Dave Taylor's African Safari: Scavengers and Decomposers - Introduction

Scavengers and Decomposers
Introduction
Page 1

Scavengers help keep the environment clean. They eat the remains of animals that have died. In the Serengeti-Mara ecosystem, they are extremely important because there are not enough predators to feed on the huge numbers of ungulates found there.

Put another way, many animals in this ecosystem do not die due to predation. Of those who do die, 70% die of old age, disease and accidents. It is not the predators that control their numbers, it is the environment. One and a half million wildebeest would seem to be at the mercy of the lions and spotted hyenas but in fact, it is the other way around. By migrating, the herds move from one predator's territory to another, causing, in turn, starvation and gluts. When the herds leave, many young and old lions starve.

In Ngorongoro Crater, however, the opposite is true. Here, the herds stay year round due to the presence of reliable water sources year round. In the crater, predators do appear to control the number of prey. Large flocks of vultures are therefore rare in the crater. There is usually not enough leftover food to feed them. The more solitary species of vultures, such as the Egyptian vulture, are present here.

Without predators killing and eating them, something has to clean up the dead that would otherwise go untouched. If some organism did not evolve to dispose of these carcasses, the land would be fouled by the stench and disease brought about by rotting bodies.

Scavengers fill that niche. As we have seen elsewhere on this CDROM, very few animals fit exactly into a given niche. Herbivores will sometimes eat meat. Most carnivores will eat plants and some eat mostly plants.

There are a few scavengers that rely entirely on finding dead animals. The Griffon vulture and the white-backed vulture are good examples of this. Other vultures and the marabou stork cannot be so easily characterized. While scavenging may provide most of their food, some of these birds do hunt and kill their own prey. Even eagles, usually thought of as birds of prey, scavenge regularly.

The line between scavenger and predator is even more blurred when it comes to mammalian predators. Lions were once believed to be hunters and spotted hyenas scavengers. Today, we know that in ecosystems like the Serengeti-Mar, lions scavenge half their meals from other predators, often hyenas. Hyenas will scavenge about half their food from other predators too (most often lions) but they also kill their own food.

Black-backed jackals are frequently seen near kills where they, too, scavenge. However, they also hunt their own food. Indeed, most carnivores will scavenge. The cheetah is the one exception. It never scavenges.

Some ungulates also scavenge meat. Warthogs and bushpigs both will root about in the remains of other animals. Grass rats and other rodents will gnaw on bones.

In the distant past, it is likely that our ancestors also scavenged off the kills of other predators.

Crocodiles prefer to scavenge. Rotten meat is easier to feed upon than fresh kills. The wildebeest crossing provides ample food resources for them without the crocodiles ever having to do any hunting. On the other hand, crocodiles are hunters and seldom pass up an opportunity to do so.

For the purposes of this book, I have assigned these animals to trophic levels based on the species' main source of food or role. Carnivores are found in trophic levels three and four.

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