Save the eagles to save the vultures?

We are used to the calls for conservation up and down a given food chain. Declines in small animals, say, could limit food supply for larger and larger animals up until the apex predators start to suffer. When it comes to predators that occupy the same general niche, logically one might see an increase in one if its competitor declines, thanks to an increase in their shared food resource. In some cases, though, it seems that such horizontal competition may actually be an important part of the health of animal populations.

Vultures, it seems, don’t just rely on their own senses to find the carrion on which they feed. They take visual cues from each other, but they also seem to follow other raptors such as eagles to the scene of the kill. Researchers from Ireland, Swaziland, and elsewhere examined vultures in the Gyps genus, and found that the arrival times at carcasses seem to follow on the heels of raptors in the area.

There are some interesting evolutionary reasons for this order of operations. Raptors have stronger beaks, so being the first to a kill works out fine because they are better at ripping through the hide of an animal. It also gives them a chance to eat first, since once vultures arrive the eagle mealtime might end; vultures are bigger and generally stronger, meaning they can force the raptors off the carcass.

When the two types of scavengers are in balance, these dynamics work out fine for both. Raptors find the meals and dig in first, then the vultures who followed them show up and kick the raptors out; both get to eat.

Previous work has shown that when vulture populations decline, it makes it harder for the remaining birds of that species to find food. They transfer information amongst each other, and with a lower density of vultures in the air they can cover less ground. But if they are also getting information from the raptors nearby, then that means population densities of those totally unrelated species are also important for survival.

The researchers used modeling to test this out, and found a significant increase in vulture foraging efficiency as raptor density increased. “Social information transfer flows within and among the vulture and raptor species,” the authors wrote in Proceedings of the Royal Society B. “In conservation terms, the resultant non-trophic interactions mean that we should shift our focus to ecosystem-based management instead of centering our attention on one species at a time.” In other words, ecosystems are complicated: Save the Particular Animal may be a misguided approach in many places. In this case, raptor populations are indeed on decline in many places, and the researchers note the subsequent decline in vultures may soon become evident.

“More generally,” they concluded, we should explore other incidences of socially acquired information transfer between species: inadvertent as it often is, this will be no easy task.” – Dave Levitan | September 16 2014

Source: Kane A, Jackson AL, Ogada DL, et al (2014). Vulture acquire information on carcass location from scavenging eagles, Proceedings of the Royal Society B. DOI: 10.1098/rspb.2014.1072.

Image:, Iakov Filimonov