Thursday, 30 June 2011

Conference insights 2

During the ATBC / SCB conference, we held a special session on the savannah / forest boundary. Organised by Prof. William Bond from University of Cape Town and Dr William Hoffmann of North Carolina State this session brough together people working on savannahs in Africa, South America, and Australia to share expertise and experience in these habitats. Many visitors to African savannahs are in this biome for the first time, and are un-aware of quite how widespread an important these biomes are to many people, worldwide. So it was fitting that the first talk of the session game some of the gloabl context. I was pretty staggered to hear that of all the primary productivity (IE all the plant growth) that happens anywhere in the world each year, 30% of it is in a savanah. In fact, savannahs cover 20% of all the land surface and are home to 20% of the world's human population too, so pretty important areas really. For visitors from the north, this is fairly extraordinary - there's little like a savannah where they come from.
Yep, it really can rain in savannahs! Wet January day at Dunia, Serengeti, 2011

We also covered the factors that drive the formation and distribution of savannah globally - primarily climate and fire (but maybe nutrients also play a part). It might not surprise those living in East Africa to hear that 85% of all fires in the world occur in savannahs, but it would be a big surprise to most visitors that this is actually a good thing! Moreover, we heard that globally, at least for areas with rainfall over 1000mm where both forest and savannah can survive, it's fire that determines whether there'll be any savannah at all. As well as existing within remarkably consistent rainfall areas around the world, the most important factor seemed to be the length of the dry season - if the dry season isn't at least 5 months long, there's unlikely to be a savannah there. This very strong dependency may, it seems, also be related to fire - a long dry season allows grass and other vegetation to dry enough to let the fire in, and it's the fire that keeps the forest back. On the nutrient front it's long been argued that soil deficiencies in Calcium, Potassium and Magnesium might also limit forest growth, allowing savannahs in the poorer nutrient areas, but we heard from Prof. Bond that, in fact, there's little evidence that at least at the soil depths where tree roots are found there's any real shortage at all - soils don't seem to be anywhere near as important as rainfall and fire in maintianing the global limits of the savannah biome, though they do have obvious impacts within it. All very intersting, and quite remarkable how similar the combination of fire and rainfall area globally in savannah areas.

I think those are probably the most relevant talks to this blog, though I did go to plenty of others too. But after the conference I had the pleasure of taking several of the conference atendees (including William and Bill) to Tarangire for a night, then for a day trip to Arusha NP where the education continued. You can read Ethan's comments on the Tangire trip over here, if you want - I'll give my own later here.

Tuesday, 28 June 2011

Conference insights

A couple of weeks ago I enjoyed spending the week at a combined meeting of the Association for Tropical Biology and Conservation and the Society for Conservation Biology's African chapter, here in Arusha. Now Arusha is not particularly a hub of scientific activity, so if you're an academic, far from colleagues any conference is going to be worth attending, and an international meeting of these two groups is probably a one-off opportunity (in fact, it's the first time ever the ATBC has met in Africa, let along Tanzania. One could wonder where the tropics really are...). So, for four days I was eyeball deep in science once more. Along side the talks, I was organising a workshop on fire and burning Serengeti, then last week I was teaching a course associated with the conference, so I decided to give talking at the meeting a miss this time (not to mention the fact that the abstract deadline once again caught me unawares...), and was able to sit and enjoy lots of interesting things. I'd have blogged about the highlights whilst they were happening, had I not been too busy with other things in the evenings. But now I've got time and am looking back over the notes, thinking I might summarise a couple of interesting facts I learnt today, and maybe some more tomorrow.

So. A talk by Damian Bell from the Honeyguide Foundation (about which all Asilia guides should already know, of course), was the first that had me reaching for my notebook. He had to hand some useful facts directly relevant to conservation, one of the 10 things I think worth talking about when there aren't any lions. Most striking to me were some figures from TANAPA's annual report (from 2007, as it happens, but I doubt they've changed much since then). TANAPA - the Tanzania National Park authority that manages all of the National Parks in Tanzania - had revenue of 69billion TSh (that was about $42,000,000 US) in 2007. This will almost all come from gate fees and bed fees - not a bad income. Of that, of course, TANAPA pay 50% in tax back to the government, but the bit that struck me was the fact that only 1.8% of the income in spent in local community related projects around the protected areas. It seems self-evident that if local populations don't see some benefit from conservation - and financial benefits are surely the most direct - then they're no going to be particularly supporting of the National Park when things are under pressure. So to see just 1.8% heading back to the communities seems extraordinarily short-sighted.

