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Several crocodile species are known to attack humans (photo courtesy of flickr user Fayes4Art)

We started out Predator Week on Monday with a study that looked at what happens when predators disappear from an ecosystem. But why do we get rid of predators in the first place? Some of them go after things we care about, like our livestock, but an even more understandable motivation for eliminating a species is that it attacks (and eats) us. Humans and our ancestors have been dealing with that problem for forever (check out the top 10 deadliest animals of our evolutionary past), and while many of us are able to live our lives without ever coming in contact with a deadly predator, there are still enough encounters to remind us that humans are not always the top of the food web. (That said, we’ve had enough reminders lately that these species are important to their ecosystems, important enough that we need to keep them around.) Here are the predators that humans had best avoid:

Cats: We’re not talking about your cute little housecat (though a nasty scratch or bite can be troublesome). Leopards, lions and tigers are the scary man-eaters of the cat world. Just this week a leopard in India was taken down after going on a rampage and mauling several people. And tiger attacks in India may be on the rise as their habitat shrinks. But when I think of man-eating cats, my mind goes to the lions of Africa, and stories like the movie The Ghost and the Darkness. If you want to avoid being eaten, a new study finds that lions take advantage of their better night vision and most often attack humans in the nights after the full moon, when the moon rises an hour or more after sunset.

Bears: Earlier this summer, a hiker was attacked and killed by a grizzly bear in Yellowstone National Park. It was the first fatal bear attack in Yellowstone since 1986. Your best option when traveling in bear country is to find out which species you may encounter, learn about them and prepare yourself with the proper knowledge and equipment so you can be ready in the case of an attack.

Sharks: What would summer be without stories of shark attacks? These attacks are real—there are several dozen each year worldwide and a few fatalities—though the hype some years is far greater than the danger. The Florida Museum of Natural History has a good list of advice for avoiding a shark encounter, much of it common sense (don’t go in the water if bleeding; don’t harass a shark). Sharks aren’t just terrifying nightmares, though; they’re smart—for fish—and many of their “attacks” may just be the shark investigating its environment.

Komodo dragons: The most famous victim of a komodo dragon attack has to be Phil Bronstein who, in 2001 when he was married to Sharon Stone, lost his big toe to one of these big lizards. These giant, carnivorous lizards, native to Indonesia, use sharp teeth, and possibly venom, to bring down large prey, such as pigs, deer and water buffalo. They’ll also attack humans and even dig up bodies from shallow graves.

Crocodiles and alligators: These are both big reptiles with pointy teeth that like to hang out in the water and wait for a meal. In the United States, we worry about the freshwater alligators (Florida’s Sun-Sentinel newspaper keeps an online database of attacks) while in Asia, Australia and Africa, the saltwater croc finds humans to be tasty meals. The easiest way to avoid them both is to stay away from waters where they may be found, and that includes the shores where the reptiles may be lying in wait for their prey.

Wolves: People who live in wolf territory often fear that these dogs will attack them or their children. In North America, wolf attacks on humans are incredibly rare, fatal ones even more so; one report counts around 20 to 30 in the 20th century. Wolves are more bold (or more desperate) in some other parts of the world, however. In Uttar Pradesh in India, wolves killed or injured 74 people in 1996 and 1997.

Hippos: Hippos are mostly herbivorous animals, but that’s a bit misleading because they seem to have a great enough dislike for humans that they’ll attack people even when the humans think they’re safe in a boat. More people are supposedly killed by hippos than by any other animal in Africa. They weigh several tons and can run as fast as, or perhaps faster than, a human on land, so it’s best to stay in the safari vehicle when traveling through hippo country.

Snakes: While poisonous snakes can kill you, tales of man-eating snakes center on species like pythons that are big enough to swallow a human child whole. Confirmed stories of such deaths, however, are extremely rare.

A leatherback turtle is just one of many predators in the ocean (credit: NOAA)

If I asked you to name a marine predator, your first answer would probably be a shark. But this category is so much bigger—sea turtles, tuna, elephant seals, whales, even birds reign at the top of the ocean’s food webs. Many of these species are at risk from challenges such as overexploitation and climate change. And scientists hoping to protect these animals have often lacked good data on their movements; it’s hard to see where creatures go beneath the water’s surface.

