Arroyo Toad

Bufo microscaphus californicus

Description

The Bufo microscaphus californicus (arroyo toad) is a small toad in the family Bufonidae, measuring 2-3 in (5-8 cm). It is a light greenish gray or tan toad with warty skin and dark spots. Its underside is buff colored and often without spots. A light-colored stripe crosses the head and eyelids, and a light area usually occurs on each sacral hump and in the middle of the back.

This taxon was originally described as B. cognatus californicus from a specimen collected at Santa Paula, Ventura County in 1915. The specimen was later shown to differ in several respects from B. cognatus and was afforded specific status as B. californicus. In the following two decades, this toad was considered a subspecies of B. compactilis and of B. woodhousei. The currently accepted taxonomy of the arroyo toad as a subspecies of B. microscaphus, the southwestern toad, is based on morphological similarities. The arroyo toad is geographically isolated from the Arizona toad by the Mojave and Colorado Deserts. Work is now in progress to determine if the arroyo toad is genetically distinct.

Behavior

The arroyo toad's movement consists of hopping rather than walking, and prefers shallow pools and open, sandy stream terraces. Its courtship vocalization is a high trill, usually lasting eight to 10 seconds and breeding occurs on large streams with persistent water from late March until mid-June. Eggs are deposited and larvae develop in shallow pools with minimal current and little or no emergent vegetation and with sand or pea gravel substrate overlain with flocculent silt. After metamorphosis, the juvenile toads remain on the bordering gravel bars until the pool dissipates. Adult toads excavate shallow burrows on the terraces where they shelter during the day when the surface is damp or during longer intervals in the dry season

Habitat

The arroyo toad is restricted to rivers that have shallow, gravelly pools adjacent to sandy terraces. Juveniles and adults forage for insects on sandy stream terraces that have nearly complete closure of cottonwoods, oaks, or willows, and almost no grass or herbaceous cover at ground level. It also uses upland habitats up to a half mile from the stream for feeding and overwintering.

Distribution

Arroyo toads were historically found along the length of drainages in southern California from San Luis Obispo County to San Diego County, but now they survive primarily in the headwaters as small isolated populations. Urbanization and dam construction beginning in the early 1900s in southern California caused most of the extensive habitat degradation. The species was formerly distributed southward along the northwestern coastal region of Baja California, Mexico, to the vicinity of San Quintin.

Most remaining populations in the United States occur on privately owned lands, primarily within or adjacent to the Cleveland National Forest. Less than 50% of the known extant populations of arroyo toad occur in areas owned or managed by the Forest Service. Due mostly to habitat destruction, only eight drainages remain where populations of this species may be viable. In 1990, only seven pairs of arroyo toads were known to have bred anywhere within the toad's range. Due to the isolation and the small sizes, almost all populations are at great risk of extinction.

Threats

Activities that could potentially result in the loss of the arroyo toad, include, but are not limited to, unauthorized collecting or capture of the species, except as noted above to momentarily move an individual out of harm's way; introduction of exotic species into occupied habitat; unauthorized destruction/alteration of the species' habitat; violation of a construction, discharge or withdrawal permit that affects occupied habitat; pesticide applications affecting occupied habitat in violation of label restrictions; or other illegal discharges or dumping of toxic chemicals, silt, or other pollutants into waters supporting the species.

Habitat destruction and alteration constitutes the most severe threat facing the arroyo toad. This toad is now confined to the headwaters of streams it occupied historically along their entire lengths. Formerly found on rivers with near-perennial flow throughout southern California from San Luis Obispo County to San Diego County, it is believed to be extirpated in San Luis Obispo County. Populations persist in Santa Barbara, Ventura, Los Angeles, Riverside, and San Diego Counties. Recent sightings of scattered individuals have been reported from Orange, San Bernardino, and southwest Imperial Counties.

The majority of the remaining populations in Santa Barbara and Ventura Counties are located on the Los Padres National Forest. This National Forest supports the majority of southern California's remaining intact large river systems and maintains five viable populations of arroyo toads. Sespe Creek in Ventura County has the largest known population. Other populations are found on the Sisquoc, Santa Ynez, and upper and lower Piru drainages.

