Thursday, 1 December 2011

Leopard Orchids

Leopard Orchid, Near Tarangire, Nov 2011
I'm just back from 2 weeks in the bush with Ethan and a bunch of other guide trainers and have a stack of photos to sort through (and loads of ideas for new posts, so there might be a flurry of activity on the blog again!). Whilst most of these are going to take a while to ponder and work though, I thought I'd start with a botanical story about the leopard orchid, since we found plants in all stages pre-flower, to ripe fruits and I got a nice set of photos.

Verreaux's Eagle Owl nesting in Leopard Orchid, Tarangire NP, June 2011
Leopard Orchids Ansellia africana are the only species in their genus, and occur widely throughout Africa, always growing as an epiphyte (i.e. growing on another plant, but not taking any nutrients from them - so not a parasite). Just like leopards, they're often seen lying on branches of big leafy trees and the flowers are yellow with brownish spots. They're very pretty, and often taken into cultivation (they're now considered threatened in the wild in South Africa, thanks to this trade), but they're also rather interesting ecologically. Three things particularly interest me: firstly, there's an association with ants - plants secrete nectar (not from the flowers though) that a wide variety of ant species drink. In return for the sugar, the plant gets effective protection from other insects. (Didn't seem to stop these Verreaux's Eagle Owls from using one in Tarangire as a nest site though!)
Leopard Orchid flowers provide no nectar to pollinators, Nov 2011

Next is that strange fact that mature flowers don't produce nectar. Actually, this isn't all that uncommon in orchids - they're well known for their 'deceptive' pollination strategies, a phenomenon noticed and described by Charles Darwin himself, in 1877. Most flowers produce nectar to encourage and reward pollinators, leopard orchids and many other orchid species don't - they rely on scent and appearance alone to persuade pollinators to visit. It turns out that leopard orchids are mainly pollinated by solitary bees, though we don't yet know if the scent the plant produces is similar to that of the pheromones produced by the bees to help them find mates - it certainly is what happens in other orchid species. In these species along comes a male bee expecting to find a female ready to mate, and instead finds a flower - in the case of the bee-orchids the deception even extends to the shape of the flower mimicing that of a female bee too - how cruel! Still, obviously works, as there were plenty of pods on one of the plants we found.
These pods could contain millions of seeds, Nov 2011

Which is where the thrid interesting fact comes in. This time it's general for all orchid species (I think?) - they are wind dispersed and have some of the smallest seeds of any plant - completely lacking any store of food for the new plant. They're so small that in some species each pod can hold over 4,000,000 seeds! I've not found the number for Ansellia, but we can be pretty sure there's lots in there. With so little energy provided, orchid seeds have no chance of germinating unless they can get it from somewhere else - and they all do it by hijacking a fungus. The seed, having settled in a crevase of a tree sits and waits - if a bit of the right fungus grows along the branch looking for something to digest (we call these little bits of fungus mycorrhiza) it infects the orchid, and instead of being digested the orchid grabs the fungus and steals it's nutrients in an act of parastism that enables the seedling to grow. In some species this later turns around and the orchid returns the favour by providing excesses sugar to the fungus (a sort of mutual parasitism if you like, or at least a symbiosis), but increasingly botanists are realising this isn't always the case and many mature plants also maintain this parasitism throughout their lives. So this stage of life is also rather interesting.

Leopard Orchid in Kigelia africana, Near Tarangire, Nov 2011
Happily these orchids still seem very abundant in much of Tanzania (though a quick google search confirms you can easily buy Tanzanian Leopard Orchids from a few suppliers at least, so how long this will last I don't know - I'm not giving you the links so you don't do it!), so you should be able to keep telling these stories for a while at least!

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