Energy Flow & Food Webs Please do not use the images in these PowerPoint slides without permission. Left: Image from Wikimedia Commons of one of the earliest known depictions of a food web, by Victor Summerhayes & Charles Elton (1923) for Bear Island, Norway Right: Provenance of “A simplified food web for Northwest Atlantic” unknown

Energy flows through the trophic levels of ecosystems Food Webs Energy flows through the trophic levels of ecosystems Nodes Taxonomic or functional categories Links Flow of material (including energy-rich molecules) Please do not use the images in these PowerPoint slides without permission. NOTE: The links (arrows) indicate flow of energy and elements through feeding relationships. Paine, Robert T. 1966. Food web complexity and species diversity. The American Naturalist 100:65-75. Paine, R. T. (1966) Am. Nat. – Food webs are the “ecologically flexible scaffolding around which communities are assembled and structured”

Energy flows through the trophic levels of ecosystems Food Webs Energy flows through the trophic levels of ecosystems Each trophic level is based on the number of feeding steps by which it is separated from autotrophs Please do not use the images in these PowerPoint slides without permission. NOTE: The links (arrows) indicate flow of energy and elements through feeding relationships or death and other loss of tissue or substances. Some food web ecologists think of detritivores occupying the second trophic level with herbivores, as depicted in your textbook’s Fig. 21.3. All organisms are either consumed or enter the detritus pool (the ultimate fate of all) Bowman, Hacker & Cain (2017), Fig. 21.3

Food Webs Another perspective: Green & Brown Food Webs Trophic levels within a simple food chain; donor levels supply energy or nutrients to recipient levels 2 Consumers “Green” or living food web 1 Consumers 1 Producers Detritus Please do not use the images in these PowerPoint slides without permission. NOTE: The links (arrows) indicate flow of energy and elements through feeding relationships or death and other loss of tissue or substances. 1 Consumers “Brown” or detrital food web 2 Consumers

Energy Flow Through Food Webs In most ecosystems, most NPP becomes detritus without passing through a heterotroph Please do not use the images in these PowerPoint slides without permission. Cebrian & Lartigue. 2004. Cain, Bowman & Hacker (2014), Fig. 21.4, after Cebrian & Lartigue (2004)

Energy Flow Through Food Webs In most ecosystems, most NPP becomes detritus without passing through a heterotroph In most ecosystems, relatively little NPP is consumed by herbivores Please do not use the images in these PowerPoint slides without permission. Cebrian, J. & J. Lartigue. 2004. Patterns of herbivory and decomposition in aquatic and terrestrial ecosystems. Ecological Monographs 74:237-259. Cain, Bowman & Hacker (2014), Fig. 21.4, after Cebrian & Lartigue (2004) Ecological Monographs

Energy Flow & Laws of Thermodynamics 2nd Law of Thermodynamics In natural thermodynamic processes, entropy never decreases Energy transformations result in an increase in entropy, i.e., only a fraction of the energy captured by one trophic level is available to do work in the next Usually only ~ 5 - 15% of the energy captured or assimilated at one trophic level is transferred to the next trophic level. Not all biomass at one level is consumed by the next level up the trophic hierarchy and biomass that is consumed is not fully converted into new biomass Consequently, food chains tend by have <5 trophic levels Please do not use the images in these PowerPoint slides without permission.

Trophic Pyramids Why are the tiers stair-stepped, as opposed to smoothly grading into one another (as in a triangular pyramid)? Please do not use the images in these PowerPoint slides without permission. For the answer – see the previous slide! Bowman, Hacker & Cain (2017), Fig. 21.5

Trophic Pyramids Bowman, Hacker & Cain (2017), Fig. 21.5 Please do not use the images in these PowerPoint slides without permission. Bowman, Hacker & Cain (2017), Fig. 21.5

Trophic Pyramids Bowman, Hacker & Cain (2017), Fig. 21.5 Please do not use the images in these PowerPoint slides without permission. Bowman, Hacker & Cain (2017), Fig. 21.5

Example terrestrial biomass pyramid Trophic Pyramids Example terrestrial biomass pyramid Please do not use the images in these PowerPoint slides without permission. Figure from: http://biology.stackexchange.com/questions/27830/in-a-typical-population-what-might-the-percentage-of-plants-be-to-predators-and Figure from Biology.StackExchange.com

Trophic Pyramids Bowman, Hacker & Cain (2017), Fig. 21.5 Please do not use the images in these PowerPoint slides without permission. Note that the example given above for the aquatic biomass pyramid is often referred to as an inverted biomass pyramid. Think about this question: Could an energy pyramid ever be inverted? Bowman, Hacker & Cain (2017), Fig. 21.5

Example marine inverted biomass pyramid Trophic Pyramids Example marine inverted biomass pyramid Please do not use the images in these PowerPoint slides without permission. Figure from: http://biology.tutorvista.com/ecology/ecological-pyramid.html This occurs when the demographic turn-over rates are especially high in the producer tier of the food web; standing crop biomass is relatively low, but productivity is very high Figure from Biology.TutorVista.com

Energy Flow Through Food Webs Amount of primary producer biomass consumed by heterotrophs is correlated with NPP Please do not use the images in these PowerPoint slides without permission. Is consumption efficiency generally higher in terrestrial or aquatic ecosystems? [See pg. 477 of your textbook.] Bowman, Hacker & Cain (2017), Fig. 21.6

