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ARTICLE IN PRESSa c t a o e c o l o g i c a x x x ( 2 0 0 7 ) 1 – 1 1

Original article

Linking small desert mammal distribution to habitatstructure in a protected and grazed landscapeof the Monte, Argentina

Solana Tabenia,*, Leandro Mastrantoniob, Ricardo A. Ojedaa

aBiodiversity Research Group (GiB), National Council for Science & Technology of Argentina (CONICET) and the Institute for Aridland

Research (IADIZA), Av. A. Ruiz Leal s/n, CC 507, C.P. 5500 Mendoza, ArgentinabDepartment of Agrarian Engineering, National University of Cuyo, Mendoza, Argentina

a r t i c l e i n f o

Article history:

Received 28 March 2006

Accepted 22 November 2006

Published online -

Keywords:

Grazing

Spatial pattern

Scales

Logistic model

a b s t r a c t

Interpretation of landscape patterns from the perspective of different species allows know-

ing the way in which they perceive landscape, and how their perception varies with scale.

We examined distribution of four small mammal species at different scales over a land-

scape including protected and grazed areas, and associated species distribution with land-

scape structure. The study was conducted in the central Monte Desert (Reserve of

Nacunan). Trap grids were set in both areas at two scales, varying their grain and extent.

To determine whether spatial patterns are random, clumped or regular, we used a point

pattern analysis. Logistic regressions were performed to relate the presence-absence of

small mammals to environmental variables. Intensity of the point pattern was not con-

stant, either in the Reserve or the grazed area. Small mammal abundance exhibited a het-

erogeneous distribution, and the existence of a first-order effect was detected for all

species. No second-order effects were detected, the point pattern was random for all spe-

cies in both areas. Both areas were differently perceived by rodent species. Habitat struc-

ture in both conditions and its variations with scale appear to be important factors

affecting distribution patterns.

ª 2006 Elsevier Masson SAS. All rights reserved.

1. Introduction

Natural and anthropic disturbances have been acknowledged

as important generators of landscape heterogeneity, as they

produce complex patterns where some areas are more affected

than others, and where severity of the disturbance varies con-

siderably even within the affected area (Turner et al., 2002).

1146-609X/$ – see front matter ª 2006 Elsevier Masson SAS. All rigdoi:10.1016/j.actao.2006.11.002

Please cite this article in press as: Tabeni, S. et al., Linking smaland grazed landscape of the Monte, Argentina, Acta Oecolo. (2

Grazing by domestic herbivores is one of the most common dis-

turbance agents in arid and semiarid ecosystems, and its ef-

fects are likely to induce changes in the spatial pattern of

landscape and alteration of multiple ecological processes by

loss of natural habitats and biodiversity (Fleischner, 1994).

Depending on the scale examined, heterogeneity caused by

grazing is in some cases increased and reduced in others

* Corresponding author. Tel.: þ54 261 428 7995; fax: þ54 261 428 0080.E-mail addresses: mariasol@lab.cricyt.edu.ar, mariatabeni@hotmail.com (S. Tabeni), lmastra@fca.uncu.edu.ar (L. Mastrantonio),

rojeda@lab.cricyt.edu.ar (R.A. Ojeda).

hts reserved.

l desert mammal distribution to habitat structure in a protected007), doi:10.1016/j.actao.2006.11.002

a c t a o e c o l o g i c a x x x ( 2 0 0 7 ) 1 – 1 12

ARTICLE IN PRESS

(Adler et al., 2001). In the central part of the Monte Desert in

Argentina, heterogeneity may be higher in grazed habitats

than in the same type of habitat patches in ungrazed areas

(Tabeni and Ojeda, 2005). If this disturbance generates varying

heterogeneity levels on different scales, the response of each

species might be variable depending on perception by each or-

ganism and on habitat requirements (Orrock et al., 2000).

In the case of small mammals, there is plenty of literature

reporting the effects of cattle raising on these species, mostly

due to changes in habitat structure (Tabeni and Ojeda, 2003).

Nonetheless, as pointed out by Adler and Lauenroth (2000),

most ecological studies focus on the manner in which a treat-

ment affects the mean value of a variable, and rarely address

changes in the spatial distribution of such variable.