It's not good on village land either - 60% of revenues generated by villages for wildlife related things goes straight to Wildlife Division, leaving only 40% for the villagers. And things are only a little better for Wildlife Management Areas - 60% of the revenue generated by a WMA stays local (40% goes to Wildlife Division), but from this the WMA has to fund all the protection and visitor access things, so it's still not clear how much will actually be felt by local villagers.

Damian also had some interesting figures on quite how much benfit the tourism industry can generate aside from conservation fees - Grumeti Reserves are currently spending $30,000 on fruit and vegetables to local farmers each month. This sort of tourism-related revenue clearly offers massive benefits, well over and above what could come from TANAPAs 1.8% investment. So it's clear that tourism can play a vital role in financing conservation - without it, there's no way we'd have the parks we do today. But, of course, tourism also needs to be controlled if its not to cause more problems than it solves. Though that's a story for another day...

The next talk that had me scribbing was the final answers to his PhD research into the Wildebeest migration into and out of Tarangire National park, by Dr Tom Morrison. Tom started by reporting the staggering decline in Wildebeest numbers from this ecosystem - between 1990 and 2000 the numbers dropped from 43,000 to only just over 5,000. Today Tom tells me there are between 2500 and 5000 remaining. Of course this is nothing to the numbers of wildebeest back in the 1960 when we know the decline began. Tom's focus has been trying to determine where, exactly, the migrant wildlbeest go and how much movement between calving areas there are. We knew that animals from Tarangire go to two main areas - via Manyara Ranch up to the nutrient rich grasslands on the way to Lake Natron, and out east of the park to the Simanjiro Plains. It has also been suggested that animals might move to Lake Manyara National Park too, so he went there for good measure. Tom didn't want to just know where one or two animals went, he wanted to know them all. And he wanted to know if the same animal might switch from place to place each year, or if they always went to the same area. So he couldn't got for the most expensive option of satellite tracking all the animals - instead he decided to take photos.
Wildebeest showing their stripes whilst on the move at Mwiba Game Rance, Feb 2011.

Just like zebra stripes, wildebeest stripes on their flanks are individually recognisable. So Tom and his colleagues made a clever computer program that will search through thousands of photos and match pictures of animals with the same stripes. All he then needed to do was take lots of pictures in Tarangire and all the breeding areas over several years and join the dots. Five years and 9000 photos later, he's managed to trace movements of 900 animals - that's a significant proportion of all the animals out there - and he discovered some interesting things. Firstly (and probably most importantly) the Tarangire population is a single population - about 18% of animals did switch calving grounds between years. Animals calving in Simanjiro one year may well calve up at Natron next year. Next he confirmed that the animals in Manyara National Park are more or less resident, living there all year around, with very little interchange with the animals calving near Natron.  But the result that seems most surprising to me is that the only thing that really determined whether or not an animal would switch calving grounds one year to another, was if it calved successfully. If I'd been asked to guess before hand, I'd suggest that an animal that calved successfully one year would want to return to the same place to calve again the next year, whereas an animal that failed might decide to try somewhere else next year - but Tom showed that the opposite occurred and more switches happen after successful calving than after unsuccessful calving. Very strange - any ideas anyone? Disease? Who knows... He also showed that successful calving has a cost to females, with them having a lower survival in years when they calve - that wouldn't be any surprise to anyone who brings up small children if they had to live in an area with lots of lions and constant distractions too!

Anyway, very interesting things, I'm sure you'll agree! Hopefully something to pass on to visitors too.

Friday, 17 June 2011

Getting intimate with snakes...

