P. A. MarquetIquique3Wv02kv4aFig. 3. The Atacama desert <strong>in</strong>nor<strong>the</strong>rn Chile. The thick l<strong>in</strong>erepresents <strong>the</strong> limit <strong>of</strong> <strong>the</strong> desertarea. Note <strong>the</strong> latitud<strong>in</strong>al <strong>in</strong>crease<strong>in</strong> <strong>the</strong> altitud<strong>in</strong>al limit <strong>of</strong> <strong>the</strong>Atacama desert (modified fromVillagrf<strong>in</strong> et al. 1983).part <strong>of</strong> Chile [Fig. 1; see also Villagr<strong>in</strong> et al. (1983), Rauh (1985) and Arroyo et al. (1988)for a general description <strong>of</strong> its limits]. Most authors divide <strong>the</strong> Chilean-<strong>Peru</strong>vian <strong>Pacific</strong> coastaldesert <strong>in</strong>to <strong>Peru</strong>vian coastal desert and Atacama (Chilean) coastal desert. Physiographically, thisregion is composed <strong>of</strong> three major units from west to east; <strong>the</strong> <strong>Coastal</strong> mounta<strong>in</strong> range, <strong>the</strong>Intermediate Depression, and <strong>the</strong> Andes mounta<strong>in</strong> range. Vegetation along <strong>the</strong> <strong>Coastal</strong> range isrestricted to <strong>the</strong> Lomas communities and <strong>the</strong> numerous dry valleys that dissect <strong>the</strong> landscapealong <strong>the</strong> east-west axis (see PCfaur 1982; Rauh 1985; Rundel et al. 1991). The IntermediateDepression, between latitudes 18" and 20°S, is characterised by <strong>the</strong> existence <strong>of</strong> forest patches<strong>of</strong> Prosopis tamarugo. Ra<strong>in</strong>fall steadily <strong>in</strong>creases on <strong>the</strong> <strong>Pacific</strong> slope <strong>of</strong> <strong>the</strong> Andes from lessthan 1 mm <strong>in</strong> <strong>the</strong> lowlands up to 400 mm <strong>in</strong> <strong>the</strong> Puna area (above 3800 m; Fig. 2). Whereas <strong>the</strong><strong>Pacific</strong> coastal desert is def<strong>in</strong>ed as a hot desert, <strong>the</strong> Puna area is a cold desert. The <strong>in</strong>crease <strong>in</strong>ra<strong>in</strong>fall with elevation is parallelled by changes <strong>in</strong> cover, physiognomy and dom<strong>in</strong>ant life formscharacteris<strong>in</strong>g plant communities. Accord<strong>in</strong>g to Villagr<strong>in</strong> et al. (1983) and Arroyo et al. (1988),
<strong>Diversity</strong> <strong>of</strong> <strong>Small</strong> <strong>Mammals</strong> <strong>in</strong> a <strong>Coastal</strong> <strong>Desert</strong>it is possible to def<strong>in</strong>e four major vegetational belts along <strong>the</strong> western slope <strong>of</strong> <strong>the</strong> Andes <strong>in</strong>nor<strong>the</strong>rn Chile: Prepuna, Puna, High-Andean and Subnival. The Puna area reaches its largestareal extent between latitudes 15" and 25"S, and narrows markedly towards nor<strong>the</strong>rn and sou<strong>the</strong>rnlatitudes (Fig. 1). The Atacama desert, on <strong>the</strong> o<strong>the</strong>r hand, <strong>in</strong>creases its altitudial penetrationtowards <strong>the</strong> south, from 1500 m at latitude 17"s up to 3000 m at latitudes 24-25"s (Fig. 3).Data and AnalysesThe database used <strong>in</strong> this paper is made up <strong>of</strong> 15 altitud<strong>in</strong>al transects, between latitudes 5"and 37"s. The transects were latitudes 5, 9, 12, 13, 15, 17, 18, 19, 22, 24, 30, 33, 35, 36and 37"s. I decided to <strong>in</strong>clude <strong>the</strong> latter five altitud<strong>in</strong>al transects, which lie well beyond <strong>the</strong>sou<strong>the</strong>rn boundary <strong>of</strong> <strong>the</strong> Atacama desert, to obta<strong>in</strong> a better picture <strong>of</strong> latitud<strong>in</strong>al species turnover,and to improve resolution <strong>of</strong> <strong>the</strong> dist<strong>in</strong>ctiveness <strong>of</strong> <strong>the</strong> <strong>Pacific</strong> desert and Puna small mammalfauna. Each transect was one latitud<strong>in</strong>al degree <strong>in</strong> width, and encompassed <strong>the</strong> area between <strong>the</strong>coast and <strong>the</strong> Puna. The presence and absence <strong>of</strong> species were recorded <strong>in</strong> each transect at fivealtitud<strong>in</strong>al belts spaced 1000 m from each o<strong>the</strong>r, from 0 m up to 5000 m. The source data wasretrieved from published studies (Mann 1945; Pearson 