The Terra Galleria Blog – QT Luong

Part 5 of 5: 1 | 2 | 3 | 4 | 5

When reading descriptions of the Sonoran Desert National Monument, you come across a lot of statements that

the national monument is the most biologically diverse of the North American deserts

for instance, from the BLM that manages the national monument, although what such a sentence exactly means is unclear. What is clear is the origin of the statement, which is the Sonoran Desert National Monument Proclamation of January 2001. As often happens on the Internet, that statement has since been copied all over the place, despite being so poorly worded.

The most literal reading of the statement making sense is that Sonoran Desert National Monument is the most diverse property amongst its peers (national monuments? protected parklands?) located within a North American desert. However, more likely, the author meant that the monument is a particularly representative area of the northeastern (U.S.) part of the Sonoran Desert, which itself is the most biologically diverse of the North American deserts. Would that be correct? In particular, is the Sonoran the most diverse of the North American Deserts? Of the four major deserts in North America, the Great Basin, Mojave, Sonoran, and Chihuahuan, only the two latter could be candidates for most biodiversity. The Great Basin and Mojave have cold winters with below-freezing temperatures, which is one of the main impediments to biodiversity.

Sonoran or Chihuahuan?

A Google search for “sonoran most diverse desert” yields 1,290,000 results, whereas “chihuahuan most diverse desert” produces 1,590,000 results. This is, of course, not an accurate way to answer, but what is interesting is the large number of results. There simply aren’t that many people qualified to answer the question. The vast majority of sites simply repeat the information that appeared in an authoritative source. The first task is therefore to try to find a few such sources.

Several of them report that the Sonoran is the most biodiverse desert in North America, for instance Encyclopedia Britannica, Center for Biological Diversity, World Wildlife Fund (WWF) and National Park Service (NPS). From the two last sources, respectively:

The Sonoran Desert has the greatest diversity of vegetative growth of any desert in the world (Nabhan & Plotkin 1994)

The Sonoran Desert is thought to have the greatest species diversity of any desert in North America

However, if we look at what those two same sources, WWF and NPS have to say about the Chihuahuan, we find respectively:

The Chihuahuan desert is one of the three most biologically rich and diverse desert ecoregions in the world, rivaled only by the Great Sandy Tanmi Desert of Australia and the Namib-Karoo of southern Africa (Olson and Dinerstein 1998)

The Chihuahuan Desert is considered the most diverse desert in the Western Hemisphere and one of the most diverse arid regions in the world

If seemingly authoritative sources do not agree with themselves on which of the two is the most diverse, maybe we need to dig a bit deeper. One way to do it would be to examine the credibility of references cited, but that is difficult for someone not in the field besides generalities such as scientific journals (ex: Olson and Dinerstein 1998) having a more stringent peer review process than scientific conferences (ex: Nabhan & Plotkin 1994). A better way is to look for hard data, that is numbers.

Searching for numbers

There are many ways to define “most diverse”, however an unambiguous one is to count the number of species. Going back to the NPS:

The Sonoran Desert is home to at least 60 species of mammals, more than 350 bird species, 20 amphibians, some 100 reptiles, and about 30 species of native fish. More than 2,000 species of plants have been identified in the Sonoran Desert.

The Chihuahuan Desert boasts as many as 3,500 plant species…The Chihuahuan Desert is home to more than 170 species of amphibians and reptiles… 110 fish species in the region … The Chihuahuan Desert supports a large number of wide-ranging mammals (more than 130 species) … The Ecoregion supports around 400 bird species

This would seem to settle it convincingly in favor of the Chihuahuan, but maybe we can try to confirm those numbers from different sources? To simplify, we will look only at the most significant, number of plant species. Surprisingly, I could not find more than a few references for the Chihuahuan. The WWF in this page confirms the NPS count of 3,500 with a reference, but in that page, it is 3,000. The only definitive way to find out the correct number would be to establish a flora species list and count the number of entries. However, I was not able to locate such a list online.

Information is more abundant for the Sonoran. Among others, the number 2,000 is also mentioned by Center for Biological Diversity, a BioScience article, and the Arizona-Sonora Desert Museum:

This desert also supports many other life forms, encompassing a rich spectrum of some 2000 species of plants, 550 species of vertebrates, and unknown thousands of invertebrate species.

