Ranunculus californicus and R. occidentalis in the Cuyamaca Lake Area

Ranunculus occidentalis

Ranunculus californicus

Fig. 1. Left: Ranunculus occidentalis. Right: Ranunculus californicus. Pictures from the Cuyamaca Lake area taken on 3 May 2018. Click on the pictures for larger versions.

On a survey for Nemophila pedunculata near Cuyamaca Lake on 23 April 2018, myself, Carla Hoegen, Fred Melgert and Don Rideout came across a Ranunculus plant with two flowers, each with five petals. When she and Fred posted the pictures at iNat, Carla alertly determined the plants as R. occidentalis, and not the R. californicus on the voucher list for this area.

This resulted in some discussion at iNaturalist about the determination of the plants in this area.

As a result, I returned to this area on 3 May 2018 with the single goal of doing a histogram of the number of petals on all the flowers I saw. Because a quick histogram from photographs from Fred and Carla showed a clear peak at five petals, I thought all the plants here were going to be R. occidentalis, and decided just to make a single histogram from all the plants I saw.

The first plants I saw had 14 flowers with five petals, and a single flower with six petals, consistent with the histogram I had done from Fred and Carla's photographs, making these plants perfect R. occidentalis. But then I found a plant with two flowers with seven petals; three flowers with eight petals; and two flowers with nine petals, which were perfect R. californicus. I'd never seen a location before that had both species together, so I was more than a little confused. I decided not to think further about the determinations, and just compile the petal histogram and see how the data sorted out in the end.

The histogram from all the flowers I saw the entire day is shown in Fig. 2.

Fig. 2. Histogram of the number of petals for all the flowers seen on all plants.

There are several possible interpretations of this histogram. The first is that this is a single population of plants that usually produces just five petals per flower, but can produce as many as 11 petals per flower. This, however, is not consistent with either R. californicus, which most floras give as having 9 to 17 petals in California (Jepson's Flora of California says uncommonly as few as 7 or 8), or R. occidentalis, which all the floras give as having 5 to 6 petals in California.

The second interpretation is that both species are present in this location. For some of my data, the plants were sparse enough that I have individual data for some plants. Fig. 3 presents the data separately for all plants which had at least one flower with five petals, and for all plants which had no flowers with five petals.

Fig. 3. Histogram of the number of petals. Top: All flowers from plants which had at least one flower with five petals. Bottom: All flowers from plants having no flower with five petals.

Clearly, there are two separate species here. The histogram for plants which had at least one 5 petal flower are perfect R. occidentalis, with all heads having only 5 to 6 petals except for a single head with 7 petals.

The histogram for plants which had no 5 petal flowers is completely different, with a peak at 8 to 9 petals and a range from 7 to 11 flowers, consistent with R. californicus. Although the floras say R. californicus is supposed to have 9 to 17 petals, other populations in southern California determined as R. californicus have 5 to 12 flowers; see the Palomar Mountain population histogrammed here. In fact, I would expect that if more plants had been surveyed separately, the number of flowers for the R. californicus population here would also have a larger range in the histogram, including a small number of flowers with 5 petals, too, on plants that also had flowers with a larger number of petals.

With such an obvious difference between these two species here, how can it be that all the plants in San Diego County have previously been determinated as R. californicus?

The answer is that the noted taxonomist Lyman Benson said so. He was the last botanist to write a monograph on the entire Ranunculus genus in North America, which included 52 taxa in its 261 pages! Benson specifically called out a voucher from Cuyamaca Lake as R. californicus.

I, and many others, have great respect for Lyman Benson, so most people probably just scratched their head and said he must have known some other reason to call these plants R. californicus instead of the obvious determination of R. occidentalis. In fact, it took me five years of puzzlement before I finally concluded that the 5 petal plants at the Santa Rosa Plateau were indeed R. occidentalis.

Apparently, the reason Benson called the Cuyamaca plants R. californicus was that he had a firm belief in his beautiful geographic distribution maps of the species, which paint a nice simple picture of the ranges of these two species. His maps have the two species completely geographically isolated except for one region 50 to 75 miles north of San Francisco Bay, with no disjuncts that would mess up the maps. When confronted by seeing specimens with just 5 to 6 petals from San Diego County, alongside plants that had a larger number of petals, he resorted to claiming the short beaks of the fruit on the 5 to 6 petal plants were an indication that the plants were R. californicus despite the number of petals clearly being those of R. occidentalis. He even speculated that the difference in the number of petals arose from independent evolution in this population!

What is the difference in the length of the beak that caused Benson to call these plants R. californicus? It is at most 0.1 mm! Benson gives the beak for R. occidentalis as 0.5 to 2.0 mm, and that of R. californicus as 0.4 to 0.8 mm.

Benson also discussed the foliage of the plants, but admitted they are consistent with either species.

He clearly had a very strong belief in the beauty of his geographic distribution maps to ignore the obvious and large difference in the number of petals, grasping at a minute difference in the beak length to prevent having to call these plants R. occidentalis and mess up his distribution maps.

A much simpler explanation is to accept that these are a disjunct population of R. occidentalis. After all, we have many other disjunct populations of other species in San Diego County. And with the recognition that R. occidentalis occurs at the Santa Rosa Plateau Ecological Reserve in Riverside County, these populations are not even as disjunct as they seemed at the time of Benson.

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Last update: 5 May 2018