(NewsNation) — Mexican drug cartels make billions smuggling humans across the southern border, but in recent years, another lucrative crime has taken off: wildlife trafficking.

In March, authorities in Texas arrested a 29-year-old Mexican man after he tried to enter the U.S. with two live howler monkeys in the back of his pickup. Less than a week later, a woman was caught attempting to smuggle 21 parrots and a keel-billed toucan into California.

It’s unclear if either person was directly tied to a cartel, but both incidents offer a glimpse into an illegal wildlife trade that is now the fourth largest funding source for criminal organizations, generating about $23 billion each year, according to the Department of Homeland Security (DHS).

In November, the International Criminal Police Organization, INTERPOL, warned that the growing issue has pushed many species “to the brink of extinction.”

Online marketplaces and social media have made the problem worse. Nowadays, it’s significantly easier for consumers to get wild animals on the black market, many of which arrive from outside the country through U.S. ports of entry.

From 2018 to 2021, wildlife trafficking surged more than 150%, according to a Moody’s Analytics report, which cited government data. The illegal trade has become a breeding ground for corruption and a major source of profit as more “structured cartels enter the space,” the report said.

Wildlife trafficking from Mexico to China has helped fuel the drug trade, with cartels trading wildlife for chemicals used to make illicit fentanyl, according to the Brookings Institution.

Latin America is especially vulnerable because of its biodiversity. Ecuador, for example, has about 1,600 species of birds, and Brazil hosts between 15% to 20% of the entire world’s wildlife diversity.

Some animals, like spider monkeys, can fetch upwards of $8,000 in the United States. Last summer, California border authorities confiscated three baby spider monkeys at the Calexico-Mexicali crossing. Just two months old, they were underfed and in poor condition but eventually nursed back to health by specialists at the San Diego Zoo Wildlife Alliance.

To combat the growing problem, DHS established a new Wildlife and Environmental Crimes Unit in 2023. That team is focused on enforcing anti-wildlife trafficking and environmental crime laws.

There’s also been a push to help trafficked animals after they’re confiscated. In October, the U.S. Fish and Wildlife Service (FWS) partnered with the Association of Zoos and Aquariums to launch the Wildlife Confiscations Network in Southern California. The network makes it easier for law enforcement to find housing for rescued animals.

In 2022, FWS special agents and other law enforcement partners investigated over 10,000 wildlife trafficking cases and collected over $11,000,000 in criminal penalties, the agency said.

A study published in April found that better wildlife screening tools, which are “severely lacking,” could help authorities crack down further.

“Currently, wildlife seizures predominately rely on prior intelligence as opposed to active surveillance methods, thus seizures reported likely represent a very small percentage of all smuggling attempts,” researchers at the University of Adelaide found.

Specifically, wildlife detection dogs are becoming more common because they can sniff out distinct scents like reptiles and birds. In fact, it was a K-9 unit that detected the nearly two dozen exotic birds heading into California in March.

For now, the U.S. remains one of the world’s largest markets for trafficked wildlife, in part because the “size and scope” of the country’s financial system makes it “ideal for bad actors to pass their illicit funds through,” Moody’s said in its report.

In that sense, addressing the illegal animal trade could be crucial in the battle against Mexican drug cartels in addition to protecting endangered species and threats to human health stemming from the transmission of disease.