Every living thing needs energy to survive. Organisms get their energy from different food sources. Plants create their own food using energy from the sun, carbon dioxide from the atmosphere, and water. Animals and insects eat plants and other animals for energy, and create waste that is used as food by bacteria and other microorganisms. This phenomenon is known as cycle matter and energy transfer, and it is best understood by studying food chains and food webs.
We’ve all heard of food chains — they’re the links that connect us to the plants and animals that we eat, the plants and animals they eat, and so on and on… all the way down to the tiniest insects and even beyond. Food webs, however, are a bit different.
Food webs are systems that consist of all of the different food chains in an ecosystem. A food web paints a more complete picture of the biodiversity that supports all of the organisms in an ecosystem. Even if one particular species of organism doesn’t consume another particular species of organism within an ecosystem, both organisms still engage in behaviors that affect the other’s ability to access food, water, and sunlight. So, even though the two organisms aren’t part of the same food chain, they’re still in the same food web.
Let’s imagine a forest with lots of different plants and animals. The foxes in the forest hunt and eat lots of smaller animals, but mostly rabbits, because rabbits stay at ground level. The rabbits eat lots of fresh grass and leaves. So in this case, the food chain consists of the grass that is eaten by rabbits, the rabbits that are eaten by foxes, and the foxes that need grass and rabbits to survive. However, there are also birds in the forest, and those birds eat grasshoppers, and the grasshoppers eat grass. If the birds didn’t eat the grasshoppers, there would be lots more grasshoppers competing for grass, which would cause the population of rabbits to decrease. Fewer rabbits mean less food for foxes.
So, in the above example, we see that although birds aren’t part of the food chain that includes foxes, rabbits, and grass, they are part of the food web because their behavior affects all of the other animals’ food supply.
Different organisms in a food web — like the foxes, birds, rabbits, grasshoppers, and grasses — are grouped into different categories called trophic levels.
What does trophic mean?
Trophic comes from the Ancient Greek word trophikos. It means anything involving nutrition, food, and eating.
Trophic levels are the different types of organisms in a food web that are categorized based on behavior. They are generally classified as producers, consumers, and decomposers.
The first trophic level is for producers, organisms that create their own food. These organisms are also called autotrophs. They make their own food through processes like photosynthesis or chemosynthesis. Photosynthesis is the process plants use to create glucose using water, sunlight, and carbon dioxide. Plants and algae are autotrophs, because they use photosynthesis.
Chemosynthesis is the process some bacteria and microorganisms use to create their own food. However, instead of sunlight, they convert inorganic compounds like hydrogen sulfide into energy. This occurs in deep ocean environments and in some active volcanoes.
Consumers are organisms that eat autotrophs, or producers. They might be animals that exclusively eat other animals (carnivores), animals that eat plants as well as animals (omnivores), or animals that eat only plants (herbivores).
Consumers are grouped into four categories: Primary consumers, secondary consumers, tertiary consumers, and quaternary consumers. Primary consumers are the organisms that only consume producers. These are called herbivores. Secondary consumers consume both producers and primary consumers, but are also consumed by other organisms. Tertiary consumers consume secondary and primary consumers, and generally aren’t hunted and consumed by other organisms except in certain ecosystems. These can be either omnivores (organisms that eat plants and animals), or carnivores (organisms that only eat animals). Quaternary consumers eat primary, secondary, and tertiary consumers, and are considered apex predators. These types of consumers are divided into the second, third, fourth, and fifth trophic levels.
Examples of primary consumers (second trophic level) include:
Examples of secondary consumers (third trophic level) include:
Examples of tertiary consumers (fourth trophic level) include:
Examples of quaternary consumers (fifth trophic level) include:
Sometimes, ecosystems only contain four trophic levels, with the tertiary consumers representing the very top environmental predators.
Decomposers and detritivores are organisms that survive on the decomposing remains of organisms. Examples of detritivores are scavengers like vultures and hyenas, which survive by eating the carcasses of dead animals.
Decomposers are organisms that use waste materials for energy and transform them into inorganic matter that enriches the soil. Fungi and bacteria are types of decomposers — they survive on dead and decaying plants and animals.
Decomposers and detritivores act as a kind of natural recycling system — they transform waste material into energy and inorganic compounds that other organisms need to survive and reproduce.
We’ve learned about food webs, so it’s time to take a closer look at food chains.
While food webs include lots of different food chains, the food chains themselves can be quite complex.
