Topic 8.2 Notes – Energy Flow Through Ecosystems

Energy Flow and Matter Cycling

Every level of biology in ecology connects to this idea, from a population of one species to a community of interacting species, an ecosystem with living and nonliving parts, and a biome at a large climate scale.

What organisms do with energy:

• Growth means making more biomass.
• Reproduction takes a lot of energy because making gametes, seeds, eggs, or offspring is costly.
• Homeostasis means keeping internal conditions stable, like body temperature or water balance.
• Organization means maintaining ordered biological structures instead of breaking down.

Energy balance matters:

• Net energy gain leads to stored energy, growth, and often more reproduction.
• Net energy loss leads to mass loss, lower reproductive output, and eventually death.

The core distinction you need to keep straight on tests:

• Energy flows through ecosystems and much of it leaves as heat.
• Matter cycles and is conserved, moving between organisms and the environment.

How Organisms Get and Use Energy

Autotrophs

Autotrophs make organic molecules using energy from nonliving sources, so they form the base of ecosystems.

• Photosynthetic autotrophs capture sunlight and convert it into chemical energy. This drives primary productivity, which is the rate at which producers store energy in organic molecules.
• Chemosynthetic autotrophs use energy from inorganic molecules. They can live where sunlight is absent, like deep-sea vent ecosystems.

Heterotrophs

Heterotrophs get energy by eating organic matter made by other organisms.

• They break down carbohydrates, lipids, and proteins for energy.
• They also use food as a source of matter, incorporating atoms from food into their own tissues.

Consumer roles

• Herbivores eat producers.
• Carnivores eat animals.
• Omnivores feed at multiple trophic levels.
• Scavengers eat dead organisms.
• Decomposers chemically break down dead organic matter and return matter to the environment.

Temperature and energy use

Body temperature strategy changes energy demand.

• Endotherms use metabolic heat to keep body temperature stable. That costs a lot of energy.
• Ectotherms depend mostly on external heat sources, so their energy demand is lower.
• Ectotherms often regulate temperature by basking, seeking shade, burrowing, or grouping together.

Trophic Levels and Energy Transfer

A trophic level is a feeding level in an ecosystem.

1. Producers
2. Primary consumers
3. Secondary consumers
4. Tertiary consumers
5. Quaternary consumers
6. Decomposers

A food chain shows one pathway of energy transfer. A food web shows many connected pathways. This aquatic example highlights producers, consumers, decomposers, and detritus feeders in the same system.

Aquatic food web and trophic levels

One very common AP trap is arrow direction. Arrows point toward the organism receiving energy.

Energy transfer pattern:

• About 10% of energy is transferred to the next trophic level on average.
• About 90% is lost, mostly as heat from metabolism, movement, and other life processes.
• Because of this, food chains usually have only 4 to 5 trophic levels.
• Biomass usually decreases at higher trophic levels.

If sunlight drops, producer biomass drops first. Then herbivores and predators usually decline after that.

The Four Major Biogeochemical Cycles

All four cycles have:

• Abiotic reservoirs such as air, water, soil, rocks
• Biotic reservoirs such as producers, consumers, decomposers
• Processes that move matter between them

Water cycle

Reservoirs include oceans, surface water, atmosphere, and living things.

Key processes:

• Evaporation
• Condensation
• Precipitation
• Transpiration

Carbon cycle

Carbon enters organisms as carbohydrates and returns to the atmosphere as carbon dioxide.

Four major processes:

• Photosynthesis
• Cellular respiration
• Decomposition
• Combustion

Nitrogen cycle

The atmosphere is the largest nitrogen reservoir, but most organisms cannot use $N_2$ directly. Soil microorganisms do the key conversions.

• Nitrogen fixation converts $N_2$ to $NH_3$, which becomes $NH_4^+$
• Assimilation brings $NH_4^+$ or $NO_3^-$ into plants
• Ammonification returns organic nitrogen to $NH_4^+$
• Nitrification converts $NH_4^+$ to $NO_2^-$ and then $NO_3^-$
• Denitrification converts $NO_3^-$ back to $N_2$

Phosphorus cycle

Phosphate comes mainly from weathering of rocks.

• Producers absorb $PO_4^{3-}$
• Consumers get it by feeding
• It returns by excretion and decomposition
• There is no major atmospheric reservoir, which makes this cycle different from carbon and nitrogen

How Energy Availability Changes Ecosystems

Changes in available energy ripple upward through ecosystems.

1. Energy available to producers changes
   • Less sunlight means lower primary productivity.
2. Producer biomass changes
   • Fewer producers means less energy entering the food web.
3. Consumer populations respond
   • Herbivores decline first, then predators.
4. Community structure shifts
   • The number and size of trophic levels can change.

Energy availability also affects reproduction:

• Seasonal reproduction often matches times of abundant resources.
• Biennial plants store energy first, then reproduce later.
• Reproductive diapause pauses development during bad conditions.
• Some organisms switch between asexual and sexual reproduction depending on energy conditions.