Populations to the south are located primarily in San Diego and Riverside Counties and are predominantly found in the vicinity of the Cleveland National Forest and on private lands within or adjacent to national forest. In San Diego County, arroyo toads have been found on the Santa Margarita, Guejito, Sweetwater, Vallecito, San Luis Rey, Santa Ysabel, Witch, Cottonwood, Temescal, Agua Caliente, Santa Maria, Lusardi, Pine Valley, Noble, Kitchen, Long Potrero, Upper San Diego, San Vincente, and Morena drainages. Populations on Temescal, Agua Caliente, Pine Valley, and Cottonwood drainages may be considered viable. Recent surveys have located very small populations of arroyo toads in four creeks in southwestern Riverside County. The single recent occurrence of arroyo toads in San Bernardino County is on Deep Creek in the San Bernardino National Forest.

Several factors presently threaten the remaining 25% of the habitat of the arroyo toad including: (1) Short-and long-term changes in river hydrology, including construction of dams and water diversions; (2) alteration of riparian wetland habitats by agriculture and urbanization; (3) construction of roads; (4) site-specific damage by off-highway vehicle use; (5) development of campgrounds and other recreational activities; (6) over-grazing; and (7) mining activities.

Dam construction was responsible for the loss of approximately 40% of the estimated original range of the arroyo toad. Twenty-six large impoundments are currently located within the range of this species, inundating over 120 mi (190 km) of suitable habitat. Additional areas have been identified as potential dam sites and, if constructed, would destroy 25% of the current range of the arroyo toad.

In addition to habitat loss through direct inundation, dams can have significant effects on habitat quality downstream. Artificial flow regulation disrupts the natural processes that produce the terrace and pool habitats required by arroyo toads. Unseasonable water releases may prevent arroyo toads from breeding due to habitat changes.

Another consequence of sustained unnatural perennial flows below dams is an adverse effect on the habitat of this species by encouraging vegetative growth in a riparian corridor, which increases ground stability and hence confines and deepens the creek channel. Water temperatures are reduced below the temperatures needed or larval development.

The arroyo toad is also sensitive to stream diversions as they cause the riparian areas to dry. Water diversions that alter normal flows have degraded habitats and adversely affected arroyo toads by leading to: the early drying of breeding pools, causing breeding failures or loss of the larval population; restriction of the period essential for rapid growth when newly-metamorphosed toads can forage on damp gravel bars; and loss of damp sub-surface soil, which may result in high adult mortality during late summer and early fall.

Development projects in riparian wetlands have caused permanent losses of riparian habitats and are the most conspicuous factor in the decline of the arroyo toad. Agriculture and urbanization have already destroyed much of the suitable arroyo toad habitat south of the Santa Clara River in Ventura County. Stream terraces have been converted to farming, road corridors, and residential and commercial uses, while the streams themselves have been channelized for flood control. Large stretches of riparian corridor habitat have also been degraded or destroyed by cattle and feral pigs.

Recreational activities in riparian wetlands have had substantial negative effects to arroyo toad habitat and individuals. Off-highway vehicles cause extensive damage to the shallow pools in which arroyo toads breed.

Streamside campgrounds in southern California national forests have frequently been located adjacent to arroyo toad habitat. In the Los Padres National Forest, each of the three campgrounds on Piru and Sespe Creeks were developed on terraces used by arroyo toads within 150-300 ft (45-60 m) of their breeding pools. On the upper Santa Ynez River, also in Los Padres National Forest, three of four campgrounds are also located in arroyo toad habitat. The placement of campgrounds is similar in the Cleveland National Forest in San Diego County; upper San Juan Creek, upper San Luis Rey River, and Cottonwood Creek all have campgrounds situated adjacent to arroyo toad breeding habitats.

The use of heavy equipment in yearly reconstruction of roads and stream crossings in the national forests has had significant and repeated impacts to arroyo toads and toad habitat.

Maintenance of the road to Ogilvy Ranch, a private inholding in the Los Padres National Forest, is likely responsible for a depressed population of arroyo toads in Mono Creek. The Ogilvy Ranch road makes 18 crossings of Mono Creek, many directly through or near arroyo toad breeding pools. In summer 1992, the Los Padres National Forest declined to open the Ogilvy Ranch road in order to protect populations of arroyo toads and other candidate amphibians and reptiles. However, the road was opened with a bulldozer in the fall. As juvenile arroyo toads were likely burrowed in the soft sand adjacent to the creek, grading the road up the creek destroyed habitat and probably killed individual toads. Regular maintenance of roads in the Los Padres National Forest negatively affects arroyo toad individuals and toad habitat on the Santa Ynez River and Piru and Sespe Creeks, as well.

