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Bufo exsul Myers, 1942(a)
Black Toad
Gary M. Fellers1
1. Historical versus Current Distribution. Black toads (Bufo
exsul) are known historically from four different spring systems in Deep Springs
Valley, Inyo County, California. Buckhorn, Corral, and Bog Mound springs are in the
immediate vicinity of Deep Springs Lake, while Antelope Spring is 7 km to the NW.
Schuierer (1962) suggested that the population at Antelope Spring might have been
introduced there in the early 1900s, but recent genetic research could not corroborate
that the Antelope Springs toads were introduced (E. Simandle, personal
communication).
A small, introduced
population of black toads occupies a flowing well near Salt Lake (Saline Valley, 65 km
SWS of Buckhorn Springs) in Death Valley National Park. In 1961, black toads were
released at Cottonwood Springs in the Owens Valley (30 km to the west of Deep Springs;
Schuierer, 1962), but there are no subsequent reports of this species in the
valley.
The entire natural
range of this species encompasses approximately 15 ha, one of the smallest ranges for any
North American amphibian.
2. Historical versus Current Abundance. Myers (1942) estimated the population at
600–700 individuals based on one visit to the site in 1940. In 1954,
Schuierer (1961) noted that this estimate was much too low and stated that there was
"an excess of 10,000 adults." Schuierer also noted that population
numbers appeared to fluctuate with fewer toads seen in 1958 compared with 1960. In
1971, Schuierer (1972) enlisted the members of a biology class to systematically count
the toads at Deep Springs. They counted 3,967 toads in the sloughs, ponds, and
bog. Difficulty of counting toads in the boggy areas likely resulted in a tally
that was too low, but he did not try to resolve the difference between his earlier
estimate of 10,000 toads and the count of approximately 4,000 toads. As part of the
same census, Schuierer (1972) reported large numbers of tadpoles: "three cubic
meters of suitable habitat may contain as many as five thousand larvae."
In 1971, the
population appeared to be "stable and maintaining itself" (Schuierer, 1972),
but he found the most accessible and frequently visited part of the toad habitat had more
smaller, presumably younger, toads. He attributed this to "collecting
pressure" from herpetologists and indicated that the population could "not
withstand the collectors, amateurs or professionals, who harvest numbers of adult
toads."
In 1999 Murphy et
al. (in preparation) evaluated population size in one area (Corral
Springs) using PIT tags and mark-recapture. They estimated the population to be >
8,400 toads, a number that is similar to that of Kagarise Sherman (1980) for the same
spring. The population at Antelope Springs has always been reported as small
(Schuierer, 1961, 1962; Stebbins, 1985), but the actual size is difficult to estimate due
to the difficulty of seeing toads in the dense vegetation. There are no population
estimates for Bog Mound or Buckhorn Springs, but the habitat at Buckhorn is more
extensive than at Corral Springs. It is likely that the total number of black
toads in Deep Springs Valley is about 24,000 individuals (J. Murphy and E. Simandle,
personal communication). Though geographically restricted, the population appears
to have been stable for the last 15–20 yr.
3. Life History Features.
A. Breeding.
Reproduction is aquatic.
i. Breeding migrations. There is no migration, but Kagarise Sherman (1980) noted
that males tend to stay near breeding sites while females move away. Black toads
lack vocal sacs; they have a release call that is somewhat higher in pitch than that of
western toads (Stebbins, 1985), but males apparently do not have an advertisement call
(Schuierer, 1961).
ii. Breeding habitat. The breeding (and non-breeding) habitat in Deep Springs
Valley includes a series of about ten springs and the associated marshes, ponds, bogs,
and sloughs. The springs are located at an elevation of 1,450–1,700 m
(4,900–5,600 ft). The valley is in an isolated, desert basin located between
the Inyo and White Mountains of northeastern Inyo County, California. Breeding
begins in late March and continues into April (Schuierer, 1961).
B. Eggs.
i. Egg deposition sites. Shallow marshes or quiet pools bordering the larger
wetlands (Schuierer, 1972). In the laboratory, eggs hatched in 4–5 d at 20
˚C (Schuierer, 1972). Egg laying takes place from late March to late May
(Stebbins, 1985).
ii. Clutch size. There are no published estimates of clutch size. Livezey
(1960) described the eggs and oviposition sites. Eggs are deposited in strings,
generally within a 30–35 cm area. The strings are entwined among grasses and
watercress. Eggs masses are either at the water surface or up to 10 cm deep.
Eggs generally are laid in the shallower parts of the marsh (Schuierer, 1961) where the
water depth ranges from 15–20 cm deep (Livezey, 1960).
C.