Tarangire National Park is known for its snakes and we’ve seen Black Mambas, Puff Adders, Green Tree-snakes, Sand-snakes, Rufous-beaked Snakes, Boomslangs, and encountered Black-spitting Cobras and watched Rock-Pythons climb trees. Considering events in the past few months, and the fear of snakes that was instilled into me as a child, why am I so fascinated by these animals? Well to start with, it might be because I’m fascinated by evolution and how animals (and plants) are related to each other.

For example, snakes belong to a group of animals known as the Tetrapods or 4-legged animals. This might sound a bit confusing but it’s because in their evolutionary ancestry, they evolved from a reptile that was probably very similar to the monitor lizards we know today. In fact, monitor lizards use their forked-tongues the same way that snakes do: to smell or shall we say taste the air. Their forked tongues pick up little particles when they flick them in and out of their mouths, and pass the particles to an organ known as the Jacobson’s organ in the roof of their mouths. The fork in their tongue allows them to tell the direction of their prey. Look at how a monitor lizard swims… you guessed it? It’s called serpentine motion and is actually a very efficient way of moving.

So as I said, I’m as interested in the different groups of animals as in the species themselves. We call these groups by different names and one of the important groupings to have an idea of, are the families. Let’s take this little one that Colin found on his walk.

It’s small, about 30 cm long, and blind. It has a big yellow stripe down its back and can you guess what it’s called? A Yellow-Striped Blind-Snake. Its Latin name is Rhinotyphlops unitaeniatus.
Rhinotyphlops unitaitensis
 Blind-snakes (family: Typhlopidae) belong to a group of snakes that are considered primitive, yet highly specialized and they are closely related to another family called the Worm Snakes (family: Leptotyphlopidae). They first appear in the fossil record 135 million years ago and since then their general body plan hasn’t changed much, it is obviously a highly successful one.
Leptotyphlops scutifrons
Blind-snakes and Worm-snakes are blind because they live under the ground and don’t need to see. Their bodies feel tight, and are cylindrical with a large scale over their foreheads somewhat like a helmet. This acts as a battering ram when they push through the soil. Their tail ends in a spike (caudal spine) that they use as an anchor for pushing through the soil. I think it’s fascinating that these two groups of snakes have special glands in their foreheads whose function no one has yet figured out.

So, enough of the little snakes, what about the big ones? If you’re lucky enough to guide in Tarangire in the dry season, you’ve got to look carefully for Pythons (family: Pythonidae) who like to hang out in trees. The dry season is Tanzania’s equivalent of winter, and so some animals do what animals in colder latitudes do: they hibernate, but here we call it aestivation. They slither up into trees and basically go to sleep for weeks, if not months. They’ve usually had a good meal, and once comfortable, they turn their metabolism down to the region of 1 Calorie per day.
Python sebea
Some people are lucky enough to get to see a python eating something and there are plenty of pictures of pythons swallowing gazelles. How do they do it? Well, snakes have lower jawbones that aren’t fused together like mammals. The python, a constrictor, kills its prey by grabbing it with its sharp hooked teeth and throwing its coils around it. It then squeezes every time the prey breathes out and eventually suffocates it. It then slowly finds the head and opens its mouth as wide as it can. The jawbones dislocate and the skin stretches as it starts to swallow its prey.

Now that is three groups of snakes covered and we haven’t even touched on venomous snakes. Well, that’s because in actual fact, most snakes aren’t venomous and even if they are, you wouldn’t react to most of them. Even the biggest family of snakes, the Colubridae family, which has over 100 species in East Africa, only has 3 species of snake that could harm you. None of these are actually aggressive and they prefer to eat birds and other small animals that they find in trees. Many of the colubrids don’t even have teeth, but the ones that do have enlarged teeth in the back of their mouths and are known to be “back-fanged”.
Philothamnus sp.
The next two families of snakes are actually the ones that give all other snakes a bad rap: the Elapids and Vipers. These two families both have fangs in the front of their mouths, but the Vipers have big fangs that fold back into their mouths, and the Elapids have smaller fixed fangs (not that I’m ever going to look at a snake’s teeth to figure out which family it belongs to). Now, the other thing which I find most fascinating about this very small group of venomous snakes is just that: their venom.

