A. PHYSICAL PROPERTIES OF SAND

Introduction

Gopher tortoises are turtles that live in dry habitats in the coastal plain of the southeastern United States. They have close relatives; the desert tortoise in southwest Nevada. Gopher tortoises eat mostly grasses and burrow in sandy soils to help them regulate their body temperature. The width of the gopher tortoise burrow is about the same as the tortoise's length, which means it can turn around anywhere in its burrow. Generally, the larger the burrow, the older the tortoise. Gopher tortoises can live at least 40 years and possibly much longer, but no one has studied them long enough to know just how long they live. The Florida scrub is home to many different species of digger wasps that build nests in the sand by raking, pulling, pushing, or carrying grains of sand. Some digger wasps can throw sand six inches or more, which keeps the sand from accumulating at the burrow entrance and helps conceal it from potential predators. Some species even make several shallow false burrows to confuse enemies such as velvet ants. Digger wasps beware! Velvet ants, which are actually wingless parasitic wasps, will slip into the undefended burrow of a digger wasp and lay their eggs on the mature larvae. When the eggs hatch, the velvet ant larvae eat the wasp larvae. Brightly colored velvet ants can be seen racing around boldly in open areas where digger wasps (and digger bees) are likely to have burrows. A gopher tortoise burrow can provide year-round shelter for over two dozen vertebrate species including the threatened eastern indigo snake, the gopher frog, and the Florida mouse. The gopher frog spends most of its life in the burrow, coming out only to feed and to find water in which to breed in late summer. Most spider babies leave home as soon as they emerge from the egg, but the Lake Placid funnel wolf spider mother keeps her young at home in her burrow until they are half adult size. She lines the upper part of the burrow with silk to prevent cave-ins and builds a sheet web at the burrow entrance. The young spiders emerge from the safety of the burrow to eat prey caught in the web. The federally endangered scrub wedge-leaf button snakeroot has a long taproot that can absorb water. Snakeroot seedlings grow under a layer of sand grains, which keeps them from drying out. Some plants that live in scrub don't depend on soil moisture or nutrients at all. Ball moss is an air plant commonly seen in scrub. It absorbs the moisture it needs from the air and gets its nutrients from rainwater, dust, and perhaps from insects and spiders that live in the base of the plants.

Sand is the soil of Florida scrub. Sometimes it’s sugary white, sometimes yellow. Living in sand can be tough. Compared to particles in other soil types, sand grains are large and hold virtually no nutrients or water. Rain drains through sand rapidly and the upper layers of sand dry out quickly. The surface can also become dangerously hot and dry. Sand is also very abrasive. Yet many species of plants and animals have adapted to these conditions and some species can live nowhere else. How do they do it?

The Advantages of Burrowing

When you walk along a sandy path in scrub, you may see a rich diversity of plants and animals. However, many animals may be in burrows and tunnels beneath your feet. Some excavations are just under the surface while others are deep (up to 10-12 ft.). Why do so many scrub animals burrow?

• Because it’s cool. Many scrub animals are nocturnal and come out only after the hot sun goes down. The surface quickly loses heat at dusk, making it less threatening for animals to walk on. But diurnal animals need a way to cool off too, especially in summer when surface soil temperatures can reach 125 degrees Fahrenheit. Even tough old gopher tortoises can get overheated and die on a hot afternoon if they can’t retreat to their burrows. Their 12-foot deep homes can be a cool 80 degrees during the steamiest part of the day—which is one reason why so many other animals like hanging out at a gopher tortoise’s place.

Because the soil cools down more slowly than the air, animals also burrow during extremely cold temperatures to stay warm.

As the temperature and humidity of the sand changes, many burrowing scrub animals travel up, down, or even sideways in the sand to find the conditions they like best. In loose sands, some animals, like the bluetail mole skink, move freely between the sand surface and subsurface.

