Saturday, 29 October 2011

Echos of extinction...

Black Rhino browsing (?!) Ngorongoro Crater, Feb 2011
Driving around places like Tarangire I often find myself wondering what impact the loss of black rhinos have had on the ecosystem. Sadly, these days there's nowhere in Tanzania with a significant rhino population (though you've got a good chance of seeing them in the Crater still) and it's easy to forget just how common they were, not so very long ago - more than 700 in Serengeti alone in 1974. Friends of mine (rather older than me!) talk of driving from the Tarangire gate to Silale and seeing over 20 rhino in a drive, and being areas they'd avoid walking in because the rhino densities were just too high to make it safe. They're big animals, at about 1500kg for a bull, and they eat a lot of food. Such large populations of a large mammal crashing about in the bush like a tank must have had massive impacts on the landscape - but there's nowhere we can go now to see what an East African savannah with a decent rhino population looks like.
Close enough to this Black Rhino! Kruger NP May 2011

Black rhino may have been important dispersers of Sausage tree seeds.
Fallen Kigelia africana flowers are a valuable food source for many species

We can speculate that perhaps some of the large, thick patches of bush of a single age now found in central Tarangire might well have been more open and varied habitats when there were rhinos there - but we really can't say for certain, because no-one's been monitoring the changes and we've nowhere to cross-check against anyway. There is one species that might, in time, show some changes - that essential savannah tree, the Sausage Tree Kigelia africana. Ever wondered what disperses Kigelia seeds? Not much eats them - baboons do when they're desperate, insects have a go and elephants will when pressed, but did you know they're a favourite of Black Rhino, and considered important seed dispersers for this species? In fact, Kigelia seeds don't germinate unless they've been eaten by something like a rhino, so mayube much of the pattern we see currently in this species related to ghosts of past rhino distribution? Who knows.

Interestingly, following the recovery of rhino populations in South Africa before the latest spate of poaching, we did discover that White Rhino are massively important in shaping landscapes - as massive grazers they create grazing lawns that are then colonised by a large number of other grazers and, impresively, but some rather rare grassland bird species that specialise on these grazing lawns. Before rhino number built up to their current levels, this sort of lawn was rather rare, and the increase in rhino has greately increased the variability of the landscape of Kruger and Hluhluwe-iUmfolozi parks, with important benefits for the populations of other species - this species, if not necessarily a keystone species, is certainly an 'ecosystem engineer'. (Why Tanzania and Kenya - despite what many will tell you - have never formed part of the range of the critically endangered Northern White Rhino is a mystery to me - so much grass here, they'd surely love it (and when not being poached they do rather well in Kenya, it seems).) Maybe they were driven to extinction by our ancestors long before modern times?
Southern White Rhino trying hard to make a grazing lawn, Kruger NP, May 2011

Anyway, all interesting things to think about when you're lucky enough to come acros a rhino, or even if just enjoying the shade of a sausage tree. Let's hope the current spate of poaching can be controlled and the increases seen earlier will continue again. (That's twice in a week I've blogged about one of the big five. Shock! Must find smaller fare for next time...)

Thursday, 27 October 2011

The landscape of fear

Serengeti Landscape of Fear - where would you feed? On the green by the river where predators hide? In the bare bits on the plain with no grass left but a good view? Or risk the woodlands somewhere in between?
Lions are often in thickets (N. Serengeti)

But sometimes on kopjes... S. Serengeti

where you might also find a cheetah! N. Serengeti
It might sound like the sort of novel you's find abandoned at a camp by passing visitors, but understanding what ecologists mean by the 'landscape of fear' - how predators have impacts on the ecology of a savannah that go well beyond their direct predation events - is such an important concept that I'm going to break my usual rule of not talking about the big five! Actually, understanding the concept is simple - it's about looking at a landscape and working out where you'd be (most) scared to be walking. Long grass? Yup, scary. Thick riverine vegetation? Not for me! Nich bushy kopjie? I'll give it a miss, thanks. You get the idea - any place you might think about looking for predators whilst on a game drive, is going to be a scary place for herbivores too. And it's not simply a function of the numbers of predators that are present, but how efficiently they might be able to hunt within that habitat - I'd be much happier walking a short grass plain with a high density of lions than I would walking through some tangled thickets with a much lower density of lions. (Obviously I'm also a bit warier of buffalo and elephant than most herbivores have to be, but if you've done a few walking safaris you'll have the idea anyway.)

