ISSN 0327-9383
Mastozoología Neotropical; 1(2): 103-112
SAREM, 1994
THE IMPACT OF AN ERUPTION OF VOLCAN HUDSON
ON SMALL MAMMALS IN ARGENTINE PATAGONIA
Oliver P. Pearson
Museum of Vertebrate Zoology, University of California, Berkeley, CA 94720, U.S.A.
ABSTRACT: Sixteen months alter an explosion of Volcan Hudson had covered parts of Santa
Cruz Province in Argentina with 5 to 15 cm of ash that had killed thousands of sheep, the
native mice were present and thriving. Eligmodontia, Phyllotis, and Reithrodon were present
in greater-than-expected numbers, and Abrothrix xanthorhinus was present in normal numbers. All were reproducing vigorously and appeared to be healthy. Molar teeth of Eligmodontia
and Phyllotis were not abraded noticeably more than in other regions.
RESUMEN: Impacto de una erupción del volcán Hudson sobre pequeños mamíferos en
la Patagonia argentina. Dieciséis meses después de que una explosión del volcán Hudson
cubrió con una capa de 5 a 15 cm de ceniza parte de la provincia de Santa Cruz (Argentina)
y produjo la muerte de cientos de ovejas, los ratones nativos estaban presentes y en expansión.
Eligmodontia, Phyllotis y Reithodon fueron encontrados en mayor número al esperado, y
Abrothix xanthorhinus fue encontrado en su nivel de abundancia normal. Todos los ejemplares
estaban activos reproductivamente y en condiciones saludables. Los molares de Eligmodontia
y Phyllotis no presentaron mayor desgaste que en otras regiones.
Key words: Argentina, volcano, mouse populations, Volcan Hudson
INTRODUCTION
Volcanic eruptions can destroy all
lifenearby and can affect plant and
animal populations hundreds of kilometers distant, but the impact of volcanic activity on small mammals has
rarely been studied. One of the most
pertinent reports traced the recovery
of small-mammal populations following a period of activity of Volcan
Parícutin in 1943 in Mexico (Burt,
1961). Most vertebrates were exterminated out to the zone where less than
about 20 cm of ash fell. Even 17
months after the initial eruption, animal life was practically absent where
die ash was 15 cm or more deep.
Another study (Andersen and Mac
Mahon, 1985; Mac Mahon et al., 1989)
documented the recovery of smallRecibido 22 Febrero 1994. Aceptado 25 Junio 1994.
mammal populations for seven years
following the brief but spectacular
eruption of Mount St. Helens in the
United States in 1980. Mammals were
trapped in many different habitats that
had suffered different kinds and degrees of volcanic mayhem. The sites
at Mount St. Helens most useful for
comparison with the South American
eruption described below were those
subjected to relatively gentle ashfall.
Pumice and ash 7-30 cm deep near
Mount St. Helens caused substantial
mortality to shrubs and forbs but not
trees; herbaceous vegetation recovered
quickly where tephrafall was less than
4.5 cm. Three species of small rodents
and one species of shrew persisted in
a high tephrafall area four months after the eruption and, in general, popu-
104
lations of those species persisted over
the next seven years while the vegetation was recovering.
On 12-15 August, 1991, a volcano
in southern Chile, Volcan Hudson,
erupted for the first time in 20 years
and sent a blanket of ash and pumice
across Argentina to the Atlantic Ocean.
By 20 August the ash was detected in
Australia (Smithsonian, 1991: N° 7),
and within a month injection of Hudson
aerosols finto the stratosphere was contributing to a substantial ozone loss
over Antarctica (Hofmann et al., 1992).
Satellite photographs on 13 and 14
August show dramatically the clearly
defined plume that dropped more than
2 km3 of ash on about 100,000 km 2 of
Santa Cruz Province, Argentina, and
contributed to the death of thousands
of sheep (López, ms). In Argentina 5
to 15 cm of ash fell along the southern
shore of Lago Buenos Aires near the
towns of Los Antiguos and Perito
Moreno (Smithsonian, 1991, N° 8;
Corbella et al., 1991; López, ms). Los
Antiguos is 130 km SE of Volcan
Hudson, so the impact of the eruption
would have been more severe closer to
the volcano in Chile. Much of the ash
was re-distributed during the week of
violent winds that followed the eruption, and even four months later strong
winds were remobilizing enough fine
ash to disrupt airport activity 1500
miles 2400 Km) away (Smithsonian,
1991, N° 11).
