Network and cable television viewership dropped last year, and at least one finger is pointing at Netflix NFLX as a major reason for the decline.

The streaming service and its rivals are churning out original shows and licensed content, giving more and more viewing options for viewers who might otherwise be surfing hundreds of cable channels. And, in 2015, Netflix’s ever-expanding user base accounted for roughly half of the 3% overall decline in U.S. TV viewing, according to a new study from analyst Michael Nathanson of MoffettNathanson titled “Is Netflix Killing TV?”

According to Variety, Nathanson arrived at his conclusion after calculating that the 29 billion hours of video streamed last year by Netflix’s 44 million U.S. subscribers equaled 6% of total American live-plus-7 TV viewing (reported by Nielsen) last year. Nathanson also predicts that the hours of video streamed on Netflix will continue to increase in coming years, growing to represent 14% of overall TV viewership by 2020.

Nathanson added that Netflix compiled more viewing hours in 2015 than small cable networks like A+E and AMC, while major broadcast networks such as ABC, CBS CBS , and NBC saw major drop-offs in viewership among households that subscribe to Netflix.

Of course, those networks aren’t the only ones facing a growing challenge, as Netflix’s streaming rivals are also gathering steam. Amazon AMZN now boasts more than 40 million Amazon Prime subscribers, though it is hard to tell how many of those members are using the subscription to watch online video content rather than just taking advantage of free two-day shipping.