Indian Paintbrush – A Pretty Parasite
Anyone who has spent much time in the Western U.S. is probably familiar with one of our most attractive wildflowers – Castilleja, or Indian paint brush. Blooming riotously in the summer, from deserts to alpine meadows, the nearly 200 species in this genus (many endemic to North America) put on quite a display – a riot of reds, oranges, yellows, pinks, and purples. Despite being maniacally difficult to tell some of the species apart, Indian paintbrush is one of my personal favorite wildflowers, and not just for it’s breathtaking colors.
There are a couple interesting things about Castilleja. First off, the colorful blooms that make it so remarkable are not actually the flowers. They are the leafy bracts surrounding the very inconspicuous and uninteresting greenish white flowers at the very tip. Much like poinsettia, all the action is in the specially formed leaves. You can kind of see this once you know what to look for, as the color sort of shades back to green as you move down the stem.
Another thing you may or may not notice about Castilleja is that you almost never see it alone. Some wildflowers like mountain bluebells or little sunflower form huge monolithic stands, but Indian paintbrush is almost always scattered through an area with other plants. The reason for this is that, although it looks just like any other green forb to the naked eye, Castilleja is actually parasitic on other plants. In fact, it comes from an entire family of plant-on-plant parasites, Orobanchaceae. Although levels of parasitism range greatly in this family, they all have a defining characteristic: haustoria. Haustoria are specialized roots that drill into the roots of other plants for the purpose of stealing their resources.
Of the parasitic plants found in this family, Castilleja is actually a fairly mild parasite, known as a root hemiparasite. Since they have fairly normal green foliage, they can photosynthesize on their own. But they get a huge boost in fitness from stealing water and other essential minerals from a host plant. This may be an adaptation to places where resources, like water, are limited, the growing season is short, or the soil is poor. And while in the wild Castilleja is almost never found without a host, they can technically survive on their own, although they are much less robust in every way if grown in isolation. Castilleja also does not kill its host plant, although it often does take a toll on its fitness and growth.
Castilleja species are generalist parasites. Although certain species do much better with and seem to “prefer” particular host species, they will usually take whatever they can get. Because the species available to them may vary greatly and because they inhabit such a wide range of ecosystems, it’s to their benefit to be flexible and work with what is around – if you’re too picky you might find yourself with no host at all. In dry areas, sagebrush is a common host plant to desert species of paintbrush. In other places, lupine species (another large genus of the western U.S. found in a range of habitats) are one of the most common hosts. In fact, Castilleja plants that are able to latch on to lupine as a host plant may be very lucky individuals.
As a member of the bean family, lupines have a symbiotic relationship with nitrogen-fixing bacteria and any plant parasitizing them is going to get increased access to nitrogen. Studies have shown that Castilleja using lupines as host experience better growth, increased reproduction, and increased pollen output. Lupines may even offer some protection from herbivory to Castilleja, as along with nutrients parasites also receive the alkaloid lupinine, which is toxic and bitter to animals. This protection may be conferred to the Castilleja, although studies have shown mixed results in whether having lupine as a host actually does decrease herbivory on Indian paintbrush or not. Either way, it’s clear there are big benefits to taking advantage of lupine, and in my area it’s not uncommon to see meadows full of almost exclusively lupine and paintbrush.
Castilleja, along with other root hemiparasites, can actually have a surprisingly large effect on community structure as a whole. Introduction of a species like this can shift the entire dynamic by helping or hurting various other competitors for resources. Say you have a plant community that is dominated by one species of herbaceous plant. There is another species present, but it just can’t compete and is barely hanging on. Now, in comes a root hemiparasite like Castilleja. It gets big benefits from parasitizing the dominant species and not much from the secondary species. But in the process of parasitizing the dominant species it greatly reduces that species’ fitness. Suddenly, the secondary species can compete! This can lead to a community going from a virtual monoculture to either a stable system with the three species (parasite, dominant, and secondary) existing in roughly equal proportions, or a constantly fluctuating system where one species becomes extremely successful for awhile before the other takes over in a repeated fight for dominance.
In addition, if the root hemiparasite does better on the less dominant species, it may actual confer a competitive advantage on the secondary species and help it to increase its range against the dominant species. While the secondary species alone may have no hope of out-competing the dominant species for limited resources, in combination with Castilleja or another hemiparasite they together may succeed in elbowing out the normally successful dominant and either sharing or completely excluding it from the system it previously ruled.
As you can see, Castilleja and other root hemiparasites are forces to be reckoned with. But there is still much we don’t know about them. Are there other ways they confer benefits to their hosts? Are some species evolving to be host-specific? How much do they actually take from their hosts? And is their color (variable within a species and location) dependent on the nutrients and minerals they receive from the host plant or is there another factor? The complicated systems in which they exist and the huge variation among species make any questions about this genus difficult to answer definitively.
Smith, D. (2000). The Population Dynamics and Community Ecology of Root Hemiparasitic Plants. The American naturalist, 155(1), 13-23. doi: 10.1086/303294.
Adler, L. S. (2003). Host Species Affects Herbivory, Pollination, and Reproduction in Experiments With Parasitic Castilleja. Ecology, 84(8), 2083-2091. doi: 10.1890/02-0542.