Near Los Antiguos, cliffs of tephra
20 m high remind one that ashfall is
not a rare event in this region; the flora
and fauna have survived other volcanic eruptions in postglacial times
(Smithsonian, 1991, N° 7).
The area in Argentina covered by ash
from the 1991 eruption is almost en-
Oliver P. Pearson
tirely semi-arid, bushy steppe devoted
to sheep grazing. The ash that fell was
not poisonous, although it was notably
high in its content of sulfur (3780 mg/
kg) and fluorine (up to 280 ppm in
plants) (Corbella et al., 1991; Lopez,
ms; Smithsonian, 1991, N° 7). A frequency distribution of particle sizes is
bimodal with peaks at 39 and 375
microns but with an appreciable percentage less than 5 microns (Corbella et
al., 1991; Smithsonian, 1991, N° 9). News
reports commented on irritation of the
eyes of humans, livestock, and wild
animals such as rheas and guanacos,
as well as severe abrasion of vehicles,
windmills, sheep shears, and other farm
machinery. Contributing to the mortality of sheep was the drying up of
small water sources in this semi-arid
region by the deposition of tephra, and
by the covering of palatable vegetation that was already scarce after several years of drought. The fleece on
many sheep became so laden with ash
that when it became wet it
overburdened the sheep so that they
could scarcely walk (Smithsonian,
1991, N° 9).
General observations
Between 4 and 7 December, 1992, 16
months after the eruption, we visited
Los Antiguos and Perito Moreno to
assess whether the 5-15 cm of ash had
affected the populations of small mammals living there. On windy days the
heavy surf on the southeastern shore
of Lago Buenos Aires was still carrying appreciable quantities of pumice
particles about 1 mm diameter, and
windrows of pumice spheres 1 to 2 cm
diameter lined the beach. These larger
ejecta had probably floated from the
Chilean end of the lake more than 60
km away. The vegetation, however,
MOUSE POPULATIONS AFTER VOLCAN HUDSON
was not covered with ash; most of the
ash had blown away in the high winds
for which Patagonia is famous. On
windy days during our visit in December, 1992, clouds of ash could be seen
blowing about in the mountains in Chile
to the west of Los Antiguos, as well as
local clouds of "dust". The morning
and afternoon sky throughout Patagonia
still had the brassy color associated with
particulate matter in the upper atmosphere.
The vegetation did not appear to have
been seriously affected. Adequate rainfall had supported new growth. Bunchgras se s had trapped appreciable
amounts of ash but were not killed,
and ash had formed drifts behind many
of the plants. The general aspect, however, was that of a normal, sandy desea
habitat with most of the vegetation
appearing to be healthy.
We saw a number of hares, pichi armadillos, tinamous, and a skunk. On
December 5 we counted five fresh hare
careases on 20 km of the paved road
along Lago Buenos Aires, and on December 7 we counted an additional five
recently killed hares on 50 km of the
same road between Los Antiguos and
Perito Moreno. Tadpoles were noted in
a stream 28 km W Perito Moreno.
A count of animal tracks in the sandy
soil suggested that animal life was
abundant. On one morning, after a high
wind during the preceding day had
erased all tracks, we recorded the number of times that animal footprints
crossed a straight line 50 m long, and
another straight line 125 m long. This
was in bunchgrass steppe at Site 2,
described below. The first line was
crossed by tracks of 2 mice, 2 hares,
birds, and 1 pichi armadillo. The second line was crossed by tracks of 39
105
mice and 4 hares. This reflects abundant populations and a large amount
of animal activity during one night.
Trapping results
Abundante: We sampled the smallmammal fauna with traps of a size appropriate for catching mice and rats
(Museum Specials and large Sherman
traps). I asume that the percent of traps
that captured an animal each night (trap
success) reflects the abundante of the
various target species. Trapping results
were as follows:
SITE 1: Lago Buenos Aires, 30 km
W Perito Moreno. On the night of 45 December, 73 traps were set in bushy
steppe habitat with an estimated ground
cover of 30 percent. Plants, listed in
approximate sequence of decreasing
coverage of the ground were: Senecio,
duraznillo (Colliguaya), bunchgrasses,
a very spiney, low cushion plant, and
bushes of molle (Schinus marchandii).
In spite of heavy rain during the night,
which sprang a number of traps, 10
mice were captured, all Eligmodontia,
and 1 Gecko. Trap success for small
mammals was 14%.
All Eligmodontia in this study are
assumed to belong to the species
morgani, in spite of the fact that their
tails average 20 mm longer than those
of the type and nine topotypes from
the nearby type locality (Alíen, 1905).
