The Remarkable Western Spadefoot Toad

The western spadefoot toad, with the scientific name Spea hammondii, is a fascinating amphibian native to western North America. This unique toad has several notable features and behaviors that enable it to thrive in its arid environment. In this comprehensive guide, we will explore the taxonomy, physical appearance, habitat, life cycle, diet, ecosystem role, conservation status, and interesting facts about the western spadefoot toad.

Taxonomy and Classification

The western spadefoot toad belongs to the family Scaphiopodidae and genus Spea. It was first described by zoologist Edward Drinker Cope in 1879 based on a specimen collected near Fort Tejon, California. The species name “hammondii” honors naturalist William Hammond, who collected the first specimens in Monterey County, California between 1855-1857 during the Pacific Railroad surveys.

This species is also known as Hammond’s spadefoot toad or the couch’s spadefoot toad. It is one of seven species in the genus Spea which are collectively known as spadefoot toads. Its closest relatives include the Great Basin spadefoot (Spea intermontana), the plains spadefoot (Spea bombifrons), and the New Mexico spadefoot (Spea multiplicata).

The western spadefoot toad is not a true toad, but rather a “spadefoot” toad lacking parotoid glands and having smoother, less warty skin compared to true toads in the family Bufonidae. The vertical pupil and spade-shaped tubercle on its hind feet distinguish it from other North American amphibians.

Its generic name “Spea” derives from the Ancient Greek word for “to dig,” referring to the burrowing nature of these toads. The common name “spadefoot” comes from the keratinized black spades on their hind feet which they use for digging.

Physical Appearance

The western spadefoot toad is a plump-bodied amphibian that grows to 1.5-3 inches long as an adult. Its skin is bumpy with small warts that secrete an irritating, milky toxin. The skin coloration can vary from brown, gray, olive, or tan, often with a pattern of light and dark splotches for camouflage against desert soils. A pale middorsal stripe may run down the back. The underside is white or yellowish, sometimes with dark mottling on the chest and belly.

Distinctive Features

Some of the western spadefoot’s most distinctive features include:

  • Large protruding eyes with vertical cat-like pupils
  • Smooth, warty skin secretions for moisture retention
  • Spade-shaped tubercles on each hind foot for burrowing
  • Parotoid glands behind the eyes that secrete irritating toxins

The vertical pupils give the toad excellent night vision for hunting and navigating in low light. The spades on their hind feet have hardened, keratinized edges that enable rapid backward burrowing into sandy soil when threatened.

Size

An adult western spadefoot toad ranges from 1.5-3 inches in length (snout to vent). Females tend to be slightly larger than males. The average adult size is around 2 inches long.

They weigh approximately 0.5-1 ounce as an adult. Western spadefoot toads grow rapidly into mature adults once they metamorphose from the tadpole stage.

Color Variations

The coloration of western spadefoot toads can vary between subspecies and populations:

  • Desert populations are paler.
  • Coastal California populations have darker olive coloration.
  • Some have yellowish or orange spots on the belly.
  • Darker morphs may be partly or largely melanic.

Their color and patterning provides effective camouflage against sandy and gravelly substrates in their natural habitat. The mottled skin blends into the dappled light and dark patches seen in dry soils.

Habitat and Range

The western spadefoot toad occupies arid regions of the western United States and Mexico. Its range extends from southern California and Baja California north to Washington state. It has been recorded at elevations up to 4,500 feet.

Geographic Range

The western spadefoot toad can be found in:

  • Most of California including the Central Valley, Coast Ranges, foothills, and Transverse Ranges
  • Southern coastal regions of Oregon
  • Southwestern Washington along the Columbia River
  • Western edge of Nevada
  • Southwestern corner of Idaho
  • Northern Baja California, Mexico

Within California, the largest populations occur in the Central Valley and surrounding foothills, as well as coastal regions from Santa Barbara south to Mexico. Scattered populations exist in suitable habitat throughout California. They are common in parts of the Mojave and Colorado deserts.

