Sarah Tew/CNET

The iPad has not just taken over as a big seller and profit driver for Apple over withering iPod sales, it's also become a major point of entry for customers into the Apple ecosystem, a new study says.

The NPD Group today released the results of its Apple Ecosystem Study, which polled more than 3,000 people in the U.S. during February. One of its main findings was that one in four iPad owners said it was their first Apple device.

"Historically, the iPod has been the introductory Apple device for consumers, with 82 percent of owners saying it was their first Apple product," NPD said in a release. "This, however, is changing as first-time Apple buyers gravitate toward other product lines."

That "introductory Apple device" reference was once dubbed "the halo effect," a term used to refer to a bump in sales of Apple's other products -- though mainly Macs -- when buyers would pick up one of Apple's iPods.

Since the iPhone and iPad, however, iPod sales have been on a decline, with buyers picking more phones or tablets. The iPad has even done the reverse of a halo effect, and led to cannibalization of the Mac, which Apple has countered by saying that those iPad sales could have gone to PCs instead.

"While over 70 percent of long-standing Apple owners began their relationship with the brand by way of the iPod, this number declines to just 57 percent among those entering the Apple franchise in the past two years," NPD said. "Newcomers to the brand increasingly turn to the iPhone or iPad as their first Apple device, which combined account for one-third of first-time Apple purchases since 2010."

Other findings from the study echo those by a CNBC study posted last month on the number of Apple products found in U.S. households. For instance, NPD said that a household with Apple products tends to own 2.4 Apple devices on average. But that 58 percent of households that have a Mac also have a PC.