SITE 2: Estancia La Aurora, 4.4 km
E Los Antiguos. This site is 21 km
west of Site 1. On the night of 5-6
December 128 traps were set in two
kinds of habitat, bushy steppe and
bunch-grass steppe, and on the night
of 6-7 December 24 traps were set in
bushy steppe. This gives a total of 152
trap-nights. Overall trap success was
27% (Table 1). The bushy steppe had
30 —
Spring
A
Autumn
P
25 —
E
E
A
R
A
P
E
0
SITE 2
Spring, 1992
II
CHILE CHICO
Spring, 1986
R
III
RIO NEGRO
Springs
P R
IV
RIO NEGRO
Autumns
R
V
CHILE CHICO
Autumn, 1986
VI
CHILE CHICO
Autumn, 1987
Fig. 1: Springtime trap success in the ashfall zone near Los Antiguos (Site 2) 15 months after the eruption (I) compared with springtime trap success nearby at Chile Chico before
the eruption (II), with springtime and autumn trap success in bushy steppe and bunchgrass steppe habitats in Rio Negro Province before the eruption (III and IV), and with
trap success at Chile Chico in two autumns before the eruption (V and VI). A= Abrothrix, E= Eligmodontia, P= Phyllotis, R= Reithrodon.
uosirad 'd Jamo
A
MOUSE POPULATIONS AFTER VOLCAN HUDSON
about 35% ground cover; the dominant plants were duraznillo (Colliguaya) up to 4 m tall, bunchgrasses, neneo
(Mulinum spinosum), paramela (Adesmia cf. boronioides), and molle
(Schinus marchandii). The bunchgrass
steppe appeared to have been burned
within the past few years, leaving it
with fewer bushes. The dominant plants
were bunchgrasses, neneo, duraznillo,
and calafate (Berberis). There had been
no grazing by livestock at either of
these two sites for many months.
It may be seen • in Table 1 that success in capturing Eligmodontia at Site
2, near Los Antiguos, was greater than
would be expected in the spring at the
beginning of the season of reproduction. The capture of numerous Phyllotis
xanthopygus was a surprise, since they
are usually caught in cliffs and rocky
places. No such habitat was nearby.
The Coney Rat, Reithrodon, is notably
reluctant to enter Museum Special and
Sherman traps, so a trap success of
5% for this species indicates a sizeable
population. The number of Abrothrix
xanthorhinus captured was close to the
"expected".
Table 1: Trap success at Site 2; 5-7 December, 1992.
Species
Eligmodontia morgani
Phyllotis xanthopygus
Reithrodon auritus
Abrothrix xanthorhinus
Number Trap
caught Success
16
12
7
4
"Expected"
Trap
Success
11%
8
5
3
2.5%
0.26%
0.43%
2.7%
27 %
5.89%
(1)
(2)
(2)
(2)
(1)Springtime trap success in bushy steppe in the Province of Rio Negro averaged over several years (Pearson et
al., 1987).
(2)Springtime trap success in bushy steppe and bunchgrass steppe habitats, averaged over several years, in the
Province of Rio Negro (Pearson, unpublished).
107
Through the kindness of Douglas
Kelt, comparison can be made with
captures he made before the eruption
in very similar habitat near Chile
Chico, less than 10 km west of Site 2.
In the spring of 1986 in 1045 trap
nights he had a trap success of 3.6%.
His catch was dominated by Abrothrix
xanthorhinus, included no Eligmodontia, but included one Abrothrix
longipilis. This catch can be compared
directly with the 27% trap success at
Site 2 in spring of 1992 (Fig. 1).The
14% trap success at Site 1 was not
included in this comparison; it is a
minimal value because heavy rain during the night sprang many traps. The
overall trap success for the four species in the spring season at Chile Chico
in 1986 (Fig. 1) before the eruption
(3.6%) was similar to that in steppe
habitat in many spring seasons in Rio
Negro Province (5.9%), but after the
eruption, trap success (27%) was seven
times greater. Eligmodontia, Phyllotis,
and Reithrodon were much more prevalent in 1992, whereas Abrothrix was
represented with about the same relative frequency in all three samples
(Panels I, II, and III in Fig. 1).
Kelt also trapped at Chile Chico in
the autumn of 1986 and the autumn of
1987. For unknown reasons, mice at
Chile Chico were spectacularly abundant in autumn of 1986 (overall trap
success 65.7%; Panel V in Fig. 1).
Overall trap success at Chile Chico in
the more-typical autumn of 1987
(20.6%) was only moderately higher
than in the autumn of many seasons in
steppe habitat in Rio Negro Province
(14.9%; Panels IV and VI, Fig. 1).
