What does your food eat? It might sound like a silly question, say if you're eating a salad, but it is still relevant. The spinach leaves in that salad made their own food using energy from the sun, water and nutrients from the soil, and carbon dioxide from the air. Earthworms in the soil broke down organic material so that the plants had nutrients. If you think about it, a whole lot went into making a single leaf.
Organic materials and the energy to build must come from somewhere. Therefore, organisms must either eat other organisms to sustain themselves or make their own energy, as plants do. The flow of this energy from organism to organism is represented by diagrams known as food chains and food webs, the subject of this article!
Energy: sustaining life on earth
Energy- or the ability of an organism to do work—must come from somewhere. For life on earth, most life is sustained via the energy produced by the sun. Sunlight can be used by organisms via photosynthesis to transfer light energy into chemical energy, which is the energy stored in the chemical bonds between molecules.
Energy stored in these bonds can then be used by organisms to carry out life processes.
What is primary productivity?
Primary production is the process by which plants or other autotrophs convert the sun’s energy into energy that can be used for food by the plants themselves or by other organisms.
The rate at which autotrophs do this is called primary productivity.
Scientists measure different types of primary productivity to help understand the flow of energy through ecosystems. The gross primary productivity (GPP) of an ecosystem is the total rate at which light energy is converted into chemical energy through photosynthesis by autotrophs or primary producers.
Net primary productivity is the total rate of primary production minus what the producers use to sustain themselves. Thus, the net primary productivity represents the available energy that may be used by the other organisms in an ecosystem.
Vast like the ocean
You wouldn’t think the open ocean would do much in the way of primary productivity, since it is dark and blue instead of green and fertile like tropical forests. However, floating in the upper layers of the ocean are phytoplankton, which can carry out photosynthesis. Because the open ocean is so large and occupied with phytoplankton, it is actually responsible for over half of the total net primary productivity on earth every year! Although, the actual rate compared to the size of the ocean is lower than in areas like tropical rainforests or coastal marshes.1
But not all organisms can use the sun’s energy directly. Some organisms must consume each other to receive energy. How organisms get their energy is the basis for learning about food chains and food webs!
Autotrophs versus heterotrophs
Before we continue to learn about energy flow through ecosystems, we should cover some common terms that scientists use to describe how an organism gets its energy.
Because autotrophs produce their own food, we call them producers as well.
As a result, we call this group of organisms “consumers”.
Food chains in ecosystems
Food chains in ecosystems are simple representations of how energy is transferred between organisms. Food chains demonstrate what eats what in sequential order from producers to different types of consumers.
For example, grass might be the start of the food chain followed by rabbits, which are eaten by coyotes.
Food chains, especially in ecosystems, often move in order from plants to herbivorous animals to omnivorous or carnivorous animals.
A food chain shows the flow of energy through organisms in an ecosystem in a simple, sequential order from producers to different levels of consumers (i.e., herbivores, carnivores, etc.).
What are trophic levels?
Each group of organisms at similar points in the food chain make up what ecologists call a trophic level. Organisms at the same trophic level get their energy from similar sources.
The different trophic levels include:
Producers (autotrophs): make their own food; examples include terrestrial plants, phytoplankton, and cyanobacteria.
Primary consumers (heterotrophs): consume the producers; examples include herbivorous mammals such as deer or rabbits and plant-eating insects.
Secondary consumers (heterotrophs): consume the primary consumers and, if omnivorous, the producers as well; examples include
Tertiary consumers (heterotrophs): consume the primary and secondary consumers; examples include apex predators such as lions or sharks.
But wait, there are more ways in which heterotrophs feed! When organisms die or excrete waste, it is recycled by other heterotrophs known as decomposers and detritivores.
- Decomposers break down nonliving biological material,
- these include organisms like fungi, worms, and bacteria.
- Whereas detritivores feed off the broken-down material,
- these include animals that are scavengers, such as vultures.
Types of food chains
Ecologists have determined that two types of food chains are the basis for most ecosystems:
Detrital food chain: In this food chain the decomposers make up the base, which then breaks down materials in the soil and detritivores feed off of this decaying material. Eventually, detritivores themselves become food for carnivores (secondary consumers and beyond).