Preferred Habitats

Western spadefoot toads inhabit a variety of arid environments with loose, sandy or gravelly soils for burrowing including:

  • Coastal sage scrub and chaparral
  • Desert scrub and washes
  • Alkali flats
  • Grasslands
  • Pine and oak forests
  • Sandy river banks and terraces

They avoid densely forested areas but may utilize streamside woodlands if suitable loose soils exist. The presence of temporary rain pools and wetlands for breeding is a key habitat requirement.

Burrows

During dry periods, western spadefoot toads spend most of their lives underground in burrows up 3 feet deep to escape extreme heat and aridity. Their specialized hind feet allow them to dig backwards into soil at remarkable speed when threatened. Deeper burrows provide insulation from temperature extremes and retain moisture.

Life Cycle and Reproduction

The western spadefoot toad exhibits an explosive breeding strategy adapted to the ephemeral wetlands they rely on for reproduction. They emerge en masse when desert pools fill with rainwater to mate and lay eggs. The larvae develop rapidly before pools dry up.

Breeding and Egg-Laying

  • Breeding usually occurs between late winter and early spring depending on rainfall, but occasionally in fall.
  • Males emerge first and form large breeding aggregations. Their distinctive mating call attracts females.
  • Females lay between 300-900 eggs in long gelatinous strands attached to vegetation.
  • The eggs hatch within 48 hours as long as pools do not dry up first.

Larval Development

  • Western spadefoot toad tadpoles are small (12-22 mm) with rounded bodies and long tails.
  • Tadpoles consume algae, organic detritus, and small aquatic organisms.
  • Cannibalistic behavior allows some tadpoles to metamorphose more quickly when resources are scarce.
  • They can complete metamorphosis into tiny toadlets in as little as 12-14 days, one of the fastest amphibian development times.

Maturation

  • Toadlets grow rapidly after metamorphosis and reach sexual maturity at 1-2 years old.
  • Adults emerge from burrows to feed and breed each rainy season.
  • Western spadefoot toads are estimated to live up to 15 years in the wild.

Their rapid growth and development is an essential adaptation to take advantage of temporary pools before they disappear. It allows new generations to emerge even during short wet seasons.

Diet and Feeding

Western spadefoot toads are opportunistic carnivores that use their long sticky tongues to catch a variety of prey. Tadpoles have a more herbivorous diet.

Adult Diet

  • Insects – crickets, grasshoppers, beetles, ants, mosquitoes, flies, moths
  • Spiders
  • Scorpions
  • Centipedes
  • Millipedes
  • Snails
  • Slugs
  • Worms

They hunt at night by waiting in ambush near the mouths of their burrows for passing prey. Their upward-facing eyes help spot overhead insects. Toads also actively forage away from burrows on humid nights.

Tadpole Diet

  • Algae
  • Plant debris
  • Aquatic plants
  • Carrion
  • Aquatic invertebrates – small crustaceans, mosquito larvae, tadpoles

Tadpoles scrape algae from surfaces with keratinized structures in their mouth. They become more carnivorous as they develop, rasp meat from carcasses, and prey on other tadpoles.

Digestion

Western spadefoot toads produce a mild toxin in their livers that aids digestion by breaking down proteins of eaten prey. The toxin is stored in their paratoid glands and secreted as a defense mechanism.

Ecosystem Roles

Western spadefoot toads occupy a variety of niches throughout their lifecycle which contribute to ecosystem functioning in many ways:

As Predators

  • Consume thousands of insects, spiders, and other
  • invertebrates which helps regulate those populations
  • Tadpoles consume algae and plant material, controlling growth
  • Provide an important seasonal protein source for predators when breeding pools dry up and tadpoles perish

As Prey

  • Eggs and tadpoles are eaten by predatory insects like beetles and mosquito larvae
  • Larval and adult toads are preyed upon by snakes, birds, mammals, and other predators
  • Their toxin secretions and burial provide defenses against predators

As Environmental Indicators

  • Population numbers signal the health of ephemeral wetlands and help guide habitat management decisions
  • Declining western spadefoot toad numbers may indicate threats like non-native species, pollution, or habitat loss

