Almost 60 percent of all described species on earth are insects. Between 700,000 to 900,000 unique insect species have been described. Many species have never been described. Estimates for numbers of insect species are difficult to make. One method of estimating the total numbers of insect species is to intensively sample a small area and determine the percent of the insects collected that are undescribed. A decades long project in the rain forests of Panama intensively sampled 0.48 hectares (about 1 acre). The small area yielded over 6000 species of arthropods. Interestingly the numbers of species in each of the subplots was correlated with the plant diversity.
Why are there so many species of insects compared to mammals? Insects have a small size and can occupy “microhabitats”. An animal the size of a cow will experience a corn field as a single habitat. Animals the size of insects can occupy multiple microhabitats within the single habitat. A cow may eat an entire corn plant. However, insects divide a corn plant into multiple niches. There are insects that specialize only on eating the seeds, or only feeding on roots, or only feeding on leaves or only drinking the pholem, etc. A corn plant that might support a small number of mammal species, may support hundreds of insect species. Other insects feed on the insects feeding on the plant. Multiply the numbers of insect species that occupy a single plant by the number of plants and the potential microhabitats for an insect species in a give area are far greater than the number of habitats for larger mammals.