Grazing food chain: This is the food chain that is easy to imagine. In this food chain, producers make up the base from which primary consumers “graze” or eat. Then energy moves onto secondary consumers (carnivores, omnivores), tertiary consumers, and so on and so forth.
Energy pyramids and energy flow through ecosystems
Between each organism in a food chain, only about 10% of the energy from the previous trophic level makes it to the next trophic level. In other words, only 10% of the energy is used to make other organic matter (also known as biomass) at the next trophic level. The rest of the energy is lost through heat or other inefficiencies in digestion, respiration, and similar life processes.
Because so much energy is lost between trophic levels, food chains are often only four or five trophic levels long!
Food chains help to visualize energy flow between trophic levels. Born from a similar idea, energy pyramids show the decreasing energy transfer between trophic levels.
The biomass of the different trophic levels decreases as you move from producer to tertiary consumer. This is reflective of what occurs in ecosystems, as energy is lost, the total biomass of organisms that make up higher trophic levels decreases. Primary producers then always make up the largest collective biomass of any trophic level, and tertiary consumers (or apex predators) make up the smallest.
Food webs in ecosystems
Food chains provide great visualization tools to understand what is happening in an ecosystem. Food webs are, however, quite simplified. In reality, an ecosystem is much more complex.
For example, a grasshopper wouldn’t be the only organism making a meal out of the grass, there might also be rabbits, deer, or other insects using that grass as a food source. Small birds may eat the grasshoppers as well, and be consumed by the snakes or hawks in turn. Additionally, you may have omnivores in an ecosystem, which can consume both plants and other animals. Once you think of all the possibilities, who eats who becomes a tangled web!
A food web is a compilation of food chains showing the multiple feeding relationships between organisms in an ecosystem. It would be almost impossible to track every interaction between organisms, but food webs help to visualize how energy is transferred through a community at a more complex level than food chains.
A food web shows complex feeding relationships between organisms in an ecosystem. It is a compilation of multiple food chains.
Differences between food chains and food webs
Food chains and food webs are both modeling systems used by ecologists to quantify and understand an ecosystem. We know that what actually happens in an ecosystem is oftentimes more complex than we can model, but it is still important to demonstrate the flow of energy. Below, Table 1 offers the main differences between food chains and food webs.
Table 1: The differences between food chains and food webs.
Type of model | Food chains | Food webs |
What do they show? | Food chains show transfer of energy from single feeding relationships from producer to consumer in sequential order of trophic level. | Food webs show the transfer of energy among organisms in an ecosystem with many feeding relationships. |
Can trophic levels of organisms be easily determined? | Yes, food chains show the tropic levels increasing sequentially. | Not always, it may be easy to tell what a producer is, but an organism may exist at multiple trophic levels in a food web. |
Can organisms exist at multiple trophic levels in the model? | No, food chains only show organisms occupying a single trophic level. | Yes, food webs show that omnivores may be both primary and secondary consumers. |
Encompasses the energy of the whole ecosystem? | No, food chains only show a single thread within an ecosystem. | Yes, food webs attempt to show the intricate relationships among organisms. |
Food Chains and Food Webs - Key takeaways
- Primary productivity is the rate at which autotrophs convert the sun's energy into energy (food) that is usable for themselves or for other organisms. Most of the life on earth is sustained by energy from the sun.
- Autotrophs produce their own food; heterotrophs must consume autotrophs or other autotrophs for food.
- A food chain shows the flow of energy through trophic levels of increasing hierarchy (producer -> primary consumer -> secondary consumer -> tertiary consumer).
- A food web brings multiple food chains together to make a more complete, complex model of who eats who in an ecosystem.
- Energy is lost through heat between trophic levels, which means only about 10% of energy gets transferred between trophic levels. Biomass of trophic levels decreases as you move from producer to tertiary consumer.
References
- Primary Productivity by the Editors of Encyclopaedia Britannica. Last updated: January 24th, 2022. Britannica.com.
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