OKAVANGO DELTA, Botswana—"Fifteen elephants right!" ecologist Mike Chase shouts above the roar of the single-engine Cessna. The plane swoops low over the herd of gray bodies as they wade single-file across the floodplain below.

We're flying over the channels and islands on the western periphery of the vast Okavango Delta, as part of the Botswana leg of an Africa-wide project known as the Great Elephant Census.

The yearlong project began in February and encompasses 18 countries—the first pan-African aerial survey of savanna elephants since the 1970s. (Forest elephants, concentrated in central and West Africa, aren't part of the census because they're mostly invisible from the air.)

Funded by Microsoft cofounder Paul G. Allen, the census will fill an urgent need, says Chase, who is founder and lead researcher for the project. "If we don't know how many are left on the continent," Chase asks, "how can we plan for the future? What is the baseline? Where do we focus our attention? Where does donor money need to be allocated?

"The truth is that the current population of elephants in Africa remains unknown," he says. "The last surveys in some areas were done 15 to 20 years ago."

According to a 2007 report, there could be anywhere from 472,000 to 690,000. But some experts believe the number may be as low as 250,000.

Founder of the Botswana-based nonprofit Elephants Without Borders, Chase has spent the past 15 years collecting unprecedented data on the status of elephants and other wildlife in Botswana, identifying cross-border elephant corridors, and discovering new migration routes.

So far, overflights of two of the 18 targeted countries have been completed, and surveys are under way in ten others.

"If we know more, then we have a chance of conserving the elephants," he says. "What we do know is that in 2013 we lost 96 elephants a day in Africa."

We also know, without any doubt, that elephants in most parts of their range are declining.

View Images VIRGINIA W. MASON, NGM STAFF SOURCES: ELEPHANTS WITHOUT BORDERS; AFRICAN ELEPHANT SPECIALIST GROUP, IUCN

Botswana: An Elephant Haven

Botswana is a happy exception to that trend. The northern part of the country, encompassing the entire Okavango Delta—an inland delta fed by the waters of the Okavango River, which evaporates in the sands of the Kalahari Desert and never reaches the ocean—has one of the few growing elephant populations left on the continent.

The best recent estimates, widely divergent and based on aerial counts in 2010 and 2012, puts Okavango's elephant numbers at 130,000 and 207,000, respectively.

Whatever the exact number, the patchwork of islands, channels, and floodplains of the Okavango are a seasonal refuge for thousands of elephants and other wildlife. This extraordinary landscape was designated UNESCO's thousandth World Heritage site on June 22, and its new status may make it an even safer haven for elephants in the years to come.

"The Okavango being a World Heritage site is a great benchmark in Botswana's natural history," Chase says. "I think [its World Heritage status] will boost the profile of the Okavango, increase tourism to a place where it's desperately needed, and inevitably I hope it helps with the conservation of this world heritage.

"It's one of the last sanctuaries for wildlife on the continent," he adds. "I'm hoping this survey can bolster the conservation efforts that it needs to safeguard its future."

View Images The Okavango Delta, with its patchwork of waterways, islands, lagoons, and channels, was recently designated the thousandth World Heritage Site. Photograph by Kelly Landen

Plane's-Eye View

I peer down at a large herd of buffalo scattered like peppercorns on a salty white island.

"Two hundred buffalo!" announces Kelly Landen, EWB's project manager. From her seat, she peers through the window between two long ski poles mounted perpendicular to the wings of the plane, framing a meter-wide field of view for the observers to spot elephants and other animals.

The census team comprises a pilot, two observers (or spotters), and one person who captures each sighting on a GPS device.

The total numbers of sightings—of elephants, as well as buffaloes, giraffes, and hippos—are crunched in a complex algorithm that also accounts for the size of the area surveyed and the number of transects flown. It's the standard methodology for strip-transect sampling, which has been widely used for aerial surveys of large African herbivores.

"We found a mega-herd of about 500 elephants last week," Landen says. "When elephants are stressed, they bunch together in large groups. The average herd size in the delta is between 10 and 20, but on the periphery, especially closer to human settlements, they face persecution, so they cluster in bigger numbers for safety."

