Spiders are beneficial to the environment because they help keep other species of insects in check. Without spiders, some pest species would become dangerously overpopulated and cause harm to people, plants and animals.
Although most species of spiders prey indiscriminately on all species of insects, they play an important role in keeping populations in check. When any particular species experiences explosive population growth, spiders catch more of them and help reduce those numbers. As a result, they help keep sensitive ecosystems in balance and prevent irreparable damage to plants. Spiders are also prey to many species of insectivores, including some bats, birds and primates. Spiders alone do not make up any one creature's diet or keep any particular insect population in check, but they play a critical role in maintaining balance in the ecosystem.
Spiders also eat nuisance species such as wasps and mosquitoes. Without spiders, these species' populations might grow to an extent that they would pose a serious risk to people and animals. Very few species of spiders are dangerous to humans or larger animals, so the risk of venomous poisoning from a spider bite is minimal. Despite widespread fears about them, most spiders simply prey on dangerous or nuisance insects.
Spiders also have beneficial effects on human industry, which can help minimize environmental damage. For example, spider webs are used to create fishing nets by some native peoples of Papa New Guinea.