Ecological Services
Southwest Region
 
Houston Toad Biology

Description

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.  Adult Houston toads are 2 to 3.5 inches (5 to 9 centimeters) long and, like all toads, are covered with raised patches of skin that resemble warts (Brown 1971).  Although Houston toads are similar in appearance to the closely-related Coastal Plains toad (B. [Incilius] nebulifer] and Woodhouse’s toad (B. woodhouseii), these species can be discerned by physical and genetic characteristics (Brown 1971, Hillis et al. 1984).  Mitochondrial DNA sequence analysis indicates that the Houston toad is a unique evolutionary unit separate from the other species (Forstner and Dixon 2000).

Range

The Houston toad is endemic to east-central Texas (Dixon 2000).  The known historical range of the Houston toad included the following 13 Texas counties (Hillis et al. 1984, Yantis 1989, 1990, 1991, 1992, Forstner and Dixon 2011, M.R.J. Forstner, pers. comm. 2014): Austin, Bastrop, Burleson, Colorado, Fort Bend, Harris, Lavaca, Lee, Leon, Liberty, Milam, and Robertson counties (Range Map).  However, range-wide audio surveys conducted from 2006 to 2011 have resulted in the detection of the species in only the following nine counties (Forstner et al. 2007, Forstner and Dixon 2011): 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 (Yantis 1991, Service 1994, Forstner 2003).  The Carrizo Sand and Reklaw formations give rise to deep sandy soils, such as the Patilo-Demona-Silstid and Axtell-Tabor soils that are often found in toad habitat (Dixon et al. 1990, Forstner 2003).


Habitat

General Habitat Characteristics

Houston toad habitat is generally characterized as rolling uplands covered with pine and/or oak forests underlain by deep sandy soils.  Although Houston toads are associated with forests and sandy soils (Kennedy 1962, p. 241; Brown 1971, p. 196), they may also breed in and move across sparsely wooded and even cleared, open areas (Dixon et al. 1990, p. 20).  It is not clear if the Houston toad requires sandy soils to persist in an area, or if its distribution is correlated to sandy soils 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 (Forstner and Dixon 2011, p. 37). 

Vegetation Characteristics

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 (Buzo 2008, p. 65; Forstner and Dixon 2011, p.37).  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 (Knutson et al. 1999, pp. 1,443-1,444).  Forests also function as a “life zone” for amphibians like the Houston toad, which Semlitsch (1998, p. 1,117) describes 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 (Porter et al. 1988, p. 916; Stiles and Jones 1998, pp. 343-344; Brown et al. 2012, p. 146). 

Herbaceous vegetation on the forest floor also plays an important role in Houston toad habitat, as it supports native arthropod species (invertebrate species, such as insects) (Harris 1984, p. 19) that comprise the Houston toad’s food supply (Bragg 1960, p. 106; Clarke 1974, pp 141-146). Studies have shown that canopy cover allowing light to penetrate the forest floor can result in increased herbaceous plant diversity (Halls and Schuster 1965, pp. 282-283).   

Breeding Habitat

Houston toads are known to breed in small pools of water and ephemeral ponds (Kennedy 1962, p. 241; Brown 1971, p. 190; Forstner 2003, p. 10).  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 (Forstner 2001, p. 2; Forstner 2003, p. 10).  Survival of eggs, tadpoles, and emerging juveniles may be low in permanent water bodies because they are more likely to harbor predators (Forstner 2003, p. 10), such as birds, mammals, snakes, turtles, fish, aquatic invertebrates, and bullfrogs (Ferguson et al. 2008, p. 452) and potential competitors and hybridizers, such as Woodhouse’s and Gulf Coast toads (Hillis et al. 1984, p. 57).  Permanent water bodies also have an increased probability of livestock usage (Forstner 2003, p. 10), which can negatively impact the quality of habitat along the edges of breeding ponds (Forstner 2001, p. 3; Forstner 2003, p. 10). 

Unsuitable Habitat Areas

Areas consisting of the following are not considered suitable habitat for the Houston toad: (1) open pastures absent of canopy cover (Swannack 2007, p. 67); (2) pastures of coastal Bermuda grass (Cynodon dactylon) or other heavy, rhizomatous mat-forming grasses that can hinder Houston toad movement (Yantis 1989, p. 6; Swannack 2007, p. 67); or (3) 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, as suggested by Yantis (1989, p. 6).  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 (Dixon et al. 1990, p. 20).  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.


Threats

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 Sanders (1953, pp. 25-47). 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 dessication, loss of breeding sites, and loss of eggs or tadpoles resulting 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 (Forstner and Dixon 2011, pp. 42-43). Both direct and indirect effects may be exacerbated due to other threats, such as habitat fragmentation and degradation (Forstner and Dixon 2011, pp. 22, 42-43).

Red-imported fire ants (Solenopsis invicta) threaten Houston toads by killing young toadlets emerging from ponds (Freed and Neitman 1988, p. 455). They have also been known to drastically reduce the abundance of native insect species (Porter and Savignano 1990, pp. 2,102- 2,103) 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 (from nitrogenous wastes, such as urine and manure), increased turbidity, decreased water quality, and an overall adverse environment for amphibian egg and tadpole development (Forstner 2001; Knutson et al. 2004; Schmutzer et al. 2008; Bull 2009).   

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 (such as wildfires and drought) and the negative effects of human-induced changes that result in habitat loss, degradation, and fragmentation (Purvis et al. 2000, pp. 1,949-1,950). Within the Houston toad’s range, it is possible that alteration of rainfall patterns due to climate change together with habitat loss may increase the likelihood of hybridization events (Forstner and Dixon 2011, p. 50) and redistribution of the species’ range (Donald and Evans 2006, p. 213).


Life History

The life expectancy of the Houston toad is at least three years, but may be longer (Price 1993). Males reach sexual maturity at about one year of age, but females require one to two years to achieve reproductive maturity (Quinn 1981, Quinn and Mengden 1984). 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 (Dixon et al. 1990, Forstner 2002a, 2002b, 2003, 2006). 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 (Hillis et al. 1984, Dixon et al. 1990). Houston toads typically breed from late January to June (Kennedy 1962, Hillis et al. 1984). Reported egg-laying dates in the field range from February 18 to June 26 (Kennedy 1962, Dixon 1982, Hillis et al. 1984). Breeding is believed to be triggered in part by rainfall and warm night time temperatures (Kennedy 1962). Other factors may also play a role in initiating chorusing activity. For example, Price (1992) found that Houston toads do not generally call during 7 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 (Service 1994). Eggs are laid in strings in the water and hatch into tadpoles that metamorphose into juvenile toadlets approximately 60 days after egg deposition (Hillis et al. 1984). After metamorphosis, juvenile Houston toads move into the surrounding terrestrial habitats where they grow and develop into adults (Forstner 2003).

Literature Cited

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Description

Range

Habitat

Threats

Life History

Literature Cited

     
Last updated: January 8, 2016