Energy
biosphere
region
landscape
ecosystem
community
interaction
population
individual
Individuals must obtain energy
and nutrients from their
environment to sustain life and
produce descendants.
Organisms use one of three main
energy / carbon sources
• Light / CO2
photosynthetic autotrophs
• Inorganic Molecules / Inorganic Molecules
chemosynthetic autotrophs
• Organic Molecules / Organic Molecules
heterotrophs
Trophic Diversity across the three
domains of life
Photosynthesis
C3 photosynthesis
C3 Photosynthesis
• C3 plants face trade-off between CO2 acquisition
and loss of water through stomata
• CO2 concentration gradient between the interior
of the leaf and the atmosphere is much less
steep than the gradient in water concentration,
especially in dry environments
• Rubisco has a low affinity for CO2
C4 and CAM Photosynthesis
• Alternative photosynthetic pathways that use a
4-carbon acid and separate the initial step of
carbon fixation from the synthesis of PGA
• C4 photosynthesis separates these two steps in
different anatomical structures, CAM
photosynthesis separates the steps in time
C4 Photosynthesis
• in mesophyll cells, CO2 combines with PEP
(phosphoenol pyruvate) in a reaction catalyzed
by the enzyme PEP carboxylase to form a 4carbon acid
• PEP carboxylase has a high affinity for CO2, so
CO2 levels in the leaf can be maintained at low
levels, which in turn increases the diffusion of
CO2 into the leaf
C4 Photosynthesis
• 4-carbon acids diffuse into specialized bundle
sheath cells deeper in the leaf
• Inside these cells, the 4-carbon acids are broken
down into pyruvate and CO2, and CO2
concentration can be raised to high levels
• Increased CO2 levels in turn increase the rate at
which Rubisco can catalyze the reaction
between Co2 and RuBP to form PGA
CAM Photosynthesis
• Carbon fixation occurs at night, when temperatures
are lower and the rate of water loss through the
stomata is reduced
• At night, stomata open and CO2 is combined with
PEP to form 4-carbon acids
• These acids are stored until daytime, when stomata
are closed and the 4-carbon acids are broken down
into pyruvate (recycled back into PEP) and CO2,
which then enters into the Rubisco-catalyzed C3
pathway with RuBP to form PGA
Heterotrophs
• Rely on carbon-based organic molecules
produced by autotrophs as source of both
carbon and energy
• Heterotrophs have evolved numerous ways of
feeding / obtaining this carbon-based energy
• herbivores
• carnivores
• detritivores
Chemical composition of life is fairly
consistent
• C, O, H, N, & P make up 93-97% of biomass
• Plants usually have a lower overall N and P
content
• C:N ratio of plants is about 25:1
• C:N ratios of animals, fungi and bacteria average
about 5:1 to 10:1
Herbivores - challenges
• High C:N ratios
• Lignin, cellulose not digestible
• Plant physical defenses
• spines, thorns, thick/tough leaves, incorporation of silica
• Plant chemical defenses
• toxins (poisons like nicotine, caffeine, atropine and quinine)
• digestion-reducing compounds (e.g., phenolics like tannins in oak)
Detritivores - challenges
• Main challenge of detritivores that feed primarily on
decaying plant matter is the extreme C:N ratio
• Plant matter has even higher C:N ratio when dead
• Dead plant matter may also retain chemical defenses
Carnivores - challenges
• Prey defenses:
• camouflage
• physical defenses (claws, spines, chemical repellents, poisons)
• evasive behavioral maneuvers (flight, burrowing, fighting,
hissing, spitting…)
Carnivores
• Prey of carnivores (i.e., meat) is usually of similar
nutritional content
• Therefore, carnivore species that are widely distributed
geographically can vary their diet to accommodate
locally available prey
• Selection of prey usually dictated most by size, as
carnivores must be able to subdue prey animals – size
selective predation
Chemosynthetic Autotrophs
• Bacteria and Archaea – use inorganic molecules as a
source of energy and carbon
• sulfer oxidizers – use CO2 as a source of carbon and oxidize
elemental sulfer, hydrogen sulfide, or thiosulfite for energy
• other chemosynthetic autotrophs oxidize compounds such as
ammonium, nitrite, iron, hydrogen, or carbon monoxide
Chemosynthetic Autotrophs – sulphur
oxidizers
