Bufo californicus Camp, 1915
Samuel S. Sweet
Brian K. Sullivan
We follow Gergus et al. (1997), Gergus (1998), and Crother et al. (2000) in separating arroyo toads (B. californicus) from Arizona toads (B. microscaphus); most recent literature refers to arroyo toads as B. microscaphus californicus. Arroyo toads were described originally by Camp (1915) as B. cognatus californicus, type locality: “Santa Paula, 800 feet altitude, Ventura County, California.” Myers (1930a) was the first to recommend species status, but in the interim, B. californicus was regarded as a subspecies of B. compactilis (Linsdale, 1940), B. woodhousii (Shannon, 1949), and most recently B. microscaphus (Stebbins, 1951). A complete synonymy is presented in Price and Sullivan (1988).
l. Historical versus Current Distribution.
Arroyo toads (B.californicus) occur in southwestern California and adjacent Baja California del Norte, Mexico, mostly on the coastal slopes, from the San Antonio River of southern Monterey County southward through the Transverse and Peninsular ranges to the Rio Santo Domingo. They also occupy a few drainages on the desert slopes of the San Gabriel and San Bernardino ranges (Jennings and Hayes, 1994a), though reported localities on the desert slopes of the Peninsular Ranges recently have been shown to involve misidentifications (E.Ervin, personal communication, 2001). The mapping format used in this volume substantially overestimates both the areal distribution and the continuity of arroyo toad populations, such that this map should not be used for any but the most general purposes. See U.S. Fish and Wildlife Service (1999a) for accurate point maps detailing the historical and current distributions of this species. The current distribution of arroyo toads is highly fragmented, largely through the alienation of coastal lowlands by development and the widespread alteration of the middle reaches of larger drainages by dams and flood control projects (Sweet, 1992; USFWS., 1994a; Campbell, 1996). Jennings and Hayes (1994a) estimated that arroyo toads had been eliminated from 76% of their historical range, though subsequent discoveries of new localities and remnant populations reduce this figure to about 65%. Many populations are now isolated, either (1) restricted to small headwater drainages above impoundments where conditions are marginal or (2) confined to narrow riparian corridors along larger drainages that are subject to extensive disturbance from water flow management practices, gravel mining, urbanization, and military training (USFWS, 1999a).
2. Historical versus Current Abundance.
Arroyo toads have declined in abundance (often to extirpation) at most sites where historical records exist. High density populations described by Sanders (1950), Stebbins (1951), and Cunningham (1962), for example, are now extirpated, and the large series of museum specimens collected by L.M. Klauber and others in the 1930s can no longer be duplicated at the great majority of their historical sites (S.S.S., personal observation). In the early 1990s, surviving populations reached a low ebb following an extended drought and several decades of adverse land-use and water management practices (Sweet, 1992). Most populations on public lands have recovered substantially following the Federal listing of arroyo toads in late 1994 (USFWS, 1994a). However, many populations remain well below apparent carrying capacity despite high recruitment, probably as a result of as-yet-unexplained high mortality during aestivation in autumn and winter (Sweet, 1992, 1993, and unpublished data). Six of eight populations being monitored on MCB (Marine Corps Base) Camp Pendleton (San Diego County) from 1996–2000 showed continuous declines of 90% (D.C. Holland, personal communication).
In general terms, population densities of arroyo toads are relatively low (about 12 adults/ha) along second- to fourth-order streams in montane and foothill areas (Sweet, 1993), where it is rare to find > 5 calling males/100 m of suitable habitat (Sweet, 1992, 1993; Ramirez, 2000). Densities are often higher along coastal streams, where a density of ten calling males/100 m is not unusual, and densities may reach 100/km locally, though there is considerable heterogeneity in density (D.C. Holland, personal communication, 2001). Still, these figures are well below reported densities for ecologically similar Arizona toads (B. microscaphus; Sullivan, 1993; Schwaner et al., 1998).
