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The Secret Life of Trees

This is another guest post by Drs. Tom Morrison and Michael Anderson  about the Snapshot Serengeti Special Edition and what their research hopes to uncover.

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Seeing the forest for the trees

First, a big THANK YOU to everyone who has helped classified images at Snapshot Serengeti, both past and present. Without the continued help of this great online community, our research would come to a grinding halt! So thank you. A number of folks (and at least one giraffe) have asked about the new study currently up on Snapshot Serengeti, so here’s a fuller explanation of this work.

Photos from our newest Snapshot Serengeti Special Season come from a camera trap experiment in Serengeti involving friends and collaborators based at Wake Forest University (US), University of Georgia (US) and University of Glasgow (UK).

One of the exciting things about these new images is that they come from some of the more remote corners of the park, far beyond where past photos (Season 1-9) were (and continue to be) collected. So, keep an eye out for different species than past surveys. For instance in the north, you might see oribi, a small and elegant ungulate with a large dark scent gland below its eye. In the south, our cameras overlap the home ranges of some of the few black rhinoceros still living in the park, and we already know there are at least a few rhino images in our pile, like this:

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We set these cameras at a slightly higher height (1.5 meters in most cases), which allows us to see species from new wider angles. Admittedly, this new experimental design makes animal classifications a bit harder because we can often see far into the distance. Our advice is to simply do your best, but don’t sweat it too much if you can’t figure it out. Better to see the forest than the trees.

Back to the research…

Speaking of trees, this new study is trying to unravel the secret lives of trees. We monitor hundreds of individually marked trees around the ecosystem and revisit them each year to measure growth, survival, disease and few other things. You may have noticed little cages in some of the camera trap photos (see giraffe above). These are part of our experiment and enclose four small native tree seedlings which we transplanted to the plots after growing them in a nursery for 6 weeks. In fact we planted over 800 seedlings around the ecosystem to study the relative importance of herbivory, fire and rainfall on seedling growth and survival. So, we need camera traps to monitor things when we’re not there.

For example, check out the following sequence captured on one of our game cameras in southern Serengeti involving one of our marked trees:

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What’s amazing about this is that not only does an elephant kill an adult tree, he does it under 60 seconds. This tree is an Acacia tortilis, or the “umbrella acacia,” named for its characteristic flat top. Umbrella acacias are one of the most common trees in Serengeti and one of our main study species. Images like these help inform our study of trees, telling us how they died, or at least how many large herbivores were in the area to potentially kill and eat them. But this begs the question: if a tree falls in the Serengeti, will anyone hear it? At least we know that there’s a small chance that one of our cameras might see it.

More results!

As I’m writing up my dissertation (ahh!), I’ve been geeking out with graphs and statistics (and the beloved/hated stats program R). I thought I’d share a cool little tidbit.

Full disclosure: this is just a bit of an expansion on something I posted back in March about how well the camera traps reflect known densities. Basically, as camera traps become more popular, researchers are increasingly looking for simple analytical techniques that can allow them to rapidly process data. Using the raw number of photographs or animals counted is pretty straightforward, but is risky because not all animals are equally “detectable”: some animals behave in ways that make them more likely to be seen than other animals. There are a lot of more complex methods out there to deal with these detectability issues, and they work really well — but they are really complex and take a long time to work out. So there’s a fair amount of ongoing debate about whether or not raw capture rates should ever be used even for quick and dirty rapid assessments of an area.

Since the Serengeti has a lot of other long term monitoring, we were able to compare camera trap capture rates (# of photographs weighted by group size) to actual population sizes for 17 different herbivores. Now, it’s not perfect — the “known” population sizes reflect herbivore numbers in the whole park, and we only cover a small fraction of the park. But from the graph below, you’ll see we did pretty well.