Larvae/Metamorphosis.
i. Length of larval stage. Schuierer (1961) states that the larval period is from
3–5 wk. Wright and Wright (1949) mention that two or three size-classes of
tadpoles were present on 12 May 1942, but there seems to be no evidence that tadpoles
overwinter because Schuierer (1972) reported seeing tadpoles of all sizes in May
1971.
ii. Larval requirements. Tadpoles are commonly found in shallow water of marshes
or pools bordering the sloughs (Schuierer, 1961, 1972). These areas are subject to
wide fluctuations in temperature (Schuierer, 1961).
a. Food. Undescribed; tadpoles probably feed on both organic and inorganic matter
suspended in the water and on the surface of plants, rocks, and other substrates.
b. Cover. Schuierer (1962) describes breeding sites as "principally in
unshaded marshes that border the sloughs." In the water, there are
"dense mats of vegetation interlaced with swamp-like sedge areas. These boggy
sites overlay a semi-soft black anaerobic mud, 1–3 feet thick" (Schuierer,
1972).
iii. Larval polymorphisms. Unknown and unlikely.
iv. Features of metamorphosis. Tadpoles reach a total length of 35 mm (Wright and
Wright, 1949). Schuierer (1961) reported the size of newly transformed juveniles as
14–19 mm. Juveniles will grow to 22–33 mm SVL by November of their
first year (Schuierer, 1961).
v. Post-metamorphic migrations. None.
D. Juvenile
Habitat. Similar to adults, juveniles generally remain in shallow marshy
areas.
E. Adult
Habitat. "Like other desert valleys to the east of the Sierra Nevada, Deep
Springs is exceedingly dry, and on its floor the vegetation consists of sparse low desert
brush (Chrysothamnus)" (Myers, 1942). Schuierer (1961) provides a
figure that details the plant associations for the alkali area, marsh, slough, stream
margins, and head of the springs. The vegetation includes Polypogon,
Juncus, Ranunculus, Carex, Lemna,
Muhlenbergia, Plantago, Scirpus, Zennichella,
Roippa (Nasturtium), Mimulus, Sisyrinchium,
Epipectis, and Salix. Toads do not frequent rocky portions of
creeks that have no vegetation (Wright and Wright, 1949). The mountains that form
Deep Springs Valley support juniper (Juniperus californica) and pinyon
pine (Pinus monophylla).
Adults are the most
aquatic members of the Bufo boreas group (Schuierer, 1961).
Black toads have never been reported > 12 m from water (Schuierer, 1961).
Black toads are
active primarily from late May to mid September (Schuierer, 1962) in conditions that
range from snowfall to hot summer days (Schuierer, 1972). Adults generally are
diurnal, but they are active at night during the late spring and early summer.
Schuierer (1961) reported toads active during the morning and early evening hours when
air temperatures ranged from 17–22 ˚C. Toads were less active when
temperatures were in excess of 25 ˚C. At temperatures above 30 ˚C, toads
were sluggish even when prodded.
"Just below the
first spring, in the water runways between tussocks, [we] began to see Bufo
exsul. The bottoms are mud, water 2–4 inches deep. These areas
more or less shaded by tussocks. Soil dark and mucky. Depressions between
tussocks 8 inches to 1.5 feet deep. The toads often dashed into holes in tussocks
or into shaded runways" (Wright and Wright, 1949).
F. Home Range
Size. Unknown, but probably small.
G.
Territories. Unknown.
H.
Aestivation/Avoiding Dessication. Black toads shift their activity from diurnal to
nocturnal during the warmer months, but aestivation has not been reported.
I. Seasonal
Migrations. None.
J. Torpor
(Hibernation). Black toads use rodent burrows located 25–50 cm above the
water level of the streams and sloughs. Toads found in burrows during November 1958
were huddled together and torpid (air temperature 12 ˚C), but became active when
handled (Schuierer, 1961). At that same time, some toads were active near springs
where water temperatures were about 20 ˚C.
K. Interspecific
Associations/Exclusions. The only other amphibian reported within the range of
black toads (Deep Springs Valley) is the western spadefoot toad (Spea
hammondii; Schuierer, 1961).
L. Age/Size at
Reproductive Maturity. The average SVL of males is 50 mm (range 44–59) and
females is 52 mm (range 46–69; Schuierer, 1961). There are no published data
on age at reproduction or on size versus age. Murphy et al. (in
preparation) note that toads at Corral Springs tend to be small, especially
compared with those at Antelope Spring (51.1 mm versus 74.7 mm SVL). The reason for
this is unclear, but they note that European carp (Cyprinus carpio) at
Antelope Spring are also small.
Murphy et al.