In 2000, marine researchers began the Tagging of Pacific Predators project as part of the decade-long Census of Marine Life. They deployed 4,306 electronic tags, which yielded 1,791 tracks from individuals of 23 marine predator species in the northern Pacific (for a total of 265,386 days of data over 2000 to 2009). The results of their study were published earlier this month in Nature.

“It is like asking, ‘How do lions, zebras and cheetahs use Africa as a whole continent?’ only we have done it for a vast ocean,” the study’s lead author, Barbara Block of Stanford University, told Nature.

The species were concentrated along two main routes: One followed the California Current, which flows southward off the U.S. West Coast, and the other along the North Pacific transition zone, the boundary running east to west between the cold waters of the sub-Arctic and the warmer waters of the subtropics.

The researchers found that the exact location of a species represented a trade-off between having access to a greater amount of ocean productivity (meaning more food) and the temperatures that the predator–or its preferred prey–could withstand. As a result, two similar predator species can occupy two different ranges without overlapping (thus avoiding turf battles between, for example, white sharks and mako sharks).

Another factor that is likely to be an important driver of predator migration is upwelling, in which cool waters rich in nutrients are brought up to the surface of the ocean. Those nutrients help microscopic organisms grow and multiply and feed bigger critters up the food web. “Using satellite observations of temperature and chlorophyll concentrations [a marker for the amount of microscopic organisms in the water], we can now predict when and where individual species will be,” study co-author Daniel Costa of the University of California, Santa Cruz, told Nature.

Some predator species, such as yellowfin tuna, salmon sharks and elephant seals, can even be found returning to the same place every year, like wildebeests of the Serengeti.

Researchers hope that this data will help them manage these species in the future. Because no one can predict what might happen to the rest of the species in the food web if these top predators are lost—and who knows what tasty seafood may become a taste of the past.

Last up for Predator Week: What preys on humans?

A stuffed solenodon in a museum (courtesy of Flickr user belgianchocolate)

For “Predator Week,” I wanted to highlight some unlikely fearsome creatures: venomous mammals. These mammals are a bizarre bunch. The male platypus has spurs on its ankles that release venom, likely to fight off male competitors during mating season. And various species of shrew and the shrew-like solenodon use venomous saliva to disable prey.

The solenodon is particularly fascinating because it delivers its poison just as a snake does—using its teeth as a syringe to inject venom into its target. Not a lot is known about these unusual mammals. There are only two solenodon species: One lives on Cuba and the other on Hispaniola (home to Haiti and the Dominican Republic). At night, they dig in the dirt with their Pinocchio snouts and long claws, looking for grub and waiting to disarm their prey—insects, worms, snails and small frogs and reptiles—with a toxic bite. The BBC has some great video footage of the strange little guys (the solenodon’s venom isn’t lethal to people but notice the handlers still wear gloves).

Based on this observation in The International Wildlife Encyclopedia, solenodons sound like little terrors:

It readily defends itself against one of its own kind, and probably attacks other animals savagely judging from the way a captive solenodon attacked a young chicken and tore it to pieces with its strong claws, before eating it.

Millions of years ago, venomous mammals may have been more common. But soon the world may lose a couple more: Like many other predators, both species of solenodon are highly endangered. Deforestation and the introduction of dogs, cats and mongooses that eat solenodons threaten to drive the critters to extinction. And in Haiti, people hunt solenodons for food.

Fortunately, the solenodon has recently become the focus of conservation efforts. It would be sad if such a unique, mysterious mammal were gone for good—although I imagine the invertebrates of the Caribbean wouldn’t mind.

Tomorrow in Predator Week: Scientists find the marine version of the Serengeti’s great migrations

A snow leopard caught in a camera trap in Afghanistan (credit: Wildlife Conservation Society)

Yesterday I asked you, the readers, about your favorite predators. Mine, well, that has to be the snow leopard (Panthera uncia). I remember seeing them at the zoo when I was a kid and being fascinated by them, so out of place in America and yet so familiar, like a bigger version of my own kitty.

