iPad mini 4. Photo: Apple

Apple's (AAPL 0.02%) share of the tablet market continues to slide. The iPad represented just 20.3% of the tablets shipped in the third quarter, according to research firm IDC, down from nearly 90% in 2010.

The tablet market as a whole has begun to contract, but Apple's share is falling faster than that of its rivals. IDC expects Apple's share to remain stable over the next four years, but continued losses could pose a challenge to the Cupertino tech giant's tablet business.

The iPad's dwindling share of the tablet market

Data from IDC tablet tracker

The iPad dominated the tablet market when it was introduced in 2010, as it faced no compelling competition. Alphabet (GOOG 1.06%) (GOOGL 1.08%) introduced Android 3.0 Honeycomb in February, 2011, the first version of Android designed specifically for tablets, and sparked a wave of innovation. Smaller, cheaper Android tablets prompted the introduction of the iPad mini in 2012, yet Apple's share of the tablet market has continued to contract while Android's has surged. Android will power about two-thirds of the tablets shipped this year, up from about one-third in the first quarter of 2011.

Windows 8, with its touch-based interface, may have also contributed to the iPad's decline, but Windows tablets are still relatively unpopular. Windows is expected to power just 8.4% of the tablets shipped this year.

IDC expects Apple's share of the tablet market to stabilize around current levels. The iPad will represent 25.9% of the tablets shipped in 2019 if IDC's projections prove accurate. Yet there's reason to doubt that outlook: in 2012, IDC believed Apple would sell about half the world's tablets in 2016. Although that's still possible, it's highly unlikely.

Not giving up on the iPad
The iPad has under-performed Apple's expectations for most of the last two years: in each of the last seven quarters, iPad unit shipments have contracted on an annual basis. Apple's management has reiterated its commitment to the product category, but it hasn't been able to stop the slide.

The iPad has consistently garnered strong reviews and high praise for its wealth of iPad-optimized apps (about 850,000). While Android tablet users have access to more apps, few are optimized specifically for tablets, leading to a substandard user experience. Continued declines in iPad market share put this perk at risk, as developers may eventually decide to ignore the platform or shift their resources to other devices if it reaches niche status.

The recently released iPad Pro has enjoyed a somewhat lukewarm critical reception, with reviewers citing (among other things) the lack of optimized apps. Re/code's Walt Mossberg wrote that "few apps [take] advantage of the greater screen real estate." This is understandable, given that the iPad Pro is a brand new device with a limited number of users. If demand for the iPad Pro picks up, this situation should change. If not, mobile developers may not waste their time updating their apps for a limited number of users.

Of course, the iPad isn't near that point yet, and may not hit it for some time. The iPhone, with global market share of around 15%, remains the preferred platform for mobile developers. But for the sake of Apple's tablet business, investors should hope IDC's projections prove accurate.