If we’re looking at evolution, how is it that a snake that eats a 30-gram mouse or bird is going to have such potent venom that it could kill me, a 78-kg human being? The most probable answer is that these snakes likely have prey that could actually cause them some harm and so they need fast-acting, powerful venom. Most of the Elapids have neurotoxic venom which acts on the nervous system basically paralyzing the prey. Most of the Vipers have cytotoxic venom which is cell destroying.
Bitis arietans
Naja nigricollis
Of course, the trends always have exceptions and for most of the venomous snakes, their venom is actually unique. Spitting cobras are elapids but they have a cocktail of cytotoxic venom with a little neurotoxic thrown in for effect, kind of like the Tabasco in a Bloody-Mary. Spitting cobras’ fangs also have a cool feature that allows them to spray their venom up to 3 metres. Instead of the venom channel exiting the fang at the very bottom, it makes a 90 degree turn near the bottom allowing the venom to shoot straight forward.  

The last family of snakes (family: Atractaspididae) is an interesting one because scientists have spent a lot of time debating what family they really belong to, or whether they are actually a family on their own. They include centipede eaters and other interesting snakes like that. One of these snakes, probably the most famous, is actually venomous and very deadly in northern African countries, but in eastern and southern Africa it doesn’t usually kill. It is a burrowing snake that kills prey a little larger than termites, like skinks (lizards) for example. You can imagine the adaptations it has to have to be able to catch skinks in the ground. First of all how does it open its mouth to bite its prey when it’s squeezing through a hole in the ground? Instead of even trying to open its mouth, it squeezes past the prey and then basically sticks its fang out of the side of its mouth like a fish hook and pulls back. Pretty ingenious isn’t it? They actually bite a lot of people who try to pick them up and are called a Stilleto-snakes or Burrowing asps.
Don't touch me if you don't know who I am!!!
I could keep going on about snakes, but it might be a better idea to come back to them when we talk about other fascinating things like mimicry or aposematic coloration. But, it wouldn’t be fair not to mention even briefly what to do if you do end up in the unfortunate position of having to deal with a snakebite from a harmful snake.

This website probably has the most comprehensive explanation for what to do, but the basic things to remember and do are:

1.     Stay calm or calm down.
2.     Mark the bite site with a pen.
3.     Immobilize
4.     Evacuate (and if you’re in Tanzania, the best place to get treated is the Meserani Snake Park).

Identifying the snake is important, but most places will treat you symptomatically which means they’ll treat you for the reactions that you’re having.

Saturday, 11 June 2011

Acacia woodland

Next on the list of major savanna habitats must be the Acacia woodlands. The sun setting behing a lat-topped acacia provides one of the iconic images of an African savanna, and many of the most interesting game drives involve meandering through Acacia woodland. In fact, there are a very large number of Acacia species in Africa - something over 150 - and they're al a little different (let's ignore the current taxonomic discussions about Australia taking the Acacia genus for it's species leaving the African's with none...) . Ecologically, Acacia species play a vital role in the savanna ecosystem, but before we think too much about that, let's start by thinking about why Acacia woodlands are found where they are.
They make great backdrops, even for Flamongos! Lake Magadi, Serengeti NP Jan 2011

First, of course, we need to identify where the Acacia woodlands are and it's obvious if you're looking for it: Acacia woodlands are typically found on the lower slopes of hills and on the flatter land at the bottom of valleys. On the ridges there are generally broad-leafed woodlands, such as Terminalia and Combretrum woodlands, or in areas with a single rainy season you might find Miombo woodlands on the ridges. Boardering rivers, of course, you often find true riverine forest, another habitat again (although one that often features Acacia species). But between the riverine forest and the broad-leaved woodlands is the area where grasslands and Acacia woodlands are commonest. And the reasons for this position are probably to be found on those four main drivers of savanna ecology: fire, nutrients, water availability and grazing pressure. On the ridges, nutrients are very scarce thanks to millenia of washing by heavy tropial rains. Lower down there are more nutrients, but in consequence the grazing/browsing pressure is going to be higher - to survive in these areas you need to be very heavily defended - like the big throns on many Acacia species. The soil moisture content is also important: like other members of the Fabaceae (peas and beans being obvious examples, of course), Acacia species have a symbiotic relationship with nitrogen fixing bacteria (called Rhizobia) that live in nodules in their roots, and these bacteria are somewhat fussy about where they live. (In fact, they can life in soil away from the plants, but they are unable to fix nitrogen in isolation.)

