The challenges of scavenging and computational maths
When avoiding his day job in stable isotope analysis, TCD’s Dr. Andrew Jackson uses computational modelling to explore the fascinating yet complex fields of animal behaviour and ecology. Most recently, Jackson and his colleagues have been looking into the trash cans of the sky; vultures.
Vultures, a polyphyletic conglomerate of carrion feeders, were once among the most abundant birds of prey. However, in less than a decade, populations of Old-world Gyps vultures have fallen to 90% of their previous population, and are critically endangered .
Though a few species may hunt to some degree when necessary, vultures are thought to be the only obligate terrestrial vertebrate scavengers . Nearly all carnivorous vertebrates exhibit some degree of scavenging, an activity that plays an important role in ecosystems. Scavenging recycles nutrients and quickly cleans away potentially toxic decomposing carcasses, helping to reduce the spread of disease. By increasing the amount of links in the food web, scavenging confers additional stability to an ecosystem . It seems like a win-win situation. Carrion feeders act as the bin-men of the biome; providing a public service as they mosey from meal to free meal.
But it’s not all coasting on thermals. In every aspect of their biology, organisms are met with trade-offs, balancing the costs with the benefits of a strategy. When it comes to finding food, this balance can be analysed through the lens of optimal foraging theory, and idea that every forager will settle on the strategy that yields the highest returns for the benefit of most interest to them. For scavengers, optimal foraging means maximising the rate at which food items are encountered .
One of the primary challenges faced by scavengers is actually finding their food. An observational study by D. C. Houston  described how Griffon vultures (Gyps Fulvus) range huge distances in search of carrion, gliding at high altitudes and relying on sight to spot food items on the ground below.
Big, obligate scavengers like Griffon vultures are completely dependent on finding the corpses of large ungulates, which are spatially “disperse and temporally ephemeral”, making them incredibly hard to locate. On the other hand, once found these carcasses contain enough food to feed multiple scavengers.
Check out the pecking order here.
Houston claimed the majority of a bird’s meals were located indirectly; through social facilitation, a phenomenon where conspecifics provide each other with information on the whereabouts of a resource . Scavengers are more likely to notice other birds spiralling down or flapping around at a carcass than to actually spot the carcass themselves. Effectively, social facilitation increases the carcass detection radius, allowing scavengers to follow others to a meal they would otherwise have missed. Since the encounter rate of carcass is both so low, and so vital to vulture success, social facilitation plays a crucial role in scavenger foraging.
To explore the mechanics of social facilitation, Jackson et al. built a simulation model to investigate the role of both carcass, and scavenger density in encounter rate. “Vultures” in the model were programed to move randomly around the simulated habitat, locating “carcasses” when they passed within a certain radius. Additionally, every time a carcass was located, it’s detection radius increased, allowing other vultures to follow the initial finder to the carcass, often producing a chain reaction attracting in many birds from a much greater distance than otherwise possible.
Though the authors themselves emphasize the approximate nature of the model parameters, a very interesting pattern emerged. It appears scavenger populations are positively density dependant; the more vultures there are, the better the vultures do.
However, the relationship between number of scavengers and incidences of carcass detection is sigmoidal rather than linear, meaning there is critical vulture density below which social facilitation won’t work. This partly explains the Gyps population crash; once poisoning and other external factors drove scavenger numbers low enough, the intrinsic problem set in. With insufficient numbers for effective social facilitation, birds find less and less food and begin to starve, further decreasing the population and driving a feedback loop that saw populations spiral to their current low.
Jackson’s innovative research combined the best of mathematical and computational models with observational field work, yielding brilliant results. Understanding the effects of foraging and ecology on vulture population dynamics is a huge step. With this knowledge, conservation efforts can be meaningfully concentrated where they will be most effective, helping us to retain the services of these majestic and intriguing creatures. This demonstrates the utility of a holistic approach to problem solving in natural science, highlighting, with the right tools, how much we can achieve.
1. Oaks, J. L., Gilbert, M., Virani, M. Z., Watson, R. T., Meteyer, C. U., Rideout, B. A., . . . Khan, A. A. (2004). Diclofenac residues as the cause of vulture population decline in pakistan. Nature, 427(6975), 630-633. doi:10.1038/nature023171.
2. Margalida, A., Campion, D., & Donazar, J. A. (2011). European vultures’ altered behaviour. Nature, 480(7378), 457-457.
3. Moleon, M., & Sanchez-Zapata, J. A. (2015). The living dead: Time to integrate scavenging into ecological teaching. Bioscience, 65(10), 1003-1010. doi:10.1093/biosci/biv101
4. Kane, A., Healy, K., Guillerme, T., Ruxton, G. D., & Jackson, A. L. (2016). A recipe for scavenging in vertebrates – the natural history of a behaviour. Ecography, 40(2). doi:10.1111/ecog.02817
5. HOUSTON, D. C., & Edward Grey Institute, Z. D., Oxford and Serengeti Research Institute, Tanzania National Parks. (1974). Food searching in griffon vultures. African Journal of Ecology, 12(1), 63-77. doi:10.1111/j.1365-2028.1974.tb00107.x
6. Jackson, A. L., Ruxton, G. D., & Houston, D. C. (2008). The effect of social facilitation on foraging success in vultures: A modelling study. Bioligy Letters, 4(3), 311-313. doi:10.1098/rsbl.2008.0038
Image credits: “Cape Vulture Stock Photos”, by Hedrus, shutterstock.