+ Chapter 22: Energy in the Ecosystem 1

+ 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. 2

+ 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 3

+ 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 4

+ 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. 5

+ 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. 6

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

+ Models of ecological energy flow A single trophic level A 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. 10

+ Energy: how many lbs of grass to support one hawk 11

+ Ocean food pyramid – roughly 2500 lbs/1136 kg of phytoplankton to support 0.5lb/0.23 kg of tuna 12

+ 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 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 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 13

+ 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 CO 2 in light (net production), production of CO 2 in dark (respiration) and gross production as their sum Radioactive carbon ( 14 C) may also determine net uptake of carbon by plants 14

+ Can use fluxes to measure productivity

+ 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 O 2 14 C approach may also be used in unproductive waters 17

+ Paired light and dark bottles used to measure aquatic phytoplankton

+ Use of Remote Sensing Satellites can use spectral bands to infer amount of chlorophyll in water or the near-infrared to red ratio on land (NDVI index) NDVI=Normalized Difference Vegetation Index

Fig , p.850

+ 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 21

+ Effects of Light and Temperature 2 Optimum temperature for photosynthesis varies with system: about 16 o C for many temperate species as high as 38 o C 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 22

+ 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 23

+ 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 24

+ Effects of fertilizer on plant growth

+ 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. 26

+ NPP among ecosystems

Pickelweed saltmarsh. Credit: © Richard Herrmann/Visuals Unlimited

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

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

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

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

Chaparral vegetation. Credit: © Richard Herrmann/Visuals Unlimited

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

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

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

+ 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. 37

+ 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 38 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) 39

+ Fundamental Energy Relationships Components of an animal’s energy budget are related by: ingested energy - egested energy = assimilated energy assimilated energy - respiration - excretion = production 40

+ 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% 41

+ 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). 42

+ 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 44

+ 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: % of net primary production in temperate forests 12% in old-field habitats 60-99% in plankton communities 45

+ 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. 46

+ Stop here 47

+ 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 48

+ Biomass Accumulation Ratios Biomass accumulation ratios become larger as amount of stored energy increases: humid tropical forests have net production of 1.8 kg/m 2 /yr and biomass of 43 kg/m 2, yielding biomass accumulation ratio of 23yr ratios for forested terrestrial communities are typically >20 yr ratios for planktonic aquatic ecosystems are <20 days 49

+ 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 50

+ 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%) 51

+ 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 52

+ 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! 53