chapter 11: flux of energy and matter through ecosystems 1

Chapter 11: flux of energy and matter through ecosystems

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“Like all biological entities, ecological communities require matter for their construction and energy for their activities.

We need to understand the routes for which matter and energy enter and leave ecosystems, how they are transformed into plant biomass, and how this fuels the rest of the community – bacteria and fungi, herbivores, detritivores and their consumers.”

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Background: Organizing Concepts

• In 1920s, English ecologist Charles Elton and others promoted a revolutionary concept:– organisms living in the same place not only have similar tolerances of

physical factors, but– feeding relationships link these organisms into a single functional

entity

• This system of feeding relationships is called a food web.

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The Ecosystem Concept

• The English ecologist A.G. Tansley took Elton’s ideas one step further:– in 1935 Tansley coined the term ecosystem, the fundamental unit of

ecological organization• the ecosystem concept: “the biological and physical parts of nature

together, unified by the dependence of animals and plants on their physical surroundings and by their contributions to maintaining the conditions and composition of the physical world.” -R.E. Ricklefs

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Some key terms

• Standing crop– Bodies of the living organisms within a unit area

• Biomass– Mass of organisms per unit area of ground (or water); usually

expressed in units of energy or dry organic matter• Primary productivity

– Rate at which biomass is produced per unit area by plants• Gross primary productivity

– Total fixation of energy by photosynthesis• Net primary productivity = GPP - Respiration• Secondary productivity

– Rate of production of biomass by heterotrophs

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GPP can be partitioned into respiration and NPP

More key terms

• Live consumer system– Proportion of primary production consumed by

herbivores – who are then consumed by carnivores

• Decomposer system– Fraction of NPP not eaten by herbivores reaches

decomposer system– Two groups responsible for decomposition of detritus

• Bacteria and fungi: decomposers• Animals that consume dead matter: detritivores

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Geographic patterns in PP

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Productivity of forests, grasslands, crops and lakes follows a latitudinal pattern

Productivity of forests, grasslands, crops and lakes follows a latitudinal pattern

NPP among ecosystems

What limits PP?

• Terrestrial communities:– Solar radiation, carbon dioxide, water and soil nutrients:

resources required for PP– Temperature, a condition, strong influence

• IF other resources were in abundant supply, radiation would be used more efficiently [eg: conifer communities only uses between 1 to 3 % of available radiation]

• Rainfall strongly correlated with productivity• Of the minerals, the one with strongest influence on

community productivity: fixed nitrogen [not atmospheric N]• May be limited by a succession of factors

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What limits PP in aquatic environment?

• Availability of nutrients (nitrate and phosphate)

• Intensity of solar radiation that penetrates water column

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Relationship between PP and SP

• Positive relationship– Secondary productivity by zooplankton, eat

phytoplankton cells, positively related to phytoplankton productivity

– Productivity of heterotrophic bacteria – also +ive with phyotplankton

– Caterpillars abundance linked to primary productivity (which is linked to annual rainfall)

– Seed-eating finch – raises more broodsIn wet years (increased plant production)

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Where does the energy go?

• In aquatic and terrestrial communities: SP is 1/10 of PP

(1) not all of plant biomass is consumed alive by herbivores

(2) not all plant biomass eaten by herbivores is assimilated and available for incorporation into consumer biomass. [what happens to the rest?]

(3) not all energy assimilated is converted to biomass [what happens to the rest?]

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Alfred J. Lotka, the Thermodynamic Concept, and Lindeman’s concept

• Alfred J. Lotka introduced the concept of the ecosystem as an energy-transforming machine:– described by a set of equations representing exchanges of matter and

energy among components, and– obeying thermodynamic principles that govern all energy transformations

• In 1942, Raymond Lindeman brought Lotka’s ideas of the ecosystem as an energy-transforming machine to the attention of ecologists. He incorporated:– Lotka’s thermodynamic concepts– Elton’s concept of the food web as expression of the

ecosystem’s structure– Tansley’s concept of the ecosystem as the

fundamental unit in ecology

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Thermodynamics and Ecology

• 1st law of thermodynamics - Energy can be neither created nor destroyed. It can only change forms.

• 2nd law of thermodynamics - spontaneous natural processes increase entropy overall– the total biomass ALWAYS decreases with

increasing trophic levels, as energy is constantly being lost to the atmosphere

– So?

