Lions finding so-called lion habitat not all that habitable.
A paper out yesterday in the journal Biodiversity and Conservation by Nicholas School graduate Jason Riggio (now at the University of California, Davis) and colleagues uses an extensive dataset and high-resolution satellite imagery to demonstrate that much of what we thought was suitable lion habitat is not. If you care about lions, that is probably pretty depressing news, but it's also news we can use -- to design more effective strategies to save the world's big cats.
Not Your Father's Savannah
The story starts with African savannahs because that's where lions make their homes.
Savannahs are generally defined as tropical and subtropical grasslands with scattered trees. They typically experience a strong rainy season and a strong dry season and are often transitional zones between deserts and jungle. (To learn more about savannahs, check out this page and this multimedia primer.)
Savannahs are especially prevalent in Africa, covering much of the non-Saharan and what we think of as Africa's tropical forests (the non-wet ones) -- in all about 5.2 million square miles or almost 50 percent of the total continent.
Good for the lions, right? Not exactly.
With the growth in human population, people have been increasingly encroaching on the savannahs at the expense of wildlife, including lions. Lion populations have been on the decline. In just a half century lion numbers have plummeted -- from around 100,000 in 1960 to about 33,000, according to the last African-wide survey (PDF) in 2006.
Have things changed much since 2006? Riggio and his co-authors set out to find out, and in so doing ran into an unexpected puzzle. Not a puzzle of numbers but one of geography.
The authors’ field observations of the savannahs that were, according to the existing maps, supposed to be prime lion habitat were actually not.*
"Existing maps made from low-resolution satellite imagery show large areas of intact savannah woodlands [where lions should have but were not inhabiting]. Based on our fieldwork in Africa, we knew they were wrong," explained Riggio. "Using very high-resolution imagery we could tell that many of these areas are riddled with small fields and extensive, if small, human settlements that make it impossible for lions to survive."
Overlaying population data on top of their own map built from high-resolution imagery -- population data, I should add, from both their work and more than 40 mainly country-specific reports since the last assessment -- Riggio et al. found that free-ranging lions inhabited only a quarter of the potential 5.2 million square miles of savannah.
This is significant because the authors believe that, as a top predator, lions serve as a proxy for ecosystem biodiversity. Areas with lions would be expected to be relatively intact. Stated differently, this means that only 25 percent of African savannah has not been disturbed, disrupted, and/or reshaped by growing human population.
And while their estimate of the total lion population -- between 32,000 and 35,000 lions -- isn't that different from previous estimates, the locations of the lion communities they found are significantly different. Because of savannah fragmentation, they are spatially much more constrained than previously thought and include very small communities that may not be viable.
The authors found that today's lions are dispersed between 67 discrete areas of which only 15 hold 500 or more lions. Of those, only 10 areas (holding about 24,000 lions in toto) are thought to have the potential to support lions for the long haul. The authors further specify that another 10,000 lions live in less viable habitats. None of the lion areas with long-term potential (so-called strongholds) are located in West or Central Africa.
The Upshot of the Research?
Using an "updated geographical framework," Riggio et al. have created a map that they believe "contains our best estimates of lion areas -- places that, as best we can tell, likely have resident lion populations." If folks are serious about saving the lions, they will need to get serious about protecting those areas.
* In this study the authors defined a savannah as various biomes areas (such as grasslands, dry woodlands, etc.) that receive between 11 and 60 inches of annual rainfall.