Savanna science in South Africa

Savanna science in South Africa!

By Andrew Tredennick

Greetings from 30,000 feet! Justin Dohn and I are currently on our way to Kruger National Park in South Africa to attend the 10th Annual Savanna Science Network Meeting. Going to conferences and meetings is one of the great perks of being a scientist. We get to go to interesting places and hear about exciting research that is currently in progress. Most of the time we read published journal articles to learn about the newest scientific findings. The cool thing about conferences is that people are presenting work that is (usually) not complete, so there is ample room for speculation, encouragement, and criticism – all things that drive science into the future. It is also a great chance to meet other people that are interested in similar topics and form potential collaborations.

Justin and I will be presenting our research as well as attending all sorts of presentations throughout the week. As an added bonus we will be going on a couple of game drives throughout the famed Kruger National Park on the border of South Africa, Mozambique, and Zimbabwe. Hopefully an elephant does not decide to overturn our rental car.

Stay tuned for updates from the Savanna Science Network Meeting. Justin and I will be picking a “Talk of the Day” each day that we will discuss and we will also describe some of our own research.

Oh… and there might be some pictures of cool wildlife as well.

South Africa, Day One:

5am. No need for the alarm we set because we went to bed at 8:30pm and our jet-lagged bodies are now wide awake. But, we were planning on this because the gates to the park outside of our guest camp open at 5:30am. The first day of talks doesn’t start until 8am so we have plenty of time for an early morning game-drive. We grab some apples for breakfast and jump into the epitome of all safari vehicles: the Toyota Corolla.

We are the first car out the gate and are ready to see some wildlife. But alas, we drive and drive with nothing but impala to show for it. Don’t get me wrong, impala are beautiful, but we want to see lions and tigers and bears (OK, just lions…the tigers and bears would be a little out of place). We find a dirt road that looks promising, but after 20 minutes of no wildlife, our morale sinks. So we turn on the radio and discuss how if we were lions we would be right here (since we know SO much about lion habitat preference, of course).

Suddenly, Adele comes on the radio and I finally look back toward the road behind to see a group of six female lions sitting in the road. We conclude Adele has mystical lion calling powers (Figure 1).

The lions are magnificent as the rising sun sets aglow their amber fur. I drive up slowly as Justin leans out the window to take pictures. Then, one stands up, and we are suddenly and acutely aware of how big a lion is…and how small a Corolla is! The spine of the lion when standing goes above the door pane where the window begins. Thus, we rapidly roll up our windows and simply watch as the lions walk away. Not a bad morning.

Figure 1. Number of Lions Seen versus Adele-ness. Correlation or causation? We may never know...

Of course, we are really here for science, not lions. So, be prepared…back to science for the next post!

South Africa, Day Two: A Primer on Savannas and Fire

Well, I wasn’t lying. Back to some science today, so hold on tight…

Fire matters in savannas. In some places fire literally keeps a savanna a savanna. But what is a savanna to begin with? Basically, savannas are ecosystems with an uninterrupted layer of grass with an interrupted layer trees. Savannas are not grasslands because they have too many trees. And savannas are not forests because they have too much grass. All three pictures below show savannas from different regions in Africa.

West African Savanna

East African Savanna

South African Savanna

So why does fire matter? The balance between trees and grasses in savannas depends on lots of things, but most people agree that rainfall, fire, and herbivory (animals eating either trees or grasses) are the most important. The coolest thing may be that the importance of these factors (rain, fire, herbivory) changes depending on where you look. This kind of change is what makes ecology interesting.

Figure 2. Percent of tree cover as a function of mean annual rainfall. Black line shows the "climatic potential" percent tree cover. The points are actual observations. Most of the points, all from savannas, do not meet the potential line. From Sankaran et al. 2005, Nature.

In some places savannas are more-or-less “set” as savannas – then amount of annual rainfall limits the number of trees even possible (see the blue panel in Figure 2). Fire and herbivory can lower the amount of trees, but the climate sets what is possible in the absence of those disturbances (see the black line that runs above the points in Figure 2). As rainfall increases though, more and more trees are possible and eventually the climate “allows” the existence of a forest – which is NOT a savanna.

