“A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature.” — Ernst Mayr

A niche in the ecological sense was derived from the Middle French word “nicher,” meaning to nest. The term ecological niche was first employed by Joseph Grinnell in 1917 in the paper he wrote about the habitat and niche of the California thrasher. 

The ecological niche idea was later refined by G.E. Hutchinson in 1957, thereafter becoming a central concept in ecological research. In essence the ecological niche is the sum of forces that defines where a species lives, its habitat, and its role within that habitat. While there is a myriad of such forces, arguably the two most prominent are climate and diet. The relevance of the ecological niche today cannot be overstated – it is how scientists can predict the fate of species as modern climate change impacts the destiny of every living animal, plant, fungus, and microbe.

A tangent from ecological niche theory is the tenet that no two species can occupy the same niche. The idea being that over time the resources that both species depend on to survive will occasionally be in short supply. Perhaps due to a prolonged drought, or too much rain, or disease, or a diminished food supply, if two species are dependent on the same resources they will compete for those resources. Only the winner of that competition will be left to occupy that “niche.”  In terms of understanding the effects of climate change on biodiversity, the concern is that a rapidly shifting climate might force species into the same space, perhaps a climate refugia – a place where species might be able to shelter against an increasingly inhospitable environment. If those species otherwise occupying the same ecological niche but in separate areas are then forced to occupy the same space, competition will ensue, perhaps leaving only one to survive. The concept for why two species cannot occupy the same niche is simple, but is it true?

Before moving to the desert in the winter of 1985-86, our family lived in the temperate rainforest of northwestern California, within an old-growth redwood forest where the annual rainfall averaged around 60 inches per year, but sometimes twice that much water fell from the sky. Much of my time during those years was devoted to studying spotted owls, a resident of those ancient forests, but whose niche was then almost entirely unknown. My task was aimed at defining that niche so that the growing antipathy between the logging industry and the conservation community could be argued with more facts and less hearsay. After moving to the desert with an average of 3 to 5 inches of annual rainfall, I thought I would expand my owl-based studies to include desert owls. In the redwoods there was one large owl, the spotted owl, and three smaller owls, western screech-owls, saw-whet owls, and sparrow-sized northern pygmy-owls.

Here in the desert, there are three large owls, great horned owls, long-eared owls, and barn owls, and there are two smaller owls, western screech-owls, and burrowing owls. Except the burrowing owls, all could be found living in desert fan palm oases. Living in potential close proximity, were some of these species occupying the same niche, defying a tenet of ecological niche theory?

Owls, along with many other bird species, regurgitate the undigestible portion of what they eat as a compact pellet. These birds lack the digital dexterity to separate food from hair, bone, or insect exoskeletons, so they swallow their prey whole. Then, in a region of their digestive tract before their stomachs, essentially their gizzard, digestive acids separate food from not food. Once that process is completed, the food continues down their digestive tract and the non-digestible material is coughed up as a compact casting or pellet. Those pellets then provide a treasure-trove of information about everything the owl eats without having to capture, or in anyway harm the owl. Dissecting thousands of pellets, I used those regurgitated castings from each of those five species of desert owls to determine how similar or different their niches were. 

All five owl species ate a lot of pocket mice. Pocket mice constituted the most frequent vertebrate prey for each owl. Kangaroo rats were also common prey among all the owls. Still there were differences. Great horned owls ate far more cottontails and wood rats than the other owls; they also ate lots more large scorpions. The most common food items for screech-owls (36%) and burrowing owls (61%) were insects and arachnids (mostly solpugids [sun spiders]). Of the two small owls, screech-owls ate more mammals and were the only species to eat a lot of cactus mice, indicating they did most of their hunting in the palm oases. The differences between barn owls and long-eared owls were slight at best. The combined pocket mice and kangaroo rats comprised 73.1% of the barn owls’ diets and 86.5% of the long-eared owls’ diets. That same combination was just 47% of the great horned owl diet, 29% of the screech-owl diets, and 30% of the burrowing owl diets. In no instance were the diets identical, but at least for the barn owls and long-eared owls they were very similar. Are those slight differences sufficient to allow coexistence? Or, since I never found barn owls and long-eared owls in the same oasis, was their interspecific avoidance allowing coexistence, but only at a distance? Some answers, but always more questions.

I still have many of the same questions, although now my focus is on lizards, not owls. At the middle to upper elevations of our desert sky islands there are as many as four species of insectivorous lizards occupying the same stretches of real estate. All eat small insects and spiders, but they do not regurgitate clean little pellets of their prey items. Side-blotched lizards are the smallest. Next are western fence lizards and southern sagebrush lizards, both similar in size. Granite spiny lizards and desert spiny lizards are largest, but similar to each other in size and both seem to prefer the largest boulders. I have only rarely found granite and desert spiny lizards together, so they mostly fit the not occupying the same niche tenet. Side-blotched lizards are more ground-dwelling than the others, which are saxicolous (rock-dwelling), so they too seem to have their own, separate niche. Western fence and southern sagebrush lizards were mostly separated by elevation, with the sagebrush lizards venturing much higher up the mountains. However, every year we are finding increasing levels of overlap. Sometimes sitting on the same boulder. 

At this point I am wondering whether the “not occupying the same niche tenet” might not apply to reptiles. Most ecologists have tried to answer this question with birds. Lizards and snakes have much lower caloric needs than do birds and mammals. Maybe those lower caloric needs result in much less potential competition. Some answers, but always more questions. And I am ok with that because the pursuit of answers, of knowledge, is really fun.

Nullius in verba                                                  

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