"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

May 2, 2012

Cuscuta chinensis

Today's post has some plant-on-plant action, featuring a species of dodder. Dodders are a group of holoparasitic plants made up of about 100-170 species. They are plants that consistent entirely of stem, with leaves that have been reduced to tiny scales. They smother the host plant in a tangled mess, and dig deep into the host tissue using modified roots called haustoria to draw out water and nutrients. We have previously featured the European Dodder on this blog, but today, we will be looking at the Chinese Dodder Cuscuta chinensis, and how it interacts with plants that are not native to its home range.

Image Credit: Jayesh Patil
There are many studies that look at characteristics of successful invasive species (an introduced species that has subsequently become a pest). Generally, plants that have become invasive after their introduction have faster growth rates and are able to utilise nutrients more efficiently, allowing them to outcompete the native flora. In addition, according to the "enemy release" hypothesis, one of the reasons why newly introduced plants and animals become so successful in their new homes is because they are freed from the burden of their natural predators and pestilence, thus allowing them to propagate unchecked across the new land. While this seems to indicate that the best way to control invasive species is to introduce their natural enemies as well, the main problem is that you are introducing yet another new species. Remember that folklore about the old lady who swallowed a fly and subsequently introduced a sequential menagerie into her body? You don't really know what cascading effects the new biological control species will have on the local ecosystem - after all, the cane toad (Bufo/Rhinella marinus) was introduced to Australia to control beetles in sugar cane plantations, but have since become a huge ecological problem.

The higher growth rate and resource-usage efficiency of these invasive plant does have a drawback though - it makes them more attractive targets to parasites. So what if a native parasite can turn the table on the invaders? What if a native parasite acquires a taste for an exotic new host?

The Chinese Dodder is a parasite with eclectic tastes, as it is capable of infecting more than 100 species of wild and cultivated plant species. To find out how well C. chinensis grows on native flora compared to their introduced counterparts, a team of researchers in China evaluated the performance of C. chinesis on 3 invasive plant species and the native equivalent from the same genus. They found that not only did C. chinensis grow much more prolifically on the introduced plants,but it also caused more damage. In fact, C chinensis is more damaging to plants that are more efficient in using their resources - the very trait which makes them so good at being invasive in the first place.

There is also another possibility - one which the researchers did not mention in the paper: Unlike the native plants which have had a long co-evolution history with the dodder and have thus evolved various means to counter the parasite's tricks blow-by-blow, the naive introduced species have never encountered C. chinesis before, which leaves them more vulnerable to attacks by the parasitic dodder. For those exotic introduced plants, it seems that the very thing which had brought them so much success in their new home may end up causing their downfall when confronted with a certain holoparasite.

Li J, Jin Z, Song W (2012) Do Native Parasitic Plants Cause More Damage to Exotic Invasive Hosts Than Native Non-Invasive Hosts? An Implication for Biocontrol. PLoS ONE 7(4): e34577. doi:10.1371/journal.pone.0034577

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