What if insects vanished?

Look closely in a forest. Beetles, ants, caterpillars and moths are everywhere!

Many animals eat insects. What if all the insects were gone? Who would miss them?

Insects are small animals all around us: beetles, ants, bees, butterflies, moths and many more. They nibble leaves, and in turn they are food for many birds and other animals.

A food web shows who eats whom. If one kind of living thing disappears, the animals that ate it lose food, and the living things it ate lose a hunter. This lab asks: what if the insects vanished from a forest food web? This is a pretend experiment. Insects have not vanished, but scientists want to know how they are doing.

Insects connect many links in terrestrial food webs. They consume plants, are eaten by birds, mammals and other insects, pollinate fruits, vegetables and nuts, control pests and weeds, and break down leaves, wood and dung.

This lab removes insects from a simplified forest web to trace the immediate knock-on effects, the start of what ecologists call a trophic cascade. It is a thought experiment carried out with a model; real insect populations are declining in many places, but they have not vanished.

In Olly's forest, insects eat leaves and berries.

Mice, songbirds and bears eat the insects. Everyone has food!

Our model forest is based on Yellowstone, in the United States. Grasses, willows and aspens, and berry bushes are at the bottom. Insects eat all three. Deer mice, songbirds and grizzly bears eat insects, along with other foods.

Higher up, owls and coyotes eat mice and hares, and wolves eat elk. With nothing removed, every animal in the web keeps its food. There are 13 kinds of living things in this simple web; a real forest has many thousands.

The model's forest web has 13 nodes modeled on Yellowstone: three plant groups; insects, deer mouse, snowshoe hare, elk and beaver as plant eaters (the mouse also eats insects); and songbird, great horned owl, coyote, gray wolf and grizzly bear above them. Insects eat grasses, willow and aspen, and berry bushes, and are eaten by the deer mouse, songbird and grizzly bear.

Links are either present or absent, and all of a species' foods count equally. With nothing removed, the model reports zero species affected.

In our pretend forest, Olly takes the insects away.

Now the mice, the songbirds and the bears have less to eat. They still find other food, like berries. The plants have fewer leaf nibblers.

Remove the insects in the model and the effects spread out in waves. First, grasses, willows and berry bushes could become more common, because the insects that ate them are gone.

Next, the deer mouse, the songbird and the grizzly bear could find less food, because insects were on their menu. None of them loses all its food, so none disappears in the model. In all, 6 of the 12 remaining species are affected.

With insects removed, the model's bottom-up rule finds no consumer left without food, so none could disappear. The deer mouse, songbird and grizzly bear each lose one food and could find less food. The top-down rule marks all three plant groups as possibly more common, since they lost a consumer. That is 6 species affected.

For comparison, removing the deer mouse affects 10 species and removing the gray wolf affects 8, through the wolf, elk and willow cascade. The model's numbers are counts of species that could change, never population sizes.

Animals that eat insects lose some of their food. Plants that insects ate lose a nibbler.

That is how a food web works: when one part changes, others change too.

Every link in a food web is a meal. Take away the insects and every animal that ate them loses part of its food. The plants that insects ate are nibbled less.

The model is very simple. It shows who could be affected, not how many animals or how fast. It counts only who eats whom, so it leaves out insects' other jobs, like carrying pollen and breaking down dead leaves. Real animals can also switch to other foods.

The model applies five qualitative rules: remove the species; consumers with no food left could disappear (iterated); species that lost a hunter could become more common (one level); their foods could become fewer; species that lost some food could find less food.

Its limits matter here. It ignores insects as pollinators and decomposers, so its 'plants could become more common' result is a feeding-link artifact: plants that depend on insect pollination could suffer instead. It ignores diet switching, competition, disease and weather, follows top-down effects only one level, and gives no population numbers.

Insects have not vanished. But in some places, there are fewer than before.

Scientists are counting them. People help insects by planting flowers and native plants and by using fewer bug sprays.

Insects really do feed birds: 96 percent of North American land birds feed their young on insects, and one chickadee family needs at least 7,000 caterpillars to raise its chicks.

Scientists have found insects declining in many places, often by about 1 to 2 percent a year, because of land clearing, farming, climate change, pollution, introduced species, pesticides and light at night. But they also say the data are patchy, and some insects are increasing. People help by growing insect-friendly plants instead of lawns, using fewer pesticides, and joining community science counts.

A 2021 PNAS special feature introduced by Wagner and colleagues reports that annual declines in insect abundance frequently fall around 1 to 2 percent, with much variation in time, space and lineage. Named stressors include land-use change, agriculture, climate change, nitrification, pollution and introduced species, plus insecticides, herbicides, urbanization and light pollution; their relative importance and interactions remain uncertain. Long-term data are meager from the tropics, and some taxa, such as Arctic arthropods and some temperate moths, have increased.

The stakes for food webs are real: Clemson Extension notes that 96 percent of North American land birds feed their young insects. Responses include pollinator policies, reduced pesticide use, community science and individual actions.