The Erwin equation of biodiversity: From little steps to quantum leaps in the discovery of tropical insect diversity

Abstract

Almost 40 years ago, Terry L. Erwin published a seemingly audacious proposition: There may be as many as 30 million species of insects in the world. Here, we translate Erwin's verbal argument into a diversity‐ratio model—the Erwin Equation of Biodiversity—and discuss how it has inspired other biodiversity estimates. We categorize, describe the assumptions for, and summarize the most commonly used methods for calculating estimates of global biodiversity. Subsequent diversity‐ratio extrapolations have incorporated parameters representing empirical insect specialization ratios, and how insect specialization changes at different spatial scales. Other approaches include macroecological diversity models and diversity curves. For many insect groups with poorly known taxonomies, diversity estimates are based on the opinions of taxonomic experts. We illustrate our current understanding of insect diversity by focusing on the six most speciose insect orders worldwide. For each order, we compiled estimates of the (a) maximum estimated number of species, (b) minimum estimated number of species, and (c) number of currently described species. By integrating these approaches and considering new information, we believe an estimate of 5.5 million species of insects in the world is much too low. New molecular methodologies (e.g., metabarcoding and NGS studies) are revealing daunting numbers of cryptic and previously undescribed species, at the same time increasing our precision but also uncertainty about present estimates. Not until technologies advance and sampling become more comprehensive, especially of tropical biotas, will we be able to make robust estimates of the total number of insect species on Earth.Abstract in Spanish is available with online material.

Agricultural intensification and climate change are rapidly decreasing insect biodiversity

“…To preserve life's variety on which we depend, for the quality of our atmosphere, clean water, soil, and our food, we face an enormous challenge. Stork (64) estimated that there were 5.5 million species of insects, but we think the number likely to be closer to 10 million or more (76). Insects are very much the little things that run our world.…”

Insect decline in the Anthropocene: Death by a thousand cuts

“…Essential time-series data on the rates, geographic scope, ecological aspects, and taxonomic nature of insect population trends are scant, relative to those for vertebrates. Most insect taxa are far more species-rich than vertebrate taxa; there are more than a million described species of insects and even the most modest estimates calculate that another 4.5 to 7 million remain unnamed (21,22). Species-level taxonomy for many lineages is challenging and often effectively nonexistent, especially for tropical faunas (which represent the majority of insect diversity); identifying tiny species can involve microscopic study, genitalic dissection, and DNA analysis, requiring equipment and expertise far beyond that needed for the censusing of vertebrates or plants.…”

“…There are still too little data to know how the steep insect declines reported for western Europe and California's Central Valley-areas of high human density and activity-compare to population trends in sparsely populated regions and wildlands. Long-term species-level demographic data are meager from the tropics, where considerably more than half of the world's insect species occur (21,22). To consider the state of knowledge about the global status of insects, the Entomological Society of America hosted a symposium at their Annual Meeting in St. Louis, Missouri, in November 2019.…”

Consideraciones sobre el estado del conocimiento de la diversidad de Coleoptera (Arthropoda: Insecta) en Colombia

“…El orden Coleoptera, cuyos integrantes son conocidos popularmente en Colombia como cucarrones o escarabajos, es un grupo de distribución cosmopolita, conformado actualmente por alrededor de 400 mil especies descritas; constituye el grupo animal más diverso del planeta, representando cerca del 25% de todos los seres vivos y 35% de los insectos (Grimaldi y Engel 2005;Rafael et al 2012;Eggleton 2020); el estimado más reciente indica que el número potencial de especies de Coleoptera en el mundo esté entre 1.700.000 y 2.100.000 especies (García-Robledo et al 2020). Los coleópteros tienen gran importancia como proveedores de servicios ecosistémicos, pues desempeñan múltiples funciones en una amplia variedad de ecosistemas: pueden ser depredadores (por ejemplo, Carabidae, Staphylinidae, Dytiscidae), fitófagos (por ejemplo, Buprestidae, Cerambycidae, Chrysomelidae), polinizadores (por ejemplo, Curculionidae, Scarabaeidae), o saprófagos (ejemplos: Dermestidae, Histeridae, Silphidae); gracias a esta diversidad de hábitos algunos coleópteros se usan como biocontroladores, o son útiles en investigaciones forenses, o pueden considerarse bioindicadores, o poseen importancia médica o económica, incluyendo algunas especies que se consideran comestibles (Marshall 2018).…”