Essays

Wild On The Prairie: Invertebrates

Harvester ants do not prevent seed bank accrual
November 11, 2001

A visitor to a program at the Sibley Nature Center talked about killing "the big red ants" on his property. When asked why, he said that he figured since they were seedeaters what little grass seed that had been produced in the last eight years of drought was disappearing into their mounds. He had spent hundreds of dollars on ant-killing products as he sought to eliminate every colony of the harvester ants. His strong opinion was expressed in a challenging way -- as if he expected us to argue with him.

Our first response that by killing all the harvester ants on his property he was indirectly breaking Texas law by also killing all the horny toads (whose diet is 95% harvester ants) went unanswered. His ride to the program was ready to leave, and he left before we could learn his name and phone number. We would have said something about how many seeds are always in the soil. To refresh our memories we went to the books and the Internet to see what research had been done about seed populations.

We first looked up information about what is known as the "seed bank." Anytime the soil is stripped bare -- be it by drought, bulldozer, overgrazing, or flood -- weeds germinate as soon as there is sufficient moisture. Basic plant ecology tells us that the first to germinate are the annual weeds and grasses. This is known as first stage plant succession. Here on the southern Llano Estacado, if as little as two inches of rain comes in the late summer, Six Weeks Grama and Fall Witchgrass magically grow and produce seed. Over a hundred species of annual wildflowers also germinate with fall and winter rains, even after years of drought. The spring of 2001 is a great example of this phenomenon.

The inevitability of this occurrence indicates that thousands of seeds have been present in the soil, waiting for the proper conditions. This occurs even in gravelly soils. Evidence of this comes from Big Bend, which had the most glorious wildflower blooms in twenty years the spring of 2001, as well.

When does germination occur for the perennial grasses that are best for grazing? We again look to plant succession. In prairie ecosystems, the presence of perennial grasses represents the climax condition, or the ultimate plant community obtainable for that ecosystem which only occurs under optimal conditions. The 1980's, when we experienced heavier than average rainfall, saw that potential reached. Third and fourth generation landowners reported having grasses "belly deep to a horse" just like their grandparents saw when cattle were first brought to the Llano Estacado.

In plant succession here on the Llano, the first perennial grasses to germinate are the short-lived windmill grass, the lovegrasses, and various panicums. Three-awn grass (or spear grass) and dropseeds are next. Both are poor grazing for livestock. Cane bluestem also appears, and is fair grazing before it sends up its blooms stalks. Blue grama and sideoats grama, the best grasses for grazing, appear last.

Different species of seeds need varying conditions for germination. The first group of perennial grasses listed can germinate if the soil surface stays moist for only two or three days, and will send roots as deep as the rain penetrated. Blue grama and sideoats grama only germinate if moisture is evenly applied to the seed coat for at least a week, and usually longer. Bare soil never stays moist that long naturally on the Llano Estacado. For the these gramas to germinate under natural conditions a thick layer of mulch, or a living thatch of other plants that can hold moisture for that length of time must be present. The seeds also must be at a temperature between 70 and 85 degrees Fahrenheit. Bare soil, in the summer, often reaches 150 degrees Fahrenheit.

On another region of the short grass prairie, in western Kansas, plant ecologists in the 1930's amused themselves counting seeds in a cubic yard of soil that had been completely denuded by that decade's multi-year drought. They performed this exercise a number of times, in a variety of soils. Each cubic yard of soil they tested had over a quarter-million seeds. Some of the seeds were buried over a foot deep. They ran germination tests, and 75 percent of the deeply buried seeds were viable. Later, by associating grass seeds with carbon dated artifacts, some of these viable seeds proved to be over a hundred years old. And, by the way, harvester ant colonies were present at every site.

According to Holldobbler and Wilson, the leading ant experts in the world, harvester ants do increase in a drought. They prefer bare soil to walk on, and they are able to find more seeds with so much exposed soil, including older seeds exposed by the blowing wind. Each harvester ant colony has a range of approximately 10,000 square feet. Within this limited area, billions of seeds wait in the seed bank. Even in a drought, hundreds of thousands of seeds are produced in the same small area almost every year, for a rainy spell will occur once a year except in the very worst of years, such as 1932, 1950, 1977, or 1998.

Humans often feel it necessary to tinker with what God has spent billions of years developing, utilizing only empirical short-term observation as our guide. One of the many great wonders of our modern society is the immediate and easy access to scientific information painstakingly gathered by millions of people in the last 15 decades. It is a pity when a person does not avail himself of such readily accessible knowledge. Sorry feller, but ants do not have much effect on seed populations!