View Images The Great Elephant Census also counts other large herbivores, like these giraffes dashing between islands in the Okavango Delta. Photograph by Kelly Landen

Elephant Refugees

"People say Botswana is not impacted by poaching, but in an indirect way, it is," Chase says. "Many of the elephants in this country are political refugees," fleeing from Zimbabwe and Zambia, where poaching is heavy, into the Okavango and southward into Makgadikgadi Pans National Park.

When Chase started studying elephants in the Okavango, very little was known about their seasonal migrations and dispersal patterns. His early research revealed that significant numbers of elephants were seasonally moving into southeastern Angola, repopulating the area after that country's civil war ended in 2002.

That insight, and knowledge of other elephant dispersal patterns, led to the establishment of the Kavango-Zambezi Transfrontier Conservation Area (KAZA), designed to link five African countries—Namibia, Botswana, Angola, Zambia, and Zimbabwe—into one huge international conservation zone.

But 15 years later, the picture has changed dramatically. "I was fortunate to study elephants during a time of peace for [them]," Chase says. "They enjoyed the freedom of Africa—they could move across international borders. This was the largest range for elephants on the continent—over 250,000 square kilometers [96,000 square miles].

"But with the recent resurgence in poaching," he says, "elephant movements have become very limited. They're not crossing international boundaries as frequently as they did five years ago. The elephants have simply moved out and sought the safety and security of Botswana."

Under the leadership of President Ian Khama, and those before him, Botswana has long been politically and economically stable.

This, coupled with a strong emphasis on conservation and ecotourism, has meant that elephants have rarely been persecuted, hardly ever poached, and—since the banning of hunting in public areas in early 2014—not killed for sport.

"Our elephants are a world heritage," Chase says, "but the habitat cannot sustain them. In Botswana they're free to roam wherever they like—into towns, communities, farmlands, and across borders into adjacent countries."

He worries that the elephants extending their ranges within Botswana will conflict with humans. "People are fed up because the population is bursting at the seams. But elephants are also the flagship species for the country's thriving ecotourism industry, so it presents a conservation quandary for the country of Botswana."

View Images Waterways, lagoons, and islands make perfect habitat for hippos—also a focus of the Great Elephant Census—in the Okavango Delta, which has one of the largest populations of these behemoths in Africa. Photograph by Kelly Landen

Nature's Transformers

Chase is also concerned that the fragile Okavango Delta won't be able to sustain the large numbers of elephants that have moved in.

Longtime Botswana ecologist Larry Patterson, who works with EWB as a vet, says there were very few elephants in the Okavango Delta in the 1970s. But now, with the population growing fast, he says they're changing the delta's ecology.

"There are fewer baobab trees. There are fewer knob-thorn trees. In the 70s you could fly along the rivers in September, when the knob-thorns were in flower, open the plane windows, smell the blossoms, and see a pale green band along the edges of the rivers. But those trees are nearly gone. So these mega-herbivores are having an impact on the delta ecology."

Given the impact of large numbers of elephants on both land and people, the Great Elephant Census comes at a critical time. Exactly how many elephants are there? And how many elephants are too many?

"This is an extremely important leg of the census," Chase says. "There's a lot of pressure on us to get it right. We've kicked off the survey by studying the marginal areas around the delta and still have much of northern Botswana to cover."

He expects the full results of the Okavango survey to be released in mid-2015.

The Cessna lurches awkwardly in the wind, leaving my stomach behind. It's hot in this airborne tin can, and the noise of the propeller is incessant.

View Images Elephants, as well as hippos, carve out watery pathways through the reed beds in the Okavango Delta. Photograph by Paul Steyn

The team still has the interior of the Okavango to cover—many hours of flying over extensive wilderness areas, such as Chief's Island and Moremi National Park, where the density of wildlife is sure to be even greater than here on the delta's western edge.

The Cessna banks, and we head back to camp for the afternoon.