HerbComparisons

Actual herbivore densities (as estimated from long-term monitoring) are given on the x-axis, and the # photographic captures from our camera survey are on the y-axis. Each species is in a different color (migratory animals are in gray-scale). Some of the species had multiple population estimates produced from different monitoring projects — those are represented by all the smaller dots, and connected by a line for each species. We took the average population estimate for each species (bigger dots).

We see a very strong positive relationship between our photos and actual population sizes: we get more photos for species that are more abundant. Which is good! Really good! The dashed line shows the relationship between our capture rates and actual densities for all species. We wanted to make sure, however, that this relationship wasn’t totally dependent on the huge influx of wildebeest and zebra and gazelle — so we ran the same analysis without them. The black line shows that relationship. It’s still there, it’s still strong, and it’s still statistically significant.

Now, the relationship isn’t perfect. Some species fall above the line, and some below the line. For example, reedbuck and topi fall below the line – meaning that given how many topi really live in Serengeti, we should have gotten more pictures. This might be because topi mostly live in the northern and western parts of Serengeti, so we’re just capturing the edge of their range. And reedbuck? This might be a detectability issue — they tend to hide in thickets and so might not pass in front of cameras as often as animals that wander a little more actively.

Ultimately, however, we see that the cameras do a good overall job of catching more photos of more abundant species. Even though it’s not perfect, it seems that raw capture rates give us a pretty good quick look at a system.

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!

 

 

 

Big Cat Wars

I’m in the process of writing up some *really* cool camera trap results from Seasons 1-6, and plan to share them here next week (as soon as I make them pretty). It would never have been possible without your guys’ help.  But in the meanwhile, this just aired again on TV, and thought you might enjoy a bit of a break! They talk about the camera traps a bit ~33 minutes in.

 

What we still don’t know

Just a pretty picture that makes me wonder what on *earth* I am doing in the polar vortex...

Just a pretty picture that makes me wonder what on *earth* I am doing in the polar vortex…

Weather.com says it’s a whopping 6 degrees outside right now, but that it feels like -14. I am really wishing I were back at the conference in California right now…

By now, both Meredith and I have gushed about all the “bio-celebrities” at the Gordon Research Conference on Predator Prey Interactions. How we got to come face to face with the scientists whose work we’ve read, memorized, admired for years. But what I think has been an even more exciting outcome of this research conference than getting to hang out with our scientific heros and listen awe-struck about everything they’ve done in the past that has led to their fame today, was a chance to sit down with them over a beer or glass of overpriced red, and talk about the future.  Not just where our various and varied subfields have been, and not even just where they are going, but where they need to go. Where the holes are in our knowledge, and what we need to do to fill them.

Much of ecology is about developing “predictive capacity.” The ability to not just describe the patterns we see in the world about us, but the ability to predict what will happen when things change. Understanding how climate change affects annual bird migrations, for example, or what losing species means for the spread of disease. We develop conceptual frameworks to tie together outcomes from different experiments and scattered observations drawn from ecosystems around the world, and these frameworks give us a way to articulate our expectations about 1) what underlying processes we think are driving the dynamics of a system and 2) a way to test those hypotheses: do the outcomes match what we predicted would happen? Or is something else going on that we need to investigate further?

One of the things I slowly worked up the courage to articulate at the conference was that I think that science surrounding predator-predator dynamics really lacks this sort of integrated, synthetic, predictive framework. We draw on a whole bunch of different sets of theories to understand the patterns of suppression and coexistence apparent in apex-mesopredator (top- and middle- predator) systems. There’s a ton of  theory out there on how species coexist when they eat the same thing, or how they coexist when they eat the same thing and also eat each other. There’s a lot of theory on how predators coexist with the things they eat. There are predictions for when we expect to see species able to coexist, when we don’t, and how these different outcomes change from, say, low productivity tundra to high productivity rainforests.

