Mastozoología Neotropical; 1(2): 113-122 SAREM, 1994 ISSN 0327-9383

DYNAMIC RESPONSES OF MAMMALS TO THE ERUPTION OF VOLCAN HUDSON

Sergio L. Saba and Daniel A. de Lamo

Departamento Biología General. Facultad de Ciencias Naturales Sede Puerto Madryn. Universidad Nacional de la Patagonia S.J.B. y Centro Nacional Patagónico (CENPAT-CONICET). Bvrd. Alte. Brown s/n, 9120 Puerto Madryn, Chubut, ARGENTINA

ABSTRACT: Ash coming from the eruption of Volcán Hudson significantly affected the densities of wild populations of mammals inhabiting an important fraction of Argentine Patagonia. The convergence of several climatic and biological factors generated a fast recovery of some populations. A significant increase in some rodent populations (mainly Eligmodontia and Phyllotis) one year alter the eruption was observed. This response may be attributed to the abundance of grass, reduced competition with livestock and diminished natural enemy densities, particularly predatory birds. The recovery of hare population was faster in the farming area of Los Antiguos, but slow in the plateau area. Large mammals (e.g. guanaco) are recolonizing the impacted land. The situation at present could be a step in the ecosystem dynamics where diversity and distribution of some wild mammalian species tends to be as was before the eruption.

RESUMEN: Respuesta dinámica de los mamíferos a la erupción del volcán Hudson. Las poblaciones de mamíferos silvestres de una parte importante de la Patagonia fueron afectadas significativamente por las cenizas provenientes de la última erupción del volcán Hudson. La convergencia de factores climáticos y biológicos generaron la posibilidad de una rápida recuperación en algunas de estas poblaciones. Se detectó un incremento significativo en poblaciones de algunos roedores (principalmente Eligmodontia y Phyllotis) un año después de esa erupción. Este tipo de respuesta puede ser atribuida a la abun-dancia de gramíneas, disminución de la competencia con el ganado doméstico y dismi-nución en la densidad de depredadores naturales, particularmente aves de presa. Res-pecto a la liebre europea, su recuperación fue más rápida en la zona de chacras de la localidad de Los Antiguos que en la estepa. Los guanacos están recolonizando el área impactada. Al presente, la situación parece constituir un paso en la dinámica del ecosistema en el que la diversidad y distribución de mamíferos silvestres tiende a asemejarse a su estado previo a la erupción.

Key words: mammals, environmental impact, volcano eruption, Patagonia

INTRODUCTION The Volcán Hudson (45° 90' S; 72° 96' W) suddenly erupted on August 8, 1991, generating a dense cloud of pumice and ash of basaltic origin that crossed Chubut Province in an ENE direction. A week later, a second larger and prolongued eruption of trachyandesitic composition took place. The tephra ejected from the volcano between

Recibido 28 Febrero 1994. Aceptado 30 Mayo 1994.

August 12 and 15, 1991, was depos-ited on a triangular area of Patagonia, close to the Chilean Pacific coast, reaching Puerto Deseado and Puerto San Julián on the Atlantic coast of Santa Cruz Province in Argentina. About 100,000 Km2 were covered by ashes (crystals and vitreous materials) fluctuating on average between 1 and 10cm thick, ejecting between 2-3 km3

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of material (Corbella et al.,1991). The eruption of 1991 seemed to be

stronger than the previous in 1971, which had produced a considerable rain of ashes (Dust Veil Index, DVI =250; Lamb, 1972).

The area covered by ash was subject to severe desertification before the eruption. Several sheep ranches were abandoned in the central and northern portion of Santa Cruz Province, attrib-utable to land degradation (by over-grazing and wood cutting) and capital loses due to the low international prices of wool (PPCD INTA, 1993).

Information about the effects of the eruption on wildlife is still scarce and fragmentary. Corbella et al. (1991) point out that a few days after the eruption, some small birds, rheas (Pterocnemia pennata) and guanacos (Lama guanicoe) were affected by some son of blindness.

In september 1991 a survey aiming to evaluate the effects of ashes on the natural ecosystem and the effects on human settlements in the Province of Santa Cruz was conducted. In this re-port we present the results of the study on the dynamic response of wild mam-mals after the eruption.