Behavioral responses of organisms to environmental

structure produce movements of individuals within patches,

which translates into different organism abundances among

patches across the landscape (Wiens, 1999). The spatial and

temporal patterns across a range of scales have been largely

reported for mammals (Krohne and Burgin, 1990; Amarase-

kare, 1994; Bowman et al., 2000, 2001; Klaas et al., 2000;

Mackinnon et al., 2001; Williams et al., 2001; Keitt et al.,

2002), but few studies explore these patterns in areas dis-

turbed by grazing (Keesing and Crawford, 2001; Schooley

and Wiens, 2001).

In the central area of the Monte Desert in Argentina, re-

sponses of small mammals to the impact of domestic

herbivores have been studied, comparing their diversity,

richness and abundance between disturbed and undisturbed

areas (Maldonado Curti 1990; Gonnet, 1998; Tabeni and

Ojeda, 2005). At broad level, comparisons between both

types of area indicate a decline in diversity and richness

of small mammals in grazed areas (Gonnet, 1998), but on

considering heterogeneity of each area, small mammal

species respond quite differently to grazing depending on

habitat type.

The presence of a particular pattern is often associated

with the operation of ecosystem-specific processes that serve

to structure the community in a non-random manner (Caylor

et al., 2003). Spatial distribution patterns in mammals can be

related to two main causes: intraspecific and interspecific

interactionsdsuch as competition, predation and reproduc-

tiondand environmental heterogeneity (Amarasekare, 1994;

Bowman et al., 2000; Schooley and Wiens, 2001). Point data

analysis can provide an insight into the scale of patchiness

and determine whether there are hierarchies of scale, and

whether spatial distribution is random, aggregated or uniform

(Gustafson, 1998).

In the present study we address the issue of how much of

the spatial patterns detected for four small mammal species

can be explained by environmental heterogeneity. Our gen-

eral goal is to analyze how these species are distributed in

space considering their eco-morphological attributes and the

heterogeneous matrix in which they occur. We hypothesized

that protected and grazed areas determine different use of

space by all four species, and we propose two objectives: (1)

analyze the spatial distribution of four small mammal species

between grazed and protected areas and habitat type, on dif-

ferent scales, and (2) relate species distribution to landscape

structure.

Please cite this article in press as: Tabeni, S. et al., Linking smaland grazed landscape of the Monte, Argentina, Acta Oecolo. (2

2. Materials and methods

2.1. Study area

The study was carried out in the MaB Reserve of Nacunan

(34�020 S, 67�580 W), located in the central part of the temperate

Monte Desert, Argentina (Morello, 1958) (Fig. 1). The climate is

semiarid and markedly seasonal, with cold dry winters (mean

temperature<10 �C) and warm rainy summers (mean temper-

ature >20 �C). Average annual rainfall is 329.4 mm (Estrella

et al., 2001).

At the beginning of the 19th century, the area of Nacunan

suffered severe exploitation from indiscriminate logging of

Prosopis woodlands, and its growing impoverishment led to in-

tensified cattle raising. The Reserve of Nacunan was created in

1961 to protect the woodlands of P. flexuosa, one of the most

conspicuous habitat types in the area. In 1986 the Reserve

was incorporated into the UNESCO Man and Biosphere Re-

serve Network (Ojeda et al., 1998) and for over 40 years cattle

have remained excluded from the area.

In this study we compared the Reserve of Nacunan with

one adjacent area under cattle grazing. The grazed field is un-

der a continuous grazing system and also presents other re-

lated activities (firewood extraction, subsistence hunting,

establishment of watering points, fences, roads and human

settlements).

The major plant communities in this area are open wood-

lands of Prosopis flexuosa that make up the tree layer along

with specimens of chanar (Geoffroea decorticans), a shrub layer

of jarilla (Larrea divaricata and Larrea cuneifolia) and piquillın

(Condalia microphylla), and a grass and herb layer. Besides,

there are Larrea shrublands, plant communities with low

tree density and dominance of jarilla and grasses (Trichloris cri-

nita, Scleropogon brevifolius and Baccharis pingraea). To a lesser

degree, there are also areas with high proportion of bare soil

and low plant cover: sand dunes (sparse woody and herba-

ceous plants) and peladal (habitat with high amounts of clay

and bare soil, with extremely scarce plant cover consisting

mostly of stunted L. cuneifolia) (Roig, 1971).

2.2. Sampling and data collection

Four small mammal species (less than 100 g) were studied: the

silky mouse (Eligmodontia typus), the gray leaf-eared mouse

(Graomys griseoflavus), the grass mouse (Akodon molinae) and

the vesper mouse (Calomys musculinus) (Rodentia, Cricetidae).