The latter reference is a chapter written by noted plant expert Mark A. Dimmitt, for the well-reviewed book A Natural history of the Sonoran Desert that appears to be the major work on this subject, conducted at the Arizona-Sonora Desert Museum. By the way, if you are in the Tucson Area and have any interest in the desert, a visit should be high on your list. It is a world-class combination of museum, zoo, and botanical garden. You can easily photograph several desert-dwelling species in natural-looking environments, and their extensive collection of desert plants is well-labeled for identification.

Desert versus Desert Regions

However, in that exact same book, we also find in the introduction by Gary Paul Nabhan:

It is home to 130 species of mammals, more than 500 kinds of birds, 20 amphibians, 100 or so reptiles, and 30 native freshwater fish. Perhaps as many as 3500 native species of plants occur within the Sonoran Desert proper

Wait, the later number seems to exceed by quite a margin the 2,000 found before. Can we find that higher number anywhere else? Friends of the Sonoran Desert does mention “4,000 species of Sonoran Desert plants”, and the Wikipedia explicitly lists 4004 species by name, which seems to be about the most unambiguous way to count them there is. Their source? Work from the Arizona-Sonora Desert Museum, supervised by the same Mark A. Dimmitt (as director of Natural History for the museum) who, as we saw, had mentioned the number 2,000.

At this point, it seemed natural to reach out to Mark Dimmitt for an explanation of this seeming discrepancy. I am grateful to him and his colleague Tom Van Devender for their comments.

• The number 2,000 originated from the meticulous research reported in the Desert Museum’s book A Natural History of the Sonoran Desert. It represents the number of plant species found in the Sonoran Desert.
• The number 4,000 represents the number of plant species found in the Sonoran Desert Region (or Ecoregion, to use a term introduced by the Nature Conservancy) which encompasses the Sonoran Desert itself plus the included and surrounding biological communities that influence it. The database in Wikipedia is not up to date and with new research the number can increase significantly. A recent publication El conocimiento florístico actual del Noroeste de México: desarrollo, recuento y análisis del endemismo by Joe Luis Leon de la Luz and others in Botanical Sciences 96(3): 555-568, 2018 finds 5,865 taxa.

Unfortunately, the Sonoran Desert and the Sonoran Desert Region are often confused, even in the publications and maps of the Arizona-Sonora Desert Museum, even by a scholar well-known in his field (the 3,500 native species of plants aren’t likely to occur within the “Sonoran Desert proper”). It is often difficult to figure out which one is actually referred to. The same could be said for the Chihuahuan Desert and Chihuahuan Desert Region. Notice how often the term “ecoregion” appears in the references previously mentioned for that desert. While drawing a line around the Sonoran or Chihuahuan isn’t hard, most of the diversity within the line would come from all the little mountain ranges (the Sky Island mountain ranges in the Madrean Archipelago) rather than from the desert itself. Patrick Alexander compared the situation to “trying to understand the composition of the dough in a chocolate-chip cookie from data about the chocolate chips”. The deserts and desert regions are not easily separated. The ambiguity of the original questions makes it difficult to answer.

The Chihuahuan Desert is not as well studied as the Sonoran Desert, which had the Arizona-Sonora Desert Museum where the research was very active and then the Madrean Discovery Expeditions. Both Mark Dimmitt and Tom Van Devender are not aware of a reliable published plant species list for the Chihuahuan Desert/Region similar to those that exist for the Sonoran Desert/Region, and an email inquiry with colleagues did not turn out new lists. The best that could be found was a 1987 unpublished summary of the flora of the Chihuahuan Desert Region by Jim Henrickson that counted 3,576 taxa. Since that number closely matches the number provided by the NPS for the Chihuahuan Desert, and given the confusion between the two, it could be that the NPS referred to the Chihuahuan Desert Region rather than the Chihuahuan Desert. Although older, that number is quite a bit smaller than the 5,865 for the Sonoran Desert Region. Tom Van Devender thinks that given its larger size, it is possible that the Chihuahuan harbors more species. This reminds me of the claim that New Hamphire’s Mount Washington has the worst weather on Earth. I can easily think of other mountains with more severe conditions, but they are not home to a weather station, so worst recorded weather may be accurate. While the Chihuahuan awaits more cataloging, for now the Sonoran Desert Region would have the largest documented diversity.

Part 5 of 5: 1 | 2 | 3 | 4 | 5