Since reproduction during spring and
summer by the four species represented
in Table 1 and Fig. 1 usually increases
autumn trap success by a factor of
108
about 1.8 in steppe and bunchgrass
habitats (Pearson, unpublished), comparison of the post-eruption, springtime population should be compared
only with springtime, pre-eruption
populations. Such a comparison suggests that even if small-mammal populations had been seriously reduced by
the ashfall, 15 months after the eruption they had recovered to levels that
seem to be higher than usual.
Reproduction: Reproductive data
confirm that the mouse populations
were thriving. Of 16 male Eligmodontia captured, 14 were in breeding
condition, and the other two were juveniles less than 3 weeks old. Of 10
female Eligmodontia, 2 were lactating
and the other 8 were pregnant carrying
an average of 6.6 fetuses. The average
number of fetuses in 20 pregnant females living in similar habitat in the
Province of Rio Negro in earlier years
was 5.9 (Pearson et al., 1987).
Of the seven Reithrodons captured,
three were sexually competent males,
one was a juvenile female, and three
were pregnant females with 4, 4, and
11 fetuses, respectively. The average
number of fetuses in 17 pregnant
Reithrodons in Rio Negro Province was
4.53 (Pearson, 1988).
Of 11 Phyllotis examined, all 5 males
were in breeding condition, 1 female was
young and entering her first estrus, 1
female was parous, 2 were lactating, and
2 were carrying 9 and 8 fetuses, respectively. These are remarkably large litters
for Phyllotis (Pearson, 1975).
Of the four Abrothrix xanthorhinus
captured, two were breeding males and
two were pregnant females, each with
six fetuses.
Health: Pyke (1984) noted the con-
Oliver P. Pearson
dition of two kinds of mice (Peromyscus and Microtus) living in meadowsteppe habitat before, during, and after
the eruption of Mount St. Helens. His
study area, 400 km NE of the volcano,
received a 2.2 cm layer of ash with
particle sizes from 0.11 to 116 microns
diameter, which is somewhat finer than
the ash at Los Antiguos. No abnormalities of the eyes were noted in mice
trapped before the eruption, but up to
10 days after the ash deposition numerous trapped Peromyscus showed
loss of fur around the eyes and two
had swollen upper eyelids. The eyes
of Microtus specimens appear to have
been unaffected. Histopathologic examination of affected specimens revealed no silicosis-like involvement of
lungs or gastrointestinal tissue. When
caged on a substrate of ash, Microtus
and Peromyscus both developed severe
swelling of the eyelids. Burt (1961)
and Andersen and MacMahon (1985)
did not note eye pathologies, but they
were trapping closer to the source of
the tephra so presumably the particles
were coarser.
All of the mice captured in Argentina appeared to be in good health.
Irritation of the eyes, which had been
reported in larger animals shortly after
the eruption (Corbella et al., 1991),
was not detected in our specimens. The
lungs of one or more individuals of
each species of mouse captured near
Los Antiguos were examined under low
magnification without detecting any
pathology.
Longevity: Skulls of all mice were
saved for examination of the teeth. Unusually great wear of the molars wouid
indicate either unusual longevity or
unusually rapid wear resulting from life
among the residual and re-distributed
MOUSE POPULATIONS AFTER VOLCÁN HUDSON
ash, which was notably abrasive
(Corbella et al., 1991). Since few mice
live more than one year, few if any of
the captured animals would have been
alive at the time of the eruption.
Judging from toothwear, as measured
by height of molar cusps (Pearson,
1975; Pearson et al., 1987), populations of Eligmodontia and Phyllotis at
Sites 1 and 2 appear to have been
composed mostly of old individuals, a
c5
—
109
few juveniles only a few weeks old,
and few or no individuals of middle
-age (Fig. 2). This indicates that the
breeding season for both species was
just resuming after several months of
sexual inactivity during the winter and
early spring. The age pyramid for
Eligmodontia at Sites 1 and 2 in December is quite similar to a pyramid
for a population in springtime in Rio
Negro Province.
ELIGMODONTIA
.30—
Cil
p.
.40 —
E.50—
.30
1
30
1
1
30 %
1
30
1
1
1
1
1
1
30%
December
RIO NEGRO
December
SITES 1&2
d
d
PHYLLOTIS
—
.21—
6 .42—
.
