How do we know how many spiders live worldwide?
This can only be estimated and one such effort by scientists is based on 65 research studies. According to these the number of spiders was calculated for all large ecosystems of the world: the tropical and temperate forests and grasslands, deserts, the arctic tundra and the agricultural areas, leading to a number of 25 million tons of live weight.
How did they estimate the mass of consumed prey?
The total amount of prey consumed by spiders was estimated by extrapolation, using two methods. The first method is based on the prey mass caught and consumed in one day per gram of spider body weight. Based on scientific publications and experience, an average amount of 0.1 mg prey per 1 mg spider was assumed, in other words: 10 % of spider net weight. Next, it was calculated on how many days spider are active and able to feed in the different ecosystems. In contrast to our temperate climates, tropical spiders are active the whole year round, but even there spiders cannot persue prey during one third of the days during a year because of rain. In total, this estimate led to an amount of 460–700 million tons of consumed prey.
The second method utilized data from 18 publications dealing with the annual prey acquisition in the different ecosystems. Important assumptions dealt with the different yields of the different amounts of prey caught by web vs. hunting spiders and differences between cultural and natural terrestrial areas. According to this approach, the annual amount of prey was estimated to be 400–800 million tons. Accordingly, both estimates are not far apart from one another.
In conclusion, we can expect 25 million tons of spiders on the planet, consuming 400-800 million tons of animal prey each year. In comparison, whales consume 280–500 million tons of meat per year and we humans consume about 400 million tons of meat and fish as well.
The ecological significance of spiders
Spiders in forests and graslands are especially significant, since together with savannas these ecosystems cover about two thirds of all land masses. 95 percent of all prey is consumed in these two kinds of ecosystems, making spiders important regulators of the ecological equilibrium: as predators of many pest species but also of beneficial species like bees. Spiders in agricultural areas consume only 2 % of the world wide production, which is a strong indication of their reduced survival rates in such heavily disturbed ecosystems, thus leading to reduced effects on the ecosystem.
Text: Kerstin Syré and Hubert Höfer, Data from:
Nyffeler, M. & Birkhofer, K. (2017): An estimated 400–800 million tons of prey are annually killed by the global spider community. – The Science of Nature 104: 1-30, DOI 10.1007/s00114-017-1440-1.