
Primary
Productivity
(GPP)
The rate of ecosystem
respiration (RP+Rs)
Net Ecosystem
Exchange
This is the carbon
that stays in the
ecosystem.
Net Ecosystem Exchange (NEE)
= Carbon absorbed or released by the entire ecosystem
(GPP – ecosystem respiration)
The rate of
carbon/energy
fixation: Gross
Primary
Productivity
(GPP)
The rate of ecosystem
respiration (RP+Rs)
Net Ecosystem
Exchange
This is the carbon
that comes out of
the ecosystem.
Carbon (energy) flow through the trophic web:
≈10%
CO2
Carnivores I
≈10%
CO2
Herbivores
≈10%
CO2
Primary Producers
Soil food chain
(detritus eaters, decomposers)
Carnivores II
Detritus, Faces, Urine
CO2
The trophic biomass pyramid:
Since all food chains are “fed” by plant biomass, a major
effort of ecosystem models is the representation of primary
production and its dependence on climate factors.
Light & CO2 limitation of photosynthesis
at the leaf level
Two types of Primary Producers: C3 and C4 plants
Maximal photosynthetic rates are temperature dependent
Gurevitch, Scheiner and Fox 2002
Stomatal conductance is regulated by many atmospheric, and
some internal factors:
PAR
VPD
Jarvis 1976
TAIR
Y
PCO2
The primary productivity of ecosystems is modeled as the
interchange of leaf-level photosynthesis and respiration, soil
respiration and the exchange of energy and carbon within the
canopy and across the canopy boundary.
To integrate primary production into the trophic web,
allocation of carbon in plants also has to be made explicit:
flowers
stems
roots
seeds
leaves
A simplified food web:
A more complex food web (arctic):
An ocean foodweb:
Cod Food Web, David Lavigne
The number of trophic levels appears to be highly conserved across
ecosystems.
Mean net primary productivity:
5-100 g m-2 yr-1
Mean net primary productivity:
1200 g m-2 yr-1
Most food chains have four trophic levels or less.
Schoener 1989
Insect webs tend to be more complex
(longer and more connected).
Schoenly 1989
Species diversity declines with increasing trophic levels.
Schoenly et al. 1991
Why are so many food chains short?
• Food chains run out of energy to support viable population sizes at
higher trophic levels?
• There could be longer food chains, but area in each of earth’s
ecosystems is too small to support another trophic level?
• Long food chains are more unstable (Pimm and Lawton 1977)?
What makes a food chain complex?
• Length of the longest trophic chain
• Number of species in the web
• Number of connections between species
Measure of stability:
•
Dynamic stability: the tendency to return to equilibrium after
perturbation (eq. is stable or unstable).
•
Resilience: how fast variables return to the equilibrium after
perturbation (e.g. determine the eigenvalues of linearized system)
•
Resistence: the degree to which a variable is changed after
perturbation.
•
Persistence of the species assemblage: the degree to which the suite
species remains the same (no species loss, no invasion).
•
Variability: the degree to which variables change over time.
•
Most stability measures are sensitive to the magnitude and nature of
perturbation and the time of observation.
Why complex food webs could be
more stable:
Why complex food webs could be less
stable:
Why complex food webs could be
more stable:
Why complex food webs could be less
stable:
The more pathways in a food chain
Predator-prey systems can be inherently
(redundancy in ecosystem function), the unstable. Linked predator prey systems
less severe would be the failure of any at least as much if not more so (May).
one pathway (MacArthur).
Limiting similarity: more species in an
ecosystem, the fewer niches left
unfilled for potential invaders (Tilman).
Robert May’s approach (1972):
1. Construct a (linear) community matrix of m species that, each on their
own would return to equilibrium (e.g. by negative density
dependence).
2. Randomly switch on interactions between species (add positive or
negative interaction parameters to the community matrix outside the
diagonal), subject to the constraints:
•
•
pick C = web connectance, the probability that any pair will
interact.
pick s = mean interaction strength
3. Determine the probability P of drawing a stable community matrix,
where (by definition) all eigenvalues have a negative real part.
Robert May’s approach (1972):
A stable outcome is favored by
• weak interactions (s)
• few species (m)
• weak connectivity (C)
Other conclusions:
•
•
species that interact with many species should only weakly interact
with them.
m species whose interactions are clustered into independent blocks of
strong interactions have a higher chance of stability that m species all
connected by weak interactions.
Pimm and Lawton’s approach:
Pimm and Lawton compared the resilience of random food
chains with different trophic levels:
Increasing tendency to return to
equilibrium after perturbation
Decreasing tendency\ to return
to equilibrium
Webs containing omnivores are generally less stable.
Increasing tendency to return to
equilibrium after perturbation
To examine the
properties of
actual food webs,
Pimm (1980)
collected information
on natural food webs,
and then randomized
trophic relationships.
p1: probability of a random food web having fewer of the same no of trophic levels.
p2: probability of a random food web having fewer of the same no of omnivores.
Summary of modeling predictions:
The more species are present in a community:
• the less connected the species should be;
• the less resilient its populations;
• the greater the impact of species removal;
• the longer the persistence of species if no species removal.
The more connected a community:
• the fewer species there should be;
• the greater the impact of species removal;
• the more resilient its populations;
• the more persistent its composition;
• the longer the persistence of species if no species removal.
Summary
Ecosystem models emphasize the concept of matter cycling and
mass balance.
Terrestrial models usually dominated by plants, herbivores and soil
microbial processes: matter cycling through higher trophic levels
often adds little to overall ecosystem dynamics.
Ecosystem models are probably the most important avenue for
investigating the potential effects of climate change (as well as
predicting climate change itself).
The current research direction goes towards the greater integration
of individual-based and ecosystem approaches.