Energy Flow and Nutrient Cycles in Ecosystems
Energy Flow in Ecosystems
The sun is the principal source of energy input to biological systems. Energy flow is not a cycle. It starts from the sun and then that energy is harnessed by plants, which are eaten by animals, which are eaten by other animals. At each step, energy is lost to the environment (for example, by heat loss).
Food Chains and Food Webs
Food chain: A chart showing the flow of energy (food) from one organism to the next, beginning with a producer.
Food web: A network of interconnected food chains showing the energy flow through part of an ecosystem.
- Producer: An organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.
- Consumer: An organism that gets its energy by feeding on other organisms.
- Herbivore: An animal that gets its energy by eating plants.
- Carnivore: An animal that gets its energy by eating other animals.
- Decomposer: An organism that gets its energy from dead or waste organic matter (i.e., a saprotroph).
- Ecosystem: A unit containing all of the organisms and their environment, interacting together, in a given area (e.g., a decomposing log or a lake).
- Trophic level: The position of an organism in a food chain, food web, or pyramid of biomass, numbers, or energy.
Food chains usually have fewer than five trophic levels because energy transfer is inefficient:
- The sun produces light; less than 1% of the energy falls onto leaves.
- Producers fix (trap) only about 5-8% of that energy because of transmission (passing through), reflection, and incorrect wavelength.
- Primary consumers only get between 5-10% because some parts are indigestible (e.g., cellulose) and not all parts of the plant are eaten.
- Secondary consumers get between 10-20% because animal matter is more digestible and has a higher energy value.
- At each level, heat is lost by respiration.
Humans eating plants is more efficient than humans eating animals. To have meat, we must feed the animal a lot of plant material to get far less meat. The plants lose energy to the environment, then the animal loses energy (throughout its whole life) to the environment and does not use up all the plant material, so it is very inefficient.
Pyramid of Numbers
Shows the number of each organism in a food chain. When moving up the pyramid, the number of individuals decreases, but their size usually increases (except when it starts from a large plant like an oak tree). The problem is that there might be thousands or more producers feeding one single tertiary or quaternary consumer, and if there is one large producer (e.g., an oak tree) and many parasites feed on the consumers, the pyramid will be inverted.
Pyramid of Biomass
A pyramid which shows the biomass (number of individuals × their individual mass). It only represents the biomass at the time of sampling. This can be misleading since the feeding levels might have organisms that reproduce at different rates.
Nutrient Cycles
The Nitrogen Cycle
- Nitrogen-fixing bacteria provide usable nitrogen for plants. These may exist in the root nodules where they live in symbiosis with the plants (nitrogen fixation), or this can happen because of lightning, or microorganisms provide them through decomposition.
- Nitrifying bacteria convert nitrogen-containing substances into better nitrogen-containing substances for the plants (nitrification).
- Plants absorb these substances and convert them into proteins.
- Primary consumers eat the plants and can make their own proteins; secondary consumers eat primary consumers, and so on.
- Death and decay happen at each trophic level, leading to stage one (the decomposers).
- Denitrifying bacteria carry out denitrification: they convert nitrogen-containing substances into atmospheric nitrogen.