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Parasite - the very word is enough to send shivers down the spine. It conjures images of blood-sucking leaches or worse, the chest burster scene from the movie Alien. We have a deep evolutionary disdain for organisms that make a living by living in or off of something else. However, I think it is due time to drop our preconceived notions of these organisms. Far from being antagonistic or detrimental, more and more we are discovering that parasites play important roles in the ecology of our planet. They can serve as indicators of ecosystem health and even promote biodiversity, something we are all scrambling to understand and preserve.
It is estimated that nearly 50% (give or take) of the lifeforms on this planet are parasites. The animal kingdom is full of them and, indeed, those are the ones we are most familiar with. However, there are plenty of parasitic plants out there as well, roughly 4,000 species actually. Some of these are subtly parasitic whereas others are so specialized that one would hardly recognize them as a plant without a bit of scrutiny. Parasitic plants are quite diverse, hailing from many different families. There is no way to generalize them all but I would like to give you an introduction to this group. At the end of this, I hope you walk away not only with a new sense of wonder for the botanical world, but also a greater appreciation for parasites as a whole.
The world of parasitic plants can roughly be broken down into two major categories - stem parasites and root parasites. As you can probably guess, this has to do with where their parasitism occurs. Stem parasites include some of the mistletoes (order Santalales) or dodder (Cuscuta spp.), which tap into their hosts tissues through their stems. The root parasites do all of their parasitizing under the soil. Their roots tap into the roots of the plants growing around them. All this is done using specialized structures called "houstoria."
There is another group of parasitic plants that do something entirely different. These are called the mycoheterotrophs. Plants like Indian pipe(Monotropa uniflora) and the coral root orchids (Corallorhiza spp.) fall under this category. These are not only some of my favorite types of plants, but they are also some of the strangest. Most of these plants have given up on the photosyntehtic lifestyle altogether. Instead, they cheat mycorrhizal fungi into forming a one-way partnership with their roots. The fungi gain nutrients from the photosynthetic plants they partner with and the mycoheterotrophs steal some of it. In a sense, these plants are indirect parasites on other plant species.
We go a bit further with categorizing parasitic plants. In doing so, we have to take a closer look at how dependent the parasites are on their host. The least parasitic of the bunch are the facultative parasites. These plants can grow with or without a host, though they usually perform much better with. The yellow rattle (Rhinanthus minor) of Europe and Asia falls under this category. On the other end of the spectrum are the obligate parasites or those that require a host. These come in two different lifestyles. Hemiparasitic plants are only partially dependent on a host plant. They derive some water or nutrients while still photosynthesizing on there own. This group includes plants like the Indian paintbrushes (Castilleja spp.), many species of mistletoe, and the witchweeds (Striga spp.).
Even stranger are the holoparasitic plants. Plants in this category have gotten rid of photosynthesis altogether. Instead, they gain all of their nutrient and water needs from their hosts. Some members of this group would hardly be recognized as plants at first glance. One of the oddest holoparasites, Hydnora africana, looks like something out of the Super Mario franchise. Because of they don't need sun, many of these species spend most of their lives underground or in the deep shade of other plant species. They only become obvious when it is time to flower.
Some of these holoparasites have even gone as far as to give up most of their "body." Instead, they exist inside their host's vascular tissues as a network of threads resembling fungal hyphae. We only become aware of their existence when their flowers burst forth from their host. Oddly enough, the species that produces the largest single flower in the world lives in this way. Native to Sumatra, the corpse flower (Rafflesia arnoldii) parasitizes vines. When the timing is right, a large bulbous growth begins to grow from the vine. This growth gradually swells like some sort of tumor until it unfurls to reveal a flower 3 feet in diameter and weighing up to 24 pounds!
Throughout all of this you may be asking yourself "what are the costs to the host?" Certainly this is worth asking. In some cases, the hosts don't suffer terribly, in others, the host is slowly drained over time. However, this is a matter of perspective. Sure, individual plants are harmed but what are the effects on the ecosystem as a whole? Taking a holistic perspective on parasitic plants paints quite a picture indeed!
More and more we are realizing the profound effects parasitic plants have on the ecosystems in which they exist. They are often keystone species as well as ecosystem engineers. Because their numbers rely on the density of their hosts, they can have a stabilizing effect, not allowing certain species to become too prevalent. This in turn opens up space for other species. Parasitic plants also alter the way water and nutrients move through the environment, creating a patchwork of habitat for other species to colonize. Time and again research is showing that parasitic plants actually increase biodiversity where they are native. What's more, many of them offer food and habitat for other organisms such as birds and mammals.
Despite their importance parasitic plants are largely ignored or, even worse, flat out maligned. Sure, they can become crop pests, however, that has more to do with the unnatural ways we grow our food rather than the parasites themselves. We can't pick favorites when it comes to conservation. Provided they are native, parasites have their place in the ecology of ecosystems around the world. It is time for a fresh perspective on parasitic plants.
Rafflesia arnoldii image by Henrik Ishihara
Matt Candeias is a plant fanatic. His current research is focused on how plants respond to changes in their environment, which takes him to the southern Appalachian Mountains where ample topography and seemingly endless plant diversity offer a window into how and why plants grow where they do. He operates a daily blog and a weekly podcast, In Defense of Plants.
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