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Ecosystems and Energy
The large fish eat the small fish;
The small fish eat the water insects;
The water insects eat plants and mud
Large fowl cannot eat small grain
One hill cannot shelter two tigers
• These three ancient Chinese proverbs preface a chapter in Animal
Ecology (1927)
• The author, Charles Elton, was a young British ecologist
• Elton took a radically different approach to understanding animal
communities
• Prior to Elton, ecology was a purely descriptive science
• To understand the community, you merely had to catalog its organisms
• In the great European tradition, natural history consisted mostly of
collecting stuff
• Once you had collected it, named it, and put it away in a museum, your
job was done
• Great museum collections, cabinets of curiosities, were very popular
with Victorians
• Elton claimed it was not enough to tally names and numbers
• To understand a biological community, you had to understand the
functional relationships between its organisms
• Biological community = all the organisms that appear in a particular
habitat that interact with one another
• What was most important in a community, said Elton, was not who you
were, but what you did
• Niche = functional role of an organism in an ecosystem (ex. nocturnal
insectivore – bat, niche = job, habitat = address)
• Elton wrote “When an ecologist says ‘there goes a badger’, he should
include in his thoughts some idea of the animal’s place in the
community, just as if he had said ‘there goes the vicar.”
• Elton was a pioneer - trying to learn the forces that determine which
organisms, and how many of them, can live in a particular ecosystem
• While a student at Oxford, Elton went on an expedition to Bear Island, a
tiny island in the Arctic Sea, near Spitsbergen, north of northern Norway
(!)
• Elton mapped out the feeding relationships on the island, published as
part of his first important paper on community structure
• Community structure = how many different species in the community +
how many individuals of each different species
• What determines community structure?
• Long standing debate in ecology - is it predation, competition, food?
• Elton felt strongly that food was the most important factor in shaping
community structure - who or what you ate, who or what ate you…
• Elton first described the food chain and the food web (what he called the
food cycle)
• Food chain = linear sequence of predator and prey in an ecosystem (who
eats who)
• Food web = interconnection of all the food chains in an ecosystem
The large fish eat the small fish;
The small fish eat the water insects;
The water insects eat plants and mud
• This ancient proverb describes a simple food chain
• Food chains represent the flow of energy through an ecosystem
• Food chains are a sequence of producers and consumers, prey and
predators
Large fowl cannot eat small grain
• Elton was struck by the fact that all the animals on Bear Island came in
discrete sizes, much bigger or smaller than others
• Elton was struck by the fact that all the animals on Bear Island came in
discrete sizes, much bigger or smaller than others
• Larger the animals, the scarcer they were
• Why did life form a Pyramid of Numbers?
• Why were big fierce animals rare?
• Why should life come in discrete sizes?
• Answer to this dilemma was not proposed until 1942
• Raymond Lindeman, young aquatic ecologist at the University of
Minnesota
• Lindeman realized that ecosystems were systems that transformed energy
• Ecosystem = all of the biological communities in a given area together
with their physical habitat
• Energy flows through ecosystems
• As energy passes from one trophic level to the next (feeding level), some
energy is lost at each level
• Herbivores eat plants, change the plant’s energy into their own energy
• Predators eats herbivores, incorporate the energy of the prey into parts of
the predator
• Herbivores eat plants, change the plant’s energy into their own energy
• Predators eats herbivores, incorporate the energy of the prey into parts of
the predator
• This transformation of energy is always inefficient - simple
thermodynamics
• Amount of energy left in the system decreases at each level
> Some locked up in maintenance and reproduction on each level
> Some lost from predator wastage (stems, bones)
> Some lost as heat energy, burned up in metabolic conversion - no reaction is
100% efficient
• Elton’s Pyramid of Numbers is explained by Lindeman’s Pyramid of
Energy
• Lindeman’s paper transformed ecology, as Elton’s book had done earlier
• The flow of energy and materials through ecosystems is the basic
organizing principle in modern ecology
• Lindeman’s paper was rejected by Ecology for being too theoretical!!
