Population explosions are an intriguing phenomenon. At times, a species will suddenly become extremely common. In some years, jackrabbits will line the highways a hundred to a mile. Or, mice might become pestiferous, too numerous to count. Arthropods such as black widow spiders, or a species of caterpillar, may be so common that it seems the world has been taken over by them. Even plants respond in such a manner.
In 1996 and 1997, the mesquite twig-girdling beetle population soared to unbelievable heights. In September 1996, virtually every streetlight in town attracted thousands each night. In the spring of 1997 the young beetles stripped most of the leaf buds from the mesquites, preventing the emergence of new growth. Many of the shrubs had no green growth above the height of a person's knee. Long-time observers had noted an invasion of the beetles in 1972 and in 1981, but not to the extent of the exponential numbers experienced in the 1990s.
In 1996, at the Sibley Pond the cattail moth caterpillars were present in mind-boggling numbers. The small, prickly, orange and gray caterpillars damaged nearly every blade of cattail, resulting in a general yellowing of the half-acre marsh. In attempting to estimate the total number, we calculated three caterpillars per blade, thirty blades per square foot, and how many square feet in a half-acre? You do the math! Mind-boggling was the correct word choice, right?
Because decaying cattail leaves have produced several tons of soil over the past eighteen years, the boardwalk is now high and dry, rather than extending over open, shallow water. We, therefore, hope the damage caused by the caterpillars is permanent. If it is not, can someone catch us a pair of nutria to put in the pond? (Nutria eat cattails.)
South of town, at the Gone Native Arboretum, there was been an unusual proliferation of whiptail lizards in 1997. The population nearly quadrupled, leading to ferocious territorial battles in the prime hunting areas. Because of drought the whiptails left the reclaimed prairie to take up residence in planted areas of the arboretum.
This left a lizard "vacuum" in the surrounding pasture, then filled by Lesser Earless Lizards which moved along the roads and through the gate from the Broken Hills ridge four hundred yards to the northeast. Lesser Earless Lizards prefer sandy soil with less than fifty percent vegetative cover. The current drought has caused the prairie lost much of its grass cover, resulting in a benefit to Lesser Earless Lizards.
Although populations of plants and animals rise and fall with the vagaries of weather, in some cases this explanation is not sufficient. At Sibley, overall lizard populations are at an all time low. Whiptails seem to be the only species remaining. Southern Prairie Lizards have not been seen at all during our annual summer camp sessions, and nor have Horny Toads. Where did they go? What happened? Did the dry times remove their food sources? (For Horny Toads this is not the case, since Harvester Ants still abound.)
A naturalist with kids observe "everything," and in 1999 kids noticed that there have been an amazing number of Roadrunner droppings along the trails, although no one ever actually sees the Roadrunners. It is likely the Roadrunner population increased when the pastureland north of Sibley was converted to nine more holes on the Hogan Park Public Golf Course. Because roadrunners eat lizards, a possible answer to the mystery does seem to have presented itself.
Still… why haven't the Roadrunners been seen? The kids theorized that the roadrunners learned, over the years, that some people find mindless and cruel fun by shooting anything that moves. They decided that the Roadrunners correspondingly changed their habits and become skulkers, Can such a theory be proved?
Or could the explanation for the dearth of lizards be the result of someone carrying a pellet gun? Has a proficient shot slipped past us, killing twenty or more lizards at a time while cruising the trails this spring? We have not found dead lizards with pellet gun wounds, and, it is hard to believe that such a person would take the lizards home to fry up!
Near the Gone Native Arboretum a few years ago, a group of kids (four boys and two girls, ages 9 to 14) was encountered carrying a bucket of dead lizards, dead snakes, and a dead box turtle. They did not have an explanation for what was going to happen to their gore (killed with sticks), other than to show off and brag to their neighbors.
Sometimes a "population explosion" is the temporary result of migration. Midlanders may remember a Cubs game called on account of a grasshopper swarm. The players could not concentrate on the game as millions of grasshoppers rode in on a cool front from the north. The ramps of the stadium became slippery from the crunched and smeared bodies left by the stampeding fans.
At least twice in the past twenty-five years, grasshopper migration has caused local ranch vegetation to be stripped until almost no green is left. The culprit is a Melanolopus grasshopper, another of the same genus as the Rocky Mountain Grasshopper, which swarmed like African Locusts in the 1880s to the early 1900s. The Rocky Mountain Grasshopper then became extinct for reasons never known. Bird grasshoppers (Schistocerca) also migrate, but rarely in numbers sufficient to cause widespread damage. In 1995, bird grasshoppers took over the shinoak in the sanddune country of Midland County.
A more benign type of migration is that of butterflies. Everybody knows about the fall migration of monarch butterflies. 1997 was the best year for huge numbers of the species in a decade. Strong south winds held the monarchs here for two weeks in late September and early October. Fewer people - perhaps only butterfly watchers - remember a Snout Butterfly migration in the late l980s. The small brownish butterflies with a long pointed nose appeared in the thousands for the annual butterfly count.
Among the big black swallowtail butterflies, often one is seen with iridescent blue on the hindwings. This is the Pipevine Swallowtail, whose larvae feeds on the pipevine plant growing along the streams and rivers to the south and southeast of Midland. The constant winds from those directions bring Pipevine Swallowtails with regularity. If you ever see an advertisement for pipevine in a garden catalog, consider ordering one. They have blooms that look like miniature curved pipes!
Population explosions occur after rain. In an earlier column, we mentioned the temporary pluvial ecosystem. Flying termites, rainbugs, tarantulas, millipedes, flying ant swarms, and toads all appear in amazing numbers. And this year although we had rain we found very few millipedes at Sibley. Why? What happened? Did the Spider Ant (Novomessor cockerelli) nests where they live between rains get too dry?
The black Spider Ants are long-legged and have large openings to their holes. Spider Ants have survived, and have increased the numbers of nests per acre. Spider Ants are omnivores, one of the Llano Estacado's custodial crew, picking up anything remotely edible, including dead animal flesh, animal droppings, seeds, rotten vegetation, dead insects and more.
Millipedes need high humidity to survive, which is one of the reasons they utilize Spider Ants' holes. They protect Spider Ants, who do not sting or bite with any force. Millipedes ooze a smelly liquid from pores along their sides. This drives the Spider Ants' enemies from the hole. This is a classic case of symbiosis, for millipedes in exchange eat mold and fungus growing on the gatherings of the Spider Ants. How long will it take for millipede populations to build up again?
Participating in the effort to understand the phenomena of the natural world humbles the observer. There are always more questions than answers.
- August 1, 1999