This relationship with Rhizobia is responsible for what is probably the most important role Acacia woodlands have in the savanna: they're incredibly important nutrient pumps. Thanks to their nitrogen fixing bacteria, Acacia species have a pretty much unlimited supply of organic nitrogen, vital to producing proteins and growth. In the generally rather nutrient poor soils of Africa, this has a massive impact. All the browsers love a snack on the nutrient rich Acacia leaves, despite their thorns and high tannin content. But most importantly, at the end of the wet season Acacia trees drop their leaves like most other savanna plants - but because they have such a plentiful supply of nitrogen, they don't bother withdrawing all the nutrient before they do so. This is immediately obvious if you drive the savanna at this time of year: Acacia trees remain greenish right until the leaves fall, other broad-leaf species withdraw as much nutrient as possible, resulting in yellow or orange leaves, before they fall. And the consequence of this is that Acacia leaf litter is much richer than the little of other species and fertilises the soil under the trees. So effective is it, that in some places Acacia litter is used as a major fertiliser for poor soils.

Again, the impact of the richer soil is immediately obvious if you go and look under an Acacia - there's a whole lot more diversity in the herb layer than under a neighbouring non-Acacia species.
Lots of herb diversity in the understory of an Acacia woodland thanks to the fertilisation effects. Near Mbalageti River, Serengeti, Jan 2011.
Not much under a Balanites but grass (oh, and a few animals) - Grumeti GR, Sep 2010
This diversity, combined with the fertilisation effect making everything rather more nutritious than elsewhere in the savanna explains the reason you spend so much of your time on game drives in Acacia woodlands: everything loves them! Of course, the problem with this is that it attracts lions and other predators who hunt much more efficiently from the cover of woodlands, so if you're a browsing animal you've got to choose: do you got for the nutrient rich woodlands and under-story of the Acacia belt but face the higher risk of being eaten yourself, or do you avoid the richer habitats and forage in safer places where you can keep an eye on predators much more easily? If you're sensible, of course, you'll probably balance the two options up and make decisions based on exactly how much you need those nutrients at any one time – early in the dry season there's plenty of forage in the open grasslands and you've had plenty of nutrients recently during the wet season anyway, so you might spend more time in the grasslands. Later in the dry season those open areas may have been grazed to nothing and you're more in need of nutrients as you might well be preparing for pregnancy, so you might decide to take the risk and forage in the woodlands: at different seasons, different strategies make most sense and within the savanna ecosystem such movements are very sensible. Of course, you might also decide to just live on the woodland margin – nipping into the woods when you're fairly sure there are no predators around, but in easy reach of the open areas if you're more worried. This, of course, means that those ecotones – the transition from one habitat (Acacia woodland) to another (open grasslands) – are going to be fantastic places to explore on your game drive.

And that, for now, is probably enough about Acacia woodlands. There's lots more to say about both Acacias and the woodlands, but as an introduction to the habitat it's not a bad place to start.
Acacias do make for nice sunsets! Here some herons roost on a bit A. tortilis near Lake Ndutu, Serengeti, Jan 2010

