Yellowstone Wolves Good For Grizzlies


The reintroduction of wolves to Yellowstone National Park, starting in 1995, has had some good effects on the food web there: The wolves keep the elk population in check, and with fewer elks to graze on the vegetation, plants including aspen and willows have rebounded. There’s further evidence that this greater availability of plants has been a boon to the beaver and bison populations in the park.

Now comes a study, just published in the Journal of Animal Ecology, that shows that grizzly bears have also gotten a boost to their diet from the wolves’ return.

Yellowstone grizzly bears are a bit different than grizzlies elsewhere — they don’t eat many berries. Studies of the bears that were conducted in the late 20th century found that Yellowstone grizzlies had some of the lowest berry consumption in the interior of North America. That was a bit weird because from July to October, female grizzlies usually gorge on the fleshy fruits. Berry calories can be easily converted to and stored as fat, perfect for bulking up for the long hibernation as well as the pregnancy and birth that usually take place during winter.

“Wild fruit is typically an important part of grizzly bear diet,” William Ripple, an ecologist at Oregon State University and coauthor on the new study, said in a statement. “At certain times of the year [berries] can be more than half their diet in many places in North America.”

Ripple and his colleagues have been studying how the reintroduction of wolves to Yellowstone has affected the Yellowstone food web, and they thought that the berry situation for the Yellowstone grizzlies might have changed with wolves’ return. The issue is more than just a note of academic interest. After the wolf population reached sufficient numbers that the government wasn’t worried they’d die out, the wolves were removed from the endangered species list in 2011, letting hunting of them start up again. If it turns out that the wolves have an impact on other endangered critters, such as grizzly bears, conservationists might have a better case for stopping the hunt.

Since asking grizzly bears to complete a survey of their diet isn’t exactly practical, the researchers analyzed bear poop, comparing scat collected from 2007 to 2009 with data from 1968 to 1987. That latter time was one when elk were increasing in numbers (they went from 3,000 in 1968 to 19,000 in 1994) and bears weren’t doing so well. The bears lost their easy meals in 1971 when the park closed all of its garbage dumps. That led to a lot more bear-human conflicts, and many bears were removed or killed. By 1975 the grizzlies were listed as threatened.

The team found that the percentage of fruit in bear scat declined from 1968 to 1987, as the elk were increasing. But after the wolves had come back into the picture, the bears began eating a lot more berries. In August, for example, the male diet was as much as 29 percent fruit and a female’s up to 39 percent.

What’s happening is that with fewer elk to munch on the park’s vegetation, the berry plants can thrive and bears can eat their fill. It’s likely that other critters, including birds and rodents, are benefiting from the recovering fruit harvest as well, the researchers say.

“Studies like this also point to the need for an ecologically effective number of wolves,” study coauthor Robert Beschta, of Oregon State University, said in a statement. “As we learn more about the cascading effects they have on ecosystems, the issue may be more than having just enough individual wolves so they can survive as a species. In some situations, we may wish to consider the numbers necessary to help control overbrowsing, allow tree and shrub recovery, and restore ecosystem health.”

Photo courtesy of Yellowstone National Park

Predatory Life On The Savanna Is Complicated (Unless You’re A Lion)


In a simple system, there’s a predator and its prey; the predator roams wherever the prey is found. Add in more predator and prey species, and where the predators live should be decided by what they eat. Real life, though, is not that simple. Just look at this new study published online by Ecology of four predator species in South Africa.

An international group of researchers studied an 85-square-kilometer, fenced-in region of South African savanna — the Karongwe Game Reserve. In this area there are four main predator species: lions, cheetahs, leopards, and wild dogs. These predators hunt 12 different ungulate species, mostly impala, blue wildebeest, waterbuck, Burchell’s zebra, warthog, and giraffe.

To figure out where the predators were going and what they were doing, many of the adults of each of the four species were outfitted with VHF transmitters during the study period of 2001 to 2005. The distributions of the prey species were determined by aerial surveys and sampling in the wet and dry seasons (respectively, November to March and April to October).

All that data was then combined. The patterns and interactions of the various species are complex, but there were definite patterns:

Lions: The big cats (above), as might be expected, are the dominant predators in this environment. That power gives them unrestricted access to pretty much anything they want. They go where the best and most vulnerable prey can be found and where they’ve got the best cover for hunting. They don’t worry about where other predator species are. Even the season doesn’t affect them much.

