Plant Rarity (or, Plant of the Week part 1)

I was going to post about the plant I studied in school, and ended up with lots of things I could say about it (imagine that). So, I thought I would start with a bit about how I chose this rare-but-not-endangered endemic, and also how plants can be rare.

I found my study plant, Physaria bellii, or Bell's Twinpod, by visiting with the Colorado Natural Heritage Program's very competent botanists and giving them sort of a laundry list of what I wanted in a study system, and they were happy to oblige: I wanted the sites 2 hours or less from my home, I wanted to work with something that at the time of the conversation (September) still had vegetation showing so I could collect leaf samples for DNA, and if possible, something that was rare and hybridizing with a more common congener. But not too rare, as I didn't want to compromise the species by sampling it.

How about Physaria bellii? Okey dokey. Physaria bellii is named after the namer's colleague, which sometimes happens in science. The genus belongs in the Brassicaceae, or mustard family, and P. bellii is just one of several narrowly-distributed species of Physaria in Colorado. It is a habitat specialist, found mostly in soils from the Pierre and Niobrarra shale formations, which make up hogbacks running along the east side of the Front Range of Colorado from Denver north to WY. It will also sometimes be found in soils associated with red sandstone. The habitat is patchily distributed, and as a result, one might hypothesize there is limited gene flow.

In the process of studying this species, I came across a simple table by Deborah Rabinowitz, which is called the Seven Forms of Rarity. When you compare the various combinations of population size (large or small) and geographic range (broad or restricted), there are seven ways a plant species can be rare. You'll notice there's an eighth combination, but that's when the factors come together and the species isn't rare (large population size, broad geographic range).

Here's the table:

Physaria bellii comes in at the upper rightmost cell. It's locally abundant in its preferred habitat, but that habitat is rare, so it is as well. Notice how plants can be rare in a variety of ways, and of course this helps when trying to figure out how to conserve rare plants.

OK, a bit about the looks of this plant. As you can see from the picture, it's a low-growing plant, and has yellow flowers. Those flowers smell very nice, as I found out when I was growing them in the greenhouse to do crosses. I guess I had just never bent down close enough in the field to get the scent. They are pollinated by generalist pollinators, and can have 50-200 flowers open at one time. The flowers mature into the "twinpod", a two-part fruit that can have two seeds per side, but usually has one. The leaves are covered by tiny hairs, perhaps as a water-loss-prevention adaptation. The leaves are variable, but all spatulate with no sinuses, and this helps distinguish P. bellii from its relatives.

Next time - hybridization, population genetics and chromosome squashing.