They say “variety is the spice of life”—a cliche clearly befitting our Sky Islands biodiversity. Within North America our region entertains more species than in any comparable area north of Mexico. More birds, mammals, reptiles, ants, bees, etc.
Why do we have so many species in the Madrean Archipelago?
A multitude of factors have conspired to create this wild cornucopia. Our relatively low latitudes, diverse topography, proximity to a good number of disparate biomes or major habitat types, and our climate patterns all factor into the impressive number of Sky Island species.
Over the great spans of evolutionary time, competition has also played a key role in diversifying various species. This truth is most evident in closely related species—ones that look so similar that we sometimes are challenged to sort them out while being left to wonder how they coexist in our region.
Our darkling or “tip-up” beetles are a prime example, with many different types making a living in local habitats. By my reckoning, no less than seven species roam Raven’s Nest Nature Sanctuary. All are black, flightless beetles that wander in plain sight over the ground. Most are scavengers—nature’s janitors—consuming organic tidbits they scavenge.
Look for a few species rummaging around red harvester ant nests at dusk. Note any differences in size, shape, or behavior. Seemingly one, more abundant, species could get the job done, yet we have many. Clearly, competition for limited resources has led to diversification of these tank-like beetles, as they diverged into species that use resources in overlapping yet different ways.
Similar look alike invertebrate species abound in our area. Butterflies, moths, ants, bees, flies, grasshoppers, and many other groups all contain “sister species” that are strikingly similar—each hammering out a living in a slightly different way.
Birds provide an easy way to see how competition has led to resource partitioning in the Sky Islands. Three species of flycatchers are emblematic in this respect. The smallest, the dusky-capped flycatcher, inhabits wooded areas from our low-elevation riparian forests, all the way up into our coniferous forests. The medium-sized ash-throated flycatcher makes a living mostly in our deserts, mesquite woodlands and riparian zones. The large brown-crested flycatcher does deserts and riparian zones.
All three species can coexist along local waterways if there is sufficient wooded cover and enough old woodpecker cavities to nest in. There, their differences in size, beak proportions, and behaviors allows them to exploit overlapping, yet sufficiently disparate subsets of our hyper-diverse invertebrate fauna. Direct competition can sometimes be witnessed, as one species of flycatcher chases another.
Mammal species have likewise been driven by natural selection to varying habitat use and behaviors that afford coexistence. Until at least the 1930s, our region hosted not only American black bears, but also Mexican grizzly bears.
Small birds able to split up resources is one thing, but how did the two widely ranging bear species work things out until people saw fit to, unfortunately, exterminate the grizzlies? For the most part, it seems that our local “grizz” exploited low to mid-range habitats, where they relied heavily upon digging up rodents with their out-sized claws. Former large colonies of black-tailed prairie dogs and other ground squirrel species were likely food sources. They also scavenged carcasses and ate a broad spectrum of plants.
Black bear also consume some of the same food, yet generally in higher habitats. There, they were able to resort to arboreal escape tactics should they directly encounter a grizzly, which rarely scale trees. Variable habitat use, differential exploitation of food resources, and discretionary retreats all played a part in affording a living to our two bear species.
Other examples of strikingly similar species abound in the Sky Islands— ones that seem redundant yet which have evolved slightly different ways of exploiting our natural resources. Two species of patch-nosed snake, three species of mud turtle, three types of spiny lizard, eight kinds of whiptail lizards, nine species of rattlesnake, 27 bat species, 28 sparrow varieties…The list goes merrily on. Clearly it’s not only governmental regulatory agencies that dislike monopolies.
The next time you venture outdoors, keep your eyes peeled for wildlife species that up till now you may have assumed are one and the same. Look for structural and behavioral differences and see what you come up with. Make it into a competition of your own!
Vincent Pinto and his wife, Claudia, run RAVENS-WAY WILD JOURNEYS, their Nature Adventure & Conservation organization devoted to protecting and promoting the unique biodiversity of the Sky Islands region. RWWJ offers a wide variety of private, custom-made courses, birding & biodiversity tours. Visit ravensnatureschool.org