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Lindeman’s Foundations of Ecosystem Ecology

• The ecosystem is the fundamental unit of ecology.• Within the ecosystem, energy passes through many steps or

links in a food chain.• Each link in the food chain is a trophic level (or feeding level).• Inefficiencies in energy transformation lead to a pyramid of

energy in the ecosystem.

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Odum’s Energy Flux Model• Eugene P. Odum popularized ecology to a generation of

ecologists.• Odum further developed the emerging framework of

ecosystem ecology:– he recognized the utility of energy and masses of elements as

common “currencies” in comparative analysis of ecosystem structure and function

• Odum extended his models to incorporate nutrient cycling.– Fluxes of energy and materials are closely linked in ecosystem

function. However, they are fundamentally different:• energy enters ecosystems as light and is degraded into heat• nutrients cycle indefinitely, converted from inorganic to organic forms

and back again– Studies of nutrient cycling provide an index to fluxes of energy.

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Simple Ecosystem

Modelenergy input from sun

nutrientcycling

PHOTOAUTOTROPHS(plants, other producers)

HETEROTROPHS(consumers, decomposers)

energy output (mainly heat)

Models of ecological energy flow

A single trophic levelA food chain

An ecological pyramid of energy

Only 5% to 20% of energy passes between trophic levels.

• Energy reaching each trophic level depends on:– net primary production (base of food chain)– efficiencies of transfers between trophic levels- More on this later -

• Plant use between 15% and 70% of light energy assimilated for maintenance – thus that portion is unavailable to consumers

• Herbivores and carnivores expend more energy on maintenance than do plants: production of each trophic level is only 5% to 20% that of the level below it.

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Energy: how many lbs of grass to support one hawk

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Ocean food pyramid – roughly 2500 lbs/1136 kg of phytoplankton

to support 0.5lb/0.23 kg of tuna

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Only 5% to 20% of energy passes between trophic levels.

• Energy reaching each trophic level depends on:– net primary production (base of food chain)– efficiencies of transfers between trophic levels

• Plant use between 15% and 70% of light energy assimilated for maintenance – thus that portion is unavailable to consumers

• Herbivores and carnivores expend more energy on maintenance than do plants: production of each trophic level is only 5% to 20% that of the level below it.

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Ecological Efficiency• Ecological efficiency

(food chain efficiency) is the percentage of energy transferred from one trophic level to the next:– range of 5% to 20% is

typical, as we’ve seen– to understand this

more fully, we must study the use of energy within a trophic level

• el

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Undigested plant fibers in elephant dung

Intratrophic Energy Transfers• Intratrophic transfers involve several components:

– ingestion (energy content of food ingested)– egestion (energy content of indigestible materials regurgitated

or defecated) (the elephant dung)– assimilation (energy content of food digested and absorbed)– excretion (energy content of organic wastes)– respiration (energy consumed for maintenance)– production (residual energy content for growth and

reproduction)

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Fundamental Energy Relationships

• Components of an animal’s energy budget are related by:– ingested energy - egested energy = assimilated energy– assimilated energy - respiration - excretion = production

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Assimilation Efficiency

• Assimilation efficiency = assimilation/ingestion • primarily a function of food quality:– seeds: 80%– young vegetation: 60-70%– plant foods of grazers, browsers: 30-40%– decaying wood: 15%– animal foods: 60-90%

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Net Production Efficiency

• Net production efficiency = production/assimilation• depends largely on metabolic activity:

– birds: <1%– small mammals: <6%– sedentary, cold-blooded animals: as much as 75%

• Gross production efficiency = assimilation efficiency x net production efficiency = production/ingestion, ranges from below 1% (birds and mammals) to >30% (aquatic animals).

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Active, warm-blooded animals – low net production efficiencies; hummingbird: <1%

Production Efficiency in Plants

• The concept of production efficiency is somewhat different for plants because plants do not digest and assimilate food:– net production efficiency = net production/gross

production; varies between 30% and 85%– rapidly growing plants in temperate zone have net

production efficiencies of 75-85%; their counterparts in the tropics are 40-60% efficient

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Detritus Food Chains• Ecosystems support two parallel food chains:– herbivore-based (relatively large animals feed on

leaves, fruits, seeds)– detritus-based (microorganisms and small animals

consume dead remains of plants and indigestible excreta of herbivores)

– herbivores consume:• 1.5-2.5% of net primary production in temperate forests• 12% in old-field habitats• 60-99% in plankton communities

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Exploitation Efficiency

• When production and consumption are not balanced, energy may accumulate in the ecosystem (as organic sediments).

