Source: Exam 1 Review Guide 2, Prof. Jensen
Difficulty: Intermediate | Prerequisites: Basic understanding of photosynthesis, cellular respiration, and food chains.
This topic addresses two fundamental questions in ecology: how does energy move through ecosystems, and how do nutrients get recycled? Energy flows in one direction (sunlight to producers to consumers to heat), while nutrients cycle repeatedly. Understanding these processes explains why there are fewer top predators than herbivores, why ecosystems differ in productivity, and why burning fossil fuels changes global climate. You should be comfortable with photosynthesis and cellular respiration before diving in.
Energy enters ecosystems as sunlight, is captured by producers, and is lost as heat at every trophic level, which is why there is less energy (and usually less biomass) at the top. Nutrients, unlike energy, can be recycled between living organisms and the physical environment. The carbon cycle is the main focus here: humans are moving carbon from fossil fuel pools into the atmosphere faster than natural processes can absorb it, driving global warming.
Primary producers
Organisms (mostly plants, algae, and cyanobacteria) that convert sunlight into chemical energy via photosynthesis. They form the base of every food web.
Primary consumers
Herbivores that eat primary producers. Think of it as the first level of consumers in a food chain.
Secondary consumers
Organisms that eat primary consumers. These are typically predators or omnivores.
Detritivores
Organisms that feed on dead organic matter (detritus). They play a critical role in nutrient recycling by breaking down dead material.
Trophic levels
The feeding positions in a food chain or food web. Level 1 = producers, Level 2 = primary consumers, Level 3 = secondary consumers, and so on.
Food web
A network of interconnected food chains showing the feeding relationships in a community. More realistic than a simple food chain because most organisms eat and are eaten by multiple species.
Grazing food web
A food web based on living plant material, starting with producers being consumed by herbivores.
Detrital food web
A food web based on dead organic matter, starting with detritivores and decomposers.
Gross primary production (GPP)
The total amount of energy captured by primary producers through photosynthesis. This is the raw total before any is used by the producers themselves.
Net primary production (NPP)
The energy remaining after primary producers use some GPP for their own respiration. NPP = GPP minus respiration. This is the energy actually available to consumers.
Respiration
The metabolic process by which organisms break down organic molecules for energy, releasing heat. This is why energy is "lost" at each trophic level.
Pyramid of production
A diagram showing that energy production decreases at each successive trophic level. This pyramid always has the classic shape: wide at the bottom, narrow at the top.
Pyramid of biomass
A diagram showing the total biomass at each trophic level. Usually follows the same shape as the production pyramid, but not always (see inverted pyramids).
Nutrient cycles
The pathways by which nutrients move between the living (biotic) and non-living (abiotic) components of ecosystems. Unlike energy, nutrients are recycled.
Pools
The locations where nutrients are stored (e.g. the atmosphere, oceans, fossil fuels, living organisms).
Fluxes
The processes that move nutrients between pools (e.g. photosynthesis, respiration, combustion, decomposition).
The carbon cycle
The biogeochemical cycle describing how carbon moves between the atmosphere, oceans, living organisms, and geological reservoirs (fossil fuels, rocks).
Global warming / the greenhouse effect
Increasing atmospheric CO2 traps more heat in the atmosphere, raising global temperatures. This results from human activities that flux carbon from geological pools into the atmospheric pool faster than natural processes can remove it.
Energy enters ecosystems as sunlight and is converted to chemical energy by primary producers through photosynthesis.
Energy flows in one direction: it cannot be recycled. At every trophic level, energy is lost as heat through respiration.
The amount of energy produced varies between ecosystems. Tropical rainforests and coral reefs are highly productive; deserts and deep oceans are not.
Gross primary production (GPP) is the total energy fixed by photosynthesis.
Net primary production (NPP) is what remains after producers use some energy for their own cellular respiration. NPP = GPP - respiration.
NPP is the energy available to all consumers in the ecosystem. Everything above the producer level runs on NPP.
Each step up the food chain loses roughly 90% of the energy from the level below (the "10% rule" is a rough approximation).
This is why food chains rarely have more than 4 or 5 levels: there simply is not enough energy left to support another trophic level.
Two types of food webs operate simultaneously: the grazing food web (based on living plant material) and the detrital food web (based on dead organic matter).
The pyramid of production always has the classic shape: more energy at the base, less at each higher level. This is a fundamental constraint of thermodynamics.
The pyramid of biomass usually follows the same shape, but can be inverted in aquatic systems. A small biomass of rapidly reproducing phytoplankton can support a larger biomass of slower-growing zooplankton at any given moment because the phytoplankton replace themselves so quickly.
Energy flows through ecosystems and is eventually lost as heat. Nutrients cycle: they are used, released, and reused.
