Tuesday, 14 February 2012

Tarangire wildebeest migration

Tarangire wildebeest on the move, Sep 2011.
Following the ATBC / SCB conference in June I mentioned a talk by Thomas Morrison on the movements of the wildebeest in Tarangire. The Tarangire migrations is, of course, tiny in comparison to the better known Serengeti migration and involves a different race of wildebeest (C. t. mearnsi in Serengeti, C. t. albojubatus in Tarangire) , but it's just as interesting to understand, and Tom and his supervisor Doug have recently published some work describing the movement that was covered in the conference talk. Until fairly recently, Tarangire was home to a large wildebeest population, though only around 6000 remain today. It's still one of my favourite places to visit though... These animals move into Tarangire in the main dry season (arriving in June) and then move out to one of two main areas for the wet season either east onto the Simanjiro plains, or north-west towards lake Natron. As with the Serengeti migration, these wet season movements are onto grasslands growing on recent volacanic soils with high nutrient content and just what is needed during late pregnancy, then when lactating after calving in February. One of the mysteries, however, is whether the population that moves to Simanjiro is the same as that moving to Natron - do the animals go one direction one year, and the other the next? And as those moving to Natron pass close to another population in Manyara, do those Manyara animals also join the movement? It's important to know the answers to these questions if we're to try and protect the animals, given that they spend around six months of the year outside the National Park system.

Wildebeest flank stripes are unique
Instead of using expensive GPS collars and tagging a few animals, Tom set out to use the individual stripes on wildebeest flanks as identification features. By photographing animals he was able to use computer software to match the pictures up and try and locate the animals at different times of year and after taking over 5600 photos he'd managed to identify some 2557 individual wildebeest (nearly 50% of the population, so a very good effort!), 150 of which he managed to photograph is successive wet seasons enabling him to identify whether individuals were returning to the same site or changing between year. And his answer is fairly simple - most animals don't change. But enough do, sometimes, that this can certainly be considered a single population from a management perspective: between 2005 and 2006 no shifts occurred, whereas between 2006 and 2007 some 20% of animals changed breeding season ranges. There was no consistent direction to these range changes - as many animals from Simanjiro in 2006 moved to Natron in 2007 as the other way around. They also found a small amount of movement between Lake Manyara populations and elsewhere (Natron, Tarangire and Simanjiro), but essentially confirmed this population as resident. More interestingly, they found that dry-season herds after a month or so in Tarangire contain an apparently random mix of animals that had spent the wet season near Natron or in Simanjiro, suggesting social ties aren't particularly important in determining the movement patterns. But the most surprising result of all, was that when considering only females, animals that had succeeded in raising a calf to at least two months old were more likely to shift the following year than those that hadn't. And unfortunately it seems as though they still haven't come up with any reasons why this might be. I'm sure Tom will be interested to hear any answers for what seems to be a fairly strong effect... (Any chance it's just are artefact of more animals shifting in 2006-7 and, perhaps, more animals not breeding in 2005-6? Not sure...)

Serengeti wildebeest are a different race to the Tarangire animals
Hopefully Tom will have more to say about this study in due course - there are plenty of other questions about populations we could use them for and I know he wants to get more funding to continue. We'll see. Meanwhile I'll be happy with the hints I'm hearing that the population in Tarangire has at last stabilised and may even be increasing slowly again, which would be good news indeed! I'll see if I can prod Tom into making some comments here too...

ResearchBlogging.orgMorrison, T., & Bolger, D. (2012). Wet season range fidelity in a tropical migratory ungulate Journal of Animal Ecology DOI: 10.1111/j.1365-2656.2011.01941.x

1 comment:

  1. Tom Morrison sent the fllowing comments: "We're still not sure what is going on with the female wildebeest in Tarangire, but you raise some good questions Colin! Our best guess is that switching wet season ranges is related to the tremendous nutritional costs associated with raising offspring (as any mother would probably tell you). If the wildebeest mothers survive this period - and our survival analysis suggests they are much less likely to survive than non-breeders - they may be at a nutritional deficit which compels them to 'explore' new areas that may have higher quality food than where they came from the year before. But it's important to keep in mind that most females still return to the same wet season range each year; of the few that do switch ranges (for example, going from the Simanjiro Plains to the Selela Plains in the North), they are most likely to be breeders.
    We're quite open to alternative explanations, so please let us know if you have any!
    Tom Morrison"