“Man, as the minister and interpreter of nature, is limited in act and understanding…”  — Francis Bacon

For much of humanity, nature appears chaotic — a jumble of plants and animals lacking pattern or reason. Naturalists see the natural world differently. We see patterns and from those patterns we develop expectations (hypotheses) of where we might find a particular plant, bird, or lizard. 

One way of conceptualizing these patterns and expectations is the “ecological niche." The term was derived from the other common use of the word, meaning a recess in a wall for placing statues, art, books, or whatever. Places where a statue, or art, or book, resides. Where it lives. Joseph Grinnell, then-Director of the University of California Berkeley’s Museum of Vertebrate Zoology, was one of the first naturalists to formalize this term in reference to nature, in his 1917 paper on “The niche relationships of the California thrasher." Grinnell’s niche was the sum of its role in the larger community in which it lives.

Later, in 1957, G. Evelyn Hutchison, a professor at Yale University, pondered how so many species could occupy the same space, and refined the meaning of an ecological niche as an amalgam of distinct components essential to a species’ survival. He reasoned that if all those species occupying the same space had the exact same requirements for survival, that they would inevitably, especially during droughts when resources might be in short supply, compete with each other. In any competition, eventually there would be a winner and a loser. To explain species coexistence, there must then be something that allows them to partition habitats so that in fact they do not have exactly the same requirements. There could be a “climate niche,” a “food niche,” a “substrate niche,” a “predator avoidance niche,” a “soil chemistry niche;" the number of niches being limited only by the creativity and depth of understanding of the naturalist. For any two species appearing to occupy the same habitat, they must differ in one or more of those components, that difference then allowing coexistence, and allowing regions to support higher biodiversity. An anecdote from the 1970s when I was attending UC Davis was that if a graduate student returned from a field season and claimed that they did not discover niche separation between two species occupying the same habitat, that their professors politely told them to go back and keep looking.

Yesterday we (a group of dedicated naturalists) were surveying lizards along a section of the Pacific Crest Trail. At first blush, most lizards are roughly the same size and shape, and most eat insects. If they occupy the same niche then, to avoid competition, there should be just one, or at best a very few species occupying any given habitat. Yesterday we found five different lizard species on our survey. They occupied the same habitat and seemed to do much the same thing. However, it was clear that certain sections of the trail were “better” for some species than others. Granite spiny lizards were only found where there were big granite boulders and big trees. We saw western fence lizards on middle-sized rocks and boulders (as well as some big ones), and on branches of small trees and shrubs. Side-blotched lizards, the smallest of these three species, were found where there were big and little rocks, but seemed to be absent where there were no rocks at all.

Those rock-less (or sparsely “rocked”) regions were where we found a couple juvenile Blainville’s horned lizards. Western whiptails were in areas of deeper leaf litter. At a coarse scale they all occurred together, but at a finer scale, while there was overlap, there was also separation. Different niches. Had we been able to delve into their diets, the timing of their activity, or how much time they spent foraging versus thermoregulating, versus avoiding predators, we would likely have found further separation. 

We were also surveying pinyon pines. Adult trees were dispersed throughout the area, but saplings and seedlings were limited to cooler, less arid north-facing slopes. This may indicate a “niche shift." Clearly there was a time when the pinyons could become established more or less randomly across this landscape, but in recent times, as it has become warmer and more arid, their ability to recruit new plants into this population is now much more limited to particular microclimates. This idea that species’ niches might be shifting is fundamental to understanding how species may be able to adapt to a warmer-more arid future, or if they cannot, perhaps go extinct.

We have three horned lizard species here in southern California that illustrate this concept of niches. All three species eat harvester ants as their primary food, and so are all members of the “harvester ant-eating” niche. Similar and diet, size and appearance, how do they avoid competition? Flat-tail and desert horned lizards occupy some of the hottest, driest regions of southern California and so share a “climate niche." However, Blainville’s horned lizard is limited to cooler landscapes, either higher mountain elevations or more coastal regions. Desert and Blainville’s horned lizards share a substrate niche — coarse sands and sparse rocks. Flat-tails are only found on fine aeolian sands and silts, usually a tad more stabilized than the loose sands preferred by fringe-toed lizards. In many ways these horned lizards are similar, but in equally important ways they are different, occupying different niches. As a result, these three species rarely if ever are found together, and so avoid competition.

However, in recent years desert horned lizards have appeared to be colonizing higher elevations — the same elevations that had been occupied by Blainville’s horned lizards. Except now, in the desert mountain regions of southern California, the Blainville’s seem to be also abandoning what were once their lower elevation habitats. Niche shifts, or at least the ability to exploit new habitats as their previous habitats become increasingly inhospitable, may be the signature of climate change survivors.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), and be safe.