Airline fares have been declining and the big three carriers have been quick to blame competition from ultra-low-cost carriers to the point that all three are making plans to introduce or expand "basic economy fares" in order to compete.

But Spirit (SAVE) - Get Report CEO Bob Fornaro said it's not the ultra-low-cost carriers, primarily Spirit and Frontier, that are ramping up the ultra-low fares; rather, it's the legacy carriers themselves.

"They've extended it beyond Spirit -- it's among themselves," Fornaro said Tuesday on Spirit's first-quarter earnings call. "It's the reason {they} are not seeing improvement" in passenger revenue per available seat mile.

Moreover, Fornaro said, once basic economy fares are implemented, they are likely to help Spirit, not hurt it.

American (AAL) - Get Report , Delta and United continue to report that PRASM is in negative territory -- which is to say that it continues to be lower than it was a year earlier. This pattern has persisted for about two years. On their first-quarter earnings calls, Delta and United said they hope to get to break-even PRASM this year.

Shortly after Fornaro spoke Tuesday morning, the Bureau of Transportation Statistics released new data showing that in the fourth quarter of 2015 the average domestic air fare fell 8.3% to $363, the lowest level since 2010. The underlying causes of declining fares have primarily been declining fuel prices; additionally, in certain sectors such as close-in bookings, demand has been spotty.

Ultra-low-cost carriers have also played a role. Fornaro acknowledged that last year increased Spirit capacity at Dallas Fort Worth International Airport, combined with Southwest's buildup at Love Field, led American to cut Dallas fares to compete.

On American's third-quarter 2015 earnings call, President Scott Kirby said Spirit overlaps American on 28% of its domestic seat miles at DFW, making it American's biggest competitor at its biggest hub.

"Given that 50% of our revenue is up for grabs in these markets, and these carriers have had so much success when they weren't matched, we know that we have to match their fares," Kirby said.

But Fornaro said that while fare cutting by major carriers may have started in Dallas, "since the third quarter and even the fourth quarter, it's spread throughout the industry.

"There's been a broadening of price competition over the last year," he said. "It's in most places. It is in Chicago, it's in the West Coast {and even} in Charlotte and other places. You see that in the legacy carrier results.. They've extended it beyond Spirit. It's among themselves.

As for basic economy fares, Fornaro said. "It's a coach seat on a legacy carrier with no frequent flier points and no seat assignment ... It's not a new product. It's as simple as taking away maybe a couple of items."

Fornaro said he fails to see the competitive benefit. In reality, he said, "the only thing that really matters is the number of seats the legacy carrier allocates to that fare class ... The number of seats they put out against us will determine the impact on us.

"If they keep the same number of seats out there, then it's neutral. {But} I say it will be neutral to positive. The more low prices a legacy carrier charges on a walk-up basis, the more difficult it is to manage the revenue."

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.