If you’ve been following my Facebook or Instagram feeds, you’ll know that I’ve spent the last several weeks road-tripping through the western states of the lower 48. Along the way, we spent some time in the deserts of the Southwest. We spent a few nights each in Santa Fe, New Mexico, the Chiricahua Mountains of southeastern Arizona, and Joshua Tree National Park, California.
The Chiricahuas and Joshua Tree National Park in particular, gave me pause. These areas were unique: Joshua Tree because of the forests of its namesake trees, and the Chiricahuas due to their status as one of the “Sky Islands”. The Chiricahuas were also home to unique, and rich strips of riparian forest along the few year-round streams. (These were dominated by beautiful stands of Arizona Sycamore.) Despite being unique, these places held no endemic species of birds. The entire avifauna of these areas was made up of either migrant birds, or residents that also occurred in other parts of North America, or at least across the region.
You see, in the South American tropics, there are hundreds, if not thousands of micro-habitats scattered through the Atlantic Forest, Amazon, and particularly in the upper Amazon and Andes Mountains. These habitats from bamboo stands in the rainforest, to high elevation Polylepis forest in the Andes each have their own communities of endemic birds. In the Southwest, similarly sized habitats like the Joshua Tree forests, and the riparian woods in the Chiricahuas lack endemic birds. I wondered why.
When I began looking for an answer, I quickly realized that I was opening a very large can of worms, that included many arguments about the nature of evolution, speciation, and its relationship to habitat diversity, stability, instability, persistence, and history. Frankly, a straight answer to my query was very hard to find, and after a few hours of tangent-chasing, I almost gave up on this post. But, I stewed on it for a week or so, and realized a few things I found seemed relevant.
First a note on endemism. Endemic species are those found in a single geographic location. While that can be looked at broadly (endemic to North America) or at a very small scale (endemic to single valley in the upper Andes of Bolivia), in this case, I’m thinking about the finer end of the scale. What causes endemism? That too is a fairly hot topic, but most evidence seems to point to high concentrations of endemics occurring in areas of climatic and therefore ecosystem, stability. I found a paper in the journal Ecography that strongly correlated the presence of endemic birds to climate stability in the Andes. In more specific terms: “Pairwise comparisons of peaks and adjacent lows of endemism provide strong evidence for linking peaks of endemism with local ecoclimatic stability.” Then went on: “Most endemics are relict populations which survived periods of global climatic change in places where these impacts were moderated.”
In other words, stability leads to endemism. That, I thought, might be the key to why the desert southwest, doesn’t contain endemic birds.
First I considered seasonality. The Southwestern United States lies at higher latitude than the tropics and experiences large temperature swings from winter to summer. Precipitation too is influenced by a seasonal, monsoonal flow out of the Pacific.
But the Amazon, and much of the rest of the tropics, also experience different precipitation from one part of the year to another. There are also a suite of endemics in the southern cone of South America where the latitude is even more extreme than the Southwest. So differences in precipitation, and temperature swings from summer to winter, didn’t yield a satisfying answer to my query.
It turns out I wasn’t thinking about the question on the right scale. I had only considered “stability” to mean how the weather is likely to change from one day to the next. But that isn’t what stability means here. Stability is being able to predict what happens not one day to the next but across centuries, millennia, or even geologic epochs.
And there it was. As the Ecography paper noted, some areas of the Andes have experienced relative climate stability since before the Pleistocene. They may experience seasonality, but that seasonality is predictable across time, meaning species have had the chance to specialize to that ecosystem, and no other. Those areas became hotspots for endemism.
In the deserts of the Southwest however, the long term patterns in climate has been one of change. One study using pollen counts in sediment cores from central Arizona found that, during the Wisconsin period (35,000-20,000 years BP), temperatures were up to 5C cooler than they are today, and wetter, allowing the growth of spruce and pine forests in what is now contiguous scrub desert.
A comparable study used ancient packrat middens to determine how the Southwest’s environment has shifted over the past 50,000 years. They found over the course of that time, that forest and xeric vegetation (desert) alternated, coming and going several times over the past fifty millennia.
The desert Southwest has been anything but stable, so endemic birds simply haven’t had the time to evolve to the constantly shifting habitats. As a result, we see species that are limited to the region as a whole (usually including much of Mexico), but not to specific habitats within the desert, as we would in the more stable areas of South America.