08 December 2017

Spiders’ Sumptuous Repast

“Spiders could theoretically eat every human on Earth in one year.” “Think spiders are freaky? Wait 'til you hear how much they eat.” Those were the headlines in The Washington Post and USA Today.

Welcome to Warren's Notice. Last March, I was all set to blog about a research study on the impact of spiders preying on insects. Then I noticed even our local Wisconsin newspaper had a few sentences on the study findings. I decided to let the topic pass.

Banana spider catching a wasp.
(photo from video
by Angelique Herring; 3:37 min)
But I didn’t delete my notes. And lately, I’ve been wondering. Maybe you missed all the reports. Maybe you’d like more detail than most news reports offered. Or maybe, seeing “spiders” in the headline was enough for you. (NB: Spiders do not dine on humans, not even for a snack.)

So, I’m back with the study. It’s not that the research will cure your arachnophobia or make you a fan of spiders. It’s just that the findings are too interesting to miss. Plus, the study complements other blog posts I've written about spiders (Spider Role Model, Web Addresses Addendum, Predator-Mimicking Moth).

Spiders Are Global Predators
Spiders are among the most common and abundant predators in the terrestrial environment. They’re found nearly everywhere, tropics to the Arctic. Over 45,000 species have been recognized thus far, and they’re all carnivores, feeding on insects and even other spiders. How much do they eat? That’s what the study was about.

Collaborating scientists from Switzerland’s University of Basel, Sweden’s Lund University and Germany’s Brandenburg University of Technology Cottbus-Senftenberg developed estimates of the weight of prey that spiders kill annually in different biome types and worldwide.

How Many Spiders Are There?
First, the researchers obtained a measure of spiders in the world. The literature provided 65 values of spider biomass (i.e., weight per unit area), which they were able to assign to seven terrestrial biomes: (1) tropical forests, (2) temperate and boreal forests, (3) tropical grasslands and savannas, (4) temperate grasslands (including old fields, permanent pastures, mown meadows) and Mediterranean shrublands, (5) annual cropland, (6) deserts and (7) Arctic tundra.

Combining the average biomass weight per unit area for each biome and the biomes’ areas, the researchers determined the total weight of global spiders to be approximately 25 million metric tons. (One metric ton is 1000 kilograms or about 2200 pounds.)  

Jumping spider attacking a fly.
(photo from video
by Andrew Huggins; 7:22 min)
How Much Do They Eat?
Next, the researchers estimated the weight of prey spiders kill annually. They used two different methods to validate their results.

The first method’s estimate was based on how much food spiders need. They combined the food requirements per unit body weight, derived from the literature, with their estimates of spiders’ weight per unit area for each biome.

The second method’s estimate was based directly on published assessments of spiders’ annual prey kill in selected biome types (e.g., prey censuses in the field combined with web density estimates).

Allowing for the range of estimates within biomes and accounting for factors such as reduced feeding on rainy days, the researchers arrived at total estimates of 460 to 700 million metric tons/year with the first method and 400 to 800 million metric tons/year with the second method. (That sentence should probably end with an exclamation point.)

Wrap Up
Putting the estimates together, the researchers concluded that the spiders’ global annual prey kill is in the range of 400 to 800 million metric tons/year. Over 95% of that is attributed to spiders in forest and grassland biomes.

So, before you swat, step on or vacuum your next spider, you’ll have to decide. Do you want 400 to 800 million metric tons of assorted insects or 25 million metric tons of spiders. As one of the other headlines from last March read, “Spiders eat twice as much animal prey as humans do in a year.” Thanks for stopping by.

Global spider prey kill study in The Science of Nature journal: link.springer.com/article/10.1007%2Fs00114-017-1440-1
Example articles on the study: