An extra 6,000 species of butterflies and other small creatures will be wiped out if their endangered plant and animal hosts go extinct, scientists say.

The addition of "affiliated species" means the biodiversity crisis is worse than thought, according to conservationists who added up the expected co-extinctions.

Researchers in Singapore, the United States and Canada used an international list of 12,200 plants and animals that are considered threatened or endangered as their starting point.

International trade in rare orchids contributes to the loss of species.

"What we wanted to learn was, if the host goes extinct, how many other species will go with it?" said biology Prof. Heather Proctor of the University of Alberta in Edmonton.

Using a mathematical model, the scientists added another estimated 6,300 dependent insects, mites, fungi and other species that rely on the hosts. These extra species weren't previously considered at risk.

Mites and parasites

The species aren't the cute and cuddly ones that tend to grab all the attention.

Many of the species facing co-extinction are mites, beetles and parasites, although Proctor noted 56 species of butterflies have already been lost because their host plants went extinct.

"It would be easy if there were always a one-to-one relationship with a host and its affiliate; however, not all parasites, for example, are restricted to a single host species," Proctor said in a statement.

"The trick was in trying to determine how many other species could act as hosts and factoring that degree of dependence into the study."

For Proctor, who said she "loves mites," the potential extinctions raise a moral reason to protect the original species.

The researchers noted species co-extinction reflects the interconnectedness of complex ecosystems.

"In view of the global extinction crisis, it is imperative that co-extinction be the focus of future research to understand the intricate processes of species extinctions," they wrote in Friday's issue of the journal Science.

"While co-extinction may not be the most important cause of species extinctions, it is certainly an insidious one."

Loss of habitat remains the main reason for extinctions, although the introduction of exotic pests and the international trade of wildlife like orchids also plays a role, Proctor told CBC Radio's As It Happens.

The study was funded in part by the Natural Sciences and Engineering Research Council of Canada.