What kind of Rat?

A Pack Rat, that’s what!

More specifically, a White-throated Woodrat, Neotoma albigula.

There are several different kinds of related Woodrats, just to make things confusing for humans. And in addition, Woodrats are not the same as the rats we usually think of, when we think of rats–those are the Old World Rattus rats.

Woodrats or Pack Rats look more like giganto mice, with their big ears and big dark eyes. Their tails are also fuzzier than the bare Rattus tails.

These amazing creatures can survive on the moisture they get from the vegetation they eat, and do not need to drink water. How’s that for a desert adaptation!

Quite a while back we talked about Pack Rat middens, the amazing homes the White-throated Woodrats build in the desert.

These folks must have special skills, to build such large defensive structures without getting nasty stab wounds, since middens often include a lot of cactus branches.

Haboob, the Desert Dust Storm

Another limiting factor in the desert arises from the ferocious dessicating and abrasive desert dust storm called the Haboob. Often associated with the seasonal monsoons, these storms can actually happen any time of year, blasting and scouring any plants and animals in their path.

The word haboob is Arabic, from the home of truly impressive sandstorms, but now is applied in any area susceptible to them.

These storms can move particles of dust and even sand, and may reach hurricane force for a few minutes at a time.

Deserts have few trees to stop the wind, and little moisture to hold down the soil. A haboob forms when warm air rises into thunderheads, then the air is cooled by rain, the storm collapses, and the cooled air falls and rushes out from the base of the storm, driving dust and sand before it, sometimes at truly remarkable speeds. Within minutes, winds can go from zero to 50 miles an hour.

Dust storms often arise suddenly, making them hard to avoid or escape. Most years in the Sonoran desert, there are traffic accidents associated with dust storms, when visibility suddenly plummets.

Desert plants need either shelter or thick skins to handle the dessication and scouring associated with a haboob or other winds.

Limiting factors in the desert

In some ways the term “limiting factor” is almost a definition of deserts, because deserts lack one or more essential ingredients for the survival of most kinds of plants and animals.

First, a desert is, by definition, a place with very little available water, either because there is little rainfall, or because it evaporates so fast. It may also be that most of the precipitation is unavailable because it is frozen most of the year, as in the far north or south, or in some high altitude regions.

Some areas may get bursts of moisture, even flooding, but it is episodic and not dependably available most of the year.

Temperature is another limiting factor. Many deserts get too hot for most organisms. Some deserts near the north and south poles are too cold. And yet others have wide swings in temperature.

Wind is another. There are few trees or shrubs to stop the wind. Desert mountains, river-courses or other sheltered areas may harbor small oases in their wind and sun shadows.

One unusual limiting factor is lack of sunlight. This can occur in deserts near the poles.

Especially interesting are the adaptations that plants and animals have developed to deal with these limiting factors. Many of them are what we talk about here.

Leafing again: the strange case of the ocotillo

One of the odder native residents of the Sonoran desert is the ocotillo bush. It typically consists of several spindly branches, vertically striped and spiny, up to 20 feet tall, that are leafless for long periods, only to sprout thick lines of small leaves within a few days after a rain.

Ocotillo branches with few leaves

Its desert adaptation of dropping leaves and playing possum is so complete that ocotillo plants for sale, in their leafless phase, with their branches tied in a bundle and roots bare, look thoroughly dead.

Ocotillo branches with many leaves

In the spring, they produce numerous red tubular blossoms at the ends of the branches that attract insects and birds of many species. They make great landscape plants because of their sparse sculptural look, and extremely low water requirements, and striking seasonal changes.

A thorny issue

Related to the last post, it is amazing how many kinds of thorns, bristles, hooks, spikes, prickers, sharp edges, pointed leaves, pointed branches, as well as claws, scales, fangs, spines, and hooves you find in the Sonoran desert, and generally in the arid western USA.

In a contrary kind of way, almost all of these indicate that there’s probably a nice, juicy, succulent hunk of flesh nearby.

But you’ll get hurt if you try to get at it!

Strangeness in the desert soil: Caliche

Here’s one way to encounter caliche. Some friends pulled over one night in the New Mexico desert and camped in their truck. Happily for the desert, there was rain in the night. Unhappily for the humans, the next morning they found themselves surrounded by a huge pond of extremely slippery ankle-deep goo. Only after a long dirty struggle were they able to push their truck back onto the road. Caliche is what allowed a mudbath to form in the middle of the desert.

Another way to discover caliche is to dig a hole almost anywhere in the desert. Soon, inches or feet deep, you’ll hit a layer of what seems to be cement, even in places where you know there’s never been any construction.

What you’ve found is caliche, a white layer of soil, rich in calcium carbonate, clay, and other minerals that have been driven down into the ground by the hard infrequent desert rains, until the minerals form an often impervious layer of soil, also known as hardpan.

Wikipedia can tell you more.

When rain falls on an area with dense caliche, rather than sinking into the soil, the water runs off, if it can find a channel, sometimes causing the surprising desert flash floods, because so little water is absorbed into the ground. Or it puddles up, creating a sea of mud.

Digging a simple hole can require a pickaxe. Since the calcium carbonate can be alkaline enough to harm plant roots, planting in an area with caliche means that you may have to add conditioners as well as nutrients to the soil– and make sure there is drainage, even if that means digging a drainage hole through the caliche.

The Arizona Master Gardener, a great source of information, gives details of how to deal with it.

Dry rain

Even the simplest things about the desert can be strange.

Rain, for instance.

When you need rain the most, where you need it most, seems to be the place you find it least.

And sometimes it comes unusually close, but never arrives.

An eerie and frustrating example is the dry rain that can occur in the desert west, called “virga.”

In the distance, you will see the clouds form, then curtains of rain finally begin to fall. But the rain disappears in the middle of the sky, between cloud and land.

Leaving you high and dry. Or low and dry.

Virga actually does happen in other regions, and can involve snow as well as rain.

It’s a sight to see, watching the rain come down, but never land on land.