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:

a population

reproduction

renewableresources

death

C (e.g. in forage)

C (e.g. in grazers)

N (e.g. in forage)

N (e.g. in grazers)

C in soil

C in atmosphere

N in soil

N in atmosphere

water vapor release

More live biomass

litter

O2 release

CO2 uptake

Absorbs light

Water uptake

Soil nutrient uptake:

N,P,S,K,…

root exudates (complex sugars, allelochemicals?,

leached N

Matter fluxes through a typical primary producer:

Matter fluxes through a typical primary consumer:

C, N, H2O, etc. in dung

C, N, H2O, etc. in milk

C, N, H2O, etc. in grass

C, N, H2O, etc. in the dead

cow

C, N, H2O, etc. in a calf

C, N, H2O, etc. in urine

O2 of air intake

CO2 of air expelled

Methane, CO2

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 CONSTRAINTIN ECOSYSTEMS

H2O

H2O

C,N,P

C,N,P

etc.

Coupled energy and water balance for terrestrial surfaces:

Surface Energy Balance Equation: SGLEHRnet

Rnet Net radiation: the difference between incoming and outgoing radiation.H Sensible (convective) heat flux: energy exchange between the

surface and the atmosphere through temperature change of air.LE Latent heat flux (evapotranspiration): energy exchange between

the surface and the atmosphere through phase change of water (evaporation, condensation, melting, sublimation, etc).

G Ground heat flux: energy exchange between the surface and the ground.S Stored heat in vegetation

RESP Water Balance Equation:

P Precipitation inputsS Changes in ecosystem water storage E EvapotranspirationR Runoff

Energy flow through living organisms are coupled to the flow of carbon:

The rate of plant

respiration per unit area (Rp)

The rate of carbon fixation per unit area:

Gross Primary Production

(GPP) Net Primary Production

into the trophic web

Net Primary Production (NPP)

Net carbon gain in biomass(= total carbon absorbed by plants (GPP) – carbon released

by plant respiration Rp)

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 ecosystem respiration (RP+Rs)

The rate of carbon/energy fixation: Gross

Primary Productivity

(GPP) Net Ecosystem Exchange

This is the carbon that stays in the

ecosystem.

The rate of ecosystem respiration (RP+Rs)

The rate of carbon/energy fixation: Gross

Primary Productivity

(GPP) Net Ecosystem Exchange

This is the carbon that comes out of the ecosystem.

Net Ecosystem Exchange (NEE)

= Carbon absorbed or released by the entire ecosystem(GPP – ecosystem respiration)

Carbon (energy) flow through the trophic web:

Primary Producers

Herbivores

Carnivores I

Carnivores II Detritus, F

aces, Urine

CO2

CO2

CO2

CO2

Soil food chain

(detritus eaters, decomposers)

≈10%

≈10%

≈10%

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 photosynthesisat 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:

VPD

PAR TAIR

PCO2

Jarvis 1976

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:

rootsstems

leaves

flowers

seeds

A simplified food web:

A more complex food web (arctic):

Cod Food Web, David Lavigne

An ocean foodweb:

The number of trophic levels appears to be highly conserved across ecosystems.

Mean net primary productivity: 1200 g m-2 yr-1

Mean net primary productivity: 5-100 g m-2 yr-1

Most food chains have four trophic levels or less.

Schoener 1989

Schoenly 1989

Insect webs tend to be more complex(longer and more connected).

Schoenly et al. 1991

Species diversity declines with increasing trophic levels.

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 (redundancy in ecosystem function), the less severe would be the failure of any one pathway (MacArthur).

Limiting similarity: more species in an ecosystem, the fewer niches left unfilled for potential invaders (Tilman).

Predator-prey systems can be inherently unstable. Linked predator prey systems at least as much if not more so (May).

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 compared the resilience of random food chains with different trophic levels:

Increasing tendency to return to equilibrium after perturbation

Pimm and Lawton’s approach:

Increasing tendency to return to equilibrium after perturbation

Webs containing omnivores are generally less stable.

De

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nde

ncy

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to

eq

uilib

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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.