3. Life History Features.
A. Breeding. Reproduction is aquatic. Arroyo toads breed in shallow streams; their reproductive biology is generally similar to that of other north-temperate species of Bufo. The advertisement call of arroyo toads is a whistling trill, averaging 8 s (4–9 s) in duration, with a dominant frequency of 1.46 kHz and pulse rate of 44 pulses/s at 15 C (Sullivan, 1992b; Gergus et al., 1997). Individual calling males are audible to humans at 300 m under ideal conditions, but the sound is easily masked by noise resulting from wind or stream riffles.
i. Breeding migrations. Sweet (1992, 1993) and Holland and Goodman (1998) report a prolonged breeding cycle for arroyo toads, beginning in February (rarely January) in coastal areas and late March or early April at montane sites and extending through July. Males begin calling about 10 d before any females respond; individual males may call almost nightly for the entire duration of the breeding cycle (Sweet, 1993). Breeding is not triggered by rain, but seems to require air and water temperatures above 11–13 C (Myers, 1930a; Sweet, 1992; Holland and Goodman, 1998); breeding activities are suspended during floods and resume when stream flow rates decline sufficiently to provide shallow edges with minimal flow rates.
Male arroyo toads do not form calling aggregations, and satellite behavior has not been observed. Females generally avoid streamside areas until they are ready to breed; they appear to select a male from a distance and approach him directly. Egg deposition occurs at the male’s calling site and typically requires > 1 hr (Sweet, 1992).
ii. Breeding habitat. Arroyo toads breed in the quiet margins of open streams and avoid sites with deep or swift water, tree canopy cover, or steeply incised banks. Males typically call in water 2–4 cm deep 1–2 m from shore, facing a low shoreline with a horizon unobstructed by nearby vegetation (Sweet, 1992). Substrates are most often gravel and sand, less frequently silt or cobble, and rarely bare bedrock or boulders. The toads do not breed in riffle areas and almost never use pools that are isolated from the flowing channel; side channels and washouts may be utilized as long as there is some flow through them, but they are abandoned as soon as this flow ceases (S.S.S., personal observations).
i. Egg deposition sites. Female arroyo toads do not transport males but instead deposit eggs at the male’s calling site. Intertwined masses of 3–5 clutches of different developmental ages are sometimes encountered, reflecting high site fidelity by males. Eggs are laid in water averaging 9 cm deep (range of 1.3–31.8 cm, n 65; Sweet, 1992) on fine sediments where there is no appreciable current; they are virtually never entangled in twigs, roots, or other submerged debris. Deposition sites are fully exposed to the sky. Detailed site data on 135 clutches are presented in Sweet (1992).
ii. Clutch size. Eggs of arroyo toads are deposited in two strands simultaneously. A sample of 40 clutches averaged 6.8 m (range 3–10.6 m) in length and contained an average of 4,714 eggs (range, 2,013–10,368 eggs). Individual fertilized ova average 1.7 mm in diameter and have a pale vegetal pole; the fully hydrated envelope is 3.3–4.2 mm in width and is relatively inelastic (data from Sweet, 1992). There is no evidence that individual females can lay a second clutch.
i. Length of larval stage. Eggs require 4–6 d to hatch at water temperatures of 12–16 C. Larvae remain associated with the degenerating envelopes for 5–6 d and do not begin active dispersal from the clutch deposition site until 15–18 d post-hatching (Sweet, 1992). Arroyo toad eggs and larvae are described and illustrated in Stebbins (1951, 1985) and Sweet (1992; reprinted in USFWS, 1999a). From hatching (Gosner stage 18, 4.0 mm TL) to stage 26 (10–12 mm TL) larvae are colored black. After stage 26, tan crossbars appear on the tailbase. By stage 30 (18–20 mm TL, 24–27 d post-hatching) the dorsum becomes tan, leaving dark crossbars on the tail base, an irregular black lateral stripe on the tail, and an opaque white venter. Larvae reach an average maximum of 34 mm TL (exceptionally to 40 mm) and require a minimum of 65 d post-hatching (most often 72–80 d) to metamorphose (all data from Sweet, 1992).
ii. Larval requirements.
