The predator of my predator is my friend.
If you’re a prey animal, you spend an awful lot of your time trying to not wind up like this:
As we’ve talked about an awful lot on this blog (here, here, and here, for example), the same holds true for a lot of predators. Just because you kill and eat other animals, doesn’t mean you don’t have to worry about being killed yourself (as this hyena so unceremoniously discovered).
But, what we haven’t talked so much about, is that the same holds true for plants. If you’re a plant, you get eaten by these terrifying animals:
But, just like prey animals and mesopredators can change their behaviour to minimise the risk of being killed, plants have a few tricks up their sleeves. They can spend a lot of energy growing big thorns, for example, that makes them less delectable.
Or! They can grow in places that their predators avoid — the places where their predators’ predators hang out. Got that? It’s a trickle-down landscape of fear, which had, until now, only been really well documented in small experimental systems with critters like spiders and grasshoppers. But researcher Dr. Adam Ford and colleagues just published an elegant paper in Science showing that leopards and African wild dogs can make the Kenyan savanna less thorny through this cascade. Basically, leopards and wild dogs eat impala. Impala eat Acacia trees. Impala much prefer to eat acacias with fewer thorns (because really, who doesn’t?) – and, if given the opportunity, impala will can eat these small-thorned acacias so much that they can suppress the acacia population.
But! Leopards and wild dogs seem to be offering these tasty small-thorned acacias a refuge. Leopards and wild dogs spend most of their time in denser thickets, where they have more cover to hunt. Impala avoid these thickets and rarely venture in — when they do, however, they have a much higher probability of being killed. And this creates this spiral – those tasty small-thorned trees survive and grow in these thickets because predators scare impala away.
So it’s a trickle down landscape of fear – a compelling and really exciting story. But, what sets Adam’s paper apart from many other attempts to document this effect in large predators, is the series of elegant experiments in which he and colleagues explicitly tested each step in this cascade. Controlling for habitat use to confirm that impala aren’t getting killed in the woods simply because they spend more time there (and in fact, they get killed more even though the spend less time there). Adding and removing thorns to acacias to see if it was really the thorns that mattered. Creating herbivore exclosures to measure whether impala could really suppress acacia density. I spent my entire time reading the article alternating between saying “This is so cool!” and “I am so jealous!” It’s an amazing story. Read more about it here (or here, or or here)!
Tidbits on wild dogs
One of our long-time Snapshot Serengeti members (thanks Reid!) sent me this NY Times article on African wild dogs. As you know, we don’t have wild dogs in the study area (though keep your eyes peeled! TANAPA did reintroduce them into the western corridor the other year, and I keep hoping we’ll catch one traveling through our grid).
But I am very interested in how dogs interact with the larger carnivore community. And these animals are just *so* cool – incredibly energetic and full of nerve. Watching a small group of dogs defend their kill against a hunting party of hyenas was one of the highlights of my trip to South Africa in June.
The article points out that wild dogs may fare better when lions fare worse (which I’ve reported on here) — and that raises some questions about questions about how to target conservation efforts. Do we have to choose between which species to protect? I’d say “not necessarily.” My dissertation research suggests that although dogs fare worse in small reserves with lions, there are places where wild dogs seem to do just fine. Selous Game Reserve (TZ) and Kruger National Park (SA), for example – big areas that have complex habitat structures. So the answer to protecting the entire carnivore guild may lie in larger, diverse reserves.
There are currently efforts in place to do create a protected area the size of Sweden that spans five southern & east African countries. If successful, according to the NY Times, the Kavango-Zambezi Transfrontier Conservation Area will be the largest terrestrial protected area in the world. Now that’s something to celebrate.
Lions and cheetahs and dogs, oh my! (final installment)
I’ve written a handful of posts (here and here and here) about how lions are big and mean and nasty…and about how even though they are nasty enough to keep wild dog populations in check, they don’t seem to be suppressing cheetah numbers.
Well, now that research is officially out! It’s just been accepted by the Journal of Animal Ecology and is available here. Virginia Morrell over at ScienceNews did a nice summary of the story and it’s conservation implications here.
One dissertation chapter down, just two more to go!
Lions, cheetahs, and dogs, oh my! Part 2.
Last week, we left off with this crazy biological paradox: lions kill cheetah cubs left and right, yet as the Serengeti lion population tripled over the last 40 years, cheetah numbers remained stable.
As crazy as it sounds, it seems that that even though lions kill cheetah cubs left and right, it doesn’t really matter for cheetah populations. There are a number of reasons this could be. For example, cheetahs are able to have cubs again really quickly after they lose a litter, so it doesn’t take long to “replace” those lost cubs. It’s also possible that lions might only be killing cubs that would probably die from another source – say, cubs that would otherwise have died from starvation, or cubs that might have been killed by hyenas. Whatever the reason, what we’re seeing is that lions killing cheetah cubs doesn’t have an effect on the total number of cheetahs in the area.
