As the number of humans on Earth has nearly doubled over the past four decades, the number of bugs, slugs, worms and crustaceans has declined by 45 percent, researchers said Thursday.

Meanwhile, the larger loss of wildlife big and small across the planet may be a key driver of growing violence and unrest, said another study in the journal Science as part of a special series on disappearing animals.

Invertebrates -- creatures without backbones -- are important to the Earth because they pollinate crops, control pests, filter water and add nutrients to the soil.

Among animals with backbones that live on land, 322 species have disappeared in the past five centuries, and the remaining species show about a 25 percent decline in abundance, said the findings.

"We were shocked to find similar losses in invertebrates as with larger animals, as we previously thought invertebrates to be more resilient," said co-author Ben Collen of University College London.

Researchers blame the decline of invertebrates on two main factors: the loss of habitat and the changing global climate.

This planet-wide decline in wildlife may be causing more violent conflicts, organized crime and child labor around the world, researchers said.

The reasons for the spike in unrest could come from food scarcity and loss of jobs, resulting in more human trafficking and crime, said the findings led by scientists at the University of California, Berkeley.

"This paper is about recognizing wildlife decline as a source of social conflict rather than a symptom," said lead author Justin Brashares, associate professor of ecology and conservation at UC Berkeley.

"Billions of people rely directly and indirectly on wild sources of meat for income and sustenance, and this resource is declining."

For instance, the study noted that the rise of piracy in Somalia arose from battles over fishing rights.

"For Somali fishermen, and for hundreds of millions of others, fish and wildlife were their only source of livelihood, so when that was threatened by international fishing fleets, drastic measures were taken," said co-author Justin Brashares.

Researchers also pointed to the rise in trafficking of elephant tusks and rhino horns as evidence of a burgeoning criminal industry linked to disappearing wildlife.

"Losses of wildlife essentially pull the rug out from underneath societies that depend on these resources," said co-author Douglas McCauley, an assistant professor at UC Santa Barbara.

"We are not just losing species. We are losing children, breaking apart communities, and fostering crime. This makes wildlife conservation a more important job than it ever has been."