Previously known as Bufo houstonensis, the Houston toad, was renamed Anaxyrus houstonensis in 2006 and is the currently accepted taxonomy. However, several publications argue for the original genus Bufo pending additional data and interpretation.
The Houston toad was one of the first amphibians federally listed as an endangered species. It was listed on October 13, 1970 under the Endangered Species Conservation Act of 1969, which was a precursor to the Endangered Species Act of 1973. Critical Habitat for the Houston toad was designated in portions of Bastrop and Burleson Counties, Texas on January 31, 1978. The Houston toad is also listed as endangered by the State of Texas.
This small, greenish-brown speckled amphibian can be distinguished from other toads by the high-pitched, trill-sounding call that males emit during breeding choruses each spring. It depends on the forests of loblolly pine (Pinus taeda) and various hardwood trees and sandy soils it inhabits for migrating, hibernating, and feeding. Shallow, temporary water sources, called ephemeral ponds, serve as breeding sites.
This species was listed as endangered in large part because of landscape fragmentation and destruction caused by agricultural conversion and urban development within the Houston toad’s forested habitat. The species is also threatened by ongoing habitat loss throughout its range, recent Texas drought conditions, and impacts from red-imported fire ants (Solenopsis invicta) and feral hogs (Sus scrofa). These threats are compounded by dwindling population numbers, which present challenge the persistence of this species.
Location in Taxonomic Tree
Like all toads, the adults of this species are covered with raised patches of skin that resemble warts.
MeasurementsLength: 2 to 3.5 in (5 to 9 cm)
Houston toads are generally brown and speckled, although individual coloration can vary considerably. The Houston toad’s underside is usually pale with small, dark spots. Males have dark throats, which appear bluish when distended.
The life expectancy of the Houston toad is at least three years, but may be longer, as documented by A.H. Price in 1993.
Males reach sexual maturity at about one year of age, but females require one to two years to achieve reproductive maturity. In mark-recapture surveys of Houston toads in Bastrop County, observed sex ratios of males to females have been highly skewed in favor of males ranging from 3:1 to 10:1. The Houston toad is an explosive breeder, appearing in large numbers at breeding ponds where the males call to attract females over a period of a few nights throughout the breeding season, beginning as early as January 18. Houston toads typically breed from late January to June. Reported egg-laying dates in the field range from February 18 to June 26. Breeding is believed to be triggered in part by rainfall and warm night time temperatures. Other factors may also play a role in initiating chorusing activity. For example, studies have found that Houston toads do not generally call during seven to 10 days prior to a full moon. However, all cues that may stimulate Houston toad breeding activity are not known.
This species tends to concentrate their reproductive efforts into producing large numbers of eggs, but each egg has less than one percent probability of survival. Eggs are laid in strings in the water and hatch into tadpoles that metamorphose into juvenile toadlets approximately 60 days after egg deposition. After metamorphosis, juvenile Houston toads move into the surrounding terrestrial habitats where they grow and develop into adults.
Although Houston toads are similar in appearance to the closely-related Coastal Plains toad and Woodhouse’s toad, these species can be discerned by physical and genetic characteristics. Mitochondrial DNA sequence analysis indicates that the Houston toad is a unique evolutionary unit separate from the other species.
General Habitat Characteristics
Houston toad habitat is generally characterized as rolling uplands that are covered with pine, and or, oak forests that are underlain by deep sandy soils. Although Houston toads are associated with forests and sandy soils, they may also breed in and move across sparsely wooded and even cleared, open areas. It is not clear if this species requires sandy soils to persist in an area, or if its distribution is correlated to sandy soils. This is because these soils within the Houston toad’s range typically support forests. Either way, it is likely that both sandy soils and the vegetation they support are key components to defining the Houston toad’s habitat.
Canopy cover appears to be a necessary component of Houston toad habitat. Most Houston toad locations are in, or very near, forested patches of habitat. As an essential component of amphibian habitats, forests help stabilize temperatures, moderate evaporation rates of aquatic habitats, contribute and recycle organic matter, and support diverse plant and animal communities. Forests also function as a life zone for amphibians like the Houston toad, which research has described as habitat that is critical for feeding, growing, sheltering and maturation, as well as survival of the entire juvenile and adult breeding populations. Not only does canopy cover provide essential habitat for the Houston toad, the loss of forested habitat can lead to red-imported fire ant infestations, which threaten Houston toad survival. Red-imported fire ants are known to select for open and edge habitats.