Food. Larval arroyo toads do not aggregate once they have become free swimming. They are substrate gleaners and feed by processing detritus and microbial mats from just beneath the surface layer of fine sediments or within the interstices of gravel deposits. They do not consume macroscopic algae or other aquatic vegetation and will not take plant material in the lab (Sweet, 1992). Larvae remain stationary for long periods and generally travel
Cover. Not needed. Larvae occupy shallow areas of open streambeds on substrates ranging from silt to cobble, with preferences for sand or gravel. Areas under tree canopies are avoided, as are portions of the streambed with submerged or emergent vegetation. Larvae are cryptic, but are strong swimmers, darting 2–4 m toward deeper water when disturbed.
iii. Larval polymorphisms. None.
iv. Features of metamorphosis. Newly metamorphosed arroyo toads may occur as early as late April or as late as early October, but metamorphosis is concentrated in the interval from late May to early July in most years. There is a positive correlation between metamorphic date and elevation in most years, but the high variability in rainfall patterns among years results in a broad range of breeding phenology. Larvae from a single cohort may metamorphose over a period of 10 d or more (data from Sweet, 1992). Forelimbs emerge 2–3 d before the tail is fully resorbed; during metamorphosis, larvae seek shallow water on exposed shorelines and become terrestrial when the tail is reduced to a stub. Newly metamorphosed juveniles are usually 12–15 mm SVL, but may be as small as 9 mm or exceptionally to 22 mm (Holland and Goodman, 1998).
Post-metamorphic migrations. Newly metamorphosed animals and juveniles remain on sparsely vegetated sand and gravel bars bordering the natal pool for 3–5 wk (Sweet, 1992). D. Juvenile Habitat.
Juvenile arroyo toads are extremely cryptic and remain within a few meters of the margin of their breeding pool for an extended period, where they forage, chiefly diurnally, on sparsely vegetated sand or gravel bars in full sun at substrate temperatures of 30–43 C. During windy or cool weather and at night, juveniles shelter in damp depressions in the gravel. Juveniles begin to actively burrow in dry sand at 20–25 mm SVL; they switch to nocturnal activity at this stage, sheltering by day in shallow burrows in stream-edge vegetation and foraging widely in the lower riparian zone after dark. Dispersal from the immediate vicinity of the stream is a prominent behavioral transition that is mediated by drying conditions and/or attainment of around 30 mm SVL, usually occurring within 3–5 wk following metamorphosis (data from Sweet, 1992). Juvenile toads 30 mm SVL disperse throughout habitats used by adults and remain active well into late summer or early fall, long after adults have stopped foraging. Many juveniles reach 45–50 mm SVL by late September to early October, and some males may attempt to breed in the following spring (Sweet, 1993).
E. Adult Habitat. Arroyo toads are closely associated with low gradient drainages from near sea level to about 1,400 m elevation (to a maximum of 2,440 m in Baja California del Norte), with most remaining populations residing in the 300–1,000 m range (USFWS, 1999a). They may locally occupy first-order drainages, but are usually associated with second- to sixth-order streams that have extensive terrace systems, braided channels, and large areas of fine sediment deposits that are episodically reworked by flooding. Vegetation reflects the frequency and intensity of flood events, with live oak, sycamore, and cottonwood groves interspersed with grasslands and sage scrub on high terraces, and patches of willows, alder, and mulefat on temporary alluvial benches adjoining the active channels. Streams may be either permanent or seasonal; seasonal streams must flow for at least 4–5 mo in spring and summer in most years to support breeding populations. Barto (1999) provided a habitat suitability model for arroyo toads that does not consider areas beyond the riparian border.
Mark–recapture (Sweet, 1992, 1993) and radiotracking studies (Griffin, 1999; Ramirez, 2000) have demonstrated that in many areas arroyo toads rarely disperse beyond the upland margins of the stream terraces, where dense hard chaparral, steep slopes, and stony soils replace fine alluvium and patchy vegetation. However, pitfall trapping work by Holland and Sisk (2000) has documented extensive use of upland grasslands and sage scrub on compacted soils in near-coastal localities, with animals being found as far as 1.2 km away from the riparian/upland ecotone. This clear regional difference in habitat use may indicate that much larger areas of coastal southern California that are now extensively urbanized were formerly inhabited by arroyo toads, and that the ecological scope of the species is underestimated by reliance on data from remnant populations.