So what? These patterns are so obvious, we don't really think about them, or think they have an important part to play in very much - but we'd be wrong. In places where top predators have been removed, we rapidly see changes in the behaviour of herbivores and, soon after, we'll see changes in vegetation. Perhaps nowhere more famously than in Yellowstone National Park in the US (described here) - when wolves were eliminated elk and bison were released from their major predator and the populations changed - they didn't change in numbers very much, becausee like Serengeti's wildebeest and zebras (and, of coruse, elephants and the rest of the mega-herbivore group) they're limited by bottom-up processes of food availability, not top-down processes like predation. But they changed in behaviour, spending much less time looking around for predators and not moving around very far from their favoured willow patches. Which mean that after 50 years of no wolf predation, those patches of willows were in a bad way - it looked possible this form of riverine vegetation would vanish forever. Until 1994, when wolves were reintroduced. Within a matter of months the female elk and bison were spending significanty more time looking around, avoided open areas and only stayed in one place for a little period before moving on. And, in time, the riverine areas started to regenerate. We'd witnessed a 'trophic cascade' - removal of a top predator had had a massive impact on vegetation and landscape, the impacts 'cascading' down from top predator through the herbivore to the basal layer.
Leopards like riverine too, C. Serengeti

And lions often hunt by rivers and small ridges, Tarangire

These Fringe-eared Oryx have spotted something from their vantage in the Tarangie plains
The same processes are in operation on our east african savannahs all the time. Lions are far, far more efficient predators in areas where they can conceal themselves (in bushes, around kopjies, along even small river lines and shelves) than in the open - and as we know they plan their hunts accordingly. Similarly, leopards are best looked for around kopjies and riverine forests, where they're both able to avoid lions and can hunt sucessfully. Even cheetahs are often around kopjies to get a good view. So these are scary places for animals and, as we've seen so often before, if you start altering grazing pressure (one of the big 4 processes in the savannah), you'll see a change in the vegetation - riverine forests get a headstart if there's a big predator population living in them, scaring all the wildlife out! There's even the possibility that this processes becomes a positive feedback - as bush increases, so does predation success, making bushes even scarier, leaving fewer herbivores to keep the bushes back, meaning the bushes spread further, etc. And on the other hand, grazers like to graze areas where they have a good view - short grass - but if there are enough of them, their very grazing ensures the grass stays short, reinforcing the benefits.

Of course, things are complex for a herbivore - you can't simply decide never to forage in a wooded area because you might get eaten, because maybe half-way through the dry season you'll have eaten all the grass on the plains, and all that's left is in those scary woods. So you can either starve to a certain death in the plains, or head into the woods and risk predation, but at least stand a chance of avoiding starvation. Animals must constantly be assessing and weighing up the costs and benefits of foraging in high reward (grass under legumes like Vachellia is often of higher nutrient content than elsewhere) but risky areas, versus the safer but less beneficial areas on the plains. Not only will seasons make these decisions change, but so too will the details vary during the day - it's far more important to be in the plains at night than it is during the day, resulting in a evening movement of animals out of woods and onto plains - woodland edges are a great place to be at sunset!
Tarangire Wildebeest treck from the woodlands to the plains every evening

Spotting predators on Serengeti's short-grass plains is easy - no fear here!
Anyway, once you've got the idea of trophic cascades and the landscape of fear, you'll start seeing how it works all the time, and I hope I've given you enough here to start thinking about at least. (You'll get a much more in-depth and very readable discussion of tropic cascades and the landscape of fear in this nice article here.)

Tuesday, 25 October 2011

Endemics, or why are some species common?