I think this might hold true for other animals, not just cheetahs. It’s a bit of a weird concept to wrap your head around – that being killed, which is really bad if you’re that individual cheetah, doesn’t actually matter as much for the larger population – but it’s one that seems to be gaining traction among ecologists who study how different species live together in the natural world. Specifically, ecologists are getting excited about the role that behavior plays in driving population dynamics.
Most scientists have studied this phenomenon in predator-prey systems – say, wolves and elk, or wolf spiders and “leaf bugs”.
What scientists are discovering is that predators can suppress prey populations not by eating lots of prey, but by causing the prey to change their behavior. Unlike many spiders, wolf spiders actively hunt their prey – sometimes lurking in ambush, other times chasing their prey for some distance. To avoid being eaten, leaf bugs may avoid areas where wolf spiders have lots of hiding places from which to stage an ambush, or leaf bugs may avoid entire patches of land that have lots of wolf spiders. If these areas are the same ones that have lots of mirid bug food, then they’ve effectively lost their habitat. Sound familiar?
Back to Africa – what does this mean for wild dogs and cheetahs? Interestingly enough, lions do not displace cheetahs from large areas of the Serengeti. We’ve discovered this in part from historic radio-collar data that was collected simultaneously on both species in the late 1980’s. Below is a map that shows average lion density across the study area. Green indicates areas with higher densities. The black “+” symbols show where cheetah were tracked within the same study area. They are overwhelmingly more likely to be found in areas with lots of lions. This is because that is where the food is – and cheetahs are following their prey, regardless of the risk of encountering a lion. The Snapshot Serengeti data confirm this – cheetahs are way more likely to be caught on cameras inside lion territories.
Unfortunately, we don’t have radio-collar data on the Serengeti wild dogs from the 1980’s. But we do have radio-collar data for the wild dogs that have been living in the larger Serengeti ecosystem for the past 8 years. As you can see in the map below, wild dogs regularly roam within just 30km of the lion study area. But they don’t settle there – instead, wild dogs remain in hills to the east of Serengeti – where there are local people (who kill wild dogs), but very few lions.
Other researchers in east and southern Africa are starting to pick up on the same patterns in their parks. From Tanzania, to Botswana, to South Africa, researchers are finding that wild dogs get kicked out of really large, prime areas by lions…but that cheetahs do not. What they’re finding (since they have all these animals GPS-collared) is that cheetahs are responding to lions at a very immediate scale. Instead of avoiding habitats that have lions, cheetahs maintain a “safe” distance from the lions – allowing them to use their preferred habitats, but still minimize their risk of getting attacked.
Carnivore researchers are only really just beginning to explore the role of behavior in driving population-level suppression, but I think that there’s good reason to believe that large scale displacement, or other behaviors, for that matter, have greater effects on population numbers of cheetahs and wild dogs, as well as other “subordinate” carnivores – not just in African ecosystems but in systems around the world. It’s a new way of thinking about how competing species all live together in one place, but it’s one that might change the way we approach carnivore conservation for threatened species.
Lions, Cheetahs, and Dogs, Oh My! Part 1.
By now it’s no secret that lions are kind of mean – and that if you are any other carnivore living in the Serengeti, you’d probably prefer a lion-less world. No tawny, muscle-bound foes to steal your food, kill your cubs, chase you around…life would be easy! You’d have plenty of food, your cubs would grow up strong, and your numbers would increase.
Or would they?
It certainly makes sense that all the nasty things that lions do to other carnivores should add up to limit their numbers. Lions are responsible for nearly 30% of wild dog deaths, and over 50% of cheetah deaths! On top of that, they steal food that cheetahs and wild dogs have worked hard to get – and might not have the energy to get again. Researchers are pretty sure that more lions means fewer wild dogs in two ways: 1) In reserves where there are more lions, there are fewer wild dogs, and 2) When lion numbers increase through time, wild dog populations decline.
The same has generally been believed about cheetahs, and some research from the 1990s suggested that reserves with more lions had fewer cheetahs. But as I started digging into the data from Serengeti, I saw a different, quite unexpected, story.
Lions, cheetahs, and wild dogs were all monitored by long-term projects for a number of years. This graph shows their population sizes since the 1960s. The increase in lions is pretty clear – lions have nearly tripled in the last 40 years, largely due to increases in wildebeest. Wild dogs disappeared from the study area. Now, their final disappearance was due in large part to disease, but it’s possible that lions didn’t help matters. In sharp contrast, the cheetah population has stayed pretty much the same. Sure, there are some ups and downs, but on average, the population has been holding steady over the last 40 years.
