Arachnologische Gesellschaft

Ecology

Spiders are predators and can be found in all terrestrial ecosystems in large numbers of both species and specimens. They live on meadows, in forests, in the mountains and in deserts, in caves, on glaciers, in the tidal zones and one species (the water spider Argyroneta aquatica) even in fresh water. Most species are generalists, feeding on insects, but also other arthropods, including spiders. They are more restricted to a carnivorous life than other predatory invertabrates, like ground beetles and centipedes. Spiders are considered and studied as a model group of terrestrial predators. Usually spiders are included in any study of food webs and energy flows in ecosystems.

Spider characters of ecological significance

With the exception of only few species, all spiders produce poison (mostly neurotoxins) in order to immobilize or kill their prey as well as in defensive behavior.

Food is digested extraintestinally, meaning outside of the body, and imbibed in liquid state, resulting in turn in only few undigested food particles and therefore only little amounts of excrements. Excretion, the expulsion of nitrogen, often takes place via the deposition of guanin within the body, for example in the European garden spider as a visible white pattern on the hind body.

All spiders produce a silk made of proteins. The different kinds of threads protect the egg sacs, encase protective retreats, make for securing threads and communication lines; many (but not all) spiders construct catching webs; and some of these species can even utilize their spinning ability to ensnare more dangerous prey.

It is still part of debate, whether spiders indeed have a lower metabolism than other invertebrates, but there is consensus on their being able to reduce their metabolism considerably in times of hunger, enabling them to endure quite long periods of food scarcity.

The combination of being able to starve, to disperse by drifting through the air on a silk thread (ballooning) and to catch prey effectively via silk webs, e.g. flying insects, make spiders successful pioneer colonizers. Virtually everywhere spiders be observed as the first animals after floods or volcano eruptions as well as on new islands; they even thrive in such places before the establishment of vegetation or a permanent fauna.

Global spider population consumes more meat than humans do

According to a recent estimate, spiders are expected to consume 400 to 800 million tons of prey each year. In comparison, humans consume about 400 million tons a year. These numbers were calculated by the two scientists Martin Nyffeler (University Basel) und Klaus Birkhofer (University Gießen) and published in the journal Science of Nature early 2017. The magnitude of this estimate is surprising and sheds new light on the significance of spiders in ecosystems. Spiders are the most widely distributed and species diverse group of predators on the planet. In many terrestrial biotopes one can find 150 spiders on one square meter, in extreme cases up to 1000. Their success is based on a highly developed sensory system, enabling them to detect prey in their vicinity very well. Spiders mostly feed on invertebrate animals, especially insects, but also prey on other spiders but only rarely on worms, snails and small invertebrates. The amongst animals unique ability of making catching webs and the use of poison makes spiders very effective predators.

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.