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, NorwayRight: Provenance of “A simplified food web for Northwest Atlantic” unknown

LinksFlow of material (including

energy-rich molecules)

NodesTaxonomic 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

Energy flows through the trophic levels of ecosystems

Food Webs

Cain, Bowman & Hacker (2014), Fig. 21.3

“Green” or livingfood web

“Brown” or detrital food web

1 Producers

1 Consumers

2 Consumers

1 Consumers

2 Consumers

Trophic levelswithin a simple

food chain;donor levels

supply energy or nutrients to

recipient levels

Another perspective: Green & Brown Food Webs

Food Webs

Detritus

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

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

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

Trophic Pyramids

Cain, Bowman & Hacker (2014), Fig. 21.5

Why are the tiers stair-stepped, as opposed to smoothly grading into one another (as in a triangular pyramid)?

Trophic Pyramids

Cain, Bowman & Hacker (2014), Fig. 21.5

Trophic Pyramids

Cain, Bowman & Hacker (2014), Fig. 21.5

Trophic Pyramids

Figure from Biology.StackExchange.com

Example terrestrial biomass pyramid

Trophic Pyramids

Cain, Bowman & Hacker (2014), Fig. 21.5

Trophic Pyramids

Figure from Biology.TutorVista.com

Example marine inverted biomass pyramid

Cain, Bowman & Hacker (2014), Fig. 21.6

Amount of primary producer biomass consumed by heterotrophs is correlated with NPP

Energy Flow Through Food Webs

Energy Flow Through Food Webs

Trophic Efficiency

Cain, Bowman & Hacker (2014), Fig. 21.7

Consumption efficiency is the proportion of NPP that is ingestedAssimilation efficiency is the proportion of ingested biomass

that is assimilated by digestionProduction efficiency is the proportion of assimilated biomass

that becomes NSP

Bottom-Up vs. Top-Down Influences

Control of energy flow through ecosystems

Bottom-up viewResources that limit NPP govern energy flow

Top-down viewConsumption plus non-consumptive species interactions,

e.g., competition, facilitation, limit lower trophic levels and govern energy flow

The “World is Green” HypothesisPredators limit herbivores and

allow plants to flourishHairston, Smith & Slobodkin (HSS)

(1960) Am. Nat.

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 levelsthat can be supported in a community; therefore NPP

ultimately dictates when top-down forces could cascade back downOksanen, Fretwell, Arruda & Niemela (OFAN)

(1981) Am. Nat.

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

Figure from Wikimedia Commons

Crosses represent a

persistent toxin whose

concentration increases up each trophic

level

Bioaccumulation

Tobacco hornworm (Manduca sexta) accumulates nicotine (a plant secondary chemical) in its body

to become toxic to many would-be predators

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

Chemical structure of DDT from Wikimedia Commons

Biomagnification

Heavy Metals

E.g., Mercury –methylmercury

biomagnifies in marine food webs

Figure from Wikimedia Commons