Wait a minute, if lions are really bad for cheetahs, then why haven’t cheetah populations declined in the Serengeti? How can they possibly be holding steady when lion numbers have tripled? What is going on???
It’s a good question. Tune in next week for an answer!
If you’ve been following Margaret’s blogs, you’ve known this moment was coming. So stop what you’re doing, put down your pens and pencils, and open up your internet browsers, folks, because Season 5 is here!
It’s been an admittedly long wait. Season 5 represents photos from June – December 2012. During those six months I was back here in Minnesota, working with Margaret and the amazing team at Zooniverse to launch Snapshot Serengeti; meanwhile, in Serengeti, Stanslaus Mwampeta was working hard to keep the camera trap survey going. I mailed the Season 5 photos back as soon as possible after returning to Serengeti – but the vagaries of cross-continental postal service were against us, and it took nearly 5 months to get these images from Serengeti to Minnesota, where they could be prepped for the Snapshot interface.
So now that you’ve finally kicked the habit, get ready to dive back in. As with Season 4, the photos in Season 5 have never been seen before. Your eyes are the first. And you might see some really exciting things.
For starters, you won’t see as many wildebeest. By now, they’ve moved back to the north – northern Serengeti as well as Kenya’s Masaai Mara – where more frequent rains keep the grass long and lush year-round. Here, June marks the onslaught of the dry season. From June through October, if not later, everything is covered in a relentless layer of dust. After three months without a drop of rain, we start to wonder if the water in our six 3,000 liter tanks will last us another two months. We ration laundry to one dusty load a week, and showers to every few field days. We’ve always made it through so far, but sometimes barely…and often rather smelly.
You might see Stan
And occasionally Daniel
Checking the camera traps.
But most excitingly, you might see African wild dogs.
Also known as the Cape hunting dog or painted hunting dog, these canines disappeared from Serengeti in the early 1990’s. While various factors may have contributed to their decline, wild dog populations have lurked just outside the Serengeti, in multi-use protected areas (e.g. with people, cows, and few lions) for at least 10 years. Many researchers suspect that wild dogs have failed to recolonize their previous home-ranges inside the park because lion populations have nearly tripled – and as you saw in “Big, Mean, & Nasty”, lions do not make living easy for African wild dogs.
Nonetheless, the Tanzanian government has initiated a wild dog relocation program that hopes to bring wild dogs back to Serengeti, where they thrived several decades ago. In August 2012, and again in December, the Serengeti National Park authorities released a total of 29 wild dogs in the western corridor of the park. While the release area is well outside of the camera survey area, rumor has it that the dogs booked it across the park, through the camera survey, on their journey to the hills of Loliondo. Only a handful of people have seen these newly released dogs in person, but it’s possible they’ve been caught on camera. So keep your eyes peeled! And if you see something that might be a wild dog, please tag it with #wild-dog!! Happy hunting!
Living with lions
A few weeks ago, I wrote about how awful lions are to other large carnivores. Basically, lions harass, steal food from, and even kill hyenas, cheetahs, leopards, and wild dogs. Their aggression usually has no visible justification (e.g. they don’t eat the cheetahs they kill), but can have devastating effects. One of my main research goals is to understand how hyenas, leopards, cheetahs, and wild dogs survive with lions. As I mentioned the other week, I think the secret may lie in how these smaller carnivores use the landscape to avoid interacting with lions.
Top predators (the big ones doing the chasing and killing) can create what we call a “landscape of fear” that essentially reduces the amount of land available to smaller predators. Smaller predators are so afraid of encountering the big guys that they avoid using large chunks of the landscape altogether. One of my favorite illustrations of this pattern is the map below, which shows how swift foxes restrict their territories to the no-man’s land between coyote territories.
The habitat inside the coyote territories is just as good, if not better, for the foxes, but the risk of encountering a coyote is too great. By restricting their habitat use to the areas outside coyote territories, swift foxes have essentially suffered from habitat loss, meaning that they have less land and fewer resources to support their population. There’s growing evidence that this effective habitat loss may be the mechanism driving suppression in smaller predators. In fact, this habitat loss may have larger consequences on a population than direct killing by the top predator!
While some animals are displaced from large areas, others may be able to avoid top predators at a much finer scale. They may still use the same general areas, but use scent or noise to avoid actually running into a lion (or coyote). This is called fine-scale avoidance, and I think animals that can achieve fine-scale avoidance, instead of suffering from large-scale displacement, manage to coexist.
The camera traps are, fingers crossed, going to help me understand at what scale hyenas, leopards, cheetahs, and wild dogs avoid lions. My general hypothesis is that if these species are generally displaced from lion territories, and suffer effective habitat loss, their populations should decline as lion populations grow. If instead they are able to use the land within lion territories, avoiding lions by shifting their patterns of habitat use or changing the time of day they are active, then I expect them to coexist with lions pretty well.