F. Home Range Size. In addition to extensive upland movements in near-coastal sites, arroyo toads show several patterns of seasonal dispersal (Sweet, 1993). The largest adult male toads may remain at a single breeding pool for an entire season, while smaller males typically move consistently upstream or downstream, calling for a few nights at each pool encountered. Between years, these males tend to travel within a 2–3 km zone along the stream and often become sedentary as they reach large size. Adult female toads are highly sedentary, with a continuously used activity area usually 100 m in diameter; such animals nearly always breed in the same pool in successive years. Older juvenile and subadult (yearling) toads make extensive along-stream movements during their extended activity season. Radiotracking studies at both near-coastal (Griffin, 1999) and montane sites (Ramirez, 2000) document qualitatively similar patterns among individual toads, although Griffin (1999) noted upstream dispersal by adult females prior to breeding.
Sweet (1992, 1993), Griffin (1999), and Ramirez (2000) showed that arroyo toads construct shallow burrows within the riparian zone where they shelter by day during the active season. Toads tend to prefer sand over finer or coarser substrates and most often burrow in relatively open microsites, but otherwise show no strong preference for available vegetation types. Ramirez (2000) found that toads construct burrows closer to stream channels as the dry season progresses, presumably to maintain water balance. Griffin (1999), Holland and Sisk (2000), and Sweet (personal observations) have noted occasional use of small mammal burrows as refugia.
G. Territories. Territorial or activity area fidelity may occur among adult female arroyo toads, and at least some male toads return to the same calling site for many nights in succession (Sweet, 1992, 1993); changes in stream level result in local adjustments in calling sites over the course of a breeding season. Other males, particularly smaller individuals, tend to “drift” upstream or downstream, calling for a few nights at each pool encountered (Sweet, 1993). Calling males display a minimum spacing of 1–8 m, with no evidence of agonistic or territorial behavior. Satellite behavior by males has not been observed (Sweet, 1992). While newly metamorphosed and juvenile arroyo toads often display patchy distributions involving some clustering, this appears to be a response to physical habitat features rather than aggregation per se (S.S.S., personal observations).
H. Aestivation/Avoiding Desiccation. Adult arroyo toads do not appear to aestivate regularly during the active season (February–July), though they may remain inactive for several days during cold or windy weather. Calling by males is greatly reduced on two or three nights preceding and following a full moon (Sweet, 1992). Subadult and adult toads, especially, become progressively less active after early July, and few adults can be found by August; juvenile and subadult toads may remain active at reduced levels into October and occasionally later in the fall following rains (Sweet, 1993; Holland and Sisk, 2000; Ramirez, 2000).
I. Seasonal Migrations. No patterns are evident beyond age- and sex-related movements described above.
J. Torpor (Hibernation). Most adult arroyo toads are inactive from August or September to February or March; subadults may remain active into early November (Sweet, 1992, 1993; Holland and Sisk, 2000). The locations and characteristics of hibernation sites are poorly known; toads are presumed to select higher stream terraces where the likelihood of severe flooding is reduced. However, Ramirez (2000) documented two individuals that ceased surface activity in early August by burrowing into the stream channel; one of these, when excavated, had produced a thin “cocoon” of shed epidermis. The extent to which toads might use upland areas for hibernation is presently unknown.
K. Interspecific Associations/Exclusions. Arroyo toads are usually microsympatric with western toads (B. boreas) and California and Pacific treefrogs (Pseudacris cadaverina and P. regilla, respectively); they also occur frequently with northern red-legged frogs (Rana aurora) and American bullfrogs (R. catesbeiana), and occasionally with western spadefoot toads (Spea hammondii; Sweet, 1992; Holland and Goodman, 1998). In the recent past they co-occurred with foothill yellow-legged frogs (R. boylii) as well, but foothill yellow-legged frogs have now been extirpated within the range of arroyo toads. Arroyo toads are eaten regularly by American bullfrogs (Holland and Goodman, 1998; Griffin, 1999; S.S.S., personal observations).
Male arroyo toads have been observed in amplexus with small, late-breeding female western toads on three occasions; hybrid larvae hatch, but have massive deformities; most die by Gosner stage 25 (Sweet, 1992). Dan C. Holland (personal communication) has also observed male arroyo toads in amplexus with large female western toads. Small male western toads are sometimes closely associated with calling male arroyo toads and will repeatedly attempt amplexus with them; male arroyo toads with an attending western toad seldom complete a call without being grasped, and this interference probably reduces mating success (Sweet, 1992).
