Food Webs

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Slide 1

In this presentation, we’re going to talk about food webs. Food webs are made up of a number of different food chains, connected together. You might remember that a food chain is made up of a sequence of organisms that eat one another. The arrows in a food chain represent the flow of energy and nutrients from one level of the food chain to the next. A food web combines a number of different food chains that occur in an ecosystem, and takes into consideration the fact that a single organism may actually be part of two or more different food chains at the same time. For example, if you ate a chicken Caesar salad for lunch, you’d be acting as both a secondary consumer (eating the chicken) and a primary consumer (eating the Kos lettuce). You’d be part of two intersecting food chains. As humans, we might be part of many different food chains at the same time. A food web can represent all the different things that a single organism might eat and be eaten by.

Slide 2

Food webs are a very important concept in ecology as they help us to understand the complex feeding relationships and the flow of energy in an ecosystem. Each food web is made up of a number of different food chains that might share one or more organisms. Food chains only represent a single path through which energy can flow in an ecosystem. A food web can represent many paths for the flow of energy and different feeding relationships. For example, in the aquatic food web shown in the picture, the phytoplankton is part three food chains: first, it can be eaten by zooplankton. Second, it can be eaten by crustaceans, and third it can be eaten by small fish. Thus it is part of at least three different food chains, that have merged together the form the food web. Theoretically, food webs can represent all the different feeding relationships and ways in which energy flows in an ecosystem. In practice, this might get a bit too complicated. As we’ve seen with the phytoplankton, a single organism might eat or be eaten by a number of different organisms. Food webs can show all of these relationships, but a food chain cannot. Consequently, a food chain can represent the different ways that energy can flow through an ecosystem.

Slide 3

Food webs are made up of a number of different food chains, joined together. For example, as humans, we form part of many different food chains. If you had spaghetti carbonara for lunch, you would eat ham and pasta (from the plant, wheat). Thus, you’d be part of two chains: one as a primary consumer, eating the wheat, and the other as a secondary consumer, eating the ham. Each food chain consists of a sequence of organisms, connected by arrows. The arrows mean “is eaten by”, or “energy flows in this direction”. Each level contains a group of organisms, representing a given feeding level. These organisms obtain energy by eating the organisms at the feeding level below them.
 
Slide 4

Each food chain in a food web consists of a sequence of organisms that are linked by predator-prey or feeding relationships. The organisms (or predators) at one level of the food chain feed on the organisms (or prey) at the preceding level. Most food chains begin with organisms called producers that gain their energy from the Sun’s radiation. These are usually green leafy plants, algae or phytoplankton which is capable of taking energy from an abiotic factor in the ecosystem (the Sun) and converting it, through a chemical reaction into a form of sugar. Energy passes to different levels of the ecosystem as the organisms at one feeding level eat the organisms at the next feeding level.

Slide 5

Scientists are interested in food webs because they provide them with an overview of the different feeding relationships that occur in an ecosystem. Each organism from the ecosystem might feed on a number of different sources, at different feeding levels of the food chain. For example, a fish might feed on phytoplankton or zooplankton, acting as a primary consumer in one food chain and a secondary consumer in another. The food web merges these food chains together so that we can gain an overview of the structure of the ecosystem, the feeding relationships between its organisms and the ways in which energy flows in the ecosystem.

Slide 6

The picture on this slide shows a simple food web in a desert ecosystem. It is made up of three different food chains that have been merged together. The producers starting the web are plants, which are capable of creating their own food. Another name that scientists give to producers is autotrophs because they are capable of making their own food. We say that producers lie at the first trophic or feeding level in the food web. Producers are generally eaten by herbivores. We call these primary consumers, and they lie at the second trophic or feeding level. You can see that the squirrel is actually part of two food chains that merge as they pass through the squirrel. As a herbivore, it plays the role of a primary consumer in both food chains. The squirrel is eaten by two different carnivores – the eagle and the fox. The eagle only plays the role of a secondary consumer in this food web, lying only on one food chain. It is situated at the third trophic or feeding level. The fox lies on two food chains. In one chain it lies at the 3rd trophic level, while in the other, it lies at the fourth trophic level. The final food chain on the left of the diagram contains grasshoppers, at the second trophic level, scorpions which eat the grasshoppers at the third trophic level and the fox at the fourth trophic level.
 
Slide 7

This very simplified food web for a desert ecosystem includes only three food chains. The first shows grasshoppers feeding on plants. The scorpions, in turn, feed on the grasshoppers, and the kit fox preys on the scorpions. In the second, the squirrel feeds on the plants, and the kit fox preys on the squirrel. In the third, the squirrel feeds on the plants, and is preyed on by the eagles. This is much simpler than usual food webs. Most food webs contain many different food chains and show organisms playing roles at different feeding levels in several of these. They involve many more different species and demonstrate both strong and weak interactions between different species in the ecosystem. For example, a full food web of this desert ecosystem might show a number of different predators that prey on the scorpion. These might include large centipedes, tarantulas, lizards, owls and eagles, bats, shrews and grasshopper mice.

Slide 8

Almost every food web starts out with organisms called producers. Producers might be plants in terrestrial ecosystems or algae and phytoplankton in water-based ecosystems. Producers are called producers because they can produce their own food. They start out with energy from the Sun, water and carbon dioxide and turn it into a form of food called a sugar. Producers specifically produce the sugar, glucose. The process of doing this is a chemical reaction called photosynthesis that occurs in the green leafy parts of plants, and can also occur in algae and phytoplankton. Oxygen and water are other products of this reaction. Consumers access the glucose when they eat the producers. They can turn it into energy through a chemical reaction called cellular respiration. It takes glucose and oxygen and turns them into carbon dioxide, water and energy.