Leopards: These cats overlap in range with lions — the best prey is found where the lions live — but they avoid the much-bigger cats. They also avoid each other. In this area at least, the biggest killer of leopards is other leopards. And during the dry season, when it’s easier to see through parched vegetation and the risk of detection by other cats is higher, leopards find it safest to move towards the smaller wild dogs.


Cheetahs: Like leopards, these spotted cats (above) are also smaller than lions, and it would be expected that they would also avoid the big cats. But, like the leopards, their range overlaps with the lions. And in the wet season, these cats tended to actually move towards locations where lions were recently roaming. The researchers theorize that cheetahs may be using other tactics to avoid lions, such as by choosing habitats, like woodlands, that the bigger cats don’t use, or being active when lions aren’t. Staying near the lions, and their high-quality prey, proved beneficial; some cheetahs were able to take down large prey like wildebeest instead of having to stick with smaller meals. But just because the cheetahs were comfortable venturing into lion territory doesn’t mean that they were entirely fearless; these cats generally stayed away from leopards.

Wild dogs: The African wild dogs — the smallest of the predators — tended to avoid all three of the cat species, but they behaved differently depending on the season. In dry times, when they could be more easily seen, the wild dogs stayed away from the other carnivores’ activity centers. In the wet season, though, when there was more cover available, they took a little more risk and only avoided areas where the other species had been recently. Constrained as they were by the movements of the other predators as well as the boundary fence they could not cross, the wild dogs had to settle for prey species they did not prefer.

The researchers’ take-away message from all these interactions is that for subordinate carnivores like the cheetahs, leopards, and wild dogs, competition with other predators can matter more than what they eat. Because if you have to eat an impala instead of a wildebeest, or even wait another day for a hearty meal, being hungry is better than getting mauled to death by another predator.

Images courtesy of flickr user Jean-Louis A

Great White Sharks Are Scavengers – Will You Think Less Of Them Now?

greatwhiteGreat white sharks have a well-deserved reputation for being fearsome predators. But a new study in PLOS One shows that they’re also crafty scavengers.

Researchers from the University of Miami in Florida, led by Captain Chris Fallows of Apex Expeditions in South Africa, documented four scavenging events over 10 years in False Bay, South Africa. False Bay is home to a breeding site of cape fur seals, and those seals attract lots of great whites — seals are a favorite great white food. But, the scientists found, when when there’s a dead whale nearby, the great whites will abandon the hunt and instead munch on a meal of whale.

“Although rarely seen, we suspect that as white sharks mature, scavenging on whales becomes more prevalent and significant to these species than previously thought,” coauthor Neil Hammerschlag, director of UM’s R.J. Dunlap Marine Conservation Program, said in a statement.

The sharks mostly eat the whale’s blubber — the part with the biggest caloric punch — but they often eat the fluke first, the researchers observed. The biggest sharks get the best bites, with littler sharks having to settle for bits of blubber that float away.

In a way, this is not a surprising study. On land, many apex predators have been observed scavenging for meals, including bears, wolves, and lions. But what is notable, I think, is that we don’t think of any of those creatures as scavengers. And we probably wouldn’t think of great whites as scavengers either, even after reading this new study.

Animals have reputations, and it’s hard to convince people that they may be undeserved. Hyenas, for example, are supposed to be lowly scavengers, but in reality, they’re predators that kill 95 percent of what they eat. Disney’s Lion King did them an incredible disservice — hyenas are actually as effective as a predator as a lion or leopard.

Why do we look down on scavenging? While picking up roadkill from the side of the road or diving into a dumpster has — at least in my opinion — a level of danger involved that makes these actions seem ill-advised, we all scavenge at some level. We eat the leftovers of catered meetings that are left in the company break room, or pick up books left in the “free” pile outside someone’s home. In New York City, you can furnish all of your apartment just from scavenging other people’s discarded belongings.

Scavenging is an effective technique for an organism to supplement its diet (or its life in general, as seen with NYC apartments). Taking advantage of a free meal, one you don’t have to expend much energy to obtain, is a smart move. And we already know that great white sharks are smart, sophisticated animals. After all, it’s one of the things that makes them so scary.

Image of great white shark off South Africa courtesy of flickr user Bring on the Photog