Mining activities are an additional threat to this species. Recreational suction dredging for gold adversely affects toad habitat and individuals. Dredging destroys breeding pools used by arroyo toads and causes excessive siltation downstream, which asphyxiates eggs and small larvae. For example, during the Memorial Day weekend of 1991, four small dredges operating on Piru Creek produced sedimentation visible more than 0.6 mi (1 km) downstream and adversely affected 40,000-60,000 arroyo toad larvae. Subsequent surveys revealed nearly total destruction of the species in this stream section; less than 100 larvae survived, and only four juvenile toads were located.

Several rivers in the Los Padres National Forest were recently temporarily closed to gold mining, and it is uncertain whether the ban will be made permanent. In December 1992, a group of miners challenged the Forest Service's authority to close Piru Creek to mining. These individuals practiced various methods of gold exraction until cited by the Forest Service. It is probable that future challenges will occur and, if successful, will threaten the population of arroyo toads on Piru Creek.

Populations of the arroyo toad are becoming so small and confined that even limited taking by campers, recreationists, and scientific researchers could adversely affect this species' viability. These toads are threatened from collecting by children near the campgrounds. No data exists on the extent of such collection activities, but it is probable that it continues to occur.

Over the past 20 years, at least 60 species of fishes have been introduced to the western U. S. States, 59% of which are predatory. The introduction of exotic predators to southern California waters has been facilitated, in part, by the inter-basin transport of water. Introduced predators had substantial impacts on the sizes of extant populations of arroyo toads and may have contributed to regional extinctions.

Virtually all rivers that contain or once contained arroyo toads support populations of introduced predatory fish, such as green sunfish, largemouth bass, mosquitofish, black bullhead, arroyo chub, prickly sculpin, rainbow trout, oriental gobies, and red shiners. All of these introduced fish prey on tad-poles and have been observed inducing high arroyo toad larval mortality in breeding pools on the Piru, Sespe, and Santa Ynez drainages. It is probable that predation by introduced fish species occurs elsewhere.

Arroyo toads occur in streams with perennial or near perennial flow. Most streams with populations of arroyo toads also have populations of introduced bullfrogs. Adult bullfrogs are highly predatory and have been observed to prey on adult arroyo toads. Habitat for bullfrogs has been enhanced within the existing range of the arroyo toad via diversions and artificially maintained perennial flows below dams. Increased bullfrog populations in these permanent water areas threaten the survival of arroyo toad populations.

Alteration of the natural intermittent flow regmes by dams has had significant adverse impacts to arroyo toads. Prior to 1992, the California Department of Water Resources, which operates Pyramid Dam on Piru Creek in the Los Padres and Angeles National Forests, frequently discharged excess flows from the reservoir resulting in the depressed population of arroyo toads on lower Piru Creek. Recent coordination among the Department of Water Resources, Forest Service, and Fish and Wildlife Service have resulted in releases from the dam that more closely mimic natural flows, benefitting the arroyo toad. Water releases of several million gallons per day from Barrett Dam on Cottonwood Creek during the period when larval arroyo toads were metamorphosing negatively affected the population in San Diego County in summer 1993.

Several other factors have also contributed to the decline of the species including drought, fire, and light and noise pollution. Additionally, there has been direct mortality of the toads due to road construction and maintenance, water inundation or drainage from dams and diversions, off-highway vehicle use, cattle and pig trampling, mining, and recreational activities.

By far, the most significant natural factor adversely affecting the arroyo toad is drought and resultant deterioration of riparian habitats. Southern California recently experienced five consecutive years of lower than average rainfall. These drought conditions, when combined with human-induced water reductions, have degraded riparian ecosystems and have created extremely stressful conditions for most aquatic species.

Drought also affects arroyo toads in another manner. Female arroyo toads must feed for at least two months in order to develop the fat reserves needed to produce a clutch of eggs. In drought years, females may find insufficient insect prey to produce eggs before males cease their courtship behavior of calling, resulting in no reproduction in that year. The extremely low reproduction of 1990 was likely due to four years of severe drought. Although rainfall patterns in 1992 and 1993 returned to near normal levels, drought is a naturally recurring phenomenon in southern California. There is no doubt that arroyo toads evolved with periodic, severe drought. However, the recurrence of this natural event combined with the many manmade factors negatively affecting arroyo toad survival remains a significant threat to the species persistence.