Saturday, 4 June 2011

Red-billed Buffalo-weavers

Red-billed Buffal0-weaver, Naabi Gate, Serengeti NP, Jan 2011
Over on my other blog I've discovered that searches for Red-billed Buffalo-weavers seem to be one of the most common routes to my blog. Very odd, as they're hardly the most inspiring of birds to look at. But despite being so common most people overlook them, they are actually rather interesting, honest! The most interesting thing about them ecologically, is that they are one of very, very few birds to posses a phalliod organ. In fact, the only other one is the White-billed Buffalo-weaver (NB some other birds - notably some ducks and ostrich - do posess pseudo-phalli, but they're not quite the same as the phalliod organ of a red-billed buffalo-weaver.). Interstingly, both males and females have them - but males are much longer (can be over 1.5cm), and those male with territories are longer still. Size obviously matter - infact, females prefer to be inseminated by males with larger organs, so it matters a lot! Unusually for birds, copulations in this species can last several minutes, and the species is very much a polyandrous breeder with both males fathering chicks in the same nest, feeding the young and defending the nest together. In fact, copulations seem to happen a lot both with females but no sperm transfer, and with other males - only a minority of copulations successfully trasfer sperm it seems, and successful copulation in the field seem to be 10 - 20 minutes long. Not bad for a bird!

The nests of this species are called compound nests, as several 'pairs' share the same walls, with typically several nests within the same overall nest structure. I say 'pairs' in quotes, because these are seriously polyandrous birds. Each compund nest is defended by a group of resident males, and each nest within the compond nest is likely to have two or more males assoiated with it (but just the one female). What's more, if you look at the genetics involved in parternity within the nests, you'll find that more often than not the actual father of some of the yound in the nest isn't even one of the resident males, but one of the non-territorial individuals hanging around the edge of the colony, so it seems that a lot of the time the female sneaks off and finds non-territorial males to father young too.
Red-billed Buffalo-weaver nests, near Arusha, Nov 2009

It follows that there's a lot of competition among males to be fathers, even though genetics again tells us that the cooperating territorial males sharing a female are at least sometimes related (though not usually close elatives). What the phalliod organ actually does isn't entirely clear - it's not the route for sperm to flow through - but apparently it needs stimulation before females can be inseminated by the male, and they're the only birds to experience anything that looks rather like an orgasm (but only after repeated copulation), and only at the point of orgasm (who's main effect seems to be pulling the female closer to the male) is sperm transferred to the female, so we think it must be related. What's more, some rather interested scientsts have demonstrated that if you manually stimulate the organ on a bird, you can induce orgasm and sperm transfer (the description in the paper isn't detailed enough to say exactly how the manual stimulation too place, but you can leave that to your imagination). 

Interesting things huh (you can read more about it here? In fact, it goes on - they've got some remarkably evolved sperm too, to deal with the considerable post-copulatory competition between sperm from different males within the female, all trying to fertilise the same egg.Can't find a picture just now, but trust me - theyre turbo-powered!

Thursday, 2 June 2011


They're the thing you probably think of first when you start thinking of savannah habitats, so probably a good place for me to start too. As we've already said, savannah is defined by grasses and all the grazing animals (by definition) depend on grass to a greater or lesser degree, so there must be something interesting to be said about grasses. Surely?! Not one of our guides in training had ever been brave enough to tell their clients about grass though, so certainly room for improvement.
Only ecologists could wander around Serengeti looking for grasses, surely?!

So, what is there to say? Well, it's probably worth first wondering why grass is so popular with so many animals, and how it survives being eaten all the time? The answer to the first part is probably as simple as there being a lot of grass out there, so clearly a niche waiting to be filled. The second is, perhaps, more interesting, even though we all know it anyway: the growing tip of grass is right at soil level, so you can eath the tips without damaging the growing point - not like most other plants at all. This is clearly a great evolutionary advantage for grass - the first grass that grew from the bottom must have had a much easier life than the other plants around. But it also raises the interesting prospect that, once evolved, the plant may actually harness grazers to keep the environment suitable for itself. The main competition grasses face on the savannah is from trees for water and (probably to a lesser degree) light. If grazers munching on grass encourage those same grazers to occassionally nibble (and thereby seriously damage) the seedling acacia nearby without suffering any serious cost themselves, they've effectively harnessed the animals to keeping the environment suitable for grasses. Neat trick for a bit of grass, huh?! Further more, we think there's a chance grasses may have also evolved flamibility for the same reason - by encouraging fire in the ecosystem (and grasses always carry the fires from place to place in the savannah), they keep seedling trees out of the picture in grasslands. As they suffer little from fire, this is again a neat trick (if it's true - there's some debate still, but the topic is an interesting one all the same!).