This Saintpaulia (African Violet), like most others is probably endemic to the Eastern Arc
As you might have guessed, I've been away again. This time I've enjoyed a few days exploring the Amani Nature Reserve in the East Usambara Mountains. It's a fantastic bit (or, really, bits) of forest, perched high above the north coast of Tanzania, and forms a key part of the fames "Eastern Arc" mountains that stretch from northern Tanzania (just catching southern Kenya in the Taita Hills) around and down the coast, cutting back inland through the equally famous Uluguru forests and down to the Udzungwa mountains. And if biodiversity is your thing, then the Eastern Arc has it - forget the savannah, the real wildlife is in these Eastern Arc forests. They form an important part of one of only 34 global biodiversity hotspots (2.3% of the global land surface, but hosting over 50% of all plant species!) identified by Conservation International (NB there are 8 of these hotspots in Africa, the same number as in Europe, Central Asia, North and Central America combined - no wonder I love to be here!). There are at least 96 vertebrate species that are endemic to the Eastern Arc forests - 10 mammal, 19 bird, 29 reptile and 38 amphibian species. A far, far greater degree of uniqueness than you'll find in any savannah. And that brings me to one of my favourite scientific questions - what makes some species common and widespread, and others rare and local? I think we'll leave for now the question of why there should be so many rare species all concentrated into such a small area (why the Eastern Arc is a centre of endemism) and focus on this more general pattern of common and rare species (mostly because it's something I've published on myself and I won't need to look so much up!).
Usambara Pitted Pygmy-chameleons Rhampholeon temporalis are incredibly restricted in range
Most people tend to ask why is such and such a species rare and then struggle to find an answer. Why should these rather cute Usambara Pitted Pygmy-chameleons be completely restricted to the Usambaras, and not hop onto the Pare Mountains just a few kilometers away, or even the forests of Kilimanjaro? No reason at all really - expect they're not there. So recently some people have started asking a slightly different question - why are some species common and widespread? This might seem a trivial piece of semantics, but I don't think it is. In fact, if you look at the areas of occupancies of species in any taxonomic group, you discover that it's not small ranges that are unusual, but large ranges - in other words, rare species are common, and it's commonness that's rare!

This Mt Kilimanjaro Two-horned Chameleon is common in Arusha and clearly related to the Usambara species, but still incredibly local in distribution
Now start looking at the problem this way and you might start wondering where a species starts from in the first place. Let's assume we have an ancestral species that's on the verge of speciation. However we want that new species to split from the ancestral population (by geographical isolation, or together with the other species by, perhaps, chaning breeding season), we're going to start with at least one rather small distribution - perhaps the founders made landfall on some distant island and evolved in isolation into a new species, restricted to this new range. But then along comes an earthquake, and the island is no longer isolated, the species is free to come back to the mainland (where, of course, it's ancestors have also continued a process of evolution and might be rather different by now). Coming back into contact with the descendants of this ancestor the new species can either compete happily and spread in range, or just might not spread at all. It looks like, for most species, they don't bother to spread, just stay nice and localised and rare. But every now and again, one of them makes the grade and becomes common and widespread. What makes this difference isn't yet clear and there's certainly no one simple answer. Rather, it seems likely that to make it big you've got to pass several tests simultaneously - you need to disperse well, breed fast, etc.
The Usambara Bush Viper is so localised and rarely seen that I can't find another photo of a juvenile to be sure of the identification here! This might be some sort of Egg-eater instead.
 On the level of individual species, though, it's still a mystery - why should Rufous-tailed Weavers have such a small range between Serengeti and Tarangire? What's wrong with the savannah that, to me, looks identical just north, south and east of this range that keeps them out? Is it competition? With what? Why can't the evolve just that teeniest bit more to let them spread further? Why is the range changing now (they're recently made it to Kenya)? It seems to be climate, but how and again, what stops that tiny bit of evolution that's needed from happening? Hmmmm. All puzzling questions really, but great to discuss whilst you're looking at some of Tanzania's endemics (especially as many of them, like the rufous-tailed weaver and ashy starline, aren't that inspiring to look at!). And, as ever, if you've got any ideas of your own, feel free to pass them on!

Oh, and do check out the Eastern Arc mountains if you want a truly unique wildlife experience!

