Harvard biologist Edward O. Wilson is the world's leading authority on ants. His book The Ants (which he wrote with Bert Hölldobler) is number twenty-seven on Modern Library's list of the hundred best nonfiction books of the twentieth century. The following is from Wilson's essay collection In Search of Nature.
At present there are about 9,500 described species of ants; this is the number so far given a scientific name. I'd venture a guess that there are in actual existence two or three times that many, and there is immense diversity within this group of hymenopterous insects. A colony of the world's smallest ant could dwell comfortably inside the braincase of the world's largest ant. One genus of ants that I've been studying, Pheidole, contains 285 named species from the New World alone. In the collection at Harvard's Museum of Comparative Zoology I have about 600 species; in other words, some 315 are new to science. More pour in from collectors every few months.
Ants are the dominant little-sized organisms of the planet -- that is, intermediate in size between bacteria and elephants. My rough estimate is that at any given moment there are about 1015, or a million billion, ants in the world. In terms of overall biomass, measured as dry weight, they are truly formidable. For example, in forests near Manaus, in the central Brazilian Amazon, ants and termites together make up more than one-quarter of the biomass -- which includes everything from very small worms and other invertebrates to the largest mammals. Ants alone weigh four times as much as the birds, amphibians, reptiles, and mammals combined. This proportion of ants is approached or exceeded in most other major types of land habitat around the world. When we consider insect biomass alone, we find that the ants and termites, the most highly social of all organisms, plus the social wasps and social bees, which rival them in colonial organization, make up about 80 percent of the biomass. These insects dominate the insect world from the Arctic Circle to Tierra del Fuego and Tasmania. In fact, ants are the principal predators of small animals roughly their own size. They are the "cemetery squad," scavenging and removing the corpses of more than 90 percent of the small animals. They are movers and enrichers of the earth, more so than the earthworms. Indeed, although the social insects as a group make up only 2 percent of all of the known described species of insects in the world, they probably make up most of the biomass.
The truth is that we need invertebrates but they don't need us. If human beings were to disappear tomorrow, the world would go on with little change. Gaia, the totality of life on Earth, would set about healing itself and return to the rich environmental states of 100,000 years ago. But if invertebrates were to disappear, it is unlikely that the human species could last more than a few months. Most of the fishes, amphibians, birds, and mammals would crash to extinction about the same time. Next would go the bulk of the flowering plants and with them the physical structure of the majority of the forests and other terrestrial habitats of the world. The soil would rot. As dead vegetation piled up and dried out, narrowing and closing the channels of the nutrient cycles, other complex forms of vegetation would die off, and with them the last remnants of the vertebrates. The remaining fungi, after enjoying a population explosion of stupendous proportions, would also perish. Within a few decades the world would return to the state of a billion years ago, composed primarily of bacteria, algae, and a few other very simple multicellular plants.
Here's how ants find food.