Complexicon: Food Webs

Dr. Jennifer Dunne explains food webs - a key model in studying complex systems.

A food web is the network of feeding interactions

among species in an ecosystem.

When you think about feeding interactions,

predators and prey are the first thing
that spring to mind,

like a cheetah chasing,
killing and eating an antelope.

But in fact, there are lots of ways
that matter and energy

get transferred from one
organism to another:

a calf consuming milk from its mother,

a vulture feeding on a dead fox,

a parasatoid wasp larva living inside
and feeding on a caterpillar,

a honey bee gathering nectar
and pollen from a sunflower,

and a katydid taking
small bites of willow leaves.

There are many ways that the existence
of one creature depends on another.

A food chain is one way of representing
feeding relationships among several species

with predators eating herbivores that eat
plants that draw energy from the sun.

An example is a hawk
that eats a rabbit that eats grass.

That's a good start, but it doesn't really
let you understand the whole complex ecosystem.

To do that we collect data
on the myriad species that co-occur in a habitat

and then figure out who they eat and who eats them.

Each species is usually in many different food chains
and those chains weave together to form a network.

That ecological network is called a food web.

Here's an example of an aquatic food web
from Little Rock Lake, Wisconsin,

that includes fishes, aquatic insects,
zooplankton, algae, plants, and parasites.

The spheres represent different species
and the links show the interactions between the species.

There are 92 tacks in this food web
with almost a thousand feeding interactions.

This image places species in the food web
based on their trophic level.

Trophic is just another word for feeding,Trophic is just another word for feeding,

and trophic level is a measure of how many times
energy and matter are transformed

as they work their way up

from species to species up the food web.

Or organisms such as plankton and algae
that generate energy from sunlight

have a trophic level of one, and they show up at the bottom of food web.

The species that feed on these, like zooplankton

and aquatic insects, show up in the middle.

Predatory fishes and their parasites
show up on the top.

You can use food webs to learn many different things about ecosystems.

For example, we can use food webs from different places

to understand whether a desert food web,

a marine food web

and a tropical forest food web

are organized in similar or different ways.

Our best science suggests that they have
fundamentally similar organization,

no matter what the habitat that is examined.

Even food webs from hundreds of millions of years ago

appear to be structured like modern webs.

The species may be very different
but their fundamental relationships are not.

We can even use food webs to understand
how humans fit into and impact ecosystems

through their roles as hunters,
gatherers, fishers, herders and farmers.

We can compare the feeding roles of humans to other species and their food webs

and learn lessons that help us to understand
ecological resilience and sustainability.

Developing the science of ecological networks such as food webs

is some of the research
we do here at the Santa Fe Institute.