Browsing by Author "GUTIERREZ, JR"

Now showing 1 - 11 of 11
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    EFFECTS OF VERTEBRATE PREDATION ON A CAVIOMORPH RODENT, THE DEGU (OCTODON-DEGUS), IN A SEMIARID THORN SCRUB COMMUNITY IN CHILE
    (1993) MESERVE, PL; GUTIERREZ, JR; JAKSIC, FM
    The effects of vertebrate predation have been monitored since 1989 on 16 replicated 0.56 ha study plots in a semiarid thorn scrub community in north-central Chile. Using fences of different heights with and without holes and suspended game netting to alter principal predator (foxes and raptors) and large rodent herbivore (Octodon degus) access, four grids each have been assigned to the following treatments: 1) low fencing and holes allowing free access of predators and small mammals; 2) low fencing without holes to exclude degus only, 3) high fencing and netting with holes to exclude predators only; and 4) high fencing and netting without holes to exclude predators and degus. Small mammal population censuses are conducted monthly using mark-recapture techniques. Degu population trends during 1989 and 1990 showed strongly but nonsignificantly lower numbers in control plots during months when densities were characteristically low (September November) for this seasonally reproductive species; since March 1991, differences have become persistent and increasingly significant. Predators appear to have greater numerical effects when their prey populations are low. Survival times of degus, particularly established adults, were significantly longer in predator exclusion grids during the 2 1/2 years of observation; thus, predation also affects prey population structure.
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    EVIDENCE FOR INTRASPECIFIC COMPETITION IN THE ACACIA-CAVEN (LEGUMINOSAE) SAVANNA OF CHILE
    (1979) GUTIERREZ, JR; FUENTES, ER
    In the Chilean savanna, A. caven plants exhibit a spatial distribution pattern in which larger individuals tend to be further apart than smaller individuals of this species. Since published models of interspecific competition in plants do not account for the phenomenon as observed in A. caven, a new model is developed in which root systems of nearest neighbors are allowed to overlap. In these circumstances resources actually available to the plants increase fairly linearly with nearest neighbors distance. The model predicts a positive linear correlation between the sum of squares of the trunk radii of nearest neighbors and the distance separating them. This prediction of the model was tested with A. caven and was able to generate the observed distribution pattern.
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    GROWING SEASONS IN CHILE - OBSERVATION AND PREDICTION
    (1979) HAJEK, ER; GUTIERREZ, JR
    Temperature data for 42 Chilean locations were analyzed and heat sums calculated (for base 5.degree. and 10.degree. C). The length, starting and ending dates of the growing seasons [for plants] were obtained. Temperature values normally found in Chilean climatological records, i.e., monthly and annual means, were related with temperature accumulations. In Chile it is possible to predict the accumulation of temperature from the annual mean temperature (r [correlation coefficient] = 0.995 P < 0.001, for base 5.degree. C and r = 0.984, P < 0.001, for base 10.degree. C) or from the mean monthly temperature of Jan. In this latter case, a good adjustment with an exponential curve is found (r = 0.76, P < 0.001, base 5.degree. C; r = 0.78, P < 0.001, base 10.degree. C). For Chile, temperature courses along the year were approximated by using a harmonic analysis. No significant differences were observed between predicted and observed values. By integrating the equations as a function of time, the accumulation of temperature and the length of growing seasons for different threshold temperatures can be determined. A significant relationship was found between these variables and the latitude, and gradients were obtained. Differences exist between littoral and continental stations in the extension of growing seasons and the accumulation of temperatures.
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    HETEROGENEOUS RESPONSES OF SMALL MAMMALS TO AN EL-NINO SOUTHERN-OSCILLATION EVENT IN NORTHCENTRAL SEMIARID CHILE AND THE IMPORTANCE OF ECOLOGICAL SCALE
    (1995) MESERVE, PL; YUNGER, JA; GUTIERREZ, JR; CONTRERAS, LC; MILSTEAD, WB; LANG, BK; CRAMER, KL; HERRERA, S; LAGOS, VO; SILVA, SL; TABILO, EL; TORREALBA, MA; JAKSIC, FM
    A prolonged El Nino Southern Oscillation (ENSO) event during 1991-1992 with three times the normal annual 85-mm rainfall was accompanied by major changes in numbers of small mammals at a semiarid Mediterranean site in northcentral Chile. Several demographic patterns were evident. Akodon olivacerrs, an omnivore, had a rapid increase in population size of more than an order of magnitude. Phyllotis darwini, a granivore-herbivore, showed somewhat delayed, smaller increases superimposed on annual oscillations. Octodon degus, an herbivore, showed a delayed response with larger increases and extended breeding in 1992-1993. Finally, Oligorzyzomys longicaudatus, a granivore, experienced increases during both a dry (1990, ppt = 32 mm) and a wet year (1992). Other species such as Aborthrix longipilis, Abrocoma bennetti, and Thylamys elegans had smaller, delayed demo,oraphic responses. Reproductive rates for the first three species were higher due to the ENSO event only in O. negus males and P. darwini as a quadratic function of time. Survival rates of all four principal species were significantly greater during the 1991-1992 ENSO. Finally, average movement between captures was lower during ENSO years, suggesting behavioral changes. Explanations for these patterns include rainfall-related increases in food-resource levels (A. olivaceus and P. darwini), the importance of source-sink processes in vagile species from more mesic, adjacent habitats (O. longicaudatus), and delayed responses to extrinsic events in species with long gestation (O. degus, A. bennetti). The heterogeneity of species responses suggests different capabilities for small mammals to respond to an extrinsic, large-scale event, and it emphasizes the importance of long-term studies in semiarid systems.
