Toxins and pollinators

Some toxic substances, such as the pesticide DDT and the metal mercury, are not broken down or got rid of by animals. They stay in their bodies and build up along food chains. This is bioaccumulation.

Each animal eats many of the ones below it, so the dose multiplies at every step. Top predators get the highest dose. In the 1950s and 1960s, pesticides such as DDT made birds of prey such as peregrines lay eggs with thin shells that broke, or killed the birds outright, and they became rare. After these pesticides were banned, they recovered.

People can be affected too. Big predatory fish such as tuna and swordfish can contain more mercury, so pregnant women are advised to limit them.

Pollination is the transfer of pollen to the stigma of a flower, so seeds and fruit can form. Bees, hoverflies, butterflies, moths and beetles pollinate many crops, such as apples, strawberries and beans. Cereals such as wheat, rice and maize need no insects: they pollinate themselves or are pollinated by the wind.

Food security means everyone having reliable access to enough safe, nutritious food. Pollinators have declined because of pesticides, loss of wildflower habitats and disease. Wildflower strips, hedges, bug hotels and using fewer pesticides all help.

How people change ecosystems	What happens
Fertiliser runs into lakes and rivers	Algae grow fast, then die. Decomposers rotting them use up the oxygen, and fish die.
Plastic waste	Animals eat it or get tangled in it. Decomposers cannot break it down, so it lasts for hundreds of years.
Habitats destroyed	Meadows ploughed, hedges removed, woods cut down, land built on: the species living there lose their food and shelter.
Climate change	Seasons shift, so food webs can fall out of step: blue tit chicks can hatch after the caterpillars they need have peaked.
Introduced species	Newcomers such as grey squirrels and signal crayfish outcompete or infect native species (see Food webs).

People can help too: nature reserves, planting hedges and wildflowers, cleaning up rivers, cutting pollution, and bringing back missing species such as beavers and pine martens. Peregrines and otters recovered once the pesticides harming them were banned.