Let’s look again at the food chain containing the fox, rabbit, and grass. While the fox is at the top of this food chain, the food chain also contains decomposers and detritivores that feed on the body of the fox after it dies. The decomposers effectively recycle the body of the fox into nutrients for the grass, where it can grow and feed the rabbits.
The other food chain in the same ecosystem consists of the bird, the grasshopper, and the grass. Because the grass is also part of the food chain containing the rabbit and the fox, it makes up a food web within the same ecosystem.
The size and the strength of the food chains within a food web depend upon what’s known as biomass.
Biomass is all of the energy in living organisms that can be used by other organisms.
In ecosystems, the biomass is first created by the producers that convert inorganic materials into the food that supports their growth. With every trophic level, the volume of biomass decreases. The lower trophic levels always contain more biomass than higher trophic levels. This makes sense, because there has to be a greater number of producers than consumers for an ecosystem to survive. In a healthy food web, there will be few secondary and tertiary consumers compared to the abundance of producers.
The amount of biomass in a food web is determined by how strong the different links within the supporting food chains are. If an element within a food chain is weakened — for example, if the grass in the grass, rabbit, and fox food chain diminishes — the entire food web is threatened.
Sometimes environmental events create a loss of biomass in an ecosystem. Let’s say that a landslide causes blockage in a river that fish species swim through. If the fish are diverted, then there will be fewer fish for the omnivores living near that river to eat. Those omnivores will then start to hunt for other prey, throwing off the balance. This causes a chain reaction through the ecosystem that might experience exponential population growth of certain species and significant reduction of others.
Human interference is a major cause of the weakening of food webs in ecosystems throughout the world. When humans construct roads and developments in wildlife habitats, they create barriers for the animals to access food and other resources they need to survive. This can cause predator animals to move to different ecosystems to hunt, compromising the food webs in their original habitats and threatening food webs elsewhere.
In addition to the development of properties in and near forest lands, the use of pesticides near uninhabited lands can cause a phenomenon known as bioaccumulation.
The introduction of man-made chemicals — chemicals from pesticides, herbicides, fungicides, as well as the industrial runoff from factories — has a tremendous effect on the food web. The pollutants that are scattered throughout a habitat accumulate on the producers. When the producers are eaten by the herbivores and omnivores, the chemicals are absorbed into the fat cells of the organisms. When the carnivores eat the herbivores and omnivores, the carnivores absorb the chemical pollutants.
The distribution of chemical toxins throughout ecosystems resulting in the wildlife absorbing the toxins in their cells is bioaccumulation. The chemicals can accumulate in the wildlife in such large quantities that they can be passed on to other organisms that were never in the original environment where the pollutants were first introduced.
The pollutants affect wildlife in a variety of harmful ways. They become susceptible to illness and disease, and spread the toxins to other ecosystems.
Fish are particularly susceptible to bioaccumulation. Fish, crustaceans, and mollusks are exposed to extremely high degrees of chemical pollutants in our oceans and waterways from the runoff from farming practices and the waste from shipping and recreational boats. The bioaccumulation in some fish is so extreme that it renders some fish poisonous. In New York harbor — once teeming with oysters that served as a dietary staple for Manhattanites — the oyster population was rendered inedible for nearly a century due to pollution.
One of the most famous — and infamous — examples of bioaccumulation was the accumulation of dichloro-diphenyl-trichloroethane (DDT) in wildlife in the last half of the 20th century.
DDT was used as a highly effective pesticide, and was credited with nearly eradicating the disease malaria throughout the Caribbean and Taiwan. However, because the slow-to-decompose DDT accumulated in the water and soil of natural habitats around the world for decades, it started to appear in the cells of wildlife within those habitats. Many species developed dangerous mutations as a result. In the United States, continual use of DDT led to birds laying eggs with shells that were too thin to support the babies, leading to the dramatic decline of bald eagles.
The presence of DDT in food webs throughout the U.S. was believed to have led to reproductive problems in animals, and was suspected to have adverse effects on humans. After the use of DDT was restricted in the early 1970s, concentrations of environmental DDT have declined, but its presence remained in trace amounts in forest soils in the Northeastern U.S.
The study of food webs in ecosystems has helped us understand how the accumulation of toxins – like DDT – in some areas affected wildlife in ecosystems in completely different locations. The study of food webs is a critically important environmental tool.
It is important for scientists to study food webs in different ecosystems so that we can understand how human interference influences the health of wildlife. To protect the health of our ecosystems, it’s essential to determine whether organisms’ behavioral changes are the result of the introduction of different manmade chemicals or natural environmental shifts.
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