back to Introduction to Edwards Plateau Vegetation and Ecology
A miscellaneous assortment of Edwards Plateau natural history
Reading the landscape (grazing, browsing, soil disturbance, soil type)
King Ranch bluestem, an invasive non-native grass
Texas winter grass and its parasite
Deer, or why murder Bambi
The density of white-tailed deer (Odocoileus virginianus) in the region is very high. Once you know what to look for, evidence of browsing is very common. Deer densities in some suburban areas of Austin are as high or higher than in ‘the country’. The effects on native plants and plant communities are probably profound, but almost unknown (but see Vegetation dynamics and management). Because of their effects on gardens and landscape planting, and because of car-deer collisions, there is some public sentiment for controlling deer numbers. However, many people feed deer and deer feed is sold at most grocery stores in west Austin. Experiments are being conducted in other parts of the country with contraceptives, but it is not a viable control option yet. Hunting is too dangerous in built-up areas, and is not necessarily effective in more rural areas. Killing does is legal during hunting season, but large numbers of does have to be killed to affect population growth rates. Many hunters prefer to kill bucks, which does not reduce population growth rates.
Fire ants, a species with no redeeming value
Newcomers to the region, watch where you step and sit! After a rain, fire ant (Solenopsis invicta) mounds are often visible, but the absence of visible mounds does not mean an absence of fire ants. If you kick a mound and ants come boiling out in large numbers, that’s a fire ant mound. If an ant bite hurts immediately, and then forms a small itching bump filled with what looks like pus, that’s a fire ant bite.
If you find a large (0.5 to 1.0 m diameter) flat cleared area with large ants going in and out, that is NOT a fire ant mound. It is a harvester (Pogonomyrmex) ant nest. Harvester ants are important native seed dispersers and are becoming uncommon due to fire ants.
Fires ants are recent invaders of this region. They are known to have a devastating effect on native ants. Their effect on other invertebrates has not been studied but is probably very large. They are known to eat the nestlings of vireos, quail, and other birds that nest close to the ground. They may be reducing the tick and chigger populations (personal observation).
Poisoning a mound often just makes the fire ant colony move.
Dr. Larry Gilbert has studied this species for many years and been instrumental in the testing and release of phorid flies, a potential biocontrol agent (http://uts.cc.utexas.edu/~gilbert/research/fireants/).
Reading the landscape: grazing
A quick way to determine if the land is still being grazed is to look at the condition of the fencing.
Species that are abundant where grazing has been very intense include Opuntia spp. (prickly pear, tasajillo), Stillingia texana (Queen’s delight), Asclepias asperula (antelope horns), Cnidoscolus texanus (bull nettle) and thistles Cirsium and Carduus species). Rocks on the soil surface may be much more conspicuous in overgrazed sites.
Technically, goats don’t graze; they are browsers, like deer. That is, they eat primarily woody plants and forbs, but little grass. Where goat densities have been very high you may see a ‘browse line’ at 1.5-2 m above the ground surface, with little vegetation other than tree trunks below it.
Reading the landscape: soil disturbance
Look for Baccharis neglecta (Roosevelt weed) as a sign of past quarrying, road bank grading, plowing, and similar soil disturbances.
Reading the landscape: soil type
Where the soil is sandy, rather than formed from the breakdown of limestone, you may find Quercus stellata (post oak) in place of Q. fusiformis and Q. buckleyi. Prosopis glandulosa (honey mesquite) is a sign of relatively deep soil.
A special kind of place: canyons
Steep sunny canyon walls may support hanging plants of Nolina spp. (beargrass). Shaded canyon walls, especially those with seeps, may have maiden-hair fern (Adiantum capillus-veneris) and columbine (Aquilegia canadensis). Travertine, a type of rock which looks a bit like small dirty cave stalactites, forms on some seeping canyon walls. Salvia roemeriana (cedar sage) is a conspicuous and beautiful canyon wildflower when it is blooming. In a few places we find palmetto in wet places on canyon floors. A beautiful local canyon accessible to the public (by guided tour) is Westcave Preserve (http://www.westcave.org/). The difference in temperature, light, and humidity between a canyon and the neighboring upland on a sunny summer day is dramatic.
