8.2 Energy Flow Through Ecosystems

• Learning objectives
• Describe the strategies organisms use to acquire and use energy.
• Explain how changes in energy availability affect populations and ecosystems.
• Explain how the activities of autotrophs and heterotrophs enable the flow of energy within an ecosystem.

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Changes in energy availability can result in …

• changes in population size.
• disruptions to an ecosystem
• affects in the number and size of the trophic levels.

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Dynamics of an Ecosystem

• biotic components of an ecosystem are living organisms can be categorized according to their food source
• Autotrophs
• Heterotrophs

• abiotic components of an ecosystem are the nonliving, inorganic properties of the environment
• Chemical elements
• Sunlight
• Water

Autotrophs

• capture energy (e.g., sunlight) using inorganic nutrients
• produces organic compounds
• Producers
• possess chlorophyll and carry on photosynthesis
• photosynthesizes
• Green plants
• Algae
• Blue-green bacteria

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Chemoautotrophs

• bacteria that obtain energy from the oxidation of inorganic compounds
• ammonia
• nitrites
• Sulfides
• synthesize carbohydrates
• found in cave communities
• ocean depths & volcanic vents

Heterotrophs

• need a source of preformed organic nutrients and consume tissues of other organisms
• consumers
• herbivores
• carnivores
• omnivores
• decomposers
• detritivores

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Herbivores

Carnivores

Decomposers

Energy Flow

• major ecosystems are dependent upon solar energy flow and finite pools of nutrients
• most cannot exist without a continual supply of solar energy
• Energy flow in an ecosystem is a consequence of two fundamental laws of thermodynamics:
• The first law of thermodynamics states energy can neither be created nor destroyed; it can only be changed from one form of energy to another.
• The second law of thermodynamics states when energy is transformed from one form to another, there is always some loss of energy from the system, usually as low grade heat.

Energy Balances

Food Webs are Examples of Energy Flow

• complex trophic (feeding) relationships that exist in nature
• grazing food web begins with leaves, stems, and seeds eaten by herbivores and omnivores
• detrital food web begins with detritus, followed by decomposers (including bacteria and fungi)
• connected to a grazing food chain when consumers of a grazing food chain feed on the decomposers of the detrital food chain
• some ecosystems, more energy may move through the detrital food web than moves through the detritus food web

Grazing & Detrital Food Web

Trophic Levels

• feeding level of one or more populations in a food web
• a food chain represents a single path sequence of organisms that form links
• organisms of an ecosystem counted in the number of steps in a food chain away from input energy (most likely sunlight)

Trophic Levels

• First trophic level
• primary producers
• Second trophic level
• all the primary consumers
• herbivores & omnivores
• Third trophic level
• secondary consumers, etc.
• carnivores and detritivores

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10% Energy movement

• about 10% of the energy at a particular trophic level is incorporated into the next trophic level
• 1,000 kg of plant material converts to 100 kg of herbivore tissue, which converts to 10 kg of first carnivores, which can support 1 kg of second level carnivores
• rapid loss of energy is the reason food chains have from three to four links, rarely five
• few large carnivores in ecosystems

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Ecological Pyramid

• trophic structure of an ecosystem
• base of the pyramid represents the producer trophic level
• apex representing the highest consumer trophic level

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Types of Ecological Pyramids

Biogeochemical Cycles

• pathways by which chemicals circulate through the biotic and abiotic components of an ecosystem
• cycles are primarily gaseous cycles (carbon and nitrogen); others are sedimentary cycles, (phosphorus)
• reservoir is that portion of Earth that acts as a storehouse for the element
• exchange pool is the portion of the environment from which producers take chemicals, such as the atmosphere or soil
• biotic community is the pathway through which chemicals move through food chains

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Water Cycle

• freshwater evaporates and condenses on Earth
• also hydrologic cycle
• evaporation
• condensation
• precipitation
• transpiration

Carbon Cycle

• photosynthesis removes CO₂ from the atmosphere
• respiration then returns CO₂ to the atmosphere.
• CO₂ from the air combines with water to produce bicarbonate (HCO₃), which is a source of carbon for aquatic producers, primarily protists.
• the reservoir for the carbon cycle is largely composed of organic matter
• calcium carbonate in shells
• Limestone
• fossil fuels
• Burning fossil fuels increases the CO₂
• More CO₂ is entering the atmosphere than being placed into reservoirs
• CO₂ is a greenhouse gas climate change due to global warming

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Nitrogen Cycle

• atmospheric reservoir of nitrogen
• N₂ is 78% of the atmosphere
• nitrogen deficiency can limit plant growth
• Nitrogen fixation occurs when N₂ is converted to a form that plants can use
• nitrogen-fixing bacteria living in nodules on the roots of legumes make nitrogen compounds for plants
• Denitrification is conversion of NO₃- to nitrous oxide (N₂O) and N₂
• Human activity through inorganic fertilizers upsets the natural balance in aquatic systems
• causes algal blooms

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Phosphorus Cycle

• weathering rock makes phosphate ions (PO4 and HPO42-) available to plants that take up phosphate from the soil
• Some of this phosphate runs off into aquatic ecosystems where algae incorporate it into organic molecules before it is entrapped in sediments
• Phosphate that is not taken up by algae is incorporated into sediments in the oceans.
• phosphate taken up by producers is incorporated into a variety of organic compounds
• animals eat producers and incorporate some of the phosphate into phospholipids, ATP, and nucleotides of DNA
• decay of organisms and decomposition of animal wastes eventually makes phosphate ions available again
• available phosphate is generally taken up quickly; it is usually the limiting nutrient in most ecosystems.
• Humans impact natural ecosystems using phosphate fertilizers causing algal blooms