Samplings of small mammals were made using trapping

grids set up on two scales. Scale is the spatial or temporal di-

mension of an object or process (Turner et al., 1989), and in-

volves two elements: grain or resolution (size of individual

units of observation) and extent (size of the area studied) (For-

man, 1995; Gustafson, 1998).

We examined two scales of perception of habitat in a hier-

archical form, based on environmental heterogeneity and on

evidence that it can be perceived by small mammals at differ-

ent habitat levels (i.e. ‘‘microhabitat’’, ‘‘macrohabitat’’) (Gon-

net and Ojeda, 1998; Corbalan and Ojeda, 2004; Corbalan,

2006; Corbalan and Debandi, 2006): (1) at small scale we

attempted to represent the environmental heterogeneity

l desert mammal distribution to habitat structure in a protected007), doi:10.1016/j.actao.2006.11.002

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Monte Desert biome

Ñacuñán Reserve

0 400 Km

Fig. 1 – Location of the study area (MaB Reserve of Nacunan) in the Monte Desert biome, Argentina.

inside the habitats, this resolution considers the available in-

formation on spatial use by these species at microhabitat

level, (2) at coarse scale the purpose was to include the differ-

ent landscape units within both areas. Although there is pre-

vious evidence of species habitat selection at macrohabitat

level, it comes from studies of fine grain. Here we assess

how small mammal species perceive the environmental con-

trast at coarse resolution, not considered thus far.

Please cite this article in press as: Tabeni, S. et al., Linking smalland grazed landscape of the Monte, Argentina, Acta Oecolo. (20

On these grounds, the coarse scale consisted of two grids of

75 ha (extent) set up in both the protected and grazed areas,

with 75 trapping stations 100 m apart (grain ¼ 1 ha). To maxi-

mize captures at each station, four traps were placed in each

cardinal direction, separated 10 m from one another. The

small scale consisted of grids of 0.25 ha (extent) each, with

25 trapping stations 10 m apart (grain ¼ 0.01 ha). Four trapping

grids were set up in each of the three main habitat types

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ARTICLE IN PRESS

(Prosopis woodland, Larrea shrubland, and sand dunes) in both

grazed and protected areas totaling 300 trapping stations per

area.

The trapping session for the coarse scale was conducted in

May 2003, operational reasons prevented us from replicating

this sampling. For the small scale, 8 trapping sessions were

carried out over 2 years: February 2001 and 2002 (summer),

May 2001 and 2002 (autumn), August 2001 and 2002 (winter),

October 2001 and 2002 (spring). For both scales each sampling

period consisted of 5 consecutive nights. Sherman live traps

were used, baited with rolled oats and peanut butter. Traps

were checked in the morning, captured animals were marked,

and their weight, sex, and reproductive condition recorded

(male: testes scrotal or abdominal, female: vulva perforate or

imperforate, pregnant or lactating).

At small scale, in order to correlate species spatial struc-

ture and environmental structure, vegetation samplings

were made within the trapping grids in each of the three

main habitat types in the Reserve and in the grazed area. A

4 by 4 m square was centered at each trapping station from

where percentages of plant cover and bare soil were visually

estimated. A total of 300 squares were established in the Re-

serve and another 300 in the grazed area.

At coarse scale we determined types of landscape

unit (cover classes) using a satellite image (Landsat 7

ETMþ231_84_10-March-2003_4x_FF) of the study area. The

scene analyzed corresponds to the same season where sam-

pling of small mammals was made at this scale. A land

cover map was derived from a supervised classification us-

ing the Minimum Distance method (Chuvieco, 1996). TM 4-

3 and 2 bands were used in the classification, with regions

of interest or training sites defined in homogeneous areas.

Verification of these sites was based on both vegetation

map (Roig and Rossi, 2001) and field data. At the post-classi-

fication stage, a confusion matrix was used to assess the ac-

curacy of the classification, by comparison with the data

recorded in the field.

2.3. Data analysis

For verifying whether different plant layers recorded at small

scale (bare soil, trees, shrubs, herbs and grasses) vary between

habitats inside and outside the Reserve, they were compared

with a Mann–Whitney test (Zar, 1999).