And so I was happy to see a bit of good news recently about these elusive Asian mountain dwellers: The Wildlife Conservation Society has found a healthy population in the Wakhan Corridor of northeastern Afghanistan, catching glimpses of the animals at 16 camera traps.

But these and many other big cats aren’t doing so well on the population level. Snow leopard numbers have declined by about a fifth in the last 16 years, according to the WCS, and the kitties are classified as endangered. They’ve been hunted for their fur and their bones—prized in Chinese “medicine”—and their prey, mountain goats and sheep, have been overhunted.

Lions are a bit better off, classified as vulnerable, but few live outside national parks or hunting preserves. Tigers are endangered and can be found in only 7 percent of their historical range. Cheetahs, the fastest land animal, have also disappeared from most areas, and scientists now worry that the remaining population lacks enough genetic diversity to remain viable. (And then there are all the smaller cat species that are threatened.)

Domestic cats may be thriving as our pets, but their wild brethren need some help. They have excellent spokespeople, though. For example, filmmakers Beverly and Dereck Joubert, who have spent years following individual kitties in the wild (their talk from last year’s TEDWomen is below). The Jouberts’ observations have shown the same thing that that yesterday’s  study did, that when these animals disappear, whole ecosystems go with them. “If we don’t take action and do something, these plains will be completely devoid of big cats, and then, in turn, everything else will disappear,” Beverly Joubert says in the video. “And simply, if we can’t protect them, we’re going to have a job protecting ourselves as well.”

Next up in Predator Week: Venomous mammals

The loss of wolves in the American West set off a cascade of changes to the region's food web. Courtesy of Flickr user WSK_2005.

Eliminating predators from an area may be seen as a good thing; you’ve gotten rid of the animal that has been killing off your livestock or even your neighbors. Others often see the loss of these species with a somewhat sad, romantic eye; how awful to never again see such a creature. But the reality of the loss of predators is far worse, say ecologists reporting in Science, and “may be humankind’s most pervasive influence on nature,” they write.

Part of that is because the worst extent of such a disappearance—extinction—is irreversible, unlike other environmental impacts, such as climate change. But it’s more because the loss, or even reduction in numbers, of predators in an ecosystem can set off something caused a “trophic cascade” in which the change in predator population has effects across the food web and ecosystem. For example, when wolves were eliminated from the American West, there were changes in the elk population and the vegetation the elk ate.

“Trophic cascades have now been documented in all of the world’s major biomes—from the poles to the tropics and in terrestrial, freshwater and marine systems,” the scientists write.

But changes to the food web aren’t the primary problem for human populations; the effects on ecosystem processes are often more dangerous. And many of these processes are big enough that even people in industrialized nations cannot protect themselves. The changes in vegetation that occur when the herbivore population is allowed to rise unchecked can change the frequency and intensity of wildfires. Infectious diseases can become more common; for example, in some parts of Africa where lions and leopards have become scare, populations of olive baboons have changed their behavior patterns, increasing their contacts with the humans nearby. Intestinal parasites have become more common in both the baboons and the people.

Then there are changes to soil bacteria, water availability, biodiversity and a host of other ecosystem features that we depend on to grow our food, keep our environment habitable and stay healthy. The scientists conclude:

We propose that many of the ecological surprises that have confronted society over the past centuries—pandemics, population collapses of species we value and eruptions of those we do not, major shifts in ecosystem states, and losses of diverse ecosystem services—were caused or facilitated by altered top-down forcing regimes associated with the loss of native apex consumers or the introduction of exotics. Our repeated failure to predict and moderate these events result not only from the complexity of nature but from fundamental misunderstandings of their root causes.

We can’t predict what will happen when a predator is lost from an ecosystem; there are too many unknown ways that species interact and the processes take place over scales of tens to thousands of square kilometers. The true effect of a loss can’t be known until years or decades after it has taken place. It’s another reason to save these incredible creatures—for our futures.

With this reminder of the importance of predators, we’ve decided to hold Predator Week here at the blog. What’s your favorite predator, either existing or extinct? Which ones would you be sad to lose forever?