Nutrients exist in pools (storage locations) and move between pools via fluxes (processes).
Key distinction: energy flow is one-way; nutrient flow is circular.
Major carbon pools: fossil fuels (largest), the deep ocean, the atmosphere, living organisms, and soils.
Major fluxes: photosynthesis (atmosphere to living organisms), respiration and decomposition (living organisms to atmosphere), fossil fuel combustion (geological pool to atmosphere), ocean absorption (atmosphere to ocean).
The atmospheric pool is small relative to other pools. Fluxing carbon from a large pool (fossil fuels) into a small pool (the atmosphere) has an outsized effect on atmospheric CO2 concentration.
Humans flux carbon into the atmosphere through fossil fuel combustion and deforestation.
Rising atmospheric CO2 strengthens the greenhouse effect, trapping more heat and raising global temperatures.
Consequences include changes in weather patterns, rising sea levels, ocean acidification (CO2 dissolving in seawater lowers pH), and shifts in species distributions.
Understanding energy flow explains why farming grain-fed beef uses far more land and energy than growing crops directly for human consumption.
The carbon cycle is the scientific foundation for climate policy, carbon markets, and renewable energy development.
Students often think energy is recycled like nutrients. It is not. Energy flows in one direction and is ultimately lost as heat. Only nutrients cycle.
Students assume the pyramid of biomass always looks like the pyramid of production. It usually does, but it can be inverted in aquatic ecosystems where small, fast-reproducing producers support a larger consumer biomass.
Students confuse GPP and NPP. GPP is the total energy captured. NPP is what is left after the producers use some for their own respiration. Consumers depend on NPP, not GPP.
Students sometimes think the atmosphere is a large carbon pool. Relative to fossil fuels and the deep ocean, the atmospheric carbon pool is quite small, which is precisely why adding carbon to it has such a large proportional effect.
⚠️ Know the difference between GPP and NPP and be able to write the equation: NPP = GPP - respiration.
⚠️ Understand why the pyramid of production always tapers (energy loss at each level) but the pyramid of biomass can be inverted.
⚠️ Be able to distinguish between pools and fluxes and give examples of each in the carbon cycle.
⚠️ Explain why moving carbon from a large pool (fossil fuels) to a small pool (atmosphere) has a disproportionate effect.
⚠️ Know the difference between energy flow (one-way) and nutrient cycling (circular).
True or False: Energy can be recycled through an ecosystem. (False: energy flows in one direction and is lost as heat. Only nutrients cycle.)
Fill in the blank: NPP = GPP minus ________. (Respiration.)
True or False: The pyramid of biomass is always wider at the base. (False: it can be inverted in aquatic systems where producers reproduce very rapidly.)
Fill in the blank: The locations where nutrients are stored are called ________, and the processes that move nutrients between them are called ________. (Pools; fluxes.)
True or False: The atmospheric carbon pool is large compared to the fossil fuel pool. (False: the atmospheric pool is small relative to fossil fuels, which is why adding carbon to it has a large effect.)
Q: Why must production decrease at each successive trophic level?
A: Energy is lost as heat through cellular respiration at every trophic level, and some energy at lower levels is never consumed or assimilated. Each level therefore has less energy available than the one below it.
Q: How can a small biomass of producers support a larger biomass of consumers?
A: If the producers have a very high rate of growth and reproduction (high turnover), they can produce enough energy to sustain a larger standing biomass of slower-growing consumers, even though at any snapshot in time, the producer biomass is smaller.
Q: Explain the difference between how energy and nutrients move through ecosystems.
A: Energy flows in one direction: sunlight to producers to consumers, with losses as heat at each step. It cannot be recycled. Nutrients, by contrast, cycle: they move between pools in the biotic and abiotic environment and can be reused indefinitely.
Q: Why does burning fossil fuels have such a large effect on atmospheric CO2, even though the atmosphere already contains carbon?
A: The atmospheric carbon pool is relatively small. Fossil fuels represent a very large pool. Moving carbon from a large pool into a small one produces a proportionally large change in the concentration of the receiving pool.
Q: Name two consequences of increasing atmospheric CO2 beyond rising temperatures.
A: Ocean acidification (CO2 dissolving in seawater lowers ocean pH) and changes in species distributions (organisms shift their ranges in response to changing climate conditions). Other valid answers include rising sea levels and altered weather patterns.
This connects to predation and community ecology because energy flow through trophic levels determines how many predators and consumers a system can sustain (top-down and bottom-up regulation). The carbon cycle and global warming connect to conservation biology and the broader theme of how human activity reshapes ecosystems. Understanding GPP and NPP also explains latitudinal gradients in diversity, since tropical regions have higher productivity.
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