Monday, 10 October 2011

Waifs and strays

Eurasian Bee-eaters at Manyara Ranch, Jan 2011
For the past fortnight or so I've enjoyed daily sight and sound of migrating Eurasian Bee-eaters whizing overhead - migration is in full flow and over the next few weeks these early bee-eaters and swifts will be joined by a whole host of other passerines and raptors. Many of these are following the movement of the rains, and will have been in Africa for many weeks already, some will stop here (like many of the bee-eaters), others will carry on further south. And among the millions of birds that get it right, we can be sure there'll be one or two that took a wrong turn somewhere, or carried on a bit too far, to turn up in unexpected places. Also moving at the moment are lots of Afrotropical species that are using the onset of the rains as a cue to move from dry season hide aways, to wet season breeding areas and with so much movement going on there too, interesting things are bound to occur. We've been in our current house for about 2 years now, but a chestnut weaver in my garden last weekend was still the first I've heard here - it's fizzing song getting going despite still being in non-breeding plumage.
Chestnut weaver in non-breeding plumage, Manyara Ranch Aug 2011

And this weekend I enjoyed a walk (in the rain - I am British, after all) around Lake Duluti where, among the regular species, a lesser flamingo was swimming around the lake in the company of this rather fine black duck. African black ducks are usually found rather rarely on highland rivers and streams, so this was quite a surprise - note it's very long body and obvious white wing marks, plus white bits on the base of the bill that separate it from other species here abouts. This and the flamingo were both new birds for me to see at Duluti, despite many visits there. The diversity of birds in Africa is truly amazing, and now, with the start of the rains, is a great time to look out for unusual things moving between their more regular locations. The forecast is for a good short rains around this part of Tanzania this year, so expect lots of breeding activing in the next few weeks too - all very exciting! Let me know if you see anything good!
African Black Duck, Lake Duluti, Oct 2011

Thursday, 6 October 2011

Red and Yellow Barbets and duets

Male Red and Yellow Barbet, Manyara, Jan 2011
There's nothing like a Red and Yellow Barbet to brighten up your morning! Even the most casual of observers wants to have a second look at this fairly common species, and well they might. Both males and females are impressively bright, you have to look past the red and yellow to the black on the throat and cap to identify the sex - male with black cap and throat (as on the right here), female just with black speckles on an orange cap (as below). And when you see one, you often find several as they tend to stick about in pairs and family groups most of the time.

Female Red and Yellow Barbet, Manyara NP, Jan 2011
Such strong pair bonds are re-inforced by their other obvious trait - the crazy duetting or even group singing in this species are thought to be mainly designed to build the pair bond, and less advertise territory occupancy to neighbours (though obviously it does that too). (If you've not heard the song, check it out half-way through this recording - crazy! I remember it when confused, because they're saying "RED (and yellow), RED (and yellow)", etc.) Evidence suggests that the calls of the two birds are so closely aligned there simply isn't sufficient reaction time for one bird to be listening and responding to the other when they do this, rather they both have some internal rhythm that they stick to, keeping each other in time together.

Dueting is actually surprisingly common in birds - at least 120 species of some 32 families are known to duet, but we don't really know why. The main features loosly associated with it are (a) it's mostly tropical birds that do it - so many visitors here won't be familiar with the concept except, perhaps, in owls, (b) it's mostly species that show rather little sexual dimorphism (like the barbets here) and (c) most of them live in rather dense habitats - though I think this species is one of the exceptions here. Certainly it's an impressive sight and sound!
Female Red and Yellow Barbet, Manyara NP, Nov 2010

Another thing you might notice about this species andthe closely related D'Arnauld's and Usambiro Barbets are their association with termites. Like so many things, these two ground barbets are rather partial to a snack on termites, given the chance, but they also rather like to nest within termite mounds - they dig a hole in the side and the termites will eventually wall their nest up, within the mound - giving lots of the benefits of termite-controlled air-conditioning to ther barbets too. Very handy. Though it might well make them vulnerable to brood parasitism from Greater Honeyguides - a species known to favour barbets and often searching termite nests for nests to parasitise.

Finally, check the toes on the top picture - classic zygodactyly (two toes forward, two backwards), perhaps an indication of their shared ancestry with woodpeckers? (Woodpeckers, Barbets and Honeyguides are all fairly closely related, within the order Piciformes - together with Toucans, but not hornbills which are more closely related to trogons and rollers, etc...).

