Sunday, 13 November 2011

Woodpeckers as keystone species

It's been a while since I posted a birdy blog and since I got some nice pictures of a Cardinal Woodpecker at the weekend, I thought I'd use it as an opportunity to talk about woodpeckers in general, since they're surprisingly important in the habitats they occupy. As usual, we'll look to answer the three questions I use to prompt me when seeing wildlife – what is it? What's it doing? And what's it's role in the ecosystem.
Female Nubian Woodpecker, Kisima Ngeda, Aug 2011

So, for identification, woodpeckers are generally fairly easy. In most of northern Tanzania and Kenya, there are four common species of savannah woodpeckers – the commonly seen Nubian, spotted all over; Bearded, the largest and with a black throat and stripy face; the rather small and neat Cardinal, with spots on the back and streaks on the front, and the very colourful Grey. Away from the dry north of Tanzania the Nubian is replaced by a number of other options – Bennett's or Speckle-throated being the obvious ones. There are plenty of other species around, of course, but they're mainly associated with forest and we'll forget about them for now. Woodpeckers in general are rather widespread, obviously similar species occur on every continent except, rather strangely, Australia. They're relatively closely related to barbets and hornbills (note the zygodactyly – two toes forwards, two-toes backwards – they share with the barbets, easily seen in some of these photos). So, that's what they are. Now what do they do?

Male Cardinal Woodpecker, Manyara Ranch, Nov 2011

Normally you'll come across a woodpecker first when you hear it, either calling – Nubians in particular are noisy – or from hearing the 'tap-tap-tap' of their beak on a tree. Calling is often done by pairs, and we can safely assume it serves the dual purpose of strengthening a pair bond and communicating to neighbours that the territory is occupied. The tapping is where it gets more interesting – most of this is exploratory, trying to find hollow bits under the bark where tasty larvae may live, some is more obviously getting at the food once they've found it, and some it again a territorial statement like calling – though this purpose seems to be less common here in Africa than in northern regions. And the most interesting of all is the hard banging they use to excavate nest holes. I'm sure (unless you're Australian!) we've all seen the beautifully neat holes woodpeckers make for their nests, often several holes in a single stem. There's two things that are particularly interesting about this to me – the first is how they do it in the first place. The speed and pressure generated in order to dig into the wood is extraordinary – the deceleration from 6-7m/sec to stationary at impact isequivalent to 1000 times the pull of gravity – the effect on humans would be similar to Usain Bolt running head-first into a brick wall at the end of his 100m sprint. Not pretty, I should think! And the adaptations they have to avoid the problems we'd get from banging out head on a brick wall are also impressive – slightly differentlength upper and lower mandibles, extra thick skull, fluid-filledshock absorbers, unusual size and shape of brain, etc., etc. Quite remarkable really!
Grey Woodpecker, Near Arusha, March 2011

But the second thing about these holes is where they get really interesting. Woodpeckers mostly use theholes they excavate only once, after which the holes are available to anything else that likes to live in holes – birds, bats, other mammals and all. In fact, there's a huge array of animals that live in holes, but can't make them themselves (though some, like barbets, may make adjustments to get the hole right for them). Over time the holes get larger and larger, allowing a whole host of species to find homes. In some northern forests, woodpeckers have disappeared (for anumber of reasons we don't need to go into), and once the holes aregone, so too do all the other species that make use of them. Thus the loss of woodpeckers has much greater impacts on the whole ecology of a woodland than the simple direct effect – the consequences cascade down through other species too. Which is exactly why some people suggest woodpeckers may be seen as keystone species – a single species that holds together a whole load of other species and have disproportionate impacts on the ecosystem. Mighty important things, woodpeckers!
This Brown-breasted Barbet is probably uing an old woodpecker hole, Nr Boma Ng'ombe, March 2011


No comments:

Post a Comment