Periodic fires may adversely affect arroyo toads by causing direct mortality, destroying streamside vegetation, or eliminating vegetation that sustains the watershed. Recent natural and human-induced wildfires had devastating effects on populations of arroyo toads. The 1991 Lions Fire on upper Sespe Creek in the Los Padres National Forest destroyed habitat containing the largest known extant population of arroyo toads including 15 known breeding pools and over 50% of the known adult population on the Sespe drainage. Surveys in 1992 revealed that the effects of the fire and subsequent flooding, erosion, and siltation caused the death of not less than 50% of the resident adult population of arroyo toads.

The vocalizations of male toads are crucial to the breeding success of this species, as their calls are the key factor to finding mates. Light and noise pollution from adjacent developments or campgrounds may also reduce arroyo toad reproductive success by disrupting the vocalization behavior of males during the breeding season. Generally, the local population of arroyo toads declines as campground use increases.

Unseasonal water releases from dams may prevent arroyo toads from breeding altogether, as discussed in Factor A, or may wash away eggs and larvae if releases are made after breeding has occurred. For example, large unscheduled releases from Pyramid Lake in May 1991 virtually eliminated all reproduction by arroyo toads below the dam in Piru Creek in what would have been the best year for reproduction following five years of drought. A proposal to convey State Water Project water from Pyramid Lake to Piru Lake via Piru Creek would also threaten arroyo toad survival on Piru Creek, if releases substantially alter natural flow regimes.

Grazing brings another potential source of mortality to this species. Horses and cattle graze in riparian areas and may trample eggs and larvae of arroyo toads. Grazing also increases levels of sedimentation in streams that can smother eggs and larvae.

Off-highway vehicle use is believed to be the primary factor responsible for the decimation of the Mojave River population of the arroyo toad. On Memorial Day weekend in 1991, a fence protecting a breeding pool on Piru Creek was cut, and off-highway vehicles had access to the creek. The disturbance destroyed a small sand bar that maintained a shallow pool, resulting in the loss of 12,000-16,000 arroyo southwestern tadpoles.

Recreational use of campgrounds is heaviest in early summer, when arroyo toad larvae and juveniles are present and most vulnerable. As the young toads are diurnal, sedentary, and live on the sand bars, they are often crushed. Recreational use has resulted in the alteration of stream and breeding pool morphology and trampling of juvenile toads. Adult arroyo toads, which forage in open areas in the campgrounds, are frequently killed on campground roads at night.

Habitat loss, high mortality, and low reproduction from all of the sources discussed above also result in the fragmentation of surviving populations into isolated subpopulations. While these subpopulations may continue to survive and reproduce over the short term, their long-term survival is not secure, because little opportunity exists for natural dispersal and recolonization following local extirpations. Habitat fragmentation increases the probability of local extirpation due to stochastic events and also likely results in reduction of genetic variability within the small, isolated subpopulations.

The recent years of extremely low reproductive success have likely been a bottleneck in the remaining populations of arroyo toads, in which few individuals will reach sexual maturity until 1995. As mature adults age and die, little recruitment into the breeding population is likely, and numerous local extinctions of already small populations are probable. As individuals may not survive and reproduce due to detrimental events such as drought or road maintenance, and, as the population numbers are low and the range is restricted, such events could cause the extinction of the species.

Conservation and Recovery

Until the Recovery Plan has been developed, recovery efforts will depend on the enforcement of regulations prohibiting the take (includes harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect; or attempt any such conduct), import or export, transport in interstate or foreign commerce in the course of commercial activity, or sell or offer for sale in interstate or foreign commerce any listed wildlife species. It is also illegal to possess, sell, deliver, carry, transport, or ship any such wildlife that has been taken illegally.

The arroyo toad has been extirpated from about 75% of its historical range and now survives in small, isolated, imperiled populations. Some critical habitat is in the Cleveland, Los Padres, and San Bernardino National Forests. This publicly owned habitat must be strictly protected from any threatening activities. Other habitat is on privately owned land and is potentially threatened by various activities. These critical habitats should also be protected. This could be done by acquiring the habitats and establishing ecological reserves, or by negotiating conservation easements with the landowners. Because of the imperilment of the remaining populations of the arroyo toad, an active program of habitat protection and improvement is required. Its populations should be monitored, and studies made of its habitat needs.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

U.S. Fish and Wildlife Service
Ventura Fish and Wildlife Office
2493 Portola Road, Suite B
Ventura, California 93003-7726
Telephone: (805) 644-1766

Reference

U.S. Fish and Wildlife Service. 16 December 1994. "Determination of Endangered Status for the Arroyo Southwestern Toad." Federal Register 59:64839-65229.