You might be wondering why President Obama is announcing Tuesday he will use his executive authority to expand the Remote Pacific Islands National Marine Monument, a vast stretch of the central Pacific Ocean. Here are a few reasons:

1. With marine reserves, bigger is often better. The original monument, established in 2009, is already nearly 82,000 square miles. But many scientists--such as Lance Morgan and Elliott Norse of the Redmond, Wash.-based Marine Conservation Institute--argue that the ecological benefits expand exponentially when sanctuaries are enlarged, both because they allow species to move freely and because they are easier to enforce. The possible expansion would encompass nearly 782,000 square miles.

2. Underwater mountains matter. Seamounts--massive mountains that lie beneath the ocean's surface--are hotspots of biodiversity. There are anywhere between 40 to 51 in the current protected area, and that number would reach between 241 and 251 if the president extends the reserve to 200 miles surrounding each of its seven islands and atolls.

3. Since it's devoid of people, animals thrive there. Almost everywhere in the world, small fish outnumber big fish. But in places such as Palmyra Atoll and Kingman Reef, scientists have found the biomass of large predators such as sharks outweighs that of smaller fish. The area--which also includes Wake, Johnston, Jarvis, Howland and Baker Islands--also features five species of protected sea turtles and 22 species of protected marine mammals as well as several million seabirds who gather there.

4. There isn't much commercial activity there, and it can move elsewhere. The fish caught in this region accounts for between 1 and 3 percent of the U.S. tuna catch in the central and western Pacific, according to the Pew Charitable Trusts, and these vessels can shift their fishing activities.  The industry may still object to the expansion, however, which is one of the reasons why the White House is seeking public input before making a final decision.

5. There is a global contest for bragging rights when it comes to creating marine reserves. At this point, environmentalists are hoping President Obama will be tempted to trump George W. Bush's record as the U.S. president who has protected the most area in the ocean. If finalized, this would become the world's largest no-take marine reserve. Britain, which currently holds the record for fully protecting the biggest swath of ocean around the Chagos Islands, is now looking at putting an area around Pitcairn Island off limits. The president of Kiribati, a small island nation in the central Pacific, announced Monday he will end commercial fishing in the Phoenix Islands Protected Area, which is roughly equal to the size of California, while the president of Palau has vowed to fully protect 80 percent of his nation's waters. Emily Woglom, vice president for conservation and policy at the Ocean Conservancy, said in a statement that Obama's announcement "shows that the President is turning his attention to his Blue Legacy."

6. It's tradition. Both Republican and Democratic presidents have used their executive authority to safeguard parts of the Pacific Ocean for more than a century. Theodore Roosevelt started it when he placed Midway Island under the protection of the Navy to stop the killing of seabirds there for their eggs and feathers, and then he helped usher through the Antiquities Act of 1906 to ensure his successors would also have the power to provide heightened protections for federal land and waters without congressional approval.  Later presidents, starting with Franklin D. Roosevelt, imposed additional restrictions. George W. Bush established the national monument Obama now intends to expand.