Arthropods, the phylum that includes insects, arachnids, crustaceans, centipedes (and millipedes), etc., are invertebrates which have exoskeletons, and they make up the majority of multicellular species on Earth. A recent study, “Arthropod Diversity in a Tropical Forest” by Basset et al., published in the journal Science, indicates that arthropods may outnumber mammals, including humans, by as much as 312 to 1.
The study, which focused on Panama’s San Lorenzo forest, involved over 100 scientists, took almost 10 years to complete, and led to the discovery of 6,144 distinct arthropod species in a 0.48 hectare parcel of the forest (about 5,749 square yards; an American football field is 4,480 square yards). “The scientists involved in the effort used 14 different techniques to collect bugs in the San Lorenzo Forest, including picking beetles out of dead wood on the rainforest floor, climbing tree trunks and deploying helium balloons to reach the forest canopy and pluck high-dwelling insects off branches, and fogging trees with insecticides to catch the flying bugs. The collection methods were repeated during three distinct seasons of the year in 12 20-by-20 square meter plots. In all, the team collected 129,494 arthropods, compared with only a few thousand in most previous studies, then spent the next 8 years identifying and sorting the bugs” (source).
By scaling up the numbers from the 12 sample sites, the researchers estimated that there are over 25,000 arthropod species in the Panamanian forest. However, some scientists question the validity of an extrapolation from such a small area to a much larger one: “Terry Erwin, an entomologist at the Smithsonian Institution’s National Museum of Natural History, warns against putting too much weight on the estimated number of species, adding that further surveys across the San Lorenzo forest could help to make the estimates of arthropod species diversity more accurate.
Erwin himself made predictions of 30 million insect species in 1982, using a very limited scope of study. He fogged one species of tree in a Panamanian tropical forest with insecticide, identifying the beetle species that dropped to the forest floor. Erwin estimated the number of tree beetles to ground beetles, and the number of beetles to insects to arrive at his final number of 30 million. Subsequent research by Andrew Hamilton of the University of Melbourne, using similar methods on several tree species in New Guinea, reduced this estimate to the current 6 million” (source).
Estimates of the total number of of arthropod species in the world are plagued by the same difficulties inherent in the San Lorenzo estimates. “One estimate indicates that arthropods have 1,170,000 described species and account for over 80% of all known living animal species. Another study estimates that there are between 5 to 10 million extant arthropod species, both described and yet to be described. Estimating the total number of living species is extremely difficult because it often depends on a series of assumptions in order to scale up from counts at specific locations to estimates for the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods” (source).
Taking such things a large step further, the task of estimating the total number of all species on Earth is an even more educated guess, at best, complicated by the lack of comprehensive studies of all species, the difficulty of accessing some species’ habitats, their very size (smaller ones are more difficult to find, never mind count), and even the definition of what a species is. Camilo Mora, et al. published an article in PloS Biology (2011: “How Many Species Are There on Earth and in the Ocean?”) dealing with this question and came up with an estimate of about 8.7 million species, give or take 1.3 million. However, “Based on current costs and requirements, the study suggests that describing all remaining species using traditional approaches could require up to 1,200 years of work by more than 300,000 taxonomists at an approximate cost of $US 364 billion.” Would it be worth it?
Yes, according to Mora: “With the clock of extinction now ticking faster for many species, I believe speeding the inventory of Earth’s species merits high scientific and societal priority. Renewed interest in further exploration and taxonomy could allow us to fully answer this most basic question: What lives on Earth?…The question of how many species exist has intrigued scientists for centuries and the answer, coupled with research by others into species’ distribution and abundance, is particularly important now because a host of human activities and influences are accelerating the rate of extinctions. Many species may vanish before we even know of their existence, of their unique niche and function in ecosystems, and of their potential contribution to improved human well-being” (source).
Article by Bill Norrington
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