Estimates of the total number of insect species or those within specific
orders are often highly variable. Globally, averages of these predictions estimate there are around 1.5 million
beetle species and 5.5 million insect species, with around 1 million insect species currently found and described. Between 950,000–1,000,000 of all described
animal species are considered insects, so over 50% of all described
eukaryotes (1.8 million species) are insects (see illustration). With only 950,000 known non-insects, if the actual total number of insects is 5.5 million, they may represent over 80% of the total, and with only about 20,000 new species of all organisms being described each year, most insect species likely will remain undescribed, unless species descriptions greatly increase in rate. Of the 24 identified
orders of insects, five dominate in terms of numbers of described species, namely
Coleoptera (beetles),
Lepidoptera (
butterflies and
moths),
Diptera (
flies and
mosquitoes),
Hymenoptera (
ants,
bees,
wasps and
sawflies) and
Hemiptera (
true bugs, e.g.
cicadas,
aphids,
leafhoppers,
bed bugs and
assassin bugs). At least 900,000 described species — about 90% of all known insects — belong to the five aforementioned orders, each of which has over 100,000 species, while the next (sixth) most diverse order,
Orthoptera (
locusts,
grasshoppers and
crickets), has just under 24,000 species. The fossil record concerning insects stretches back for hundreds of millions of years. It suggests there are ongoing background levels of both
new species appearing and
extinctions. Very occasionally, the record also appears to show
mass extinctions of insects. The
Permian–Triassic extinction event saw the greatest level of
insect extinction, with the
Cretaceous–Paleogene being the second highest. Insect diversity has recovered after past mass extinctions, due to periods where new species originate with increased frequency, though the recovery can take millions of years.
In the Holocene Several studies seemed to indicate that some insect populations are in decline in the late 20th and early 21st centuries, and has also been popularized as the
windshield phenomenon. For many studies, factors such as abundance,
biomass, and
species richness are often found to be declining for some, but not all locations in many studies; some species are in decline while others are not. Every species is affected in different ways by changes in the environment, and it cannot be inferred that there is a consistent decrease across different insect groups. When conditions change, some species easily adapt to the change while others struggle to survive. Concerns of
declines in insect abundance in the holocene have been attributed to
habitat loss from land use changes such as
urbanization or agricultural use,
pesticide use,
invasive species, and
artificial lighting. The use of increased quantities of
insecticides and
herbicides on crops have affected not only non-target insect species, but also the
plants on which they feed.
Climate change and the introduction of exotic species that compete with the
indigenous ones put the
native species under stress, and as a result they are more likely to succumb to pathogens and parasites. For 203 insect species that had
IUCN-documented population trends in 2013, 33% were in decline with variation in documented species across orders. Most scientific and public attention has been focused on the conservation of larger,
charismatic vertebrates, and relatively fewer studies have been done on insect groups, especially
Diptera,
Orthoptera and
Hemiptera. Data from the past from which to calculate trends is largely unavailable, and what does exist is mostly related to
Western Europe and
North America. Insect population assessments that have been undertaken were largely concentrated on the more popular insect groups, butterflies and moths, bees,
dragonflies and beetles. though ecologist Manu Sanders notes that many of these findings are often biased limited to specific geographic areas and specific groups of species. The methodology of the Sánchez-Bayo & Wyckhuys study has been questioned; the search string used to sift through the scientific literature was "(insect* + decline* + survey)". This meant that the authors identified studies finding insect declines, but may have missed those that found increases in insect populations or stability. Entomology professor
Simon Leather suggested that media reports of an "Ecological Armageddon" may be exaggerated and advocated for more funding to allow better collection of long term data on the decline. Claims of pending mass insect extinctions or "insect apocalypse" based on a subset of studies have been popularized in news reports, but often make claims extrapolated beyond the study data or
hyperbolize study findings. The
Entomological Society of America has stated there are not sufficient data to predict an imminent mass extinction of insects. For some insect groups such as some butterflies, bees, and beetles, declines in abundance and diversity have been documented in European studies. Other areas have shown increases in some insect species, although trends in most regions are currently unknown. It is difficult to assess long-term trends in insect abundance or diversity because historical measurements are generally not known for many species. Robust data to assess at-risk areas or species is especially lacking for
arctic and
tropical regions and a majority of the
southern hemisphere. ==Conservation==