Energy Limitation
• The rate at which organisms can acquire energy from
their environment is limited by both external and
internal (e.g., physiological) constraints
Photosynthetic autotrophs
• energy intake primarily limited by internal,
physiological constraints
• light energy packaged into photons – a plant’s
photosynthetic rate rises proportionally with the
number of photons available…to a point, Pmax
• the density of photons necessary to produce the
maximum rate of photosynthesis, Pmax, is Isat (the
irradiance required to saturate photosynthesis)
Photosynthetic autotrophs
• Plant species differ in their Pmax and Isat values
• Shade-adapted plants generally have lower
maximum photosynthetic rates and they achieve
them at lower levels of irradiance (photon flux
density: umol photons/m2/sec)
Animal Functional Response
• Animal (heterotroph) energy intake is primarily
limited by:
• time spent searching for food
• time spent handling / processing food
Three Theoretical Animal Functional
Response Types
• Type I functional response: food intake rises
linearly with food density, then abruptly levels
off at a maximum feeding rate
• consumers that require little to no processing /
handling time
Three Theoretical Animal Functional
Response Types
• Type II functional response: food intake rises
linearly with abundance at low food
densities, then increases more gradually at
higher food densities until it plateaus at
some maximum rate of intake
• most animals show this response type
• similar to response shown by plants: in both
cases, energy intake eventually limited by
internal constraints
Three Theoretical Animal Functional
Response Types
• Type III functional response: food intake increases
very slowly at low food densities, then very rapidly at
higher food densities, then eventually levels off at a
maximum feeding rate
• at low densities, food items are better protected, harder to
find…most predators will ignore uncommon foods and
focus on more abundant types until the food reaches
some threshold density
• animals may also require learning to exploit particular
food source, and at low abundance, they may not have
sufficient exposure to the food to develop searching and
handling skills
Optimal Foraging Theory
• Models how organisms obtain energy as an
optimizing process that maximizes or minimizes
a particular quantity
• Assumes energy is limited and must be allocated
to optimize some functions at the expense of
others
Optimal Foraging Theory - Animals
• Can be used to predict what consumers will eat,
and when and where they will feed
• For instance, knowledge of search time, handling
time, and energy content of different size prey
items can be used to predict what size prey will
be sought by a predator
Optimal Foraging Theory - Animals
• E/T = energy intake of a predator
• For a single prey species:
E/T = Ne1E1-Cs / 1+Ne1H1
• For two prey species:
E/T = (Ne1E1-Cs) + (Ne2E2-Cs) / 1+Ne1H1+Ne2H2
Optimal Foraging Theory - Animals
• In general, optimal foraging theory predicts that
predators will continue to add additional prey items
to their diet until the rate of energy intake (based
on the previous equation) reaches a maximum.
• If increasing diet breadth by an additional prey
species costs more (in terms of search and handling)
than focusing on a single prey species, the predator
will specialize on a single prey species
Optimal Foraging Theory - Plants
• In plants, foraging occurs via growth
• Growth above ground maximizes light
interception, rate of photosynthesis and energy
intake
• Growth below ground maximizes water and
nutrient intake
• Competing demands on resources from these
two growth dimensions
Optimal Foraging Theory - Plants
• Theory predicts that plants will allocate energy
to growth in a way that compensates for the
more limited resource
• In water and/or nutrient-poor environments
with abundant light, plants allocate more energy
to the growth of roots
• In low light environments with ample water and
nutrients, plants invest more energy in the
growth of stems and leaves
ladybug = predator in both these curves
Ozone dose (µl l-1 *h)
(Fraxinus excelsior = ash tree; Fagus sylvatica = beech tree)
(a-f refer to shell
size of predatory
snail)
cockle shell height (mm)