(in preparation) found the sex ratio skewed significantly in favor of
females (62.7% of population). This was unexpected since Kagarise Sherman (1980)
found fewer females than males. Murphy et al. believe that the shift in sex ratio
may be due to changes in habitat.
M. Longevity.
Unknown.
N. Feeding
Behavior. Stebbins (1951) examined the stomachs of several dozen black toads and
found the remains of beetles, ants, and lepidopteran larvae. Livezey (1961)
examined the food habits of 23 adult and 24 juvenile toads and provided a detailed table
listing number and percent frequency of each food type. He noted that
"…juvenile toads eat more small food materials than the adults, as well as
more larval stages of dipterans and coleopterans…juvenile toads consumed
considerably greater absolute numbers of food items." The primary food items
were Diptera, Coleoptera, Hymenoptera, Homoptera, with lesser numbers of Hemiptera,
Odonata, Collembola, Corrodentia, fairy shrimp (Anostraca), spiders, mites, and
mollusks.
Schuierer (1961)
examined the stomachs of 26 toads and found Hymenoptera, Diptera, Coleoptera, and small
numbers of Hemiptera, Gastropoda, and Arachnida.
O. Predators.
None reported, but eviscerated toads have been observed suggesting predation from Common
Ravens (Corvus corax; E. Simandle, personal communication).
P. Anti-Predator
Mechanisms. Black toads are "more aquatic than other members of the
Bufo boreas group. Adults are often found resting on the surface
of the water near water cress clumps or along the margins of the sloughs and ditches in
tussocks of the dwarf bulrush. When disturbed, they swim to the bottom where their
coloration blends with that of the dark substratum. Their swimming is more
frog-like than toad-like. Locomotion on land is by walking, with the body raised
high off the ground" (Schuierer, 1961). "When prodded, they progress by
short, ineffective hops" (Schuierer, 1961). "Unless pressed, the toads
seemed to prefer to walk rather than to hop" Stebbins (1951).
"The brownish
black ground color with yellow-white mottling blends in with the substratum, both on land
and in the dark-colored water courses…." (Schuierer, 1961).
As with other
Bufo, black toads produce skin secretions that are toxic and function as a
deterrent to predators.
Q. Diseases.
None reported.
R. Parasites.
"Some toads were parasitized by an undetermined intestinal nematode.
Occasionally leeches were found on toads" (Schuierer, 1961).
4. Conservation. Schuierer (1961) noted that "the annual recanalization of
the water courses for irrigation has notable effect upon the population. When
stream modification occurred after oviposition, the marsh area dried before metamorphosis
was completed." He also noted that collecting had affected the
population. Subsequently, Schuierer (1972) reviewed the status of black toads in
1971 and concluded that the population appeared to be in good condition (compared with
1962). Schuierer indicated that the primary threat to the population was from
"collectors, amateurs or professionals" who were removing toads from the
population. His conclusions were based on the relative lack of large adult toads
from the most accessible areas. Schuierer (1972) also noted that the toads seem to
have "adjusted to various climatic conditions and periodic droughts, as well as
changes in the irrigation pattern of its habitat."
Busack and Bury
(1975) reported that livestock grazing at the springs and canals was a potential problem
for the toads. Bury et al. (1980) made five specific conservation recommendations
that addressed issues of spring management, introduced predators, livestock grazing,
marsh burning, channel modifications, and management of the area by both state and
federal government and Deep Springs College.
All native habitat
for the black toad is owned by Deep Springs College. Cattle graze throughout Deep
Springs Valley, but the college fenced the springs in the early 1970s and currently
allows only brief, seasonal grazing. This has resulted in an increase in
vegetation, especially sedges (Scirpus sp.) and cattails (Typha sp.;
Murphy et al., in preparation). The college has also eliminated
the practice of diverting water from the springs for irrigation and no longer allows the
raking and burning of vegetation. All these management practices are in keeping
with the recommendations of Bury et al. (1980).
As with any species
with a highly restricted distribution, there is concern that a catastrophic event or
introduction of a disease or predator could eliminate the entire species.
Black toads were
listed as Rare by the California Fish and Game Commission in 1971, but that status was
changed to Threatened in 1984.
Acknowledgments. I thank John Murphy and Eric Simandle for valuable
comments based on their extensive experience with black toads.
1Gary M. Fellers
Western Ecological Research Center, USGS
Point Reyes National Seashore
Point Reyes, California 94956
gary_fellers@usgs.gov
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
Citation: AmphibiaWeb: Information on
amphibian biology and conservation. [web application]. 2010. Berkeley, California:
AmphibiaWeb.
Available: http://amphibiaweb.org/.
(Accessed: Feb 9, 2010).
AmphibiaWeb's policy on data use.
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