They found that there have been 164 bills aimed at removing protections for endangered species since 2011, or 33 per year, on average. That’s a marked increase from the previous 15 years, when lawmakers introduced 69 bills or riders, or an average of five per year.

For the study, Pang and Greenwald reviewed every stand-alone bill and budget rider introduced since 1996 aimed at undermining the Endangered Species Act or attempting to remove protection for individual species.

According to the website that accompanies the report, seven riders to the House Interior Department and environment appropriations bill have been introduced this month. If passed, the riders would block funding for the listing and protection of four endangered species: the Sonoran desert tortoise, the lesser prairie chicken, the northern long-eared bat and the Preble’s meadow jumping mouse.

“Since January of this year, there have already been 66 legislative attacks on endangered species — the most in any of the past 20 years,” authors Jamie Pang and Noah Greenwald write in the report. Since 1996, 233 bills and riders targeting the ESA have been introduced.

The Endangered Species Act (ESA) is under threat this year from a record number of bills and legislative riders that could undermine protections for dozens of species, according to a report released Tuesday by the Center for Biological Diversity .

Congressional debates over amendments to the Interior appropriations bill gave Greenwald and Pang the idea for the review. “It just became clear that the rate and number of attacks had grown substantially in recent years,” said Greenwald, endangered species director at the Center for Biological Diversity. “And it’s become more and more clear that there’s a well-funded industry campaign against endangered species,” which he attributes in part to the 2010 Supreme Court decision Citizens United, which opened the door to unlimited corporate campaign contributions.

Oil and gas industry contributions to Congressional lawmakers jumped from $10.5 million to $25 million from 2004 to 2014, corresponding to an uptick in ESA attack bills, the authors conclude.

According to the report, Republican lawmakers account for 93 percent of the bills and riders introduced since 1996, with five members of Congress responsible for nearly a quarter of ESA-related legislative efforts since 2011.

Rep. Ken Calvert, R-Calif., has introduced 9 bills since 2011, including riders to permit the import and export of ivory, as well as bills threatening protections for gray wolves, greater sage-grouse, northern long-eared bats and valley elderberry longhorn beetles. Since 2011, he has received more than $660,000 in campaign contributions from Big Agriculture and over $250,000 from oil and gas interests.

Sen. John Cornyn, R-Texas, whose bills have targeted four species of threatened Texas salamanders and the dunes sagebrush lizard and taken more general shots at the application of the Endangered Species Act, has received close to $3 million in campaign contributions from the oil and gas industry and more than $1.6 million from Big Agriculture, according to the report.

Neither Calvert nor Cornyn responded to requests for comment before time of publication.

Still, the report notes that many of the bills and riders are unlikely to pass. “I think, largely, it’s a scattershot approach, where they’re thinking some of them are going to stick,” said Greenwald. “Unfortunately, the one that maybe has some traction is targeting gray wolves. A rider passed in 2011 to delist gray wolves in Idaho and Montana, and that opened up a flood of hunting. Now they’re seeking to remove protections for wolves in Wyoming and the western Great Lakes.”

The ESA was passed by Congress in a unanimous vote in 1973 and still has the overwhelming support of the American public. According to the Fish and Wildlife Service, 99 percent of species protected by the act have avoided extinction.