But around the world, top predators suppress populations of smaller predators (called mesopredator suppression). It’s not because the top predators are eating up all the food, and it’s not because the top predators are eating the mesopredators. It seems to happen because the bigger guys chase, harass, and kill the smaller guys. This is bad enough, but it also creates a “landscape of fear” in which that the smaller guys change their behaviors to try and avoid these aggressive encounters. There are lots and lots of ways in which mesopredator suppression can happen…but we (as a community of ecologists) don’t have a good, integrated framework for making predictions about when we expect to see mesopredator suppression vs. when we don’t. We don’t have a set of expectations about how these patterns change with different behaviors or different types of environments. We don’t have a solid understanding of what mesopredator suppression means for other small predators, prey animals, and the plants that the prey animals eat. We have lots and lots of examples of all sorts of complex things happening…but we don’t yet have the ability to predict how these things play out in new systems.

And that, to me, is one of the most exciting “holes” that needs filling. How do we  tie together our knowledge from all of these disparate studies, where lions suppress wild dogs but not cheetahs, or coyotes kill foxes left and right but aren’t actually the reason that fox populations are low.  I guess my PhD is trying to fill a tiny, tiny bit of that hole. But it’s a damn big hole and sometimes it’s hard to see how one PhD will cover a whole lot of ground.  I guess what was so exciting at the GRC is just how many other people are also trying to fill those holes…and with all of us working together, we just might actually be able to understand the world around us that much better.

The curious case of the giraffe and the oxpecker

As you all ready know Snapshot Serengeti’s thousands of camera-trap images are part of an ongoing study into predator interactions by Ali. There are few projects that use camera-traps as extensively as Snapshot Serengeti and of course Ali has her hands full analysing the bits relevant to her. The cameras work around the clock recording details of daily and nightly life in the Serengeti and do not discern between the stuff Ali does and doesn’t want. That’s why, Ali’s sanity aside, they are such perfect tools. Those same cameras providing Ali’s data could also be the basis of a future ecologist’s research.

One of the most striking asides for me is the case of the giraffe and the oxpeckers.

Oxpeckers are small birds that feed on ticks and other parasites that they glean from the bodies of large mammals. Most usually they are seen riding along on large mammals such as buffalo, wildebeest and giraffe whilst they search their hosts for ticks or open wounds. This in itself is not an unusual occurrence and most of you will have hit the bird /other button with these guys. Much more unusual are the shots of giraffe at night time with these birds using them as roosting spots. There are two species of oxpecker, the red-billed (Buphagus erythrorhynchus) and the yellow-billed (Buphagus africanus) both of which are found in the Serengeti.

According to research carried out by M. Stutterheim and K. Panagis that looked at the roosting habits of both species the red-billed oxpecker roosts in trees but the yellow-billed was often found roosting on their preferred host species. Apparently red-billed oxpeckers feed on a wide range of host species where as yellow-billed oxpeckers are much more picky preferring buffalo and giraffe. It is thought that the habit of roosting at night on their favourite host species is an adaptation to save the birds time looking for the right animal the following day. Given that buffalo and giraffe are prone to walking large distances this is probably very sensible.

From most of the images we have of oxpeckers on giraffe at night it is hard to tell which species they are but there are one or two where you can see the tell-tell yellow bill confirming that they are indeed yellow-billed oxpeckers. The images also show that the birds seem to prefer settling between the hind legs of the giraffe. This must be a nice warm spot in winter and keeps them safe from any nocturnal predators.

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Perhaps the behaviour is not so unusual after all but rather little documented. Getting photographic evidence of birds at night on mobile roosts is obviously not easy. Looks like our camera-traps have excelled themselves again.

Why does the zebra have stripes?

While procrastinating on this lovely Sunday afternoon, I stumbled across this incredible video of a octopus camouflage in action:

Now, we don’t have anything quite that camouflaged in the Serengeti, but in watching that video my thoughts turned to one of our more strikingly colored species: the zebra. Their starkly contrasting black and white stripes have puzzled researchers and naturalists for a long time.

