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.
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!
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.
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…
Craig (my adviser and the Director of the Lion Project) sometimes jokes that I wandered into his office looking to study tigers. It’s actually sort of true. I had been at the University of Minnesota to interview with a tiger researcher – but fell in love with the science that Craig’s team was conducting. Six months later I became the newest addition to the Lion Lab.
As part of the Lion Lab, my dissertation research focuses on how lions coexist with other large carnivores – hyenas, leopards, and cheetahs. Understanding how species coexist is a really big question in ecology. When two species eat the same thing, the species that eats (& reproduces) faster can exclude the slower species from that area. A lot of ecology is devoted to understanding the conditions that allow for coexistence in the face of such competition. The natural world is an incredibly diverse place, and it turns out the plants and animals have all sorts of strategies to survive together – though we’ll have to dive into those details another day.
Carnivores throw a bit of a wrench into our understanding of coexistence – even when they don’t eat exactly the same prey, they harass each other, steal food from each other, and even kill each other – and these aggressive interactions can result in dramatic suppression or even complete exclusion of certain species. For example, there’s a fair bit of evidence that wild dogs have a tough time surviving in areas with lots of lions and hyenas – not because lions and hyenas kill wild dogs, but because they steal food from them. Since wild dogs expend so much energy hunting, they simply can’t afford to lose those calories to scavengers. These patterns aren’t actually unique to large carnivores – a lot of animals, from bugs to birds, interact this way. However, since carnivores range over such large areas, it can be challenging to understand their dynamics.
That’s where the camera traps come in. The long term lion research project provides incredible amounts of detailed data on what lions do, where they are, and how successful they are at reproducing. By adding the camera survey on top of the lion study area, I can collect information about the other carnivore species and integrate it with the detailed lion data to ask bigger questions than could be answered with one dataset alone. Unfortunately, there aren’t any wild dogs left within the study area, but I can still investigate how lions coexist with leopards, cheetahs and hyenas. It’s a bit gruesome when you get down to it — lions tend to dominate all the other species when it comes to one-on-one interactions, stealing their food or even just killing them for no apparent reason. For example, lions kill somewhere between 25-55% of cheetah cubs! And you can see here Stan’s photos of lions just killing…and leaving…a leopard.
Lions will also kill hyenas, but enough hyenas can be a pretty solid threat to lions – able to steal carcasses or kill their cubs. Leopards sometimes kill and eat lion cubs. We don’t yet know if hyenas and leopards do this at a rate that actually hurts lions in the long-term, but we’re hoping to find out.
One of the key things I’m trying to find out (with a lot of green coffee and evening sessions) is how these species use their habitat with respect to each other. Research in other ecosystems shows that smaller carnivores (those that usually lose a fight) can get pushed out of large areas, existing sort of in the ‘no-man’s land’ between top carnivore territories – and when this happens, their numbers can plummet. However, if the smaller carnivore can just avoid the larger one within its territory, they might be able to coexist. A lot of this depends on the habitat complexity – for example, in open areas, it’s harder for the smaller guy to hide.
The camera traps let me evaluate these different patterns of avoidance to understand how lions, hyenas, leopards, and cheetahs all coexist in Serengeti National Park. Once we understand their dynamics in Serengeti, we can hopefully understand why they do or don’t coexist elsewhere. It’s a pretty cool science question – and it’s also an amazing adventure. I head back to Serengeti this January for my final field season, and am looking forward to sharing the adventure with you on this blog.