Differences in peak breeding season, oviposition sites, and larval behavior serve to minimize interactions between arroyo toad and western toad larvae (Sweet, 1992); larval habitat overlap is greatest between arroyo toads and California treefrogs.
Some potential for competitive interactions between arroyo toads and western toads exists during the juvenile stage. While western toads usually metamorphose earlier in the year than do arroyo toads and are predominantly nocturnal, normal variation in breeding phenology can result in temporal and spatial overlap with arroyo toads. In laboratory tests, groups of size-matched individuals of both species grew at similar rates when reared separately, but arroyo toads grew at only half the rate of western toads in trials where the two were housed together. Juvenile western toads consumed prey (crickets) at three times the foraging rate of arroyo toads (Sweet, 1992). There is, however, no evidence for substantial interference competition in the field.
L. Age/Size at Reproductive Maturity. In the population studied intensively by Sweet (1992, 1993), calling male arroyo toads ranged from 51–67 mm SVL, while gravid or breeding females ranged from 66–78 mm SVL. A few males marked in the late summer of their natal year were mature and calling the following spring, but none of these individuals was observed to mate. Most males (and a small number of females) mature in their second year, with most females breeding for the first time as they reach age 3. Calling males do not grow during the breeding season (many, in fact, shrink), and thus males that forgo breeding until age 2 are substantially larger. Because females strongly prefer the largest males in a local chorus and satellite behavior is unknown, there is no evident advantage in the early maturation displayed by some male arroyo toads (Sweet, 1993).
M. Longevity. Mark–recapture studies suggest that few arroyo toads survive into their fifth year, and that these are predominantly females (Sweet, 1993). In the absence of American bullfrogs, adult arroyo toads have a high survivorship during the active season, but suffer 55–80% mortality as they overwinter (Sweet, 1993; Holland and Sisk, unpublished data). American bullfrogs target calling male arroyo toads and are associated with sex ratio biases of up to 1:19, leading to local extirpations (Sweet, 1992).
N. Feeding Behavior. Newly metamorphosed and juvenile arroyo toads feed mostly on ants and small flies, while larger individuals eat a wider range of invertebrates (Cunningham, 1962; Sweet, 1992). Adult toads feed predominantly on ants, especially nocturnal, trail-forming tree ants (Liometopum occidentale). Ant foraging columns follow the same trails for many weeks, and most individual arroyo toads will return to the same site each night, consuming up to 25% of their body mass in 1–1.5 hr of feeding (Sweet, 1992, 1993). Feces of adult arroyo toads typically contain 95% Liometopum exoskeletons by mass and superficially resemble feces of horned lizards (Phrynosoma sp.), which contain only diurnal ant taxa (Sweet, 1992).
O. Predators. Arroyo toad eggs and young larvae have few or no predators (Sweet, 1992), though mosquitofish (Gambusia affinis) and crayfish (Procambarus clarkii) have been observed to remove and consume individual eggs (D.C. Holland, personal communication). Two-striped garter snakes (T. hammondii) have been observed to repeatedly ingest then reject egg strands (Sweet, 1992). Free-swimming larvae are actively hunted by garter snakes and are subject to high predation by introduced fishes (especially green sunfish (Lepomis cyanellus) and prickly sculpins (Cottus asper; Sweet, 1992).
Juvenile arroyo toads may suffer heavy mortality locally from killdeer (Charadrius vociferus) and are often essentially eliminated by trampling by humans on streamside flats at popular recreational sites (Sweet, 1992). Trampling may also occur where cattle frequent the riparian zone, but there is a larger negative effect from enhanced evaporation because the microrelief of the surfaces of sand and gravel bars is greatly increased by trampling (Sweet, 1992).
Predation intensity declines as juvenile toads become nocturnal and disperse, and subadult and adult toads show very high survivorship during the active season at sites lacking American bullfrogs; two-striped garter snakes and common garter snakes (T. sirtalis) occasionally attempt to consume mature arroyo toads with variable success (though some toads that escape die later of injuries sustained; Sweet, 1993, Griffin, 1999). Other than killdeer, no predation by birds or mammals has been documented, though adult toads occasionally display injuries consistent with attempted predation by shrews.