• Because (frequently) it’s more humid. Not only can the sun be hot, but it can broil you to a crisp if you’re an insect without protective armor. Like many forest or wetland arthropods, many scrub arthropods have a thin layer of protective wax covering their bodies to keep them from drying out. But because sand can scratch up the waxy coating, many burrowing scrub arthropods are also hairy. The hairs protect the wax by holding sand up off the surfaces of their bodies.
• Because (sometimes) it’s safer down there. The scrub is full of predators. A burrow is one of the best ways to protect yourself from red-tailed hawks, bobcats, or raccoons if you happen to be an eastern indigo snake, a Florida mouse, or a camel cricket. However, if you are a termite, grub (beetle larvae), ant, or grasshopper egg, the sand may not be the same safe haven. It’s another world down there and most creatures are in constant danger of being eaten--even underground.
• Because there’s food down there. Many predators are well equipped to hunt beneath the surface of the sand. Moles and shrews tunnel for grubs, caterpillars, and other insect larvae that graze on the fine hair-like roots of some scrub plants. Sand skinks live in the sand and search for termites that munch on buried leaves and twigs. Scrub pygmy mole crickets prefer to eat microscopic algae that grow between the grains of sand just under the surface in sunny locations. Ant lions make pits to trap ants and other insects, or they hunt in " doodlebug" fashion. Almost all of these predators leave trails that can be seen on the surface!
• Because it’s a good place to raise young. Babies can be easier to protect in an enclosed space underground. Mice do it, ants do it, digger wasps do it, and so do scrub burrowing wolf spiders. Some animals, like the rosemary grasshopper and scrub firefly, lay eggs in the sand but don’t make a burrow for them. Most likely, animals dig their home burrows deeper than the tunnels they make while looking for food.
• To escape fire. Fires occur naturally and frequently in scrub. Although intense, the heat from these fires only penetrates several inches below the surface of the sand. Animals can survive fire by finding refuge in a nearby gopher tortoise burrow, a rodent burrow, or in their own burrows.
• Because they can! Scrub animals that live and hunt underground are especially adapted for life in sand. Most have body parts specialized for digging. Most likely, animals adapted for life in sand would not survive in habitats with other soil types.

Some Scrub Plants Are Like Icebergs

Many scrub plants have adapted to the dry, sandy conditions and the frequent fires that occur in scrub habitat by exposing only parts of themselves. Rather than stretching big limbs out above the ground, oaks that occur in scrub, such as Chapman’s, myrtle, sand live oak, and scrub oak, have underground branches that often look a lot like roots. Only 25 percent of a typical scrub oak is above ground. The other 75 percent is underground, giving it lots of roots to take up water and enabling it to sprout back after a fire.

Some scrub plants, such as the scrub wedge-leaf button snakeroot, pigeon-wing, and scrub buckwheat, have adapted to dry soil conditions by having very long taproots. Other scrub plants like palmettos, prickly pear cactus, and Florida rosemary have a network of fine, shallow roots that can quickly absorb rainfall before it drains through the sand.

For more information on adaptations that help scrub plants conserve water.

The Background Scoop on Sand

The sand of Florida scrub was formed millions of years ago as the southern Appalachian mountains were eroded by frost and gravity. Rivers carried the sand to the sea. Coastal currents transported the sand south to the region of Florida. As the sand accumulated, dune islands were created. When the ocean receded, these islands became connected together and formed ridges. Today, Florida scrub is found on these ridges.

The Mississippi River delta and the floodplain of Louisiana were deposited in a similar way yet do not appear as hills, like the uplands of Florida. That’s because sand and mud behave very differently. One of the strange, fluid-like features of sand is that it can be formed into waves, called dunes. Mud does not do this. Mud can be ground to dust and blown away to form an even layer, but it almost never forms dunes of dust. When dust does form little dunes, they quickly erode under the force of rain, but sand does not.

Sand is a bizarre and sometimes confusing material. When dry, sand pours like a liquid, but under a sudden shock, dry sand acts like a solid and can support a great amount of weight. When wet, sand acts like a solid and can be easily shaped. Water creates surface tension that holds the sand grains together.

Small burrowing animals that live in scrub benefit greatly from these simple facts of physics. Sand grains spread the force from shock or pressure away from the point of contact and help prevent a burrow from caving in. Surface tension keeps the sides of a small burrow from collapsing. Surface tension also allows small burrowing animals, such as ants, to bring up sand in little clumps rather than grain by grain.

Clay is probably a better soil for building burrows than sand is, but excavating sand is much easier for most small animals than digging a burrow in clay. And although sand is more easily displaced than clay, an animal can build and rebuild its underground chambers and runways in sand with a minimum of effort. Some of the special animals and plants we see in the scrub such as gopher tortoises, sand skinks, and wedge-leaf button snakeroot are probably there because of sand.

The activities in the unit are designed to help students understand the characteristics of sand and adaptations of scrub plants and scrub animals that burrow in sand.

digger wasp	scarab beetle	gopher tortoise	pygmy mole cricket
Digging feet of some sand-dwelling animals
A. Physical Properties of Sand     I.A.1     I.A.3 B. Animal Tracks in the Sand     I.B.1 C. Glossary     D. Questions for Student Evaluation