Herbaceous vegetation on the forest floor also plays an important role in Houston toad habitat, as it supports native invertebrate species, like insects, that comprise the Houston toad’s food. Studies have shown that canopy cover allowing light to penetrate the forest floor can result in increased herbaceous plant diversity.
Houston toads are known to breed in small pools of water and ephemeral ponds. They also have been heard calling from or have been captured in ditches, lakes, puddles in roads, moist areas in yards, flooded pastures, potholes, streams, stock tanks and permanent ponds. Survival of eggs, tadpoles and emerging juveniles may be low in permanent water bodies, because they are more likely to harbor predators, like birds, mammals, snakes, turtles, fish, aquatic invertebrates and bullfrogs. They are also more likely to harbor potential competitors and hybridizers, like Woodhouse’s and Gulf Coast toads. Permanent water bodies also have an increased probability of livestock usage, which can negatively impact the quality of habitat along the edges of breeding ponds.
Unsuitable Habitat Areas
Areas consisting of the following are not considered suitable habitat for the Houston toad:
- Open pastures absent of canopy cover
- Pastures of coastal Bermuda grass (Cynodon dactylon) or other heavy, rhizomatous mat-forming grasses that can hinder Houston toad movement
- Forested areas with a dense, woody understory and low light availability that can hinder the growth of herbaceous vegetation on the forest floor and the prevalence of arthropods.
Although not considered suitable habitat, it may be possible for Houston toads to move through such areas while dispersing to suitable habitat areas or breeding sites. Therefore, Houston toads may still be found within these types of habitats at any given time, but it is likely they do not persist in these locations for long periods.
Habitat loss and alteration in the forms of fire suppression, conversion of forests to agricultural pastures, residential development and artificial impoundments have contributed to a very different ecosystem and landscape than when the Houston toad was first described by O.T. Sanders in 1953. The suppression of wildfires also has led to a dramatic increase in the understory density and decrease in natural forest canopy health within the range of the Houston toad.
Drought has been an additional stressor for the Houston toad for many years. Direct effects of drought on this species include desiccation, loss of breeding sites and loss of eggs or tadpoles, which results from pond evaporation. Indirect consequences of drought include decreased prey availability and increased predation pressures, as the overall productivity of the ecosystems declines under stress. Both direct and indirect effects may be exacerbated due to other threats, such as habitat fragmentation and degradation
Red-imported fire ants (Solenopsis invicta) threaten Houston toads by killing young toadlets emerging from ponds. They have also been known to drastically reduce the abundance of native insect species that may serve as the Houston toad’s food source.
Livestock wading and feral hog use can prevent vegetation from establishing around a Houston toad breeding pond’s perimeter and result in high levels of nitrates that come from nitrogenous wastes, like urine and manure. Other negative impacts include increased turbidity, decreased water quality and an overall adverse environment for amphibian egg and tadpole development.
The Houston toad’s distribution appears to be restricted naturally as the result of specific habitat requirements for breeding and development. These natural restrictions along with their diminishing population sizes make them particularly vulnerable to stochastic events, like wildfires and drought, and the negative effects of human-induced changes that result in habitat loss, degradation and fragmentation. Within the Houston toad’s range, it is possible that alteration of rainfall patterns due to
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.
Learn more about climate change together with habitat loss may increase the likelihood of hybridization events and redistribution of the species’ range.
A dense growth of trees and underbrush covering a large tract.
Areas such as marshes or swamps that are covered often intermittently with shallow water or have soil saturated with moisture.
The Houston toad is a small, brown amphibian found in a narrow range in central Texas. The toad has been listed as endangered under the Endangered Species A
The Houston toad is endemic to east-central Texas. The known historical range of the Houston toad includes the following 13 Texas counties: Austin, Bastrop, Burleson, Colorado, Fort Bend, Harris, Lavaca, Lee, Leon, Liberty, Milam and Robertson counties. However, range-wide audio surveys conducted from 2006 to 2011 have resulted in the detection of the species in only the following nine counties: Austin, Bastrop, Burleson, Colorado, Lavaca, Lee, Leon, Milam and Robertson. There is a high correlation between the occurrence of the Houston toad and outcrops of the Eocene Epoch Sparta Sand, Weches, Queen City Sand, Recklaw and Carrizo Sand formations. The Carrizo Sand and Reklaw formations give rise to deep sandy soils, like the Patilo-Demona-Silstid and Axtell-Tabor soils, which are often found in toad habitat.
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