Chapter 22: Energy in the Ecosystem

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.

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

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

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.

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.

Simple Ecosystem Model energy input from sun PHOTOAUTOTROPHS (plants, other producers) nutrient cycling HETEROTROPHS (consumers, decomposers) energy output (mainly heat)

Models of ecological energy flow Eugene Odum’s “universal” model of ecological energy flow. (a) A single trophic level. (b) Representation of a food chain. The net production of one trophic level becomes the ingested energy of the next higher level. A food chain A single trophic level

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.

Energy: how many lbs of grass to support one hawk

Ocean food pyramid – roughly 2500 lbs/1136 kg of phytoplankton to support 0.5lb/0.23 kg of tuna

Primary Production: reminder Primary production is the process whereby plants, algae, and some bacteria (primary producers) capture the energy of light and transform it into the energy of chemical bonds in carbohydrate: its rate is referred to as primary productivity 6CO2 + 6H2O  C6H12O6 + 6O2 for each g of C assimilated, 39 kJ energy stored The rate of primary production determines the rate of energy supply to the rest of the ecosystem: gross primary production = total energy assimilated by primary producers net primary production = energy accumulated (in stored form) by primary producers gross - net = respiration, the energy consumed by producers for maintenance and biosynthesis

GPP can be partitioned into respiration and NPP

Measurement of Primary Production 1 How much energy has been accumulated by net production? harvest techniques determine dry mass accumulated (net production) gas exchange techniques determine net uptake of CO2 in light (net production), production of CO2 in dark (respiration) and gross production as their sum Radioactive carbon (14C) may also determine net uptake of carbon by plants

Measurements of Carbon Dioxide flux in dark and light can provide an estimate of GPP

Measurement of Primary Production 2 Aquatic systems pose special problems: harvest approach is not practical for small organisms, such as phytoplankton carbon is too abundant for practical measurement of small changes Alternatives for aquatic systems: light and dark bottles may be used to determine changes in O2 14C approach may also be used in unproductive waters

Many factors influence primary production

Effects of Light and Temperature 1 Plants are not usually light-limited in full sun. Shading (by other leaves or plants) may reduce photosynthetic rate below its maximum. Overall, photosynthetic efficiency of the ecosystem is typically 1-2%: remaining energy is either reflected or absorbed and dissipated Leaves reflect 25 to 75% Molecules other than photosynthetic pigments absorb remainder – converted to heat and radiated, or conducted across leaf surface, or transpired Photosynthetic efficiency Percentage of the energy in sunlight that is converted to net primary production during the growing season

Effects of Light and Temperature 2 Optimum temperature for photosynthesis varies with system: about 16oC for many temperate species as high as 38oC for some tropical species Rate of photosynthesis increases with temperature, up to a point: rate of respiration also increases with temperature net assimilation may thus decrease at high temperatures

Water limits primary production (reminder) Photosynthesis in terrestrial systems is water-limited: under water stress, stomates close and gas exchange ceases, stopping photosynthesis Transpiration or water-use efficiency: typically 2g production per kg of water transpired (4g for drought-tolerant crops) ecosystem-level efficiency may be an order of magnitude poorer (0.2 g/kg) Most precipitation is not taken up by plants

Nutrients stimulate primary production – terrestrial and aquatic. Terrestrial production may be nutrient -limited: fertilizers stimulate crop production N is the most common limiting element Aquatic systems are often strongly nutrient - limited: especially true of open ocean inadvertent addition of nutrients may stimulate unwanted production

Nutrient use efficiency (NUE) Ratio of dry matter production to the assimilation of a particular nutrient element – usually expressed as grams per gram NUE of natural vegetation vary widely depending on other limitations on plant production (…H20… nutrient availability…)

Measuring NUE Vitousek: wanted to pinpoint influences of env factors on production in tropical and temperate forests by comparing the cycling of individual elements with the flow of energy through each system as a whole. Estimated the total dry mass of litter produced each year per unit of area How? By collecting falling leaves and branches Measured amounts of several nutrient elements in the litter Element most strongly correlated with litter production probably limited overall plant production Found: nitrogen content of litter more nearly constant than that of phosphorus or calcium What does that mean? production paralleled nitrogen assimiliation

Primary production varies among ecosystems. Primary production is maximum under favorable combinations of: intense sunlight warm temperatures abundant rainfall ample nutrients On land, production is highest in humid tropics, lowest in tundra and desert.

NPP highest in humid tropics

NPP in terrestrial ecosystems Increases with temperature Increases with precipitation – up to a point

NPP among ecosystems

Credit: © Richard Herrmann/Visuals Unlimited 205379 Pickelweed saltmarsh.

Credit: © Theo Allofs/Visuals Unlimited 283044 Temperate Rainforest showing moss-covered trees and ferns, Olympic National Park, Washington.

Credit: © Adam Jones/Visuals Unlimited 212904 Fall foliage and view of Mt. LeConte, Great Smokey Mountains National Park, Tennessee.

Credit: © Beth Davidow/Visuals Unlimited 301419 Northern Boreal Forest of Spruce and Aspens and tundra ponds.

Credit: © Joe McDonald/Visuals Unlimited 300241 African Lioness (Panthera leo) and African Elephants, Masai Mara Game Reserve, Kenya.

Credit: © Richard Herrmann/Visuals Unlimited 205342 Chaparral vegetation.

Credit: © Steve Maslowski/Visuals Unlimited 210424 A Bison herd on the prairie.

Credit: © Patrick J. Endres/Visuals Unlimited 301450 Arctic tundra biome in summer, Alaska Range Mountains, Denali National Park, Alaska.

Credit: © Richard Thom/Visuals Unlimited 307010 Sonoran Desert scene with Creosote Bush, Saguaro, Cholla, and Paloverde.

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.

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 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)

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

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%

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).

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

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

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

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

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

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

Ecosystem Energetics Comparative studies of ecosystem energetics now exist for various systems. Many systems are supported mainly by autochthonous materials (produced within system). Some ecosystems are subsidized by input of allochthonous materials (produced outside system). autochthonous production dominates in large rivers, lakes, marine ecosystems allochthonous production dominates in small streams, springs, and caves (100%)

Cedar Bog Lake Lindeman’s study of a small lake in Minnesota uncovered surprisingly low exploitation efficiencies: herbivores: 20% carnivores: 33% residual production of plants and herbivores accumulates as bottom sediment

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!