But, if you look at the brownish panel in Figure 2 you will see that very few points (these points are actual observations of tree cover in savannas) actually meet the line (which shows what is possible in terms of rainfall). Why not? Fire.

Figure 3. Bi-stable and stable savannas in Africa. Vertical hashed regions are the bi-stable savannas. This means they could potential be forested. From Sankaran et al. 2005, Nature.

And some other things, well, lots of other things…but that would ruin my big finish. So fire can keep savannas as savannas instead of forests. That is the take-home message. And we can actually see these “bi- stable” savannas (that is, savannas that could be either savanna or forest) in Africa in Figure 3 and for the globe in Figure 4.

OK, that was a lot of information. But I wanted to set you up for the next post talking about a presentation by Carla Staver at Princeton University on modeling fire spread in savannas. Plus, think about the great conversation piece you now have!

Figure 4. Bi-stable and stable (deterministic) savannas throughout the globe. Of main interest here are the "Bistable, currently low tree cover" regions. From Staver et al. 2011, Science.

South Africa Day 2 Cont.: Fire and Abstractions of Reality

Ann Carla Staver from Princeton University gave a very interesting presentation on modeling fire spread in savannas. As we saw in the last post, fires are a big deal in savannas. And when we as ecologist recognize that something (like fire) plays a large role in an ecosystem (like savannas) the next step is to figure out how it works. Think about and old car, circa 1970s. You probably know that the carburetor is important in making the car go, but you may not know why or how it actually works. Ecologists are mechanics of ecosystems – we want to know how things work. Not necessarily to fix the system like a mechanic does with a car, but more to understand because that is the goal of science. More importantly in some cases, a deep understanding of an ecosystem can help guide its management. So to guide management in savannas we need to understand fire.

Models are key tools for ecologists. A model is an abstraction of reality – just like a Van Gogh painting or a road map of the USA. We abstract so we can understand and we leave out details to appreciate the whole…otherwise we are right where we began, just staring out at a fire in a savanna. Consider this quote from Sylvie and Bruno Concluded by Lewis Carroll:

____________________________

“That’s another thing we’ve learned from your Nation,” said Mein Herr, “map-making. But we’ve carried it much further than you. What do you consider the largest map that would be really useful?”

“About six inches to the mile.”

“Only six inches!”exclaimed Mein Herr. “We very soon got to six yards to the mile. Then we tried a hundred yards to the mile. And then came the grandest idea of all! We actually made a map of the country, on the scale of a mile to the mile!”

“Have you used it much?” I enquired.

“It has never been spread out, yet,” said Mein Herr: “the farmers objected: they said it would cover the whole country, and shut out the sunlight! So we now use the country itself, as its own map, and I assure you it does nearly as well.”

____________________________

Figure 5. Spheres in barrels that limit percolation. If we pour jelly on these, which one end up with more jelly at the bottom?

Carla presented an interesting idea of modeling fire spread in savannas as percolation. Savanna fires are carried by grass, not trees. Trees can actually stop the fire. So, Carla visualized trees as barriers to fire spread and so the fire must percolate through the trees. For example, take a look at Figure #. Imagine each of those “barrels” represents a confined space where “spheres” of some kind are floating in space. Now imagine you pour a bowl of raspberry jelly over the barrels. In the first bowl the jelly would fall through lots of open space and maybe rest on some spheres. For the most part though, a lot of jelly makes it to the bottom of the barrel because there are so many spaces for the jelly to go through, or percolate. The second barrel has many more barriers to spread, so the percolation of jelly would stop sooner, and less would make it to the bottom of the barrel.

This is basically how Carla set up her model of fire spread, but the barrels are some area of savanna and the spheres are trees of different sizes. The more trees you have the less fire spread. Pretty cool idea. I think borrowing from other disciplines (as Carla did here, borrowing percolation equations from physics) is a great way to gain new understanding. Sally Archibald, Carla Staver, and Simon Levin discuss some of the ideas from this talk in a really cool paper titled “Evolution of human-driven fire regimes in Africa” (Archibald et al. 2012, PNAS).

And just for fun…we went on another game-drive, so here a cool picture of a giraffe peaking over at us!

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