For starters, the stripes seem like they would be terrible camouflage. I mean, how much more could you stand out from the open plains of waving gold grass? But at dawn and dusk, especially from a distance, the stripes seem to bleed into gray, making them look a surprising lot like elephants (no joke), or rocks, or even nothing at all. Still, up close they still look like bright black-on-white zebras, and it’s hard to imagine that any lion lurking in the thickets nearby would be fooled.

Some researchers have mused that the bold patterns disrupt the perception of predators, and that when the zebras run en masse from an attacking lion, they become a confusing jumble of stripes into which the initial target disappears. Others have pointed out that every zebra has a unique set of stripes, and that these stocky equids  might use these patterns to identify herd members, mates, or even mothers (if you’re a hungry foal).

One of the my favorite explanations has always been that the stripes protect against the savanna’s most fearsome creature: the tsetse fly.  These blood-sucking insects are not only vectors for some nasty diseases (such as sleeping sickness), but also hurt. A lot. (Having spent more time than I care to remember in the woodlands where these terrible, terrible creatures thrive, just the thought of tsetses makes me shudder. I have spent many hours hurling expletives (fruitlessly) at the tiny terrors.) Tsetse flies suck. A lot. And if wearing stripes were a way to fend them off, I’d have gone out in a zebra suit every day. There are in fact stories of one intrepid researcher back in the day dressing up in a stripey suit and attempting to test whether zebra stripes deter tsetses. But there’s only so much that one man in a zebra outfit can do, and field experiments are notoriously difficult…and so this remained a buried rumor until last year.

Last year, Swedish researchers discovered that horseflies (a close cousin to the terrible tsetse) don’t like stripes. And they tested this on an experiment useing  number of fake, plastic zebras painted solid black, solid white, and various things in between. Turns out that the flies really like dark colors over light colors, but still like solid light colors over stripes. And while in the real world, there are things (such as smells) that may attract tsetses to stripey animals despite their off-putting pattern, this study is pretty exciting. And next time I have to venture into the savannah woodlands? You can bet I’m wearing that zebra-striped shirt.

March of the Elephants

When you think of elephants, you may immediately think of their defining characteristics: trunks, big ears, tusks. Or you may think about how they live in large family groups and are very social. You may even think about the story of the blind men and the elephant. You probably don’t think about them as engineers of their ecosystem. But they are.

Elephants are native to the Serengeti ecosystem, but Serengeti elephants were likely all killed off for ivory in the 1800’s. At least, there weren’t any recorded there until the middle of the twentieth century when they started moving back in again. In the 1960’s they migrated in from both the north and the south, and by 1970 there were over 3,000 elephants in the Serengeti. Things got rocky for elephants again in the 1980’s as severe poaching reduced their numbers in Serengeti National Park to around 500. In 1988, elephants were given CITES endangered species status and worldwide trade in ivory was banned. This was good news for Serengeti elephants and their numbers rebounded again into the thousands.

These ups and downs in elephant population have allowed scientists to study the impact elephants have on their environment. I’ve written before about how the rainfall patterns in the Serengeti affect grasses, and about the role that fire plays. Elephants have their greatest impact on trees. Elephants eat both grasses and trees, but depend on trees for food during the dry season.

In the first half of the twentieth century, the number of trees per hectare was slowly declining across the Serengeti. But starting in the 1970’s, the number of trees rapidly increased. Scientists believe that the initial decrease in trees was due to the the disease rinderpest. Rinderpest killed off the majority of Serengeti’s wildebeest, allowing the grass to grow tall, and fueling huge, strong fires. These fires killed most tree seedlings, meaning that as trees died, they were not being replaced. When rinderpest was halted, the wildebeest population exploded, and the wildebeest kept the grass short and the fires smaller, allowing trees to gain a foothold once more.