MATERIALS AND METHODS The area affected by pumice and ash from the Volcan Hudson in Argentina, belongs phytogeographically to the Provincia Patagónica, Distrito Occiden-tal y Central (Soriano, 1956). The mean annual rain (m.a.r.) is about 150 mm. The temperature ranges between 1-3° C (July), and 14-17° C (January). Los Antiguos is in a relatively rainy loca-tion (254 mm m.a.r) and the dominant winds are from the W (De Fina et al., 1968). Three field trips, designed to

maximize information retrieval were conducted. That is 4, 18, and 28 months after the eruption (m.a.e.). The first was in December 10-13, 1991. The second, between February 25-28, 1993, and the third one between No-vember 29 and December 3, 1993. These trips will be called Survey I, II and III (SI; Sil; SIII), respectively. Several rural dwellers were interviewed on SI, in order to obtain information about the effects of ashes on their live-stock and on some conspicuous wild species. This information served as a base on which to place a sampling design aiming to evaluate the ecosys-tem changes after the eruption.

Sampling was based on: animal ac-tivity determined by strip transects (in SI, SII and SIII), nocturnal censuses of hares (Lepus europaeus) by line transects (in SI; SII and SIII) and by live trapping of small rodents (in SII and SIII). Four sampling sites were defined. Site 1 corresponds to a con-trol area, not affected by the eruption. The other three sites were located in a highly impacted area (Site 2) and in moderately affected areas, respectively (Sites 3 and 4, Fig. 1). Four strip transects (50 m long; 2 m wide) were analyzed in each site. Signs of animal activity, such as feces, tracks, burrows or direct evidences were recorded every meter.

Site 1 corresponds to a plateau. It is a steppe with bushes and grasses domi-nated by Mulinum spinosum, Stipa speciosa and Poa lanuginosa, located to 20 km of Río Mayo. The vegetation cover is about 40%. Phytogeographi-cally, it belongs to the Ocidental Dis-trict. Since no evidence of ashes were found in this area, it has been consid-ered as a control site for operational purposes.

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Site 2 is an alluvial plain of the Jeinimeni River, characterized by Mulinum spinosum, Adesmia campes-tris, Senecio filaginoides and Stipa humilis. The vegetation cover is about 25%. The deposition of ash fluctuated between 19 cm on vegetation mounds, and 27 cm between the mounds. This site showed the highest impact of those surveyed in this study.

Fifty four km south of Perito Moreno, north to Ea. La Vizcaína, there is a glacier valley corresponding to Site 3. Phytogeographically it be-longs to the Central District, subdistrict

Santacrucence. It is an East oriented hillside with gentie slope, character-ized by the presence of Nassauvia glomerulosa, Verbena sp., Stipa chubutensis, Stipa ameghinoi and Poa duseni, with a vegetation cover of 25%. Ash concentrated on mounds (8 cm high) and one layer of 3 cm thick was measured on the intermounds.

Site 4 is located 80 km from Bajo Caracoles, on Ea. La Frisia. The land-scape is characterized by gorges de-scending from a volcanic plateaus of Jurassic age (Barrio, 1993). Phytogeo-graphically it belongs to the Central

Fig.1: Map of strip transect Sites (1-4) and areas A and B where rodent live traps were set after the eruption of the Hudson volcano. 1: Site 1 (control area); 2: Site 2 (high impacted area), between the towns of Los Antiguos (Arg.) and Chile Chico (Ch.); 3: Site 3 (medium impacted area); 4: Site 4 (medium impacted area). For a description of each area, see text.

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District, subdistrict S antacrucence whose dominant species is Verbena tridens. The vegetation cover is about 60%. The deposition of ash fluctuated between 11 cm on mounds and 4 cm deep on the vegetation intermounds.

Line transect counts of hares were performed in a pick-up truck travel-ling for 3 consecutive nights, over 2 round trip transects (i.e. 4 line transects). Two of them (4.8 km long each) were located around the farms, in the basin of the valley River Los Antiguos, a high productivity area under irrigation. The other two line transects (10 km long each) were checked close to Los Antiguos, a semi-arid zone with low primary produc-tivity used for extensive sheep breed-ing. Every hare observed was recorded.

