Vultures are more valuable than you may think, or at least they were.

In the 1980s, more than 40 million vultures existed throughout India, where they ate about 12 million tons (11 million metric tons) of rotting flesh each year, according to the environmental writer Tony Juniper. Today, however, vulture populations have been reduced to only a few tens of thousands, and three of the most important species are listed as critically endangered by the International Union for Conservation of Nature (IUCN).

They have declined largely because ranchers started giving their cattle an anti-inflammatory drug called diclofenac that is toxic to the birds, which eat dead cattle, Juniper writes. Without vultures to eat this festering morass, wild dogs have taken their place and populations have boomed. The dogs have, in turn, spread rabies by biting humans, killing an estimated 50,000 people in the last couple decades, according to Juniper.

"Deaths of course have much more than financial consequences, but they do have financial consequences, not least for the families of the dead" and the Indian economy, writes Julian Champkin at Signifance, the bimonthly magazine and website of the Royal Statistical Society. This cost is an eye-watering $34 billion, according to Juniper.

Once described as the world's most plentiful raptor, the critically endangered white-rumped vulture (Gyps bengalensis) has declined by more than 99.9 percent in just 15 years, the IUCN reports.

Vultures, disliked as they may be for their motley appearance and association with death, once provided billions of dollars worth of services for humanity. The same is true for other animals, and other aspects of the ecosystem (like rain forests), writes Juniper in his new book "What Has Nature Ever Done For Us? How Money Really Does Grow on Trees" (Profile Books, 2013). He argues that ecosystems have created all of the things that humans value, including humans themselves and our markets; therefore, economic values are meaningless if they don't consider environmental impacts.

"The truth is that the economy is a wholly owned subsidiary of ecology, not the other way around," he said.

There are several captive breeding programs to reintroduce vultures to the wild, according to the IUCN. 

Reach Douglas Main at dmain@techmedianetwork.com. Follow him on Twitter @Douglas_Main. Follow LiveScience on Twitter @livescience. We're also on Facebook & Google+.

Related on LiveScience and MNN:

This story was originally written for LiveScience and is reprinted with permission here. Copyright 2013 LiveScience, a TechMediaNetwork company.