Tuesday, January 4, 2011

Mistletoe: the kissing parasite

For many years now, every time I see mistletoe hanging-up around Christmas time I find it sort of humorous.  Most people think of mistletoe as the "kissing" plant, I find it as a complex parasitic organism.  Yes, mistletoe is a parasite that is pretty common in western forests and deserts.

A clump of mistletoe growing in the center of a juniper tree.

However, parasitism is only the beginning of the story.  Even more interesting is how the mistletoe got on the tree in the first place.  Mistletoe produces red or white berries which are possibly toxic to humans but extremely tasty and nutritious to birds of all types.  Many types of birds will gorge themselves on the berries and as a consequence carry the seeds to new locations.  In-fact, a southwestern bird known as the pheinopepla was found to eat around 1100 berries a day when berries were available.   Eating all those berries means a lot of seeds being transporting to new host plants.

Phainopepla, found to eat around 1100 mistletoe berries per day when berries were available.
Seeds are transported in the birds digestive tract but also on their beaks.  Mistletoe berries are covered with a very sticky substance causing seeds to stick to the birds beak, which the birds wipe off onto trees and shrubs where a new plant can grow.  The sticky seeds also pass through the digestive tract of birds and when defecated on a plant can germinate and parasitize the new plant very quickly.

Desert tree severely parasitized by mistletoe.

From all this you may think that mistletoe is a severe problem taking over and destroying our forests, but things to not always as they first appear.  In many cases mistletoe actually benefits the forest.  First of all the berries provide food for bird species that live in the area, increasing the number of birds and number of bird species an area can support.  Secondly, some trees, such as the junipers, when parasitized actually produce more of their own seeds.  This also increases the food available for birds and animals, thus supporting greater numbers of animals and greater diversity as well.  Parastized trees also form deformed 'witches brooms' which many birds and animals prefer for nesting sites.

So the next time you see mistletoe hanging in the doorway wow your "kisser" with this knowledge and they may never look at mistletoe in the same way.  They may not want to kiss you after their new found knowledge either though..

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