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Lecture 4

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Biological Sciences
Marc Cadotte

Lecture 4: Itʼs all about energy: Primary production Itʼs all about energy ▯ - Nothing in ecology makes sense if you donʼt understand energy ▯ - Energy = currency for organism interaction ▯ - Energy for most organisms come from sunlight (which bombards Earth with energy) ▯ - Energy is very limiting; need to be able to locate energy sources (find a prey) ▯ - We only see a couple predators because there isnʼt enough energy available for a huge ▯population ▯ - Organisms obtain energy from sunlight, from inorganic chemical compounds, or through the consumption of ▯ organic compounds ▯ - Organic compounds = eating other things; photosynthesis (turning solar energy into useful energy). Inorganic ▯ chemical compounds = such as sulfur in geothermal vents ▯ - Autotrophs photosynthesis; chemosynthesis ▯ ▯ - The energy is converted into chemical energy stored in carbon-carbon bonds of organic molecules ▯ ▯ - Most autotrophs obtain energy through photosynthesis. Sunlight provides the energy to take up ▯ ▯ carbon dioxide and synthesize organic compounds ▯ ▯ - Autotrophy: 6 Carbon Dioxide + 6 Water - -(Light)--> 6 Sugar + 6 Oxygen ▯ - Energy in ecosystems orginates with primary production by autotrophs ▯ - This is the basis by which we we build a system (through energy input) ▯ - All interactions can only happen because there is an input of energy. Energy moves in one direction Primary Production ▯ - Energy assimilated by autotrophs is stored as carbon compounds in plant tissues; carbon is the currency ▯ used for the measurement of primary production ▯ - Primary productivity is the rate of primary production ▯ - How much carbon are the autotrophs fixing and turning into biomass (sugar) = primary productivity ▯ - Gross primary production (GPP): total amount of carbon fixed by autotrophs in an ecosystem ▯ - GPP depends on the influence of climate (precipitation, temperature, etc) on photosynthetic rate and the leaf ▯ are index (LAI) - leaf are per unit of ground area▯ ▯ - LAI = how much the group is covered by leaves ▯ - LAI varies among biomes: ▯ ▯ - Less than 0.1 in Arctic tundra (less than 10% of the ground surface has leaf cover) ▯ ▯ - 12 in boreal and tropical forests (on average, there are 12 layers of leaves between the canopy and ▯ ▯ the ground) ▯ - LAI=1 = 100% of ground is covered by leaves. 12=12 layers of 100% leaf covering ▯ - A single layer canʼt capture all the radiation coming through, a lot actually passes through the single layer ▯ - Additional layers = allow you to capture more energy ▯ - The top layers are more efficient than the bottom layers (higher uptake of carbon dioxide) (because bottom ▯ layers are farther away from the sunlight) ▯ - Net primary production (NPP): NPP = GPP - respiration ▯ ▯ - NPP represents the biomass gained by the plant ▯ ▯ - NPP is the energy left over for plant growth and consumption by detritivores and herbivores. NPP ▯ ▯ represents the storage of carbon in ecosystems ▯ ▯ - NPP is high in warm regions and low in dry/colder regions ▯ - Measuring belowground NPP is more diffult ▯ ▯ - At any given time, you can only see a fraction of the NPP below ground ▯ ▯ - Roots turn over more quickly than shoots; that is, more roots are “born” and die ▯during the growing ▯ ▯ season ▯ ▯ - Roots may exude a significant amount of carbon into the soil, or transfer carbon to mycorrhizal or ▯ ▯ bacterial symbionts ▯ - In biomes where competition for light is important, a smaller percentage of NPP is allocated to roots. In ▯ nutrient-poor biomes, such as tundra and grasslands, over 50% of NPP is allocated to roots. You can only see ▯ a small fraction of the plant, and therefore their ▯ NPP Primary Production: measuring: ▯ - Go to an area, cut out the plant, dry, weigh, get biomass (carbon) = good for small areas ▯ - Harvesting plant biomass is impractical for large or biologically diverse ecosystems ▯ - Solution: measure chlorophyll concentrations ▯ - Done using satelite imaging: use different filters to get different wavelenghts of light (interested in visible light and infrared Primary Production: ▯ - NDVI (normalized difference vegetation index): uses the difference between visible light ▯(VIS) and near- infrared (NIR) reflectance to estimate the absorption of light by chlorophyll ▯ - high NDVI = lot of chlorophyll. NDVI = (NIR - VIS)/(NIR + VIS) ▯ - NDVI can change over seasons (less in winter, more in summer) ▯ - Phytoplankton turn over much more rapidly than terrestrial plants (only flow around for a couple of days, then ▯ they settle on the ground and die), so biomass at any given time is low compared with NPP; harvest ▯ techniques are not
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