A popular myth: ball moss
Ball moss (Tillandsia recurvata) is common in Austin but rare outside of Austin. It may be that the city ‘heat island effect’ allows it to grown in Austin. This is a member of the bromeliad family, closely related to Spanish moss.
It is an epiphyte, not a parasite. There is no evidence at all that it harms trees, contrary to local myth. Indeed, since it is an epiphyte, it is difficult to see how it would harm trees. The myth probably arises from the tendency of sick and dying trees to have conspicuously large ball moss populations. This is likely due to the lack of new (hence ball moss-free) growth and also to increased light levels within the canopy of an unhealthy tree.
Comic relief: armadillos
That loud scuffling you hear when you are out doing fieldwork, that sounds like a herd of elephants, is one armadillo (Dasypus novemcinctus) under the trees. Armadillos dig large holes under the trees, to which they retreat when scared. They make smaller holes, often just scrapes, wherever the soil is not too rocky or dry, as they forage.
If you make too much noise, however, you won’t see a live armadillo. You will be restricted to seeing dead armadillos in the road and the many armadillo knickknacks for sale all over Austin. Jim Hightower, a local liberal Democrat, once said - in the context of politics - that the only thing in the middle of the road is yellow lines and dead armadillos.
Armadillos eat invertebrates in the soil, and are also strongly attracted to buried peatpots. They will dig up an entire experiment, only to discard every peatpot and the plant in it (personal experience - and it wasn’t funny at the time!).
An interesting insect: harvester ants
The nests of harvester ants (Pogonomyrmex) are conspicuous because of the flat bare area that they clear. Their nests typically have a central bare area covered with coarse sand or gravel, surrounded by a ring in which the only plant species are species of three-awn grass, Aristida spp. It is not known whether this is the result merely of selective weeding, or differential discarding of germinated seed, or something else. You can also see their trails running off for long distances. They are not aggressive and can be picked up by letting them run up a stick or even your hand. Their sting is painful, however, so don't antagonize them!
They are seed eaters but also eat dead insects (personal observation). They are now absent from areas (including Brackenridge Field Laboratory where they used to be abundant) because the fire ants have eliminated them. 'Pogos' are seed eaters, especially grass seeds. They were the principal food of the horned lizard (Phrynosoma cornutum), which was common in this region 50 years ago but is now extinct in the Austin area.
An invasive grass
Bothriochloa ischaemum (King Ranch bluestem) is native to Eurasia. It is a perennial bunchgrass. It is extremely common along roadsides, in part because it has been widely planted by the highway department for erosion control. It is found in savannas throughout the eastern Edwards Plateau. It was once widely recommended to ranchers, but was then found to be of low forage value.
It is unusually plastic. When heavily grazed or mowed, the tillers grow horizontally and the stand may resemble a golf course in height. Unmown and ungrazed, it grows upright and is often 20 or 30 cm tall. In some sites it forms pure stands that replace the native plant species. It can successfully invade and dominate ungrazed sites as well as grazed sites, and burned as well as unburned sites. However it, like most of the native grass species, does not tolerate heavy shade.
Texas winter grass and its parasite
A native grass and its parasite
Nassella (formerly Stipa) leucotricha (Texas winter grass) is our only common perennial C3 grass. It has both chasmogamous seeds (on the conspicuous inflorescences) and cleistogamous seeds (down among in the leaf bases). It is often infected by Atkinsonella texensis, an epiphytic fungus. This fungus sterilizes the plant, preventing it from forming both chasmogamous and cleistogamous seeds. Instead, an infected plant forms a fungal fruiting body at the end of what would have been the flowering stalk (culm). This fungal fruiting body looks exactly like a bird dropping. The fungal fruiting body is cheaper (less dry weight) than the inflorescence and its seeds would have been, so infected plants actually grow faster than uninfected plants.
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