I have canoed extensively throughout the 30,000 Islands of Georgian Bay, Ontario, and have marveled at the unusual abundance of spiders found on the outer islands such as the McCoy Islands southwest of Pointe au Baril. Many varieties of spiders inhabit the islands, and in far greater concentrations than on the mainland. Webs festoon virtually every branch of every tree on the islands, and spiders crawl all about the ground, ready to lay egg sacs on any item left unattended for just 30 minutes! Since the outer islands generally have fewer insects that could be food for spiders due to windy conditions there, I am at a loss to explain why so many more spiders inhabit the islands compared to mainland areas. Birds that prey on spiders can easily access the islands too, so there should not be a lack of predators. Any thoughts from your contacts with entomologists?
I would guess it has to do with an exceptional abundance of small prey for the spiders.
I should think that a wide diversity also allows for an abundance of spiders, but smaller stuff like planthoppers and springtails that live lower down are probably a good food source as well.