Community Structure

Lecture 37: Trophic Structure

Reading: None.

Energy Flow in Nature

Productivity per unit standing crop biomass also varies with different types of autotrophs:

P:B ratio: 0.042 Forests (due to dead biomass of wood xylem)
0.29 Other terrestrial autotrophs
17.0 Aquatic and marine autotrophs

Conversion of PAR to biomass varies for different plants and regions, but has a maximum value of approximately 10% among crop plants.

Conifer forest 1%- 3%
Deciduous forest 0.5%-1%
Desert plants 0.01%-0.2%
Crop plants 3%-10%

Measuring Primary Production

Harvest methods
Gas exchange methods
Methods are similar for terrestrial and for aquatic/marine ecosystems.

Harvest Method: Estimating community primary production

B = B₂- B₁

B is the biomass of a reprsentative plant sample and B₂ and B₁ are biomass values at time t₂ and t₁.

In annual plants and crops started from seed, B₁ = 0 at time t₁.

Assuming no losses in biomass other than respiration, then B = NPP during the time interval t₂ - t₁.

If we let L = biomass loss by death of plant or plant parts and

G = biomass loss to consumers (herbivores) then:

NPP = B + L + G

Biomass can be converted to energy terms:

leaves: 4,229 cal/g dry weight
roots: 4,720 cal/g dry weight

Gas Exchange Methods

In terrestrial systems:

Oxygen release or carbon dioxide uptake during daylight measures NPP directly (for daylight hours).

Carbon dioxide release or oxygen uptake during night darkness measures respiration directly (for dark hours).

If respiration is assumed to be constant (or if rate is corrected for day-night changes in temperature) we can estimate GPP, NPP + respiration.

In aquatic and marine systems:

Light bottle/dark bottle method (clear glass bottle, black opaque bottle)

Paired bottles are filled with water and organisms from a given depth.

Initial dissolved oxygen concentration is measured in each bottle, then the bottles are stoppered, and lowered to the collection depth (or held in a plant growth incubator).

After 24 hours (or some other fixed time interval) the oxygen concentrations are measured again in each bottle.

Change in oxygen concentration in the light bottle represents GPP-R = NPP during the elapsed time interval. Actually this is NPP minus respiration by consumers.

Change in oxygen concentation in the dark bottle represents respiration alone by all organisms in the water sample.

The sum of the rates of oxygen concentration change in both the light and dark bottles (expressed as positive values) will be GPP or gross community production during the time interval between oxygen concentration evaluations.

Biological Oxygen Demand (BOD)

The BOD test uses techniques similar to the gas exchange methods for energy flow measurement. The BOD test is used to measure the biological impact of organic matter in water.

The use of oxygen by bacteria that occurs in a sample of water in a closed bottle is a function of the organic matter concentration in the water. Organic (non-toxic) pollutants (municipal sewage, agricultural runoff, industrial wastes) in water can be evaluated in this manner. Such organic pollutants can cause oxygen depletion of water as a consequence of bacterial metabolism in natural environments. In a BOD test, the oxygen concentration of water samples is measured before innoculation with bacteria and after a standard period of incubation. Oxygen depletion rates are compared to those of water containing known concentrations of carbohydrates.

Indirect Measures of Primary Production

Oxygen concentration in flowing waters (rivers and streams)

Chlorophyll concentration

remote sensing of chlorophyll

Direct measurement of carbon dioxide uptake

open systems with flow-through chambers

Food Chains - Trophic Levels

Trophic Level	Activity	Type of Organisms
1	primary producers	autotrophs
2	primary consumers	herbivores
3	secondary consumers	carnivores, parasitoids
4	tertiary consumers	higher carnivores

One organism can occupy more than one trophic level, and food webs characterize trophic interactions.

Potential Interactions = , n = #of species in an ecosystem

Food webs are not too complex.

As the number of species increases, connectance decreases.

Characteristics of Food Webs

Food chain length	short chains
Extent of omnivory	infrequent
Position of omnivores	short jumps in chain
Compartments	chains interconnected not completely separated

Secondary Production and Ecosystem Energy Flow

Energy flow is unidirectional and requires continuous inputs.

Ecosystem energy processes conform to the thermodynamic laws.

Energy cannot be made or destroyed
Energy transformations are not perfect and energy is lost in each transformation in the form of heat.

Energy is transfered from primary producers to primary consumers, then to secondary consumers, then to tertiary consumers, and at each trophic level energy is lost as respiratory heat.

The remains of organisms, and fecal and urinary wastes contain energy that is used by decomposers.

The energy that transfers to the decomposers eventually is lost as heat.

Unlike energy, matter is recycled in nature, so materials move through a food chain (or web), are processes by decomposers, and are eventually available to producers again.

Within a given organism, and therefore within a given trophic level, energy is partitioned, so only a fraction goes to net production (growth and reproduction) which is available to the comsumers of the next trophic level (after Krebs, 1994, p 633).