Lecture 20 Slides Movement of Energy_Student Version.pptx

Number of trophic levels

Aquatic ecosystems typically have more trophic levels (often five)than terrestrial ecosystems (three to four) due to differences inecological efficiencies. Aquatic=more trophic, up to 5 , 3 or 4 max for Terrestrial, cannot consume in terr ecosystems , corn Terrestrial plants contain many defense compounds, and many havetissues that cannot be consumed (e.g., wood). Aquatic algae is small, has no defenses, and is easily consumed;therefore a higher fraction of an ecosystem's energy can move up thefood chain. no algae defense, In addition, consumers in aquatic systems are only slightly larger thanthe diet they consume; terrestrial systems contain many largeproducers and herbivores, whales exception

Efficiencies of energy transfer

Ecological efficiency (food chain efficiency): the percentage of netproduction from one trophic level compared to the next lowertrophic level Ecological efficiency incorporates consumption, assimilation, and netproduction.Because energy is lost at each of these steps, ecological efficiency isusually low, ranging from 5% to 20%; 10% is used as a rule of thumb.Low ecological efficiencies make it difficult to have long food chains;there is little energy to support higher trophic levels. each transfomraiotn losing energy,y low eco efficiency , 10%, hard to keep long food chains

Stoichiometry

Ecological stoichiometry: the study of the balance of nutrients inecological interactions, such as between an herbivore and a plant. The nutrients that a species requires depends on itsbiology. Example:Fast-growing water fleas (top) have a lower ratio of nitrogento phosphorus than slow-growing copepods (bottom). Fastgrowth requires high concentrations of phosphorus tosynthesize proteins necessary for rapid growth. If the ratio of ingested nutrients to requirednutrients is low, consumers must eat more food. Higher consumption leads to higher rates ofexcretion, which lowers ecological efficiency

Residence times

Energy residence time: the length of time that energy spends in agiven trophic level; the longer the residence time, the greater theaccumulation of energy in that trophic level Biomass residence time: the length of time that biomass spends in agiven trophic level Average residence times for primary producers range from morethan 20 years in forests to less than 20 days in aquatic ecosystems. colder Enviro longer residence time , forests longest residence time algae will be dead, tree still there

Primary productivity

Most energy that moves through ecosystems originates as solarenergy that powers photosynthesis. Where sunlight is not available (e.g., deep ocean thermal vents), producers rely on chemosynthesis as their source of energy. Producers harness energy and form the basis of food webs. Producers use energy for respiration, growth, and reproduction. The energy used for growth and reproduction is the energy availableto consumers. Primary productivity: the rate at which solar or chemical energy iscaptured and converted into chemical bonds by photosynthesis orchemosynthesis

Measuring primary productivity

Other techniques can be used to measure CO2 uptake and release Researchers can place a leaf into a sealed container with added CO2 that contains trace amounts of a rare carbon isotope (e.g., 14C). To measure NPP, they track the net movement of 14C from the air intothe plant tissues and back into the air. On a larger scale (e.g., a forest), researchers measure CO2 uptake andrelease by using towers that sample CO2 concentrations at differentheights above the ground. The differences in CO2 within the forest and in the atmosphereprovides an estimate of photosynthesis and respiration in an area In aquatic systems, measuring CO2 does not provide a good estimateof NPP because CO2 is rapidly converted into bicarbonate ions .Since producers release O2 during photosynthesis and take up O2 during respiration, we can estimate NPP and GPP by measuring changes in concentrations of O2. The process is identical to a light-dark bottle experiment in a sealedair chamber, except it is done in sealed water chambers .In a bottle exposed to light, the net increase in O2 is the combinedresult of photosynthesis and respiration by algae (NPP) .In a dark bottle, the decrease in O2 is a result of algal respiration. The sum of NPP and respiration is equal to GPP.

Primary productivity

Photosynthesis is not a very efficient process.

Measuring primary productivity

Primary productivity is a rate; the choiceof how to measure this rate depends onthe particular ecosystem being studied .NPP can be measured as the change inproducer biomass over time. Researchers harvest plants to determinemass of growth over a period of time. Substantial amounts of herbivory ortissue mortality will lead to anunderestimation of NPP. Researchers may estimate the biomasslost to herbivory or tissue mortality

Lecture Concepts

Primary productivity provides energy to the ecosystem. Net primary productivity differs among ecosystems. The movement of energy depends on the efficiency ofenergy flow.

Measuring primary productivity

Researchers typically only harvest above-ground plant growth. The amount of below-ground biomasscan be substantial (e.g., rhizomes, roots). Harvesting below-ground tissues ischallenging because they are deep andtend to break off when harvested. In addition, fine roots frequently die andare replaced, making it difficult toestimate the biomass accumulation. Light-dark bottle experimentsPlants also send energy to mycorrhizalfungi; this energy is included in total NPP

Light-dark bottle experiments

Since producers take up CO2 during photosynthesis and produce CO2 during respiration, we can measure NPP by recording the rate of CO2 exchange from the leaf.Leaves are placed in a sealed chamber with aCO2 sensor. When a leaf is exposed to light in the chamber,photosynthesis and respiration occur; the netuptake of CO2 represents NPP. When a leaf is concealed in the dark, only CO2respiration occurs. We can then measure GPP

Primary productivity

Standing crop: the biomass of producers present in a given area ofan ecosystem at a particular moment in time. Ecosystems with high primary productivity may not have a highstanding crop; consumers may eat it as quickly as it grows. Gross primary productivity (GPP): the rate at which energy iscaptured and assimilated by producers in an area. Net primary productivity (NPP): the rate of energy that is assimilatedby producers and converted into producer biomass in an area;includes all energy that is not respired: NPP = GPP - Respiration Both GPP and NPP are expressed in units of Joules (J) / m2 / year

Trophic pyramids

Trophic pyramid: a chart composed of stacked rectangles representing the amount of energy (i.e., pyramid of energy) or biomass (i.e., pyramid of biomass) in each trophic group -amount of energy/biomass useful way looking at energy or biomass