Lines of 100 rodent live traps were set in two study areas near Los Antiguos (areas A and B, Fig. 1), covered by ashes. Area A is a border of a glacier valley, located on a pla-teau 7 km E from Los Antiguos. Vegetation is characterized as a bushy steppe with Colliguaya intergerrima, Mulinum spinosum and Stipa humillis with a vegetation cover of about 45%, showing a strong anthropic impact associated with overgrazing and fire. Area B is in a fluvio-glacier plain located 2.3 km from Los Antiguos, on the road to River Zeballos. Vegeta-tion is a bushy steppe with Colliguaya intergerrima, Mulinum spinosum and Stipa humilis with about 50% of veg-etation cover. Both areas, phytogeo-graphically belong to the Central Dis-trict, subdistrict Santacrucence.

Sherman traps (27 x 9 x 7.5 cm) were used with standard bait. One hundred traps were set for two con-secutive nights in area A and one night

in area B during SII. During SIII they were activated for 2 consecutive nights in each study area. The animals cap-tured were sexed, measured and their skulls and pelts were prepared.

RESULTS As was informed by ranchers, the main initial consequence of the volcano erup-tion was a reduction of livestock num-bers either by selling them or due to mortality attributable to lack of re-sources. On the other hand, ranchers informed that birds and some conspicu-ous mammals, notably hares and gua-nacos emigrated from the affected ar-eas. It coincides with the fact that we found few corpses in the explored area.

From our periodical sampling, we obtained the following results:

a) Animal activity signs detected by strip transects in Sites 1-4 respectively, are presented in Table 1. In the con-trol area (Site 1), an important animal activity was evident during SI, mainly guanacos, hares, and sheeps. The ac-tivity was reduced in SII and SIII, and it may correspond to a recolonization of the areas affected by the ashfall.

Table 1: Frequency of animal activity signs obtained from strip transects during surveys I, II and III and Site 1 (control area), Site 2 (high affected area) and Site 3 and 4 (medium affected areas).

Guanaco Hares

1 2 3 4 1 2 3 4

11 0 1 2 40 11 9 2 6 0 0 2 0 75 33 12 1 0 0 6 2 39 11 22

Fox Rodents

1 2 3 4 1 2 3 4

1 0 0 0 1 0 0 2 0 0 0 2 O 14 25 11 0 1 0 0 1 1 7 6

Species Sheep

Sites 1 2 3 4 Surveys

I 88 0 2 2

II 21 O 12 1

III 6 0 0 0

Species Armadillo

Sites 1 2 3 4 Surveys

1 0 0 0

II 4 0 2 0

III 1 2 8 4

IMPACT OF THE VO HUDSON ERUPTION 117

In Site 2, only few signs of hare activity were recorded during SI. Since it is situated in an international border area, sheep breeding is restricted. Dur-ing SII, several signs of animal activ-ity were found, particularly from hares and rodents, which diminish during SIII (see Table 1).

In Site 3, during SI, few animal signs were found. The scarce activity was mainly from hare and in a lower de-gree from sheep and guanaco. Animal activity increased 18 months after the eruption (SII), particularly with sign& from hares and rodents (burrows, fe-ces and tracks). Slight signs of activity are from Edentates (armadillos, prob-ably Zaedyus pichiy). This last element from the fauna increases in SIII, reach-ing a similar level to those found for hares and rodents, which showed an evident decrease compared to SII.

In Site 4 the pattern is similar to Site 3 with respect to rodents and arma-dillos; however, hare and guanaco activity increased from SI to SII and to SIII. General wild animal activity had similar magnitudes in Sites 1 and 4, but the trends were inverse (54, 10 and 5 animal activity signs, total 69 in Site 1 vs. 6, 27, 38, total 71 in Site 4; SI, SII and SIII respectively). Activity was higher in Sites 2 and 3 with simi-lar trends through time (11, 89, 43, total 143 and 10, 60, 26, total 96, for Sites 2 and 3, respectively). A peak of high animal activity was detected in the Sites affected by ashfall (2, 3 and 4) 18 m.a.e. (27, 176, and 107 animal activity signs in SI, SII and SIII re-spectively).

b) Nocturnal hare censuses by line transects: during SI we did not record any observations of hares. Relative

abundance of hares, considered as the sum of individuals observed every night over the distante driven, begins to be important in February 1993 (mean =0.183 ind/km, SD =0.288), increas-ing in December of the same year (mean =0.356 ind/km, SD =0.269). The difference between both observations is not significant (T test, p =0.123). Separating the observations by land-scape unit (two line transects in the farms and two in the plateau), it may be seen that the highest relative density of hares was reached 18 m.a.e. in the farming area, being low in the plateau. Similar results were obtained 28 m.a.e. for both types of environment (Table 2). Rela-tive density of hares calculated for the plateau 28 m.a.e. was significantly higher (T test, p =0.018) than that calculated for the same environment 10 months earlier.

