Spiders scare off insects and help plants boost seed production in exchange for dollops of sugary nectar, say scientists in New Jersey.
Many species of plant produce enticing nectar bonuses known as extrafloral nectaries (EFNs). The EFNs attract ants which defend the plant against leaf-eating insects in return for the sweet treats. Some tropical tree species can die without the ants' protection.
Only a few spiders are known to take an interest in EFNs. To find out more, Steven Handel and Scott Ruhren from the State University of New Jersey in New Brunswick studied the interactions between jumping spiders from the genera Eris and Metaphidippus, and a yellow-flowered legume (Chamaecrista nictitans) common to the eastern US.
Greenhouse experiments showed the spiders jumped onto plants with active EFNs six times more often than those without, and regularly ate the extra nectar. The plants also benefited from the spiders' presence in the field, where seed production rose by 8 per cent (Oecologia, vol 119, p 227). "This is the first evidence that spiders can increase plants' fitness in terms of seed production," says Handel. "It's a completely unexpected link in nature."
Most spiders sit and wait for their prey, says Handel, but jumping spiders move around and are aggressive-just like the ants that protect trees. Handel suspects the spiders play as vital a role as ants in protecting plants. "Most people hate spiders. Now there is finally one reason to like them," he says.
Author: Matt Walker
New Scientist magazine issue 15th May 1999
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