"Lost Worlds: Life in the Balance" is a timely reminder of the importance of biodiversity and an occasionally breathtaking demonstration of the IMAX format.

Veteran documentarian Bayley Silleck ("Cosmic Voyage") has recruited some formidable talent to help explain why urban dwellers shouldn't take the world outside their cities for granted. In addition to some informative commentary from Venezuelan scientist Margarita Lampo, Harrison Ford's familiar voice guides viewers on a 40-minute tour that stretches from Alaska to New York City to the jungles of Brazil.

The most compelling moments in "Lost Worlds," however, are those where the camera does the talking. "Lost Worlds" opens in what's left of the Mayan city of Tikal in modern day Guatemala. Even though no one has occupied the pyramids for centuries, these buildings still have a formidable presence, which seems amplified on the IMAX screen. Despite the fact that thousands buildings once stood in the area, the forest has now reclaimed most of the city.

The film then moves to New York, and asks if the current metropolis can avoid the same fate. While the city's concrete and metal seems self-contained, a computerized tour through the city's plumbing reveals that the water that sustains New York actually originates for the Catskills, nearly 100 miles away.

Without the naturally cleaned water, the city would have to depend on expensive purification systems. The script by Amanda McConnell and Sugith Varughese then explains that maintaining the natural balance is imperative for urban survival.

A good portion of the rest of the film follows Lampo as she visits the Tepuis Mountains, which are found in Venezuela, Brazil and Guyana. She's on a quest to see how toads are surviving in the region. The animals can be an important barometer for how the rest of the ecological system is doing.

The idea of searching for toads might seem rather dull, but the Tepuis (or Table Mountains) prove a gorgeous and surprisingly lively subject for IMAX photography.

Spectacular cliff sides dart past the screen, and many of the animals and plants that live there can be found nowhere else. It's not surprising that the same cameras that captured the world's tallest mountain in the IMAX hit "Everest" were used to make this one.

In addition to making thumb-sized toads and insects appear to be the size of SUVs, the 70mm film really vividly captures the bizarre colors that many of the creatures have.

At times, one wonders if "Lost Worlds" might have benefited from having no sound at all. Ford has donated millions of dollars from his movie earnings to the cause of conservation, so it's not surprisingly that he sounds earnest throughout the film. Nonetheless, narration is not the actor's strong suit (remember the awful voiceover that used to accompany "Blade Runner?"). He's not helped by some of the cornier passages ("Food comes from a supermarket, and water from a sink. Or does it?").

It also might have been interesting to explain how nature makes it possible for New Yorkers to drink the water rather than simply moving on to another locale. Nonetheless, the makers of "Lost Worlds" can be forgiven for trying to make their words as spectacular as their images.