Where the two species co-occur, American bullfrogs are major predators on arroyo toads. Focusing mainly on calling males (but also consuming pairs in amplexus), individual bullfrogs can essentially eliminate local populations of arroyo toads (USFWS, 1994a, 1999a).
P. Anti-Predator Mechanisms. Eggs and small (black) larvae appear to be distasteful, but larger larvae are readily eaten by various predators without evident discomfort (S.S.S., personal observations). The cryptic appearance, behavior, and flight responses of larger larvae are effective against foraging garter snakes but are ill-adapted to attacks by introduced centrarchid fishes (Sweet, 1992). When disturbed, larger larvae usually dart towards deeper water, making an arc that often contacts the surface. Green sunfish are proficient at intercepting fleeing larvae and have been seen to roll laterally to take them even in shallow water (S.S.S., D.C. Holland, personal observations).
Juvenile arroyo toads are extremely cryptic, with pattern elements that closely match the color frequencies and patch sizes of the damp gravel with evaporite deposits where they forage, and they remain motionless when approached. On land at night, subadult and adult toads will move rapidly into dense vegetation, and calling males will retreat underwater in response to movements by a human observer about 25 m away. No specialized postures or visible release of parotoid gland secretions in response to capture or handling are known (S.S.S., personal observations).
Q. Diseases. No instances of disease are known among wild populations, but a chytrid fungal epidemic killed all juvenile arroyo toads being reared in lab in 1991. Symptoms appeared too soon after collection for a lab-acquired origin, but no unexplained mortality was observed in the wild source populations during the remainder of the season. This fungus could not be maintained for study in a lab colony of boreal toads (Sweet, 1992).
Egg clutches swept into deeper and cooler water by changes in water level are usually attacked by fungus (Sweet, 1992; see “Conservation” below).
R. Parasites. Larval arroyo toads are often heavily infected with encysted metacercariae of an unidentified trematode whose definitive hosts are birds. At metamorphosis, the cysts become concentrated in the groin and around the urostyle as the tail is resorbed. A small, unidentified cestode is regularly present in the body wall musculature. No adverse effects attributable to parasitism have been noted (Sweet, 1992).
Egg clutches are subject to high mortality from changes in water level; losses either by stranding as stream flow declines or displacement by even minor flooding events are commonplace. In particular, late rains and associated minor flooding or water management via dam operations can minimize survival of clutches and small larvae across entire drainage systems in some years (Sweet, 1992). Most clutches that survive changes in water level hatch, except that those portions swept into deeper and cooler water are usually attacked by fungus (Sweet, 1992).
Arroyo toads have declined in abundance (often to extirpation) at most sites where historical records exist, and were federally listed as Endangered in late 1994 (USFWS, 1994a). Jennings and Hayes (1994a) estimated that arroyo toads had been eliminated from 76% of their historical range, though subsequent discoveries of new localities and remnant populations reduce this figure to about 65% (see “Historical versus Current Distribution” above). Surviving populations reached a low ebb in the early 1990s following an extended drought and several decades of adverse land-use and water management practices (Sweet, 1992). Many of these populations are now isolated. Most populations on public lands recovered substantially following the federal listing; however, the high-density populations described by Sanders (1950), Stebbins (1951), and Cunningham (1962) are now extirpated, and many populations remain well below apparent carrying capacity (Sweet, 1992, 1993, unpublished data). These populations continue to face threats due to disturbance from water flow management practices, gravel mining, urbanization, and military training (USFWS, 1999a).
Acknowledgments.S.S.S. acknowledges contract support and logistical assistance from the U.S. Forest Service, Los Padres National Forest, invaluable field assistance by former USFS biologist Nancy Sandberg, and permits and encouragement from the U.S. Fish and Wildlife Service. B.K.S. acknowledges support of the Arizona Game and Fish Department and the assistance of Rob Bowker, Mike Demlong, Matt Kwiatkowski, Erik Gergus, and Keith Malmos. We thank Dan C. Holland for comments on the manuscript and permission to cite unpublished data and observations.
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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