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In this Washington Post article, Norman Platnick of the American Museum of Natural History is quoted as saying the following:

If spiders disappeared, we would face famine.

Spiders are primary controllers of insects. Without spiders, all of our crops would be consumed by those pests.

I also found this article that says the following:

If we took spiders off the Earth, we would all die in less than 5 years.

Without all those hungry, carnivorous spiders, insect populations would explode, food crops would be decimated, ecological balances would be ravaged, and humans would probably starve within a matter of months – if they hadn’t already died from insect borne diseases.

The idea that humans would go extinct if spiders disappeared is bolstered by Platnick in an interview in which he questions if humans would even exist without spiders:

Norm: [...] And they are in fact the dominant predators of insects, and without them we'd be in dire straits. In many cases most of our crops, for example, would be totally destroyed by the insects that already do take a large toll on our crop production, but the spiders help control them.

Dr. Biology: [...] What if there were no spiders on Earth, what would the world be like? Would many of the other living things still be here? Would we even be here?

Norm: I'd say it's questionable whether we would be here.

Are spiders really that important to crops and humans?

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    This is an interesting question but not well formatted. There are two separate claims (famine and extinction) from two different sources. The famine claim seems credible - source is credible, plenty of evidence spiders are important, plus any major ecosystem change will cause drastic change in agriculture. The extinction claim seem to be pure hyperbole and comes without any support. – Colin Pickard Jul 23 '14 at 12:14
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    We can't answer a question about the future. – user5582 Jul 23 '14 at 16:06
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    Because it is possible to spray pesticides on food crops, albeit expensive, there would still be some supply. Also note that many insects are also predatory, and once the spiders aren't competition, many of these predatory populations would theoretically boom, to fill the void. Note that many predatory insects are actually sold in bulk as an organic insect control, but spiders aren't – J. Musser Jul 24 '14 at 3:00
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    Many birds eat both spiders and insects. If one source goes, they would be forced to focus on the other. Basically, spiders are a big, but not the only insect predator. If one predator disappears, others will fill its place, growing quickly with the large supply of food. Note: This is all my train of thought. As far as I know, there aren't any scientific studies to back me up. – J. Musser Jul 24 '14 at 3:29
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    I edited it back because the "is there evidence for" part is implied in every question here. – Oddthinking Jul 24 '14 at 18:16
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Nope. If insects ran rampant and ate everything, we could just eat them.

Could Less Meat Mean More Food?

“Nutritionally, it is excellent food,” says Arnold van Huis, an entomologist at Wageningen University in the Netherlands. “It’s the same or even better than conventional meat, fish, or poultry.” Just 100 grams of caterpillars can provide all of an adult’s recom- mended daily protein, along with iron, B vitamins, and other essential nutrients, he says.

Insect cuisine has become quite trendy, though the push is in its infancy.

There are bug cooking shows and even attempts to brew insect beer,

Insects provide plenty of nutritional value.

The nutritional value of fourteen species of edible insects in southwestern Nigeria This study revealed that some of the insects which are pests also have high nutritional qualities. Protein content of insects especially caterpillars has been studied from Central Africa (Richards, 1939), South Africa (Quinn, 1959; Dreyer, 1982) and South America Dufour, 1987)... The results of this study confirm the fact that insects are indeed a good source of protein and other nutrients. The consumption of non-toxic insects therefore, should be encouraged. Insects are traditional foods in most cultures, playing an important role in human nutrition and have much nutrient to offer. They can be reared for their high nutritional qualities and sold to the populace that regards them as delicacies.

The added protein would help because most countries suffer from a protein deficiency. In addition, the insect harvest would help contain or reduce the plant matter consumed, leaving enough to add variation to the human diet.

Like with everything else, humanity would adapt.

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    This answer assumes that we could control the bug population effectivly. If all predators that prey on bugs where removed this would likely lead to a maltheusian collapse - Of course that is not the case in the current discussion. However avoiding the bug population overshooting what is sustainable would be a serious difficulty. – Taemyr Oct 9 '14 at 14:31
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    Your references show that some insects are nutritious. They don't show that a viable population of humans would likely sustainably remain if spiders disappeared. – Oddthinking Oct 9 '14 at 15:48
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    Should one be asked to prove that Russel's Teapot doesn't exist? The question relies on the premise that humans must eat crops to survive. By offering evidence that insects could partially or completely assume the role of those crops, I've effectively pointed out that the very problem is its own solution. While there would be a slight decrease in caloric content over the plants themselves, there's no reason to assume that a shift in dietary sources would spell doom. The more far-fetched notion bears the greater burden of proof. – Don Kindred Oct 9 '14 at 16:57
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    What do you mean by inedible? Sure, many well fed people would turn their nose up at an apple bored through, but that doesn't mean that the unaffected parts are inedible. It also doesn't mean that the affected parts are. In ecological efficiency, the grasshopper is the same as the doe and the spider the same as the lion. – Don Kindred Nov 10 '14 at 16:11
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    Why do reasonable scientists not study auras? Because it's a stupid waste of time. Norman I. Platnick's livelihood depends upon spiders being relevant. The story is designed to get him funding from sensationalists that don't understand skeptical inquiry. A "mysterious mystery" is emotional BS. Data is truth. There is no data supporting this wild claim, so the only solution is to dismiss it out of hand until such evidence presents itself (as any sane person would). Sometimes, the right answer to a question isn't tons of evidence, rather a simple affirmation that the premise is really stupid. – Don Kindred Nov 10 '14 at 16:31

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