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
Food Webs
Energy flows through the trophic levels of ecosystems
Nodes
Taxonomic or functional categories
Links
Flow of material (including
energy-rich molecules)
Paine, R. T. (1966) Am. Nat. – Food
webs are the “ecologically flexible
scaffolding around which communities
are assembled and structured”
Food Webs
Energy flows through the trophic levels of ecosystems
Cain, Bowman & Hacker (2014), 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
1 Consumers
1 Producers
1 Consumers
2 Consumers
Cain, Bowman & Hacker (2014), Fig. 21.3
“Green” or living
food web
“Brown” or detrital
food web
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)
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
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
Trophic Pyramids
Cain, Bowman & Hacker (2014), Fig. 21.5
Trophic Pyramids
Cain, Bowman & Hacker (2014), Fig. 21.5
Trophic Pyramids
Cain, Bowman & Hacker (2014), Fig. 21.5
Trophic Pyramids
Cain, Bowman & Hacker (2014), Fig. 21.5
Energy Flow Through Food Webs
Amount of primary producer biomass consumed by
heterotrophs is correlated with NPP
Cain, Bowman & Hacker (2014), 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
Cain, Bowman & Hacker (2014), 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.
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.