The Living Dead: Time to Integrate Scavenging into Ecological Teaching

1. What they did

Carrion (dead animals) and scavenging (eating carrion) have long been left out of classic ecology. Food webs were drawn as producers, plant eaters, meat eaters and top predators, and later detritus feeders, with no scavengers. The authors argue that this no longer matches the science, and that university teaching has not caught up.

• Textbooks. They checked the most recent editions of 20 ecology textbooks commonly used by students and found in university libraries.
• Research trend. They searched the Web of Science for papers in Ecology journals with “scaveng*“,”carrion” or “vulture” in the title, and compared the share published in 2004 to 2013 with papers on detritus feeding, decomposition, predation and plant eating.
• Review. They summarised recent evidence on why scavenging matters, with two examples in boxes: whale falls on the deep-sea floor and scavenging at carcasses in the African savannah.
• Proposal. They set out what ecology textbooks should cover on scavenging.

2. What they found

Textbooks

• Scavenging never appeared in any table of contents.
• Scavenging, scavenger and carrion were almost absent from the indexes: scavengers or scavenging vertebrates appeared in five books, and carrion consumption in one.
• Where scavenging was covered, it took one paragraph to barely two pages, always descriptive and tucked into sections on decay, other interactions or succession.

Research trend

• 64 papers had “scaveng*” in the title, 73% of them published from 2004 to 2013. For “carrion” (149 papers) and “vulture” (188), 52% and 65% came from that decade.
• For comparison, the shares for detritus feeding, decomposition, predation and plant eating were 50%, 39%, 52% and 35%.

What recent research shows

• Most, if not all, carnivores scavenge when they get the chance, including animals long thought never to eat carrion, such as snakes.
• Scavenging happens in all land and water ecosystems (table 1) and moves energy between land and water.
• Food webs underestimate scavenging links. More energy passes per scavenging link than per predation link, which has led to predation being overstated.
• Because predators also scavenge, scavenging can affect the populations of the animals eaten (top-down effects), and predation and scavenging are closely linked.
• Carrion plays a part in disease, scavenging networks can be highly structured, and scavenging may help stabilise ecosystems.
• Scavenging by early hominins was decisive in our own evolution, and scavengers have provided many services to people (such as disease control and ecotourism).
• Scavenging is under threat: changes in where and when carrion is available (from human food subsidies, new diseases and sanitary rules) are changing the behaviour and survival of species from insects to vultures. Research now offers guidelines for vulture feeding stations, sanitary rules, and reducing deaths at power lines and wind farms.

Whale falls (box 1)

• There is a fresh whale carcass on the sea floor every 16 to 36 km. In the first stage, which lasts months to about 5 years, hagfishes, sleeper sharks, amphipods and crabs remove about 90% of the soft tissue at about 1.7 to 2.5 kg per hour.
• Whaling has cut whale numbers and so whale falls, and some specialist whale carrion feeders have gone extinct.

African savannah (box 2)

• In a camera study of carcasses from 2 kg chickens to 4-tonne elephants, carcass size shaped the scavenger community.
• Larger carcasses took longer to eat but were eaten faster (0.14 kg per hour for small carcasses, 4.45 kg per hour for large ones). They were found sooner, and a smaller share was consumed.
• Vultures, lions and spotted hyenas ate nearly 50% of small carcasses and nearly 100% of medium and large ones. Only horns, a few hard bones and shreds of skin remained, after 1.6 to 419.1 hours. Insects and microorganisms played almost no part.
• Large carcasses drew more scavenger species (4.75 per carcass on average). Small carcasses drew the most species overall (14), probably because the dominant scavengers found them less easily; with fewer apex predators around, genets and mongooses could feed there.
• Figure 4 shows a 2000 kg white rhino, killed by another male in a fight, eaten by spotted hyenas, lions and Nile crocodiles until only a few hard bones remained three and a half days later.

3. What they concluded

• This is the “golden age” of scavenging research. Scavenging is a widespread, non-random and large pathway for energy, with effects from individuals to ecosystems and links to other processes.
• There is a gap between the science and what ecology students are taught. Food webs should include carrion and scavengers, both obligate (such as vultures) and facultative (predators and many others).
• Including scavenging could improve energy flow models and help with global change; for example, apex predators can smooth out swings in carrion supply that climate change is increasing.
• Box 3 proposes textbook content: basic concepts; when and where carrion is available; the evolution of scavengers, including humans; the structure and functioning of scavenger communities; what scavenging does (including nutrient cycling); links with predation; effects on coexistence and stability; global change impacts; and managing scavenging and conserving scavengers.

6. Other technologies, skills or innovations

• Motion-sensing cameras at carcasses (box 2, figure 4) and manned submersibles and remotely operated vehicles at whale falls (box 1). The authors note that better technology has helped recent scavenging research.

7. Implications for conservation (from the paper)

• Vultures and large predators are in serious danger worldwide from poisoning (intentional and unintentional), shooting, power lines and wind farms, road kills, lead poisoning and habitat loss. Conserving them will be extremely difficult without broad recognition of their ecological role and the services they provide.
• Science-based guidelines now exist for supplementary feeding stations, sanitary rules, and reducing scavenger deaths at power lines and wind farms.
• Many charismatic scavengers, especially vultures and large predators, are globally threatened (figure 3).