Parasitic plants - a Commiphora mistletoe

Plicosepalus meridianus growing on Commiphora, Mwiba Ranch, Aug 2011

In the middle of the dry season out here there's precious little colour to break the greys and browns, but look closely at the Commiphora and you might be surprised. Not only do they flower themselves in the dry season - a small rather unexciting flower, but many of them (I think mainly C. schimperi, but it's tricky for me without leaves!) around Mwiba had been parasitised by Plicosepalus meridionalis, a shockly pink flower that on first glance appears to grow straight out of the tree's bare branches. (I don't know a common name, I'm afraid.)

So, how would we interpret this 'wildlife' sighting? Start by identifying it - I'm fairly sure this is what I've photographed (certainly this genus), but some of you are probably better botanists than me and might correct me. It belongs to the family Loranthaceae, most of which are parasites and which also includes all the African mistletoes (though not the European and North American ones). It's worth pointing out it's only growing on Commiphora too - like many, it's a pretty host specific parasite.

Note how it grows all along the host!

Then explain what it's doing. You probably don't need to say it's flowering - that should be obvious! But have a close look and you should spot a little green at the base, and have a fiddle with the Commiphora bark and you might find some more where the plant holds tight to it's host. It's green because, like most mistletoes, it can photosynthesize a bit - though it does get most of it's nutrients from it's host. So its also (probably) photosynthesizing and stealing nutrients and water from the Commiphora - though probably not much at the moment, as the host is in a dormant phase for the dry season. Find the plant in the wet season and the story is different - mistletoes use both active and passive mechanisms to tap into and extract nutrients and water from their hosts. This is clearly not to the hosts' advantage, but the impacts probably aren't so strong that it threatens the health of the host - a closely related species P. acacia (guess what it's host is!) has been found to only thrive when the host itself is thriving, a sick host means a sick parasite with few flowers and low growth.

That's what it's doing, but what about my third question, what's the role in the ecosystem? Well, in this case we can speculate a bit. The obvious thing it's doing right now, is flowing in the middle of a period where there are few other flowers available. (It can probably do this and remain active during the dry season because it has a ready source of water from it's host, the succulent Commiphora.) And the flowers are pink. So stick around a few minutes and see what happens - you'll almost certainly see a sunbird nip in to feed. Pink, red, orange and yellow flowers are often signals for birds (which have good colour vision), and during the dry season there are precious few flowers around. Some sunbirds move away for the dry season - Coppery Sunbirds are a well known migrant, for example - but many stay and make use of the few specialised flowers available during this lean season. And those few flowers that are available, of course, must get visited very regularly, with excellent pollination chances. So you could argue the plant is helping to maintain the pollinator community during the lean period - certainly if there weren't a few species during this, all the sunbirds would have to migrate to greener areas during the dry season.

Scarlet-chested Sunbirds (here Arusha, March 2011) require dry season flowers

Even more interestingly, we could talk about some of the general impacts of parasitic plants on biodiversity at large. As we've mentioned, parasitised plants aren't (usually) killed by other plants, but they are weakened. And if you were going to be a parasite you'd probably want to parasitise something fairly vigorous and strong that there's lots of, to ensure the next generation can also find some hosts to parasitise. Now, if you have one particularly strong and domiant plant in an ecosystem, it's likely to out compete all the others and you'll end up with a rather low diversity system. But if parasites are more common on these strong plants because it's in their interests to do so, then they'll weaken that dominant plant and allow other species that aren't usually able to compete to survive too, boosting the biodiversity of the ecosystem - a pattern that has been demonstrated experimentally in grasslands, using grass parasites. So although I don't know it happens with this species and Commiphora, you can certainly wheel the story out again in April when the Cycnium are flowering everywhere (you know them, the little white and pink flower that love short grass, I've not got a photo though, I'm afraid. You'll have to look here for one if you don't know it!), as these are also hemi-parasites (hemi-parasites being parasites that don't steal all the requirements they need from their hosts, but also make some nutrients themselves) on grass. Certainly plant parasites are interesting things with important impacts!