Browsing by Author "MEDEL, RG"
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- ItemBODY SIZE, ENERGETIC AND FORAGING MODE OF RAPTORS IN CENTRAL CHILE - AN INFERENCE(1988) BOZINOVIC, F; MEDEL, RGAn inferential analysis of the foraging mode (opportunist or mediate by prey selection) of a taxonomic assemblage of raptors in central Chile was conducted. The analysis of energetic aspects such as daily requirements of predators and energy supplied by the preys, with estimations of prey relative abundances and their incidence in the diet of raptors, led us to conclude that contrary to the opportunistic hunting mode suggested by others authors, these predators apparently present prey selection (on a biomass basis). This phenomenon is particularly evident in raptors of small body size.
- ItemCOMPARATIVE ECOLOGY OF DARWIN FOX (PSEUDALOPEX-FULVIPES) IN MAINLAND AND ISLAND SETTINGS OF SOUTHERN CHILE(1990) JIMENEZ, JE; MARQUET, PA; MEDEL, RG; JAKSIC, FMUsing comparable techniques we studied the abundance, habitat use, and diet of Darwin's fox (Pseudalopex fulvipes), as well as prey availability in two constrasting settings of southern Chile: on the mainland in the Nahuelbuta ranges, and in Chiloe Island. This fox lives in sympatry with its congener P. griseus in Nahuelbuta, but is the sole fox on Chiloe Island. We estimated that Darwin's fox is about twice as abundant in Chiloe than on the mainland. The structure of the habitat where Darwin's fox was found was remarkably similar between the two study sites despite floristic differences. Its diet, however, was markedly dissimilar, with mainland foxes preying extensively on mammals and reptiles and little on insects, and island foxes preying primarily on insects and amphibians, and little on mammals. The low consumption of mammals in Chiloe may be attributed to the low densities reached by that prey on the island as compared to Nahuelbuta (estimated at about double the insular density). Fruit consumption in Chiloe was almost three times higher than in Nahuelbuta, suggesting a lower energy diet for insular foxes. We also report behavioural observations on Darwin's fox provided by knowledgeable sources.
- ItemDISCOVERY OF A CONTINENTAL POPULATION OF THE RARE DARWIN FOX, DUSICYON-FULVIPES (MARTIN, 1837) IN CHILE(1990) MEDEL, RG; JIMENEZ, JE; JAKSIC, FM; YANEZ, JL; ARMESTO, JJThe rare Darwin''s fox Dusicyon Fulvipes (Martin, 1837) was thought to be restricted to Chiloe Island in southern Chile, and to be a subspecies of Dusicyon griseus. We report the findings of a continential population of D. fulvipes, 600 km north of its known insular ranges and in sympatry with D. griseus. We document for the first time the food habits of Darwin''s fox, add new information on its morphometry, and discuss the systematic, zoogeographic and ecological implications of the discovery of the continental population.
- ItemEXPERIMENTAL-EVIDENCE THAT HIGH POPULATION FREQUENCIES OF LIZARD TAIL AUTOTOMY INDICATE INEFFICIENT PREDATION(1988) MEDEL, RG; JIMENEZ, JE; FOX, SF; JAKSIC, FMFrequency of autotomized tails in lizard populations has been taken as an indicator of predation pressure upon those populations. However, recent correlational evidence points to autotomy as reflecting lizards'' escape efficiencies and/or predators'' attack inefficiencies. We report experimental evidence on the relative inefficiencies of three predator species (a teiid, a snake, and a falcon) as autotomy-producing agents, and on the relative escape efficiencies of three congeneric lizard species under laboratory conditions. The falcon was the least inefficient lizard predator (100% of successful attacks) whereas the teiid and snake were more inefficient (10-20% of the lizards escaped by autotomizing their tail). Lizards that successfully escaped predation by the widely-foraging teiid had relatively longer tails than unsuccessful ones. No difference in this feature was detected between successful and unsuccessful lizards attacked by the sit-and-wait snake. All three lizard species were equally efficient at escaping the three predators through tail autotomy. Our observed differences of comparative inefficiencies of lizard predators give experimental support to assertions that tail-loss frequencies do not adequately serve as indices of presumed predatation pressure. High rates of tail-loss among lizard populaions could instead reflect attempts at predation by inefficient predators.
- ItemMICROHABITAT SHIFTS OF LIZARDS UNDER DIFFERENT CONTEXTS OF SYMPATRY - A CASE-STUDY WITH SOUTH-AMERICAN LIOLAEMUS(1988) MEDEL, RG; MARQUET, PA; JAKSIC, FMThe Iguanid lizard Liolaemus tenuis is shown to be a rock and trunk dweller (apparently preferring perches between 0-30 cm height) in a central Chilean locality where it coexists with a single ground-dwelling congener. In its southern distributional ranges L. tenuis is sympatric with another tree-dweller, L. pictus. Habitat shift is demonstrated in this latter case by L. tenuis concentrating on tree trunks, and at modal heights 30-60 cm. Liolaemus pictus occupies lower (apparently more favorable) perches, actively interferring with its congener.
- ItemNICHE RELATIONSHIPS BETWEEN 2 SYMPATRIC LIOLAEMUS LIZARDS IN A FLUCTUATING ENVIRONMENT - THE LEAN VERSUS FEAST SCENARIO(1989) NUNEZ, H; MARQUET, PA; MEDEL, RG; JAKSIC, FMLiolaemus fuscus and L. lemniscatus are two similarly-sized, completely sympatric species of central Chile. In some localities they are also syntopic, sharing the same microhabitat (rocks). We studied the microhabitat, time, and food use of these two species throughout one year, computing standard niche metrics. Between-species similarities on these three niche dimensions were very high at all times of the year (mean = 82% for all three microhabitat, time, and food overlaps), slightly decreasing during winter. The "feast" seasons of plentiful food, when dietary overlaps reached 90 and 99% (spring and summer, respectively) were associated with the two species overlapping broadly in microhabitat (77 and 81%, for spring and summer, respectively) and activity time (92 and 91%, respectively). "Lean" times were associated with decreased overlap in both diet (59 and 81% for fall and winter, respectively) and activity time (87 and 58%, respectively), but not in microhabitat (71 and 97%, respectively). Our data provide limited support for the view that interspecific niche relationships in fluctuating environments may be understood within the "lean" versus "feast" times scenario.
- ItemOBJECTIVE RECOGNITION OF GUILDS - TESTING FOR STATISTICALLY SIGNIFICANT SPECIES CLUSTERS(1990) JAKSIC, FM; MEDEL, RGPresently, no standard protocol for objective guild recognition is consistently used by ecologists. Apart from intuitive designations of guild membership, four statistically-based protocols are currently available: those of Colwell (1977); Holmes (1979); Lawlor (1980); and Adams (1985). The first is based on nearest-neighbor variance in overlap, the second on multivariate statistics, the third on clustering techniques, and the fourth on psychometric analysis. We propose a fifth approach, first developed by Strauss (1982) for purposes other than guild recognition. We advocate the use of bootstrap procedures to resample any given empirical matrix of consumers by resources, within constraints set by either of four different randomization algorithms. Subsequently, pseudovalues of similarity in resource use between the consumers are computed and their frequency distribution is displayed in a histogram. The overlap pseudovalue that exceeds percentile 95 may be considered statistically significant and chosen as the cutoff point that identifies significant species clusters (guilds) in the original (empirical) similarity matrix. We exemplify use of this approach with the food-niche matrix obtained for a predatory assemblage in California [USA], and discuss its implications for the general analysis of guild structure.