• Finally accepted and published, thanks to G.E. Hutchinson, influential
ecologist
• Autotroph = self-feeder, autotrophic organisms produce their own energy
(photosynthesis)
• 6CO2 + 12H2O + light => C6H12O6 + 6H2O + 6O2
• Autotrophs are the producers, herbivores and carnivores are the
consumers
• Plants, cyanobacteria, some protists transform light into chemical energy,
stored in the chemical bonds of glucose
• How efficient is photosynthesis?
• Efficiency of photosynthesis is actually less than 2% !!!
• Less than 2% of the light that strikes green leaves gets captured as
chemical energy
• About 25% of this captured energy goes into growth, maintenance, and
reproduction of the plant
• The remaining 75% is available to the next trophic level (herbivores)
• Plants are primary producers
• The solar energy assimilated by plants is called their gross primary
production (GPP)
• Subtract from this GPP the fraction that the plant uses for itself (~25%),
what’s left is called net primary production (NPP)
• Net primary production is relatively easy to estimate, = total dry weight
of new biomass added over the course of one year (g/m2/yr)
• NPP gives us a rough yardstick to compare the productivity of different
types of ecosystems
• Highest NPP - wetlands, tropical forests, estuaries, coral reefs
• Lowest NPP - desert, open ocean
• Open ocean is a biological desert!! - most of the ocean’s productivity is
in the plankton, the upper few inches
• Conversion of solar energy to plant tissues is very inefficient
• Conversion of plant tissue to herbivore tissues is very inefficient
• Conversion of herbivore tissue to carnivore tissues also very inefficient
• Heterotrophs = fed by others, heterotrophic organisms eat other
organisms to survive
• Heterotrophs burn (oxidize) glucose (and other compounds) to recover
the energy stored in the chemical bonds (respiration)
• C6H12O6 + 6O2 + 6H2O => 6CO2 + 12H2O + energy
• 90% of the energy at any trophic level is lost going to the next trophic
level
• Only ~ 0.03% of the original solar input remains by the time you reach
the end of the food chain
• Most ecosystems have 4 trophic levels, some have 5 - 6 (secondary,
tertiary carnivores)
One hill cannot shelter two tigers
• Because of the inevitable loss of most of the solar energy that enters the
ecosystem, only a tiny portion remains when you reach the peak of the
pyramid of energy
• But - could one hill have sheltered two Tyrannosaurs??
• That depends - on whether or not T. rex was warm-blooded or coldblooded
• Warm-blooded animals (endothermic) need more energy to sustain
themselves that cold-blooded animals (ectothermic)
• The laws of thermodynamics have not changed since the Cretaceous
• The pyramid of numbers must have applied to extinct communities as
well
• How can we estimate the way energy moved through a vanished
ecosystem?
• Many lines of circumstantial evidence that at least some dinosaurs were
warm blooded
• Many lines of circumstantial evidence that at least some dinosaurs were
warm blooded
> Erect stance
> Rapid gait (run, gallop) - trackways
> Feathered dinosaurs - insulation
> Predator/prey ratios
• Calculate the number and relative mass of predators and prey in the
ecosystem
• Total biomass predator / total biomass prey
• Ectothermic communities have high PPR
• Calculate the number and relative mass of predators and prey in the
ecosystem
• Total biomass predator / total biomass prey
• Ectothermic communities have high PPR
• Endothermic communities have low PPR
• How can we calculate this ratio for dinosaurs??
• Pick one formation, all dinosaur fossils from that formation are about the
same age
• Estimate mass per species, calculate PPR
• Dinosaurs are at the upper end of the range for modern mammalian
communities
• Estimates have a built in bias - what is it??
• Probably overestimating the number of large carnivores
• Collection bias in favor of big carnivores
One hill cannot shelter two tyrannosaurs