Background

Insects, which are a key component of the Earth's ecosystem and the largest group in the animal kingdom, have long been shrouded in mystery regarding their total number of species. For the past 40 years, the academic community has maintained a standard estimate of approximately 6 million insect species on Earth. This figure originated from early research in the 1980s, which involved collecting insects from rainforest trees and extrapolating the data.

However, existing prediction methods have clear limitations. Traditional morphological classification methods struggle to distinguish cryptic species, which are morphologically similar but genetically distinct. Furthermore, the lack of physical time and manpower limits the accuracy of calculations. With the accelerating rate of species loss due to climate change and habitat destruction, securing baseline ecological data has become more urgent than ever.

Key Findings

The study published in the Proceedings of the National Academy of Sciences (PNAS) began with the extensive ecological data collected from the Área de Conservación Guanacaste (ACG) in Costa Rica. Robert K. Colwell, Professor Emeritus at the University of Connecticut, Laura Melissa Guzman, Professor at the University of Southern California, and Dirk Steinke, Professor at the University of Guelph, jointly analyzed the insect genomic information collected from the ACG over more than 40 years. The research team utilized a sample of over 1.6 million specimens. They analyzed the cytochrome c oxidase subunit I (COI) region, which contains genetic information (DNA barcoding) from these specimens, and identified a total of 53,945 genetically distinct species.

The research team noted that even extensive sampling only captures a portion of the total insect population and developed a mathematical model to estimate the proportion of unidentified species. In the analysis, the parasitoid wasp Microgastrinae, which is used as an ecological indicator, was used. This parasitoid wasp exhibits extreme host specificity, laying its eggs only on specific butterfly or moth larvae, making it suitable for representing insect diversity. The research team compared the number of parasitoid wasp species captured in 15 Malaise traps (tent-shaped insect collection devices) installed in the study area with the proportion of parasitoid wasp species actually inhabiting the entire ACG. Based on this, they estimated that the 'undersampling ratio,' which includes undiscovered species not captured in the traps, is approximately 2.4 times higher.

This ratio was then extrapolated to the entire insect population of the ACG, and the correlation between global plant distribution and forest area and insect diversity was applied to estimate the total number of insects on Earth. The analysis revealed that the number of insects on Earth is estimated to be between 14 million and 20 million, which is 2 to 3 times higher than the previously accepted figure of 6 million.

Significance and Prospects

The fact that there are at least 14 million insect species on Earth means that humans have currently identified less than 10% of the total insect species. Only about 1 million insect species have been officially described. The remaining 90% or more of species are at risk of disappearing due to climate change and development before they can be registered in the scientific community. The study is valuable in that it provides a crucial benchmark for understanding the actual scale of biodiversity loss.

However, the research model also has limitations. It projects data from a specific region, the Costa Rican rainforest, and the undersampling ratio of a single taxonomic group, parasitoid wasps, to the entire world. In ecologically different regions, such as arid or temperate zones, the diversity proportion of parasitoid wasps or the proportion of undiscovered species may vary. To ensure global reliability, additional validation work in key ecosystems on each continent is needed.