Disney's Animal Kingdom turns 20 this morning. The animal-themed park opened to mixed reviews on Earth Day in 1998, over time evolving into a full-day destination worthy of critical praise. A few years ago, it passed Disney's Hollywood Studios -- a park that is now in the process of a massive makeover -- to become Disney World's third most visited park.

The park has come a long way. Disney's Animal Kingdom attracted more than 10.8 million guests in 2016, according to industry tracker Themed Entertainment Association. In a few weeks, we should get the estimates for what will be a record 2017 haul, following the debut of Pandora-The World of Avatar. Turnstile clicks bottomed out at 7.3 million in 2003.

However, the unwritten storyline in Animal Kingdom turning 20 is that Disney (DIS 1.54%) hasn't introduced another new theme park in Florida in that time. Disney World was christened with the 1971 opening of the Magic Kingdom, now the world's most visited theme park. It would go on to open a new park every 11 years or less -- until now.

Na'vi River Journey opened in May 2017 at Disney's Animal Kingdom.

Image source: Disney.

Five is the charm 

There's been plenty of speculation and online chatter over the years about a fifth theme park. Disney certainly isn't smarting for space given the massive size of its resort, and Disney World has built enough new on-site resort hotel accommodations to justify additional park capacity. Ticket prices have gone up every year under Disney's Animal Kingdom tenure. 

Right now, there may not be anything inherently wrong with its four-park approach. Disney's theme parks business was the only segment to grow revenue and operating income for the entertainment giant last year. However, with rival Universal Orlando closing the attendance gap since 2009 -- and likely to announce at least one new theme park itself in the coming years -- Disney may not have much of a choice.

Universal Orlando parent Comcast (CMCSA -0.52%) has been snapping up land just outside of its Florida resort over the past two years, and it now has more undeveloped land outside of Universal Orlando than it does in its current resort that features two theme parks, a high-tech water park, and what will be six hotels this summer. Comcast isn't playing around. 

Disney will be busy adding major new attractions to its four parks in the next three years, building up the resort in anticipation of a blowout 2021 when Disney World turns 50. However, tourist attention will then turn a few miles north to Universal Orlando, where Comcast will likely be building up a still-unannounced new theme park and several more hotels to go with the eight that will be open by 2021

Today, the spotlight is on Disney's Animal Kingdom, and rightfully so. The park that opened to mixed reviews is now more than just a cult fave. It has made smart ride, attraction, and dining decisions. The next three years also shouldn't be a problem as Disney puts out one e-ticket ride after another. However, once the last of those 50 candles go out for the Magic Kingdom, it will already need to be well into the development of its fifth theme park. Comcast is gaining on Disney, and you can't ignore the objects that are growing in your rearview mirror.