And then, despite first apperances, there's grasses and there's grasses. Which is to say, not all grasses are the same. In fact, there are somewhere between 9 and 10,000 grass species, depending on who's counting. And that includes (in only eight grass species) 70% of all crops - maize, wheat, sugar, millet, rice, etc. are all grasses. So there's somthing interesting to begin with!

Given the incredible diversity of grass species out there, it's no surprise that for grazing animals there are some pretty serious differences too, particularly in the dry season. One of the most important ecological divisions of grasses is into the two groups or "Sweet grass" or "Sour grass" - the familiar sweetveld and sourveld for our South African colleagues. This fundamental distinction is based on what the grasses do to the nutrients they've captured during the dry season. During the dry season all grasses stop growing and most above ground parts die, life remaining only in and near the roots. Indeed, for annual grasses, the entire plant dies during the dry season. But plants preparing for the dry season have two options - either they withdraw all the nutrient they can into the roots, leaving a very poor cellulose only dry matter in the dry season, or they don't and the nutrients remain available above the surface during the dry season. Obviously, grazing animals will prefer the latter. In general grass species do one of these or the other, but a few adapt their strategies slightly depending on how nutrient rich the soil is, being less careful about the nutrient that gets left to grazing animals in areas where the soils are richer.

This fundamental distinction between strategies for preparing for the dry season has huge impacts in grazing animals. Indeed, there's some evidence that the (now largely extinct) large mammal migrations of South Africa were primarily driven by animals leaving the sourveld in the dry (winter) season and moving to areas with better dry season grazing options. The same impacts can be seen in many areas of East Africa too.

This is far from the only difference between grasses, however, as even during the wet (growing) season there are considerable differences in the nutritional content of grasses growing in different soils. In particular, grasses growing on the recent volcanic soils associated with the Rift Valley (famously the short-grass plains of southern Serengeti and Ngorongoro, less widely know being the Simanjiro plains north east of Tarangire NP, etc.) are particularly rich in nutients. Indeed, it is largely the search among pregnant and lactating female wildebeest and zebra for grasses rich in Calcium (needed for growing healthy bones in particular)  and Phosphorus (needed for growth in general) that drive the famous migrations, with calving occurring precicely when the animals can use the nutrient rich grasslands in these areas.

Grasses can also be beautiful!
The other main variable that determines how palatable a particular grass might be is, of course, how well defended it is. Grasses use silica, a natural glass, to defend themselves - it's what makes grasses sharp enough to cut you as you walk past some. But not all species are as well defended as others, and (perhaps even more importantly) at different stages of growth the same plant may be better or worse defended. Silica takes time to produce and deposit once growing has begun, so new growth - as well as being rather nutrient rich - is typically rather poorly defended. Which is great for grazers! So you often find that once an area has attracted a number of grazers and they've kept it free of old, well defended course grasses, a grazing lawn develops where, just as in our garden lawns, as soon as the grass gets a bit high (and starts to defend itself), something comes along and chops the top off. In East Africa these grazing lawns ocur comonly on particularly rich grasslands (indeed, the whole of the Serengeti's short-grass plains may be a grazing lawn), but also on a smaller scale around local nutrient sources, like termite mounds. It's incredibly easy to find them at the end of the wet season, where most grass is tall, except at the grazing lawns. And a nice feature to point out, once you start seeing them. Interestingly, in areas with Hippos, grazing lawns are really common by rivers, they're very good at forming them. But in the few places where White Rhino are still relatively abundant they also play a very important roll in opening and maintaining grazing lawns that are then also utilised by many other species - this is a process that is now (sadly) lacking from many parts of Africa, and we can only wonder about the overall impact.
Grazing Lawn in Kruger, May 2011 - note characteristic very short grass among larger patches of tall, less palatable grasses. White Rhinos at work?!

Anyway, that's enough for now. I hope I've persuaded you that there are some interesting things to say, even about grass! And I'm sure there'l be more to come too...