Monday, 3 October 2011

Dung Beetles

Continuing the theme of small but rather important pieces of the savannah jigsaw puzzle, I thought I'd write a little about dung beetles. As usual, I'll try and follow my three questions for interpreting wildlife sightings - what is it? What's it doing? And what's it's role in the ecology of the environment?
Scaning for the route?!

So, let me first confess that I've never even identified a single dung beetle to species level. Unless you're a real specialist, I think you can forget it. Our dung beetles are are insects of the order Coleoptera and, as a rather prominent biologist (J.B.S. Haldane) once (may have) said when asked what we can learn about the Creator through studying His works: "He must have an inordinate fondness for beetles". In fact, the latest estimates  - published here in August - of terrestrial biodiversity are around 8.7 million species, of which about 7.8 million are animals (we've only described about 1.7, though). It's estimated that about 80% of all species are insects, and of these about 40% are Coleopteran beetles, which would suggest there are about 2.5 million species of beetles out there. In one tiny corner of the Serengeti plains alone, over 100 species were recorded in a relatively small study. As only a tiny proportion of these are already described I conclude (a) if you want to discover a species new to science, look at beetles, and (b) there are far too many beetles to spend time trying to identify them specifically. Still, most of our dung beetles belong to the Scarabaeidae family, and most people will have heard of Scarabs, especially if they know anything about the ancient Egyptians, who considered them sacred (holy), since it's clear that the world must be kept in motion by a giant dung beetle rolling it about.

So, that's what they are, identified as far as I feel the need. But what are they doing? Well, when we usually notice them they're rolling balls of dung along the track. Why? Because they eat it. Yumm. Most of the ruminants feeding on grass only extact about 50% of the nutrients from their forage, and obviously much less for hind-gut fermenters like elephants, so there's still significant resource left in the dung of these animals. And dung beetles love it for everything - they eat it themselves (some species eat it on site, some under the dung and some roll it off to snack on elsewhere), they roll it off and eat it as part of their courtship procedure (if you find two on the same ball, they might off on honeymoon with a nice snack to keep up their energy...), and they take it away to provision their young. For breeding, often the males dig large holes where they'll store several dung balls, a female laying a single egg on the top of each one (and in some cases coating the balls with a layer of clay that hardens around the dung ball. These broods are, in turn, a favourite food of honey badgers and some mongooses.
Dung beetle nest predated by honeybadger or mongoose, Lake Manyara, July 2010

The really exciting thing about dung beetles (I promise!) is, however, the impact they have on the ecology. First, let's appreciate the task they perform in tidying up dung. Your average zebra produces about 4.1 kg of dung per day, and a Grant's Gazelle about 0.75kg (never let it be said I'm not full of useful facts!), so let's assume about 200,000 zebra and 1.4M wildebeest for Serengeti, and guess that wildebeest, being bigger than Grant's Gazelles do about 2kg per day, and we're looking at a massive 1.3 Million tons of dung per year in Serengeti - nearly 3620 tons per day! It's just as well there's an army of dung beetles out there, just waiting for their meals (as the Australians learnt, when they started cattle ranches and the Australian dung beetles, used to a fine quality product from kangaroos, turned their noses up at the offerings from cows, with a massive fly problem the result - and they had to import African dung beetles to clean the mess up!). And, of course, we all know that dung is a pretty good fertiliser - whilst most of what they bury they also eat, the sheer numbers ensure that huge amounts of nutrient cycling are carried out by these beasts. What's more, they a bt picky about where they dig - it must be moist enough for them to dig, so that's why they're out there rolling balls long distances, looking for somewhere suitable. And as not everywhere is suitable, they tend to concentrate the dung in certain areas, creating a nutrient hotspot. Which, of course, attracts more wildebeest, to produce yet more dung, which is immediately returned locally - a major source of heterogeneity in the Serengeti plains. So, absolutely critical for nutrient cycling in the savannah - in fact, the huge volumes of dung involved alows you to realise that, thanks to dung beetles removal and burying, almost all the soil you walk on in Serengeti must, at one stage not that long ago, have been a dung ball. Lovely thought...