To save the scavengers, open up vulture restaurants
Animals are born, they eat, they breed, and they die. That’s how it’s supposed to work, at least if you believe the lyrics of a Disney movie featuring cartoon lions. In the best cases, reality is not too far off. But sometimes animals are born and they never get the chance to grow up. Or they grow up, but die before they’re able to successfully rear their own young. The savannah is unforgiving, and the struggle to find enough food to simply survive is what defines life for the African white-backed vulture.
You’d think that as obligate scavengers, African white-backed vultures (AWBVs) would actually find the savannah to be a smorgasbord. When other animals die, their carcasses become carrion, a tasty meal for the magnificent eaters of death. But if the animals that AWBVs feast upon are dying faster than they can reproduce, then eventually the birds will starve.
In sub-Saharan Africa, the main threats to these magnificent birds are habitat loss and food reduction. One popular measure implemented for their conservation has been a set of “vulture restaurants,” sites where vultures are provisioned with carcasses. They have the effect of adding to the carrion already available to the birds, allowing more food to be distributed across the population, as well as making the availability of food more predictable both in time and space. Vultures, after all, can’t predict just where and when the animals they eat will die.
At 300 breeding pairs, Swaziland’s population of AWBVs is the densest nesting population of the species in the world. During the latter part of the 20th century, this population actually increased, despite the overall decline of the species. But most of the vulture restaurants were closed around the year 2000 – out of seven, only one still remains – and the population remains healthy. It suggests that their home range in Swaziland has enough resources to sustain the population without any human aid. That makes them an ideal group to study in order to understand the effects of fluctuating food availability on the strength and size of populations of vultures.
Zoologist Adam Kane from Trinity College London, along with colleagues from the University of Swaziland, the University of Pretoria in South Africa, the University of Leide in Spain, and the University of Bern in Switzerland, turned to this group of carrion eaters to understand their nutritional needs in order to inform conservation measures. How long can the population sustain its current level of growth? When will food run out?
It’s not as simple a question as it sounds, because the birds’ needs change over time. More nutrition is needed during the breeding season and while the chicks are still in the nest, because not only do the adults have to eat, but they need to feed their young. And the duties associated with parental care means that the adults can’t fly very far away to find food. During the breeding season, they’re restricted to a foraging radius of just 45 kilometers, with their nest in the center. During the rest of the year their radius expands to 260 kilometers.
What that means is if they’re faced with a food shortage during the breeding season, both the adults and chicks risk death. Meanwhile, if there’s a food shortage during the other months of the year, the vultures simply have to fly farther to find enough to eat.
Combining this information with the ungulate biomass available to sustain the current population within the ecosystem, the researchers determined that through most of the year, there is indeed enough food available for everyone to eat. (Most of the carrion is comprised of impala, blue wildebeest, and zebra.) It’s only in November through April (wet season), at present, that there is not enough food nearby. But during that time of year, the vultures can simply fly a bit farther away to eat.
The researchers then projected their models out for twenty years, the expected lifespan of an AWBV. By year 5, there will be a food shortage in July and August (the breeding season) and by year 13, there will be a food shortage in September and October (the start of the wet season). May through June (the start of the dry season) does show a shallow decline over the next two decades, but nowhere near severe enough to seriously impact food availability (yet).
What that means is that while Swaziland’s 300 pairs of AWBVs are doing just fine right now, they’re teetering on the precipice of sustainable eating. In less than a decade, they’ll face food shortages at precisely the time when they must forage as true locavores, sticking close to home.
The tale of Europe’s bearded vultures provides an important warning. After the 2001 outbreak of bovine spongiform encephalopathy (mad cow disease), carcasses were destroyed in plants, reducing the amount of food available to vultures. That led to a delay in egg laying, smaller clutch sizes, and reduced breeding success and chick survival. Unless something is done for Swaziland’s AWBVs, then they might face the same fate in the coming years.
Given the predictions of their model, the researchers propose that Swaziland’s Hlane-Mlawula-Mbuluzi reserve network (which contains 202 of the 300 nesting pairs) re-institute vulture restaurants. By ensuring that there is no deficit of carrion at any stage of the year, the birds would have their best chance to thrive. If provided with enough carrion at restaurants, then the population could perhaps grow big enough to seed other, new populations in nearby areas.
And it isn’t only the AWBVs that would benefit; restaurants would also serve white-headed vultures, marabou storks, tawny eagles, and bateleurs. In fact, the only species that they predict would not benefit is the Lappet-faced vulture, but that’s because it’s only got one local breeding pair to begin with.
There is one more advantage to the opening up of a series of vulture restaurants. As the vultures became accustomed to visiting, researchers would have a better opportunity to capture, mark, and tag the birds. High-resolution movement and demographic data is lacking on AWBVs, both of which will become increasingly necessary for conservationists and policymakers.
Kane acknowledges that opening up a chain of vulture restaurants is not without drawbacks; the birds could be conditioned to rely on restaurants, rather than using them as supplements. But a “well-managed vulture restaurant could minimize these issues while maximizing the benefits,” he argues. – Jason G. Goldman | 15 October 2014
Source: Kane A., A. Monadjem, M. A. Colomer & A. Margalida (2014). Carrion ecology modelling for vulture conservation: are vulture restaurants needed to sustain the densest breeding population of the African white-backed vulture?, Animal Conservation, n/a-n/a. DOI: http://dx.doi.org/10.1111/acv.12169
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