So what have we seen so far? Stay tuned – I’ll share some preliminary results next week!
Map adapted from: Kamler, J.F., Ballard, W.B., Gilliland, R.L., and Mote, K. (2003b). Spatial relationships between swift foxes and coyotes in northwestern Texas. Canadian Journal of Zoology 81, 168–172.
Big, Mean, & Nasty
I recently gave a talk at the Arusha-based Interpretive Guide Society – a really cool group of people interested in learning more about the natural history of Tanzania’s places and animals. I’ve taken a few clips from the presentation that describe in a bit more detail how lions bully their competitors.
Looking at the photos above (all nabbed from the internet), how many of you would like to be a wild dog? A leopard? A cheetah? There’s no doubt about it – lions are big, and mean and nasty. If you are any other carnivore species in the Serengeti – or across Africa, lions chase you, steal your food, even kill you. So what do you do? How do you survive? That’s essentially what my dissertation seeks to answer. How smaller “large carnivores” – hyenas, leopards, cheetahs, and wild dogs — live with lions. Under what circumstances do they persist? Under what circumstances do they decline or even disappear?
There are a handful of ways in which these species interact, but what I’m most interested in is aggression and it’s repercussions. As the above pictures suggest, lions tend to dominate aggressive interactions.
The relationship between lions and hyenas is one that has wormed its way into the public psyche through nature documentaries such as “Eternal Enemies.” While such movies play up the frequency of such interactions, they certainly do happen. Lions not only kill a number of hyenas, but steal their hard-won kills. Dispel any notion of lions as some noble hunter — they in fact steal a lot of their food from other carnivores. In fact, research from Kay Holekamp’s group in Masaai Mara indicates that lions can suppress hyena populations just because they steal food from them! It’s actually a similar story for wild dogs – lions kill wild dogs too, but since wild dogs expend so much energy hunting, that if lions steal just a small fraction of the food that wild dogs catch, wild dogs simply cannot recover. They would have to hunt for more hours than there are in a day to make up for this caloric loss.
It doesn’t stop there. We don’t know how much food lions steal from cheetahs or leopards. We also don’t know how often lions kill leopards, but lions kill cheetah cubs left and right. Studies from Serengeti indicate that lions may be responsible for up to 57% of cheetah cub mortality!
So how do hyenas, wild dogs, leopards, and cheetahs survive? Well, that’s what I’m trying to figure out. But what I can tell you is that not all of these smaller carnivores sit back and take their beating quietly. Take hyenas. They’re about 1/3 the size of a lion, but they live in groups. Big groups. Much bigger groups than lions. And if there are no male lions around, if hyenas have strength in numbers, they can steal food from female lions, and even kill their cubs. While leopards don’t live in groups, they can easily kill (and eat!) a lion cub that has been hidden while mom is away hunting.
Unfortunately, what we don’t know is whether this reciprocal aggression by leopards and hyenas has any measurable affect on lion populations, and whether it’s this aggression that allows hyenas and leopards to coexist with lions. The cameras behind Snapshot Serengeti will provide the first-ever map of leopard and hyena distributions within the long-term lion study area – by comparing lion reproductive success (which we know from >45 years of watching individually identified animals) to leopard and hyena distributions, we can see if lions do better or worse in areas with lots of hyenas or leopards – and whether this is due to getting less food or producing fewer cubs.
What about cheetahs and wild dogs? Even though wild dogs, like hyenas, live in groups, there’s no evidence that this helps them defend themselves or their kills against lions. And cheetahs, well, there’s no record of them killing lion cubs, but who knows?
So how do these guys live with lions? To be honest, wild dogs don’t tend to do very well in places with lots of lions. In fact, it’s generally believed that wild dogs have failed to recolonize Serengeti, despite living *just* a few km from the border, because lion populations are so high. For a long time, researchers and conservationists believed that cheetahs also couldn’t survive in places with lots of lions – but that perception is beginning to change, due, in part, to data coming in from Snapshot Serengeti! It seems that cheetahs not only do just fine in reserves with lots of lions, but use the same areas within the park as lions do. I have a sneaking suspicion that how cheetahs use the habitat with respect to lions, how they avoid encountering lions even though they’re in the same places, holds the key to their success. Avoidance, combined with habitat that makes avoidance possible (read: not the short grass Serengeti plains you see below).
I’ll write more about avoidance and habitat another day. In fact, I’m currently revising a paper for a peer-reviewed journal that addresses how cheetahs and wild dogs differ in the ways they avoid lions – if accepted, it will be the first appearance of Snapshot Serengeti data in the scientific literature! I’ll keep you posted…