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
“Green” or living
food web
1 Producers
Detritus
1 Consumers
2 Consumers
“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
Why are the tiers stair-stepped, as opposed to smoothly grading
into one another (as in a triangular pyramid)?
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
Example terrestrial biomass pyramid
Figure from Biology.StackExchange.com
Trophic Pyramids
Cain, Bowman & Hacker (2014), Fig. 21.5
Trophic Pyramids
Example marine inverted biomass pyramid
Figure from Biology.TutorVista.com
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.
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
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
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