Okay, but what about elephants? Well, elephants eat trees — especially small, tender saplings and regrowth from trees damaged by fire. In the 1980’s, while poaching was particularly severe on the Tanzanian side of Serengeti (Serengeti National Park), the Kenyan part of Serengeti (Maasai Mara) mounted a strong anti-poaching effort and kept its elephant numbers high. Across the Serengeti, the trees were increasing, but in the Maasai Mara there were also a lot of elephants. It turns out that in the Maasai Mara, the trees didn’t increase like they did across the border in Tanzania where the elephants had been greatly reduced. Instead the high number of elephants in the Maasai Mara is keeping tree numbers down, despite the reduction in fire intensity.

So elephants are key players in maintaining what scientists call “alternative stable states” in the Serengeti. While there are plenty of elephants once again in the Tanzanian part of the Serengeti, they don’t reduce the higher tree numbers; only fire can do that. But on the Kenyan side of the border, tree numbers remain low because elephants there have been continuously eating saplings; the tree population cannot increase because of the constant elephant pressure. The key difference between the two areas is simply their history.

I think the fourth blind man should get extra credit.

The Fourth reached out an eager hand,
And felt about the knee
“What most this wondrous beast is like
Is mighty plain,” quoth he:
“‘Tis clear enough the Elephant
Is very like a TREE!”

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”.

Wolf spider. Photo from Wikipedia.org.

“Leaf bug” from the Miridae family. Photo from Wikipedia.org.

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.

Lion density is mapped per 1km x 1km grid cell. High density areas shown in green, lower in pale orange/gray. Cheetah locations are the black +'s.

Lion density is mapped per 1km x 1km grid cell. High density areas shown in green, lower in pale orange/gray. Cheetah locations are the black +’s.

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.

DogMapcrop

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.

Complex Landscapes

This past week I’ve been reworking a paper about a study with Anna Mosser and Craig. The study asks the question: How did lions come to live in groups? It doesn’t seem like group-living in lions would be something you would spend much time thinking about – until you realize that lions are the only cat that regularly lives in groups. What’s special about lions?

Craig’s work over the past decades has shown that seemingly intuitive ideas about why lions form groups are wrong. Lions don’t form groups in order to hunt more efficiently. Lions don’t form groups to cooperatively nurse their young. Lions don’t form groups to protect young against aggressive outsiders. Instead, it appears that the primary purpose of lion groups is to defend territories against other groups of lions.

So territorial defense appears to be the key to group living in lions. But is territorial defense the only thing that matters? That’s what we set out to investigate. We created a computer model that simulates a bunch of lions living on a landscape. The model is a simplification of what happens in real life, but it contains some essential aspects of lion living.

First, we have complex landscapes. Previous research suggests that group territoriality is more likely in complex landscapes because there are highly desirable areas that are worth defending. If you had a landscape where everything was more or less the same, then you wouldn’t need to fight your neighbor over some small patch of it; you could just wander off and find your own patch that would be more-or-less the same quality as your neighbor’s.

Second, we have various behaviors that we can turn on or off in our simulated lions. For example, we can tell them that they can live together in a territory, but they can’t cooperate to defend it. We can also tell them whether or not they can live in a territory with their parents when they grow up. And we can tell them whether they’re allowed to make their territory bigger if they recruit more lions into their group.

By manipulating the types of landscapes and the various behaviors, we explored how often our simulated lions formed groups. Our results suggest that while territorial defense is important, it’s also important to have complex landscapes with high-value real estate. If the landscape isn’t very complex, then it’s easy enough to find an area to set up a territory without fighting for it. And if the landscape is complex, but doesn’t have any areas with high value, then there’s nothing worth fighting for or defending. It’s also important that lions be able to pass their valuable territories on to their offspring, for without inheritance, the benefits of all that fighting and defending are gone in a generation.

Lions evolved on the savannas of East Africa, where the landscape is complex with patchy areas of high value (near where rivers come together, for example). Humans did too. It’s possible that the same sorts of savanna landscapes that shaped group living and territorial defense for lions did so for people, as well.

Our simulation model

The simulation model. White areas are high-value real-estate, while black areas are low-value. Red shapes show where lions have formed a group territory, while blue shapes show where there’s a territory defended by just a single lion.