• Exploitation efficiency / trophic transfer efficiency = ingestion by one trophic level/production of the trophic level below it.

• To the extent that exploitation efficiency is <100%, ecological efficiency = exploitation efficiency x gross production efficiency.

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Reminder of key terms

• Consumption efficiency (CE)• Assimilation efficiency (AE)• Production efficiency (PE)

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Some General Rules

• Assimilation efficiency increases at higher trophic levels.

• Net and gross production efficiencies decrease at higher trophic levels.

• Ecological efficiency averages about 10%.• About 1% of net production of plants ends up as

production on the third trophic level: the pyramid of energy narrows quickly.

• To increase human food supplies means eating lower on food chain!

• [virtual water]

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Virtual water

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Virtual waterhttp://environment.nationalgeographic.com/environment/freshwater/embedded-water/

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Virtual waterhttp://environment.nationalgeographic.com/environment/freshwater/embedded-water/

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Virtual waterhttp://environment.nationalgeographic.com/environment/freshwater/embedded-water/

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Virtual waterhttp://environment.nationalgeographic.com/environment/freshwater/embedded-water/

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Virtual waterhttp://environment.nationalgeographic.com/environment/freshwater/embedded-water/

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LIVE CONSUMER AND DECOMPOSER SYSTEMS: GENERAL PATTERNS OF ENERGY FLOW

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Live consumer and decomposer systems: general patterns of energy flow

DOM: dead organic matter

LCS: live consumer system

Relative sizes of boxes and arrows are proportional to the relative magnitude of compartments and flows

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Process of decomposition

• Immobilization – when an inorganic nutrient element is incorporated into organic form, primarily during the growth of green plants [carbon dioxide becoming carbohydrates, eg]

• Mineralization – conversion of elements from organic back to an inorganic form

• Decomposition – the gradual disintegration of dead organic matter by both physical and biological agents

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Who decomposes?

• Bacteria and fungi: begin the process of decomposition. Use soluble materials (amino acids and sugars)

• Microbial specialists: break down residual resources (structural carbohydrates and complex proteins)

• Some specialist microbivores feed on bacteria and fungi– Microbivores: group of animals that operate alongside the

detritivores; minute animals that specialize at feeding on bacteria or fungi but are able to exclude detritus from their guts

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What do they eat?

• Plant detritus– Two of the major components of dead leaves and

wood: cellulose and lignin– Lacking cellulase enzymes, majority of detritivores

depend on production of cellulases by associated bacteria or fungi or protozoa

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What do they eat?

Feces and carrion [decaying flesh of dead animals]

• Carnivorous vertebrates: poor quality dung [feces, manure]. Why?

• Elephant dung within minutes eaten by adult dung beetles feed on the the dung, bury large quantities along with their eggs to provide food for their larvae

• Without those beetles, though…

• Cattle dung. Cow pop increased from 7 in 1788 to 30 million in 1988 – producing 300 million cowpats/day – in Australia– Lack of native dung beetles loss of 2.5 million ha/year/ under

dung. So introduced 20 species of beetles

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Elephant dung? Into paper?

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Energy moves through ecosystems at different rates.

• Other indices address how rapidly energy cycles through an ecosystem:– residence time measures the average time a packet of energy resides in

storage:• residence time (yr) = energy stored in biomass/net productivity

– biomass accumulation ratio is a similar index based on biomass rather than energy:• biomass accumulation ratio (yr) = biomass/rate of biomass production

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Residence Time for Litter

• Decomposition of litter is dependent on conditions of temperature and moisture.

• Index is residence time = mass of litter accumulation/rate of litter fall:– 3 months in humid tropics– 1-2 yr in dry and montane tropics– 4-16 yr in southeastern US– >100 yr in boreal ecosystems

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Biomass Accumulation Ratios• Biomass accumulation

ratios become larger as amount of stored energy increases:– humid tropical forests have

net production of 1.8 kg/m2/yr and biomass of 43 kg/m2, yielding biomass accumulation ratio of 23yr

– ratios for forested terrestrial communities are typically >20 yr

– ratios for planktonic aquatic ecosystems are <20 days

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