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    INTRASPECIFIC AND INTERSPECIFIC COMPETITION BETWEEN MATORRAL SHRUBS
    (1981) FUENTES, ER; GUTIERREZ, JR
    The model of Gutierrez and Fuentes, which adequately predicted sizes between neighbor distances in the Acacia caven savanna, is applied to the more diverse matorral community of central Chile. The model essentially predicts a positive linear relationship between the sizes of nearest neighbors and the distance separating them. This prediction is tested experimentally by cutting the nearest neighbors of individuals involved in species-pairs of known successional status. The experiments sustain the competition model; matorral shrubs [Lithraea caustica, Quillaja saponaria, Colliguaya odorifera, Muehlenbeckia hastulata, Baccharis rosmarinifolia] compete intra- and interspecifically; and matorral shrubs tested seem to fit into a model of hierarchical competitive dominance in their successional pattern.
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    SEASONALITY OF CONCEPTION IN HUMAN-POPULATIONS IN CHILE
    (1981) HAJEK, ER; GUTIERREZ, JR; ESPINOSA, G
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    SEASONALITY OF MORTALITY IN HUMAN-POPULATIONS OF CHILE AS RELATED TO A CLIMATIC GRADIENT
    (1984) HAJEK, ER; GUTIERREZ, JR; ESPINOSA, GA
    Mortality of human populations in Chile was studied from a latitudinal and seasonal perspective, and related to climate and main diseases. Approximately 3,000,000 deaths were analyzed. Seasonality considered along a latitudinal and climatic gradient is almost absent in the northern portion of the country; in the central part peaks are evident in summer, at latitudes of 40.degree. S in winter and at 45.degree. S in winter and spring. Seasonality along the years of analysis shows a different behavior for the different zones of the country and no unique pattern is evident. A clear relationship between the air enthalpy and seasonality of mortality is detected. Circulatory diseases appear as the most important causes of deaths, exception made of latitude 40 where respiratory diseases are the significant causes of deaths. Age groups below 1 and above 65 yr are mostly affected.
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    SPATIAL-DISTRIBUTION OF SOIL NUTRIENTS AND EPHEMERAL PLANTS UNDERNEATH AND OUTSIDE THE CANOPY OF PORLIERIA-CHILENSIS SHRUBS (ZYGOPHYLLACEAE) IN ARID COASTAL CHILE
    (1993) GUTIERREZ, JR; MESERVE, PL; CONTRERAS, LC; VASQUEZ, H; JAKSIC, FM
    Soil nutrients and density and biomass of annual plants underneath and outside the canopy of Porlieria chilensis shrubs were measured at the end of the growing season in a protected arid coastal site in Chile. Levels of soil nitrogen, phosphorus and organic matter were significantly higher underneath than outside the canopies of shrubs. Almost 4 times as many plants occurred outside than underneath shrubs, but no significant differences in total aboveground biomass were found. Several species had higher densities and/or biomass outside rather than underneath shrubs, whereas others showed the opposite trend. Species richness was lower underneath P. chilensis canopy. The spatial microdistribution of ephemeral species may be explained by differential water and nutrient requirements. Comparison of the patterns observed in our protected site versus surrounding unprotected areas supports the generalization that man, by removing shrubs and trees, has changed a previous heterogeneous spatial distribution of nutrients to a more homogenous one.
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    TEMPERATURE EFFECT ON AUTOCHORY IN COLLIGUAYA-ODORIFERA (EUPHORBIACEAE)
    (1980) GILIBERTO, J; GUTIERREZ, JR; HAJEK, ER
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    VEGETATIVE RESPONSES TO DEFOLIATION OF 2 CHILEAN MATORRAL SHRUBS
    (1980) TORRES, JC; GUTIERREZ, JR; FUENTES, ER
    Two common matorral species: Lithraea caustica and Colliguaya odorifera were experimentally defoliated, L. caustica and C. odorifera are the matorral species previously shown to exhibit the highest and lowest levels of natural defoliation by browsing. Experimental defoliation was performed at 4 levels: 0% (controls), 25, 66 and 100%. C. odorifera exhibited vegetative responses stronger than controls only in the 2 highest levels of defoliation (66 and 100%); L. caustica responded vigorously at the 25 and 100% levels. The results are related to the observed position of both species in the matorral community and their relative defoliation risks by natural and introduced herbivores.