Spiders and birds share insect declines

A black and yellow garden spider

Spiders and birds face some of the same problems in the world we have shaped.

You probably are unaware of spider problems. You probably don't think of spiders at all until you see one in your house. The collapsing world of insects, however, poses the same threat to spiders as to birds.

"Spiders of the World, a Natural History," a new book from Princeton University Press, gives a thorough overview of the world's spider species. They are estimated to number about 48,000, with perhaps as many yet to be identified and named.

"Spiders are among the most dominant predators in almost all of the earth's terrestrial ecosystems," according to the book's introduction.

They are estimated to consume between 400 million and 800 million tons of prey — mostly insects — each year. Hundreds of millions of tons.

Spiders, however, by their size and reclusive nature cannot easily, or perhaps at all, be tracked to determine the extent of loss. And, most spiders, all of them predators, are generalists, eating other creatures as well.

Asked about spider declines, Dr. Gustavo Hormiga, one of six authors contributing to the book, believes insect losses certainly have touched the spider world. He said in an email, "It is virtually impossible that the insect declines do not have an associated decline in spider populations."

The decline in spider numbers is due not only to the importance of insects as a prey item, Dr. Hormiga said. Like birds, deforestation, pesticides, large industrial-scale monocultures, and other issues contribute to the insect loss that impacts spiders.

How large is the decline? What species are involved? It's difficult to know.

"For the immense majority of spider species we know very little beyond the original species description," Dr. Formiga wrote.

Although spiders hunt in many ways, flying insects are a large part of the diets of many spider species. For that reason, the pattern of decline is probably more evident in spiders that use webs for capture, he said.

The book does not address the insect issue. It's an intriguing and illustrative introduction to and overview of the spider world. It uses as examples members of the 115 spider families.

Minnesota has nine spider species that build webs and seven species that don't, according to the University of Minnesota extension service.

One of the most beautiful is the black and yellow garden spider, a large builder of webs. The cellar spider is common, resembling what many of us call daddy longlegs.