Energy

region

biosphere

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 Moleculeschemosynthetic autotrophs

• Organic Molecules / Organic Moleculesheterotrophs

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 4-carbon 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)

cockle shell height (mm)cockle shell height (mm)

(a-f refer to shell size of predatory snail)