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Population Explosions

Photo: Public Domain, via Wikimedia Commons.

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.

Although populations of plants and animals rise and fall with the vagaries of weather, in some cases this explanation is not sufficient. Sometimes a “population explosion” is the temporary result of migration. Midlanders may remember a Cubs baseball 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 MidlandCounty.

A more benign type of migration is that of butterflies. Everybody knows about the fall migration of monarch butterflies. Fewer people – perhaps only butterfly watchers – remember a Snout Butterfly migration in 2005. The small brownish butterflies with a long pointed nose appeared in the thousands.

Population explosions occur after rain, when the soil temperature is 68 degrees or more, and an ephemeral ecosystem exists for couple of weeks. The basis of such an ecosystem is the presence of grass root termites. When walking through the pastures, one often notices small mud tubes encasing dead plant matter. Sometimes the mud tubes extend as high as eighteen inches along the upright stalk of a dead plant.

Many insects, arthropods and birds eat the alates (the winged males and females) when they emerge to mate in the air. Following a thundershower hundreds of thousands of alates per acre fill the sky near sundown. Nighthawks, scissortail flycatchers and kingbirds go berserk at the bounty, sweeping and swooping with mouths wide open, beaks clacking at every gulp. Tarantulas open their burrows and lurk at the rim of the hole for passing victims. Red mites (known locally as rainbugs) emerge from their burrows, the morning after a rain to feast on the dying male termites that litter the ground. The first three weeks after a significant rain event bring uncountable changes to the arid landscape.

Participating in the effort to understand the phenomena of the natural world humbles the observer. There are always more questions than answers. What population explosions will we have this year?

Nature Notes is sponsored by the Dixon Water Foundation and is produced by KRTS Marfa Public Radio in cooperation with the Sibley Nature Center in Midland, Texas. This episode was written by Burr Williams of the  Sibley Nature Center.