Energy Flow Through Food Webs Trophic Efficiency Consumption efficiency is the proportion of NPP that is ingested Assimilation efficiency is the proportion of ingested biomass that is assimilated by digestion Production efficiency is the proportion of assimilated biomass that becomes NSP Please do not use the images in these PowerPoint slides without permission. Bowman, Hacker & Cain (2017), Fig. 21.7

Bottom-Up vs. Top-Down Influences Control of energy flow through ecosystems Bottom-up view Resources that limit NPP govern energy flow Top-down view Consumption plus non-consumptive species interactions, e.g., competition, facilitation, limit lower trophic levels and govern energy flow The “World is Green” Hypothesis Predators limit herbivores and allow plants to flourish Hairston, Smith & Slobodkin (HSS) (1960) Am. Nat. Please do not use the images in these PowerPoint slides without permission. Wikipedia “Alpine tundra” page; accessed 16-XI-2014 [By Zehnfinger 16:11, 7 March 2007 (UTC) (Own work) [CC-BY-SA-2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons] Hairston, Nelson G., Frederick E. Smith & Lawrence B. Slobodkin. 1960. Community structure, population control, and competition. The American Naturalist 94:421-425. [Origin of the trophic cascade idea] Photo from Wikimedia Commons

Bottom-Up vs. Top-Down Influences We should always start with a bottom-up template: “the removal of higher trophic levels leaves lower levels present (if perhaps greatly modified), whereas the removal of primary producers leaves no system at all” Hunter & Price (1992) Ecology “Break the food chain and creatures die out above the link” John McPhee’s (1998) Annals of the Former World, pg. 84 Potential reconciliation: NPP determines the number of trophic levels that can be supported in a community; therefore NPP ultimately dictates when top-down forces could cascade back down Oksanen, Fretwell, Arruda & Niemela (OFAN) (1981) Am. Nat. Please do not use the images in these PowerPoint slides without permission. Fretwell, Stephen D. 1977. The regulation of plant communities by food chains exploiting them. Perspectives in Biology and Medicine 20:169-185. Hunter, Mark D. & Peter W. Price. 1992. Playing chutes and ladders: Heterogeneity and the relative roles of bottom-up and top-down forces in natural communities. Ecology 73:724-732. Oksanen, L., S. D. Fretwell, J. Arruda & P. Niemela. 1981. Exploitation ecosystems in gradients of primary productivity. The American Naturalist 118:240-261.

3-tier vs. 4-tier Trophic Cascades Secondary carnivore + + _ + Primary carnivore + + + _ + + + Herbivore Please do not use the images in these PowerPoint slides without permission. Note: In this case, arrows represent positive and negative species interactions; solid arrows = direct effects; broken arrows = indirect effects. 3-tiered autotroph and top carnivore are indirect mutualists, but 4-tiered autotroph and top carnivore are indirect antagonists. _ + _ + Primary producer Bowman, Hacker & Cain (2017), Figs. 16.12 & 21.11

Bioaccumulation & Biomagnification Bioaccumulation – the accumulation of a substance (toxin, heavy metal, etc.) in an organism; the rate of uptake is greater than the rate of loss Biomagnification – the increasing concentration of a substance from one trophic level to the next Crosses represent a persistent toxin whose concentration increases up each trophic level Please do not use the images in these PowerPoint slides without permission. Wikipedia “Biomagnification” page; accessed April 14, 2015 ["Biomagnification" by Sballesteros15 - Own work based on:. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Biomagnification.svg#/media/File:Biomagnification.svg] Figure from Wikimedia Commons

Bioaccumulation Tobacco hornworm (Manduca sexta) accumulates nicotine (a plant secondary chemical) in its body to become toxic to many would-be predators Please do not use the images in these PowerPoint slides without permission. Wikipedia “Tobacco hornworm” page; accessed 2-IV-2015 ["Tobacco Hornworm 1" by Daniel Schwen - Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Tobacco_Hornworm_1.jpg#/media/File:Tobacco_Hornworm_1.jpg] Photo of tobacco hornworm from Wikimedia Commons

Biomagnification Environmental Toxins E.g., DDT (dichlorodiphenyltrichloroethane) – used as an insecticide in the early 20th century; is lipophilic and biomagnifies, especially in birds of prey; causes eggshell thinning; banned from agricultural use in the U.S. in 1972 Please do not use the images in these PowerPoint slides without permission. Wikipedia “DDT” page; accessed April 14, 2015 ["P,p'-dichlorodiphenyltrichloroethane" by Leyo - Own work. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:P,p%27-dichlorodiphenyltrichloroethane.svg#/media/File:P,p%27-dichlorodiphenyltrichloroethane.svg] Chemical structure of DDT from Wikimedia Commons

methylmercury biomagnifies in marine food webs Biomagnification Heavy Metals E.g., Mercury – methylmercury biomagnifies in marine food webs Please do not use the images in these PowerPoint slides without permission. Wikipedia “Mercury in fish” page; accessed April 14, 2015 ["MercuryFoodChain-01" by Bretwood Higman, Ground Truth Trekking. - Transferred from en.wikipedia; transferred to Commons by User:Shizhao using CommonsHelper.(Original text : From www.groundtruthtrekking.org:Source URL: http://www.groundtruthtrekking.org/Graphics/Large/MercuryFoodChain-01.pngSource Page: http://www.groundtruthtrekking.org/Graphics/MercuryFoodChain.html). Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:MercuryFoodChain-01.png#/media/File:MercuryFoodChain-01.png] Figure from Wikimedia Commons