.63—
.84—
E 1.05 —
1.36 —
30
1
1
q
December
SITES 2
1
30 %
30
1
1
1
I
1
I
December
NORTHERN PATAGONIA
I
50 %
Fig. 2: (Aboye). "Age" pyramid of Eligmodontia caught at Sites 1 and 2 at the beginning of the breeding season
compared with that of Eligmodontia caught near the beginning of the breeding season in the Province of Rio
Negro in earlier years. The December pyramid for Rio Negro was modified from Figure 6 of Pearson et al., (1987)
by increasing the amount of toothwear in November by .05 mm, which corresponda to the rate of abrasion per
month shown in Fig. 4 of that report.
(Below). "Age" pyramid of Phyllotis at Site 2 at the beginning of the breeding season compared with that of the
same subspecies near the beginning of the breeding season in earlier years in the Provinces of Neuquen, Rio
Negro, and Chubut in Argentina. Cusp heights of these earlier specimens have been advanced in age an average
of 22 days to bring them aIl to December 5, comparable to the pyramid for Site 2 in the fallout crea. The amount
of this advance assumed abrasion at a rate of .007 mm per day (estimated from Fig. 2 in Pearson, 1975).
110
The pyramid for Phyllotis at Site 2
is compared with a pyramid of a population of the same species constructed
from 21 available specimens collected
in the spring (25 October - 1 December) in the Provinces of Neuquen, Rio
Negro, and Chubut, Argentina.
Toothwear of the Site 2 animals is only
slightly greater than that of the sample
from farther north, captured before the
latest eruption of Volcan Hudson.
DISCUSSION
Sixteen months after the eruption, three
of the species captured were those
captured before the eruption during a
much greater trapping effort by Kelt.
No Abrothrix longipilis, an ominvore,
were captured after the eruption, but
the habitat is too arid to support many
individuals of this species. Kelt had
not captured any Eligmodontia before
the eruption, whereas this seed and
vegetation eater was common (11%
trap success) after the eruption.
Phyllotis, a seed and vegetation eater,
and Reithrodon, an eater of green vegetation, were notably abundant after
the eruption. The abundance of
Abrothrix xanthorhinus, a seed-eater,
was approximately at the level it had
been during a springtime sample before the eruption and in similar habitat
elsewhere in Argentina. Reproduction
of Reithrodon and Phyllotis had ceased
during the winter, as is usual, and was
proceeding vigorously at the time of
our spring collection.
Fossorial, herbivorous, pocket gophers (Cratogeornys and Thomomys)
survived the eruptions of Parícutin and
1VIount St. Helens in appropriate habitat that received less than 15 cm of
tephra (Burt, 1961; Andersen and
MacMahon, 1985). A rancher at our
study site at Estancia La Aurora in-
Oliver P. Pearson
formed us that tuco tucos (Ctenomys),
a fossorial herbivore similar to pocket
gophers, had been present on the
Estancia before the eruption in small
numbers. We found no evidence of
them during our post-eruption visit.
Kelt captured specimens of Ctenomys
before the eruption near Chile Chico,
which is only a few kilometers west of
Estancia La Aurora. Since the habitat
at both of our study sites appeared to
be suitable for tuco tucos, and since
their digging activity makes their presence easily detectable, it is possible
that the 5 to 15 cm of ashfall reduced
the number of tuco tucos. Unfortunately we lack post-eruption data from
Chile Chico and quantitative pre-eruption data from our sites.
With the available data it is not possible to determine with certainty
whether life in the abrasive ash from
Volcan Hudson caused unusually rapid
wear of the teeth, as it did in certain
Mexican species after the eruption of
Volcan Parícutin (Burt, 1961). Most
of the Eligmodontia and Phyllotis at
Sites 1 and 2 had greatly worn teeth,
but they were obviously over-winter
survivors of considerable age, and
comparable populations from other
regions also contained a high percentage of individuals with worn molars
(Fig. 2). It seems unlikely that abrasion by ash from Volcan Hudson advanced molar wear in adult mice by
more than a month or two. No pathologies attributable to tephra were
detected.
We conclude that despite possible immediate effects of the eruption and
ashfall, 16 months later all four of these
species of mice living in the ash zone
were abundant, healthy, and reproducing vigorously. Eligmodontia, Phyllotis, and Reithrodon were more abun-
MOUSE POPULATIONS AFTER VOLCAN HUDSON
dant than expected. Possibly, their
predators had not recovered, and possibly the amount and quality of food
for mice had improved after the eruption because of reduced competition
from sheep and other herbivores such
as guanacos.
ACKNOWLEDGEMENTS
I am grateful to Douglas Kelt for generously sharing his trapping data from
Chile, and to Javier Bellati and Carlos
Lopez of INTA for providing important bibliographic sources.
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