c) Live capture of Rodents: during SII, from a total of 300 trap nigths activated in both aneas, 32 were sprung by intensive wit,c1Then, 173 trap nigths were active at study area A (2 consecutive nights) and 95 at study area B (1 night). A total of 26 animals were captured in area A (trap success 15.03%) and 9 animals in area B (trap success 9.47%). The capture composi-tion is presented in Table 3.

Table 2: Hare (Lepus europaeus) relative abundance (ind/km) by landscape unit, obtained from SII and SIII (during SI no count were recorded).

Survey II Survey III

Farm landscape

Mean: 0.28 Mean: 0.31 S.D.: 0.39 S.D.: 0.315

Plateau

Mean: 0.03 Mean: 0.38 S.D.: 0.05 S.D.: 0.30

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During SIII, no animals were caught at area B (2 consecutive nights, 142 effective traps nights) and only 3 ani-mals were captured at area A (2 con-secutive nights, 183 effective trap nights, 1.64% trap success). The spe-cies are Reithrodon auritus (young male) and Abrothrix xanthorhinus (1 young male; 1 adult male).

A detailed description of captures from SII and SIII are presented in Fig. 2, as capture success (%) per species.

DISCUSSION The high faunistic activity measured by strip transects in the control area, agrees with the information obtained by ranchers from the region. Wildlife migrated from the impacted area after the eruption. As time passed, range condition of the affected areas tended to be like that of the non affected ones, and animal activity on the control area decreased.

Ash from the volcano may have af-fected large (guanacos), medium size (hares, armadillos) and small mammals (rodents) as well. However, 18 months after the eruption, a strong increase in the relative abundance of those mam-

mals was observed. In the high im-pacted zone (Site 2) and in one of those of medium impact (Site 3), the activ-ity of the mammalian fauna was re-duced more 28 m.a.e. than the activity measured 18 m.a.e.. In the other area with medium impact (Site 4), guanaco and hare activity was still increasing while rodents showed a steady state. Armadillos increased their presence continuously between 4 and 28 months after the eruption.

Generally speaking, the abundance of hares decreased in the summer com-pared to the winter. The relative abun-dances calculated in Surveys II and III are still lower than calculations for the Precordillera in Río Negro Province (5.1 ind/km July; 3.6 in August; 3.8 in September and 2.6 in October; Amaya, 1981). The relative density of hares seems to be stable in the farming zone (chacras) 18 and 28 months after the eruption, increasing in the platean dur-ing the same period of time.

Amaya et al. (1979), in a study on 1,293 hares, described that the repro-ductive season for this animal in Patagonia extends from late August to late January. In other words, the depo-

Table 3: Rodents captured during SII, with description of sex and reproductive status.

Reproductive stage (1) Juveniles

males females

Eligmodontia morgani 16 2 Phyllotis xanthopygus 2 3 Ctenomys sp. (3) 1 Reithrodon auritus Mus musculus

Adults males females

2 3(2)

2 2

1 1

(1) Determined by reproductive tract development. (2) 1 lactating, 1 pregnant carrying 6 fetuses (3/3, right and left horn respectively)

and 1 pregnant carrying 7 fetuses (5/2). (3) Probably C. coyhaiquensis, new species in D.A. Kelt and M.H. Gallardo (1994).

14

E

E

P

R il

6

B TC A SII

G 12

10

o 8

Q. 4

2

o A

SIII

IMPACT OF THE VO HUDSON ERUPTION

119

sition of ash in 1991 must have af-fected the repoductive cycle for this species. Burial of the ash by plowing and cleaning activity in the area around Los Antiguos may have reduced the impact of the eruption on the hare population regularly inhabiting the farming zone. This fact could have gen-erated a faster re-colonization of the farms compared to the plateau areas. In this last environment, relative den-sities of this Lagomorph measured 28 m.a.e. were similar to those calculated for the same species in the farming zone 18 and 28 m.a.e..

