Under the mistletoe
An experiment on a parasitic plant proves an important ecological concept
Jul 14th 2012 | from the print edition
THE magical properties of mistletoe will be familiar to any fan of Asterix. The hero of the series of comic books by René Goscinny and Albert Uderzo gains his superhuman strength from a mistletoe-containing potion brewed by the local druid. In the real world, mistletoe is less well regarded (except by those seeking someone to kiss at Christmas). It is a parasite that can damage trees. But an experiment conducted recently in Australia suggests this is only part of the story. For, in the woodlands the study investigated, mistletoe did indeed have wondrous properties. If it was removed, many other species vanished, too.
The study in question, just published in the Proceedings of the Royal Society, was conducted by David Watson and Matthew Herring of Charles Sturt University, in Albury, Australia. Dr Watson and Dr Herring looked at patches of woodland in Billabong Creek, New South Wales, and sorted them into two groups: those where mistletoe was present, and those where it was absent. They then divided the former in two. Half the patches they cleared of mistletoe, and half they left untouched. Then they waited, to see what would happen.
Actually, that is not quite true. Before the experiment got under way, they surveyed the bird population in every patch of woodland each season for two years—in other words, eight times in all. Then, when three years had passed after the removal of the mistletoe from some of them (during which time they continued to cull any resurgent bunches of mistletoe from the trees in the experimental patches), they carried out another extended bird survey.
The results were intriguing. As would be expected, the one species that feeds almost exclusively on mistletoe berries, the aptly named mistletoebird, vanished from the culled woodlands. Species that make their nests in bunches of mistletoe also tended to disappear. But lots of other birds disappeared, too: around a fifth of them, compared with what had been in the culled patches before the culling.
Indeed, things were worse than that. In the patches of woodland where mistletoe was left in place, the number of bird species actually increased, probably because of the end of a prolonged drought in the area. Compared with the species diversity in these patches when the final count was made, the number of species in the culled patches of woodland was down by about a quarter. Mistletoe is thus acting as what ecologists refer to as a keystone species: it supports an entire ecosystem, and removing it has drastic effects for the whole.
Clearly, those effects are much greater than just a loss of berries and nesting sites. Most likely, mistletoe is enhancing the food supply of other birds by increasing the number of creepy crawlies available for them to eat. That could happen in two ways. First, mistletoe weakens (and sometimes kills) its host trees, opening them to infestations by wood-boring insects and similar critters. Second, mistletoe sheds a lot of leaves, which add to the depth of the leaf litter in a wood. Deep litter favours insects, and thus insectivorous birds.
Whatever the mechanism, though, this study provides what no previous study ever has: evidence from a controlled experiment that keystone species really exist. Remember that next time you catch somebody under the mistletoe at Christmas.