Thursday, 8 September 2011

The roles of elephants...

Elephants, Tarangire NP, Aug 2011
As you've probably guessed, I've been away again, so thanks to Ethan for sharing his discoveries whilst I was away. I'm sure we'll come back to the migration again soon (especially as I'm hoping to be in the thick of it again this weekend!), it's such a fascinating subject. Meanwhile, part of my travels took us to Tarangire where the elephants can never fail to impress, so in a rare forray into the world of the 'big 5', here's a post about elephants... The Tarangire elephants are a population fast recovering from the poaching of the 1980s (though I'm sure some still goes on at times) - in 1960 there were only 440 animals in the park, by the last full census I can find numbers for in 1996 there were 2000. Many of these early arrivals migrated into the park from outside to escape the even heavier poaching in peripheral areas and have since become resident (or semi-resident) within the boundaries.  But since 1993 the closely monitored population in the north of the park has continued to increase at about 7% per year (pretty close to the maximum theoretically possible, given gestation rates, etc.), which is rolled out over the whole park to 2011 would give about 5500 animals. A not unreasonable estimate I'm told. As the park has an area of aroud 2850km2, that gives a density of nearly 2 animals per square kilometer. Compare that to the densities during periods of regular culling in Kruger NP of around 0.4 animals in the same area, and you can see the extremely high densities present in Tarangire.
Eles love to wallow - digging waterholes as they do and removing up to 1 m3 of soil a time.

In actual fact estimating densities of any animal is trickier than you might imagine - they're certainly not unifrom across the landscape, with local concentrations in certain areas, or in different seasons. So it's fairly hard to make direct comparisons of densities across different National Parks, but it's pretty clear that Tarangire is certainly among the top two or three elephant parks in Africa. So the question I'm innevitably asked, is what is the impact of these elephants on the landscape? Weighing in at around 3000kg and eating as much as 200kg of food per day, elephants can have a massive impact on the landscape - add to that the fact they're pretty good a toppling tasty looking trees (generally across the quieter tracks I like to frequent, it seems!) and there's a lot going on. In some corners of Africa it is certain that they've had massive impacts on vegetation - creating rather unsightly bare areas around permanent waterholes and rivers. However, whilst tourists might not like these places, increases in elephants are often associated with similar increases in buffalo and impala, and the biological impacts are not all negative. It's also difficult to discuss issues of elephant density from a well informed basis as we don't actually have any real idea about the starting conditions before massive hunting for ivory - most of Africa's elephants were hunted out alongside the slave trade in the 1800s, so even in the high density areas of today we really don't know how this compares to densities of even only 200 years ago, nor do we know what the environment looked like particularly well back then.

Tarangire Elephants deep in the swamp keep the water open. Aug 2011
In Tarangire, however, the elephant population increase has occurred at the same time as the density of trees in the park has increased (for reasons we might ponder in a later post), and whilst they certainly leave their mark on the baobabs, there's little evidence of major vegetation changes as a consequence of incredibly heavy elephant browsing. So, for now I'm going to skip discussions of potentially negative impacts of elephants and will illustrate just one of their particularly beneficial aspects that was plainly on view in Tarangire - their role in keeping waterholes open.

Open water created ideal habitat for water birds: Silale Swamp, Tarangire
From the picnic site I counted more than 380 elephants enjoying the Silale swamps - they were there for food and water, of course. But in the process, they keep the edges of this swamp free from vegetation. Elsewhere along the river their rolling and wallowing  was keeping pools of water open much more than would be possible without them (each animal can walk off from a mud bath with up to 1m3 of mud attached, a volume that takes me a serious effort to move!) - indeed, elephants are capable of digging in sand rivers to access the water (and at times salt) well over 1m below ground. So they create waterholes and maintain open, vegetation free areas in swamps (a role often also played by hippos, but not in Tarangire). They're also great fun to watch splashing in the water, and as well as explaining how fussy they are about the cleanliness of the water they like, it's also worth talking about how their activities benifit all the other animals around that need water too. They are certainly worth of the name 'eco-system engineer' as well as that of a 'keystone species'.

When you're tired of elephants you'd better stop guiding... Tarangire May 2011

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