As the global market for tablet computers continues to grow, research firm IDC is reporting that Apple’s share has shrunk from 59.7% last year to 50.4% today.

Devices from Samsung, Amazon and Asus, representing the biggest producers of Android tablets, have moved in to take a larger slice. The trend mirrors that seen in the smartphone sector where the iPhone’s once unassailable dominance has been superseded by phones using Google’s operating system.

However, the tablet market overall is continuing to grow and the Cupertino-based company is continuing to ship more iPads.

The IDC report says the entire tablet market grew 49.5% between the third quarter of 2011 and 2012. The iPad itself saw 26.1% growth in sales in this period compared to the new Asus-manufactured Google Nexus 7, which grew by 242.9%.

“Competitors are turning up the pressure on market leader Apple,” Ryan Reith, program manager at IDC said in a statement. “With the recent introduction of a number of Windows 8 and Windows RT tablets, consumers now have a third viable tablet platform from which to choose.”

Apple has dominated the tablet computer market after it virtually reinvented the industry with the release of the first iPad in 2010. However, companies such as Samsung have been quick to release similar devices, and today the South Korean firm controls 18.5% of the market, according to IDC.

That’s a massive 325% growth in shipment compared to this time last year for the firm.

Apple also faces challenge from companies producing much smaller devices. Both Amazon’s new Kindle Fire and the Nexus 7 are 7-inches, compared to the iPad 4, which is 9.7-inches measured diagonally across its screen.

The smaller devices are seen as much more portable and are priced far below the iPad. The cheapest Kindle Fire, for example, costs only $159 and the Nexus 7 comes in at $199. By contrast, the iPad 4 starts at $499.

IDC speculates that some of the fall in market share for Apple was down to people holding out for the iPad Mini, which at 7.9-inches looks to directly compete with the smaller tablets.

“Now that the new mini, and a fourth-generation full-sized iPad, are both shipping we expect Apple to have a very good [fourth] quarter,” IDC’s tablet research director Tom Mainelli said.

There is plenty of room for Apple to maneuver as it goes through its latest upgrade cycle. As long as there is great enthusiasm that turns into sales for products like the iPad Mini, Apple will keep its share, although it will dip and then rise from time to time.

However, there's one thing to note - the cheapest Mini starts at $329. So it remains to be seen whether Apple can still claw its way back with its latest offering.