Interactions with Other Species

  • Coexist with other aquatic species in temporary pools including fairy shrimp, mosquito fish, newts, and frogs
  • Compete for food resources and predate tadpoles of Pacific treefrogs and California red-legged frogs
  • Some garter snakes may synchronize breeding with spadefoot toads as a food source for their young
  • Maintaining high biodiversity in wetlands protects ecosystem functions and benefits sympatric species

Overall, western spadefoot toads provide trophic connections in food webs, affect energy flow and nutrient cycling, indicate habitat changes, and interact with numerous sympatric species throughout their complex life history. They are an important component of biodiverse western aquatic and terrestrial ecosystems.

Threats and Conservation Status

While still widespread across the western states, some western spadefoot toad populations have experienced declines, especially in California. They face a variety of threats from human activities and habitat changes.

Major Threats

  • Urbanization and agricultural development that destroys or degrades breeding pools and upland habitat
  • Livestock grazing and off-road vehicle use that collapses burrows and damages pools
  • Predation and competition from introduced fish, bullfrogs, and crayfish
  • Climate change altering rainfall patterns and drying of ephemeral wetlands
  • Road mortalities during explosive breeding migrations
  • Pollution from agriculture and urban runoff impacting wetlands

Conservation Status

  • Considered a Species of Special Concern in California due to population declines
  • Petitioned for federal listing under the Endangered Species Act in 2012 but not listed
  • Classified as Near Threatened on the IUCN Red List of Threatened Species

Conservation Strategies

  • Protecting existing breeding pools from development and monitoring known populations
  • Public education about driving carefully in rain events when spadefoot toads are migrating
  • Restoring disturbed ephemeral wetlands by removing invasive species and revegetating
  • Constructing artificial breeding pools if insufficient natural vernal pools exist nearby
  • Fencing off pools from livestock access and protecting adjacent uplands habitat

Ongoing conservation efforts by state and federal agencies, land managers, biologists, and citizens aim to maintain western spadefoot populations and wetland habitats. Their unique explosive breeding behavior provides opportunities to see these remarkable amphibians in action when desert pools fill with rain.

Interesting Facts

The western spadefoot toad has many fascinating behaviors and adaptations for survival in its harsh, arid environment:

  • Their explosive breeding aggregations in desert rain pools give them the nickname “rainpool toads.” Hundreds rapidly converge when heavy rainfall hits.
  • Tadpoles develop hardened mouth parts for scraping algae and rasping flesh from carcasses. This facilitates their carnivorous eating habits.
  • If picked up, western spadefoot toads secrete a smelly, milky white toxin from their paratoid glands that irritate mucous membranes and deter predators.
  • The spadefoot’s keratinized spades on their hind feet allow rapid backward burrowing by using their feet as shovels. They can disappear underground in seconds.
  • Western spadefoot toads can remain dormant in deep burrows for up to 10 months. In dry conditions, they may estivate for years waiting for rain.
  • Their upward-facing eyes allow them to spot overhead prey while remaining concealed at their burrow entrance. Their vertical pupils also aid hunting at night.
  • Tadpoles have been documented metamorphosing from eggs to tiny toads in under 12 days – one of the shortest development times for amphibians.
  • Western spadefoot toads have one of the most rapid life cycles among North American amphibians due to the ephemeral wetlands they rely on.
  • They inhabit an elevational range from sea level near the Pacific Ocean up to 1,370 m (4,495 ft) in the Sierra Nevada mountains.
  • The longest recorded lifespan of a western spadefoot toad in the wild is estimated at 15 years.

Conclusion

The western spadefoot toad is a unique and fascinating amphibian that has evolved many specialized adaptations for exploiting temporary wetlands in highly arid environments. Understanding its complex life history provides insights into maintaining biodiversity in vulnerable aquatic ecosystems. Protecting existing vernal pools and grasslands will be essential for the long-term viability of western spadefoot toad populations. Everyone can contribute by driving carefully during rains, removing invasive species, and reporting observations to local agencies. With increased awareness and habitat conservation, the western spadefoot toad may continue thriving across its western range.