1951, 1982; K<strong>of</strong>ord 1954; Greer 1965;Dorst 1971; Meserve and Glanz 1978; Pearson and Ralph 1978; P<strong>in</strong>e et al. 1979; Pizzimenti andDeSalle 1981; Reise and Venegas 1987) and <strong>the</strong> author's unpublished data. Both univariate andmultivariate statistical methods were used to <strong>in</strong>vestigate patterns <strong>in</strong> this data set.In order to reveal additional trends with<strong>in</strong> <strong>the</strong> data, an ord<strong>in</strong>ation technique (Pr<strong>in</strong>cipalComponents Analysis; PCA) based on <strong>the</strong> covariance matrix was employed. The nature <strong>of</strong> <strong>the</strong>data (i.e. presence/absence) is likely to cause a 'horseshoe effect' (Williamson 1978), render<strong>in</strong>g<strong>in</strong>terpretation <strong>of</strong> <strong>the</strong> analysis difficult. In this study <strong>the</strong> considerable overlap <strong>in</strong> taxon compositionamong sample units has elim<strong>in</strong>ated this anomaly (see also Duigan and Kovach 1991). O<strong>the</strong>rexamples <strong>of</strong> <strong>the</strong> use <strong>of</strong> PCA as applied to <strong>in</strong>cidence data can be found <strong>in</strong> Jolliffe (1986) andKeller and Pitblado (1989). An additional analysis <strong>of</strong> clusters (not reported here) obta<strong>in</strong>ed with<strong>the</strong> UPGMA (Unpaired Group Mean Average) algorithm applied to a similarity matrix calculatedwith Jaccard's <strong>in</strong>dex, corroborated <strong>the</strong> pattern obta<strong>in</strong>ed by <strong>the</strong> PCA ord<strong>in</strong>ation technique.Results and DiscussionPalaeobiogeographyA key element to understand<strong>in</strong>g current patterns <strong>of</strong> diversity and distribution <strong>of</strong>sigmodont<strong>in</strong>e rodents <strong>in</strong> <strong>the</strong> coastal desert and adjacent Puna area is <strong>the</strong> history <strong>of</strong> arrivaland dispersal <strong>of</strong> <strong>the</strong>se rodents <strong>in</strong> South America, as well as <strong>the</strong> palaeoclimatic history <strong>of</strong><strong>the</strong> study area.The time at which sigmodont<strong>in</strong>e rodents entered South America and <strong>the</strong> pac<strong>in</strong>g <strong>of</strong><strong>the</strong>ir southward migration is a controversial issue. There are three major hypo<strong>the</strong>ses; oneadvocates <strong>the</strong> arrival <strong>of</strong> <strong>the</strong> ancestral stock dur<strong>in</strong>g <strong>the</strong> Late Pliocene, ano<strong>the</strong>r suggests atime <strong>of</strong> arrival by Early Miocene, and yet ano<strong>the</strong>r an Upper Miocene migration (see Webb1985 for a review). Marshall (1979) proposed a palaeobiogeographic model suggest<strong>in</strong>gthat sigmodont<strong>in</strong>e rodents arrived at South America by waif dispersal across <strong>the</strong> BolivarTrough dur<strong>in</strong>g a drop <strong>in</strong> sea level sometime between seven and five million years beforepresent. After this <strong>in</strong>itial dispersal event, <strong>in</strong> <strong>the</strong> Upper Miocene, <strong>the</strong>se rodents underwenta major adaptive radiation <strong>in</strong> <strong>the</strong> nor<strong>the</strong>rn Andean area. In a similar ve<strong>in</strong>, Reig (1986)identified <strong>the</strong> Puna area as <strong>the</strong> major centre <strong>of</strong> diversification <strong>of</strong> both phyllot<strong>in</strong>e andakodont<strong>in</strong>e rodents, supported by palaeoclimatic evidence and current patterns <strong>of</strong> diversityand endemism. This is <strong>in</strong> agreement with evidence presented by Vuilleumier and Simberl<strong>of</strong>f(1980) for Puna birds. As <strong>the</strong> issue stands, <strong>the</strong>re is no doubt that sigmodont<strong>in</strong>e rodentshad potential access to <strong>the</strong> Puna area and to <strong>the</strong> <strong>Pacific</strong> coastal desert throughout most <strong>of</strong><strong>the</strong> Pleistocene. Fur<strong>the</strong>r, <strong>the</strong> palaeoclimatic evidence and <strong>the</strong> current patterns <strong>of</strong> diversity,as po<strong>in</strong>ted out by Reig (1986) and Vuilleumier and Simberl<strong>of</strong>f (1980), characterise <strong>the</strong>Puna area as a major centre <strong>of</strong> diversification for both birds and sigmodont<strong>in</strong>e rodents.