Slide 9

Food webs describe the relationships between the different organisms in an ecosystem. Each organism may form part of many different food chains that lie inside the food web. It then will have different relationships with the other organisms in each of those food chains with respect to energy flow, and the way its presence in the ecosystem affects the populations of the other animals in the food web.

The diagram shows part of a food web for the Australian grasslands. The grasslands have a number of different producers including grasses, wattle and gum trees. Termites are one of the primary consumers in the grasslands. They form the main energy source for echidnas, one of several energy sources for magpies, and one of many sources of food for lizards. So the flow of energy from termites to echidnas is much more important than the flow of energy from termites to magpies or lizards. The dingo is a top predator in this food web, being able to eat many different animals including echidnas, lizards and emus. Each of these animals only provides a small part of the dingo’s energy requirements. So, some relationships are far more important than others in dictating the energy flow through the ecosystem. Some relationships are more important than others in determining the size of populations of different species in the ecosystem. The influence a species has on the populations of the species it eats depends on how many organisms of that species there are in the ecosystem, and what proportion of its diet is made up by the different organisms it eats. For example, if there was a population explosion of echidnas in the Australian grasslands, they would have a greater short-term effect on the population of termites than they would at normal population levels. In the long term, however, their numbers would dwindle because they’d run out of termites for food.

Slide 10

An American scientist called Robert Paine was intrigued by the different species that occurred in the rocky intertidal zone of Washington State on the west coast of the USA. He observed the different ways that species influenced each other, both in energy flow and influences on population,

and came up with three ways to represent these relationships using food webs. The three different types of food webs he came up with were connectedness webs, energy flow webs and functional webs. In the next few slides we’ll look at each these types of food webs individually.
 
Slide 11

The first type of food web is the connectedness web. Another term for them is the topological food web. Scientists who draw connectedness webs are interested in the feeding relationships between species. They indicate these using links in a food web. However, they do not have any way of quantifying the energy flow from one organism to another. Connectedness webs are the food webs that we’ve been showing you until now, like the one in the diagram on the right. Of course, only part of that food web is shown here.

Slide 12

Energy flow webs contain a little more information than connectedness food webs. As well as showing the feeding relationships between species, they are able to quantify the amount of energy that flows from one species to another. The thickness each arrow tells you how much energy flows between the species at either of its ends. In other words, it reflects the strength of the relationship between the two species. The diagram on the right shows part of an energy flow food web. Of course, this one has just been made up for this presentation.

Do you notice the different thicknesses of the arrows joining the termites to the echidnas, the lizards and the magpies? The one from the termites to the echidnas is the thickest, indicating that the largest part of the energy found in the termites flows to the echidnas. The second thickest is the arrow from the termites to the lizards. So, the second largest amount of energy found in the termites flows to the lizards. The smallest arrow joins the termites to the magpies. This shows that the smallest energy flow is from the termites to the magpies. So, the echidnas have the strongest feeding relationship with the termites, followed by the lizards, and the magpies have the weakest.

Slide 13

The third type of food web is the functional web. Another word for a functional food web is an interaction food web. They tell us how important each species is in maintaining the ecosystem. They show all organisms of each trophic level at the same height in the diagram. Each link reflects the influence of the species it joins on the population levels of the species at the opposite end.

Slide 14

The first purpose of a food web is to indicate the direct relationships between species. These are indicated by the arrows in the food web.

We can also use the food web to classify species, depending on the level and relationships that are shown in the food web. The species at the bottom of the food web are called basal species. These are species that can produce their own food. Scientists call them autotrophs, and they include things like green leafy plants and phytoplankton. In the middle are the herbivores and the carnivores that are eaten by other carnivores. They are called intermediate species, and include herbivores such as the kangaroo, and intermediate level carnivores such as the echidna. Finally, there are the top predators. These are species that eat other species of animals, but don’t form prey for anyone else. The dingo is a top predator in Australia.

Slide 15

Food webs also indicate indirect relationships between species. There won’t be arrows drawn between these species, but you will be able to follow along arrows from one to the next. For example, echidnas eat termites, and dingoes eat echidnas. Our food web shows

An arrow from the dingo to the echidna, and another arrow from the echidna to the termite. There’s a path from the dingo to the termite, but not a direct arrow. The termites indirectly influence the dingoes.


 
Slide 16

The food web can be viewed from the bottom up or from the top down. The species at the bottom of the food web indirectly control the populations and energy of all species above them in the food web. For example, if there were no grass in the grasslands, the termites couldn’t eat, and so there wouldn’t be any. The echidna would then have no termites to eat, and the dingo would have no echidnas to eat. The productivity of each level of the food web depends on the productivity of the level below it. If populations at one level are depleted in the long term, then the populations at higher levels will also suffer. This is what we call bottom-up control.

Working from the top down, if a predator’s numbers are depleted, then this may lead to an increase in the population of its prey. This will then have effects on lower levels of the food chain.

Slide 17


The way that energy flows may be very different in aquatic and terrestrial environments. Food webs, and particularly energy flow food webs, can be used to reveal these differences.
 
Slide 18

Food webs are made up of many different food chains, joined together. There are lots of different paths in a food web, connecting different species of plants and animals. Many consumers, such as the magpie in our grassland food web feed at more than one trophic level. The magpie at the termites, making it a secondary consumer and the grain, making it a primary consumer. Another example is provided by humans. When we eat salad or vegetables, we are primary consumers. When we eat beef, we are secondary consumers as the cow ate grass. Finally, if we eat salmon, we are tertiary consumers. Salmon feed on small fish, small fish feed on zooplankton, and zooplankton feed on phytoplankton. As we’ve seen in this presentation, food webs contain a wealth of information about the relationships between the organisms of an ecosystem.