Live capture of small mammals dur-ing Survey II, mainly Eligmodontia (n

=23) and Phyllotis (n =9) is signifi-cantly higher for this son of environ-ment (see Pearson, 1994). The popula-tional increment of these seed and green eaters could be explained by the higher supply of herbs, short gramin-eous and ephemeral plants. Revegeta-tion in the impacted zone by regrowth, tillering, elongation of meristems aboye the ash level, and recruitment of new plants has been reponed (Oliva et al., 1993a). On rodent capture sites, 18 months after eruption, a significant cover of Poa lanuginosa was observed (Bertolami, com. pers.) that could be an indicator of better range condition.

On the other hand, in survey I and II

Sergio L. Saba and Daniel A. de Lamo Dynamic responses of mammals to the eruption of Volcan Hudson

Fig. 2: Rodent live capture success (in percentage) for study areas A and B and total capture (TC) during Survey II (SII: February 1993) and area A in Survey III (SIII: December 1993). In study area B no captures were recorded in Survey III. E: Eligmodontia morgani, P: Phyllotis xanthopygus, M: Mus musculus, C: Ctenomys coyhaiquensis, R: Reithrodon auritus, A: Abrothrix xanthorhinus.

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no predatory birds were recorded at the capture sites, in agreement with Serret' s (1992) report for the highly impacted areas. During survey III, di-rect observations of cinereous harrier (Circus cinereus), red-backed hawk (Buten polyosoma), american kestrel (Falto sparverius), black-chested buz-zard eagle (Geranoaetus melanoleucus) and burrowing owl (Athene cunicularia) and great horned owl (Bubo virginianus) regurgitated pel-lets were registered (Saba et al., in prep.). In our study, relative abundante of another rodent predator, the grey fox (Pseudalopex griseus) was scarce. We consider that our sampling design was small to detect patterns of activity of this territorial animal, which usu-ally has a wide honre range and low densities. Any interpretation of the few signs found may be confusing.

We understand that a series of fac-tors such as food availability, absence of predators and reduction of live-stock may explain the surprising popu-lational peak observed in Eligmodontia and Phyllotis.

Twenty eight months after the erup-tion, the environmental conditions be-tween impacted and not impacted areas seemed to be similar. Capture success of different rodent species with regu-lar densities for the summertime (see Pearson, 1994) were obtained in this study in SIII.

CONCLUSIONS Information about the response of wild mammals to volcanic catrastophic events is scarce. The eruption of Volcán Paricutín (México) in 1943 and Mount St. Helens (USA) in 1980 are examples of these studies. The eruption of Volcán Hudson in 1991 affected an arid to

semi-arid environment in southern Patago-nia and was a unique opportu-nity to follow the response of mam-mals in this sort of ecosystem. In spite of the lack of information about the condition before the eruption, we detected a sequence of events that al-lowed us to give a description and to formulate an interpretation of the facts.

The eruption of Volcán Hudson generated an acute impact over an area that had been previously altered by a severe process of degradation of anthropic origin. The mammalian fauna show signs of fast recovery from the impact of the eruption.

At first, a reduction of wild mammal populations and a drastic reduction in the number of sheep was the result. This situation, plus a higher precipita-tion regime during 1992 (235.01 mm vs. a mean for 1951-1960 =116 mm, Perito Moreno, Servicio Meteorológico Nacional) on a layer of ash acted as an adequate substrate for the establish-ment of annuals and rhizomatous grasses (Oliva et al., 1993b). The con-junction of biotic and abiotic factors may have favored a pronounced veg-etation recovery. Then, in a second stage, a fast population growth of mammals (particularly rodents), took place. The demographic increase and the amazing species diversity of small mammals could have been facilitated by the lower degree of competition with domestic herbivores and a time lag in the recovery of natural preda-tors, particularly birds.

The re-entrance of natural predators and the lower availability of forage caused by inadequate rain during 1993 (101.9 mm, Perito Moreno, Servicio Meteorológico Nacional), could have produced conditions similar to those

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OLIVA, G., C. CHEPPI, L. MONTES y G. CLIFTON. 1993b. Las cenizas del Volcán Hudson: su impacto sobre la vegetación. P. 55-67. En: Efectos ecológicos y socio-económicos de las cenizas del Volcán Hudson. Informe fi-nal de la primera etapa (G. Clifton y J.C. Costa, eds.). Convenio CFI-Prov. Santa Cruz, Río Gallegos, 199 p.

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