“There are worse sins for a scientist than to be wrong. One is to be trivial” – Robert MacArthur

Scientifically speaking, one way to be to be trivial would involve collecting data (“facts”), but then doing nothing toward fitting those facts into a hypothesis that, if ultimately proven, would help fill in pieces of the jigsaw puzzle of how nature works. Collecting facts alone is no different than stamp collecting, which may mean many things to a collector, but it is not science. One of the puzzle pieces that MacArthur addressed was “why are there so many species?” His advisor at Yale University, G. Evelyn Hutchinson, had developed what is now referred to as “Niche Theory,” the idea that each species occupies a unique niche in nature, defined by the multitude of things each species needs and does to survive. Part of that niche theory was that no two species could occupy the same niche, eat the same things, in the same place, at the same time. Otherwise, since resources (food, space, habitat) are not unlimited, those two species would then compete for those resources, and one of them would ultimately thrive while one would not, resulting in fewer, not more species. MacArthur tackled the problem of there being so many warblers occupying northeastern oak-hickory-maple woodlands of the U.S. and Canada. Warblers all eat small insects and are all about the same size, but there could be as many up to a dozen species in one woodland. At first blush, they all occupy the same niche, and so must compete for the same resources, flying in the face of one of the primary tenets of Hutchinson’s theory. What MacArthur discovered was that those warblers divide up those trees in subtle ways, some species forage on large branches, some near the trees’ trunks, some near the top of the tree, some in the lower canopy. While it looked like they must compete for the same food, it turned out they divide the trees up like giant pizza slices, each species getting its own slice. 

I think about MacArthur’s warblers when we, our community of naturalists, are out counting lizards. Our goal is to understand how species are responding to climate change, and part of that broader thinking includes the idea of species’ niches and whether or not climate change might result in niches changing or colliding. For instance, we can think about a vegetarian niche. Chuckwallas, desert iguanas, and desert tortoises are all primarily vegetarians living at low to mid elevations across much of California’s deserts. However, there are differences. Desert iguanas and tortoises “mostly” live on desert flats, washes, and alluvial fans. Chuckwallas are more restricted to rocky outcrops. Both iguanas and chuckwallas are very capable of climbing into shrubs to eat leaves and flowers of deep-rooted desert plants, plants that are at least partially buffered from periodic, temporary droughts, whereas tortoises are not built to climb trees.  

As climate change results in more frequent and severe droughts, meaning fewer annual plants in those dry years, what does that mean for desert vegetarians? At lower elevations, where drought effects are most acute, each of these vegetarian species is being hit hard. I would have predicted those that could climb into small trees and shrubs should do better, but just this week, hiking in a low elevation canyon we saw both a female chuckwalla and a juvenile iguana that were just skin and bones. No signs of any tortoises; their population in that area has experienced catastrophic declines. Arguably different niches, but at least at low elevations none are doing well. At mid elevations we have seen male and female chuckwallas looking fat and contented, but even there one female chuckwalla on the mid-elevation 49 Palms Oasis trail in Joshua Tree National Park was also skin and bones. Sample size of just two, but it is very possible that both female chucks had recently laid eggs, putting literally everything they had into a possible next generation (none of the male chucks looked thin at all). So, their emaciated appearance may have been as much the result of a typical breeding cycle, at least not entirely due to a dearth of food. At this point it looks like tortoises, being limited to just annual plants, are suffering the most from what climate change hath wrought so far.

At higher elevations I have wrestled with identifying niche boundaries, or if niche boundaries even exist, between an insectivore group of lizards: granite and desert spiny lizards, western fence lizards, southern sagebrush lizards, and side-blotched lizards (a recent climate change immigrant from lower elevations). These species are all regularly seen, sitting on boulders basking in the sun along the same trails, at the same elevations. Sure, side-blotched lizards are smaller and granite spiny lizards pretty large, which may (or may not) translate to different food-size selection. However, granite and desert spiny lizards are very close in size as are fence lizards and southern sagebrush lizards. I have seen both species pairs sitting on the same boulder at the same time. There are other subtle differences. Granite spiny lizards tend to be on bigger boulders, southern sagebrush lizards on mid-sized boulders, fence lizards on logs and branches, and side-blotched lizards on the smaller rocks or on the ground. These are all tendencies, not rigid boundaries. Are they enough to define separate niches for each species? Do slight differences in size or preferred basking sites really mean that these lizards do not compete with each other for the same insects? 

My heretical and as of yet unproven view is that deserts, sky islands, and ectotherm reptiles and insects (body temperatures derived from ambient conditions) do not necessarily follow the same rules as eastern deciduous forests and endotherm (self-heating) birds and mammals. Those eastern forests rarely deal with droughts severe enough to limit leaf (and so insect) production; farmers there reliably depend on annual rainfall to grow corn without irrigation, and the forest trees have the same expectations. Those warblers are like tiny inefficient engines that depend on huge quantities of insects to fuel maintaining their constant body temperature, frantic flights in search of food, and raising one or more broods of chicks over the spring and summer months. A relatively consistent spring-summer rainfall regime results in consistent food expectations and allows the warblers to maximize reproduction and population growth. Potential competition between species for food should thus not be surprising, nor should the niche partitioning observed by MacArthur aimed at minimizing competition and allowing more species to occupy the same forest. 

Deserts and sky islands do not follow those same patterns. Drought is common and severe drought is increasingly common here. Plant production as well as associated insect numbers vary widely between years. Lizards conserve energy by allowing air temperatures and the sun to maintain optimal body temperatures. Once eggs are laid, lizards rarely expend energy in additional parental care. Even with their comparatively laid-back lifestyle, when food supplies are particularly low most lizards will simply crawl into a rock crevice or burrow and wait out those famine conditions for up to a year or more. The result is that lizard population numbers often lag a year behind peaks in food resource abundances, and so competition between species may not be so acute as to catalyze sharp niche boundaries. The high biodiversity of deserts and sky islands may then be the result of the assembly and re-assembly of species during and following repeated ice ages, not due to dividing the desert up like giant pizza slices.

Just a thought. Being wrong is not a sin. Tragically Robert MacArthur died at a too early age from cancer. He was considered one of the brightest ecological minds of his time and it would have been so helpful to have him weigh in on the contrast between different biomes and whether tenets of Hutchinson’s Niche Theory should apply equally in each. In the eulogies for MacArthur there was a litany of praises for his acumen as a mathematician and theoretician. Hutchinson concluded his eulogy by saying that “MacArthur knew his warblers,” underlining that MacArthur was also a naturalist, and it was the insights gained from weeks and months of watching warblers that allowed him to understand the subtle differences missed be so many others before him.

Nullius in verba
Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.