1. Energy Flow & Food Webs 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

2. Links Flow of material (including energy-rich molecules) Nodes Taxonomic or functional categories 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

3. Energy flows through the trophic levels of ecosystems Food Webs Cain, Bowman & Hacker (2014), Fig. 21.3

4. “Green” or living food web “Brown” or detrital food web 1  Producers 1  Consumers 2  Consumers 1  Consumers 2  Consumers Trophic levels within a simple food chain; donor levels supply energy or nutrients to recipient levels Another perspective: Green & Brown Food Webs Food Webs Cain, Bowman & Hacker (2014), Fig. 21.3

5. 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

6. Cain, Bowman & Hacker (2014), Fig. 21.4, after Cebrian & Lartigue (2004) Ecological Monographs In most ecosystems, relatively little NPP is consumed by herbivores Energy Flow Through Food Webs In most ecosystems, most NPP becomes detritus without passing through a heterotroph

7. Energy Flow & Laws of Thermodynamics 2 nd 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

8. Trophic Pyramids Cain, Bowman & Hacker (2014), Fig. 21.5

9. Trophic Pyramids Cain, Bowman & Hacker (2014), Fig. 21.5

10. Trophic Pyramids Cain, Bowman & Hacker (2014), Fig. 21.5

11. Trophic Pyramids Cain, Bowman & Hacker (2014), Fig. 21.5

12. Cain, Bowman & Hacker (2014), Fig. 21.6 Amount of primary producer biomass consumed by heterotrophs is correlated with NPP Energy Flow Through Food Webs

13. Trophic Efficiency Cain, Bowman & Hacker (2014), Fig. 21.7 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

14. 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. Photo from Wikimedia Commons

15. 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.