Let me begin, before I offend some folks, that I was born and raised in South Dakota where the Ring Neck Pheasant is the state bird and I spent a lot of years hunting Pheasant, in addition to Sharp Tailed Grouse and Greater Prairie Chicken (both species collectively referred to as "prairie grouse" in SD).
Pheasants have minimal impact on Sharp Tailed Grouse populations, mostly because a Sharp Tailed Grouse will almost always kick Pheasant butt if the two come in conflict and get in a fight over forage or breeding areas.
However, a Greater Prairie Chicken *always* loses a fight with a Pheasant with the result that Pheasants have played a significant role in the decline of Greater Prairie Chicken populations.
Interestingly, Sharp Tailed Grouse are very tolerant of the Greater Prairie Chicken, and conflict between the two species is extremely rare. Meaning Sharp Tailed so not have any significant impact on breeding and forage access of the Greater Prairie Chicken. They *could* but they *don't* as they are not intolerant jerks like the Pheasant.
The point here being that despite its widespread popularity, the Ring Neck Pheasant does have a negative impact on the native Greater Prairie Chicken and does meet the definition of a invasive species. It's just that state game management agencies all look the other way as there's big money in pheasant hunting.
Personally, I don't enjoy hunting Pheasants more than "prairie grouse" and I'd be just as happy if the Ring Neck Pheasant had never been introduced in the US or were eliminated. We'd have more grouse and in particular more prairie chicken to hunt and life would still be good.
Flame away....