Data on small mammals are point pattern-like spatial data

(Cressie, 1993). A number of methods have been used for

quantifying the characteristics of a point pattern. First-order

statistics describe the intensity l of a point pattern in the

study region, a typical approach is to use the mean point-to-

point distance to derive a mean area per point as in nearest-

neighbor methods. In contrast, the second-order property of

a point pattern is related to the joint density of the occurrence

of two points at a given distance, and describes the small-

scale spatial correlation structure of the point pattern (Peliss-

ier and Goreaud, 2001; Wiegand and Moloney, 2004). Some of

the second-order statistics such as the O-ring statistics or

Ripley’s K function describe characteristics of point processes

at many distance scales. These statistics characterize the

number of points encountered in the neighborhood of a ring

or circle, respectively, of radius r centered on an arbitrary

Please cite this article in press as: Tabeni, S. et al., Linking smalland grazed landscape of the Monte, Argentina, Acta Oecolo. (20

point of the pattern, allowing to interpret spatial structure in

terms of interaction processes (aggregation, inhibition etc.)

(Pelissier and Goreaud, 2001; Wiegand and Moloney, 2004).

The O-ring statistic O(r) is a scale-dependent neighborhood

density and can be defined as the expected density of points

of the pattern within a distance r from an arbitrary point of

the pattern.

To determine whether spatial patterns of the small

mammals were random, clumped, or regular, we conducted

a series of univariate O-ring analyses. The total number of

individuals captured in each trap (without recaptures) was

used in this analysis as value of the pattern in the grid

cell of the trap. From our analyses we expect first-order ef-

fects in the spatial distribution of small mammals due to

environmental heterogeneity. To factor out the first-order

effects, and thereby study the second-order effects of

interest, we used a heterogeneous Poisson null model that

distributes random points in accordance with the non-

constant first-order intensity (Wiegand and Moloney, 2004).

To construct the first order-intensity l we used a circular

moving window estimator of l with radius r ¼ 1, 2 or 3 cells,

which are the scales where we expected second-order ef-

fects. At either scale, small or coarse, analyses were con-

ducted separately for the Reserve and the grazed area. The

significant departure from the null model was tested by con-

structing confidence envelopes using 99 Monte Carlo simula-

tions of the heterogeneous Poisson null model (Wiegand and

Moloney, 2004).

Logistic regressions were performed to relate the presence-

absence of small mammals with environmental variables at

different scales.

At small scale, variables used in the regression were treat-

ment (Reserve and grazed area), habitats (Prosopis woodland,

Larrea shrubland and sand dunes), and the proportion of

each type of layer (trees, shrubs, grasses, herbs and bare

soil). At coarse scale, variables used were treatment and cover

classes.

Colinearity among explanatory variables was determined

by the Pearson correlation matrix, and only uncorrelated vari-

ables were entered into a logistic regression at 0.05 level of sig-

nificance. The minimum model that best explained the

dependent variable without including unnecessary explana-

tory variables was found by applying the stepwise procedure

using backward elimination until all remaining predictors

were significant at the 0.05 level.

Contingency tables of data observed vs. values predicted

by the model were constructed for different cut-off values.

Percentages of correctly classified occurrences (sensitivity)

and of correctly classified absences (specificity) were esti-

mated. The least error cut-off value was determined for

each model, thus maximizing sensitivity and specificity.

We also used the receiver operating characteristic (ROC) as

a threshold or cut-off independent measure of model accu-

racy. A ROC was obtained by plotting the true positive pro-

portion of correctly predicted occurrences (sensitivity) on

the y-axis against the false positive proportion of correctly

predicted absences (specificity) on the x-axis. The area under

the ROC curve (AUC) was used to test for greater significance

than the area under a random model, with AUC ¼ 0.5

(Schadt et al., 2002).

desert mammal distribution to habitat structure in a protected07), doi:10.1016/j.actao.2006.11.002

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3. Results

3.1. Habitat variables

At small scale, all plant species recorded were categorized

into four layers: trees, shrubs, herbs and grasses. The per-

centage of the different layers varies between Reserve and

grazed area depending on habitat type. Grazed sand dunes

had lower cover of herbs (U ¼ 3062, P < 0.001), shrubs

(U ¼ 2530, P < 0.001) and grasses (U ¼ 3858, P < 0.05) compared

to the Reserve. In the grazed Larrea shrubland, both

herbs (U ¼ 3435, P < 0.001) and grasses (U ¼ 2147, P < 0.001)

decreased significantly, whereas bare soil showed a signi-

ficant increase (U ¼ 2804, P < 0.001). In the Prosopis woodland,

both trees (U ¼ 3877, P < 0.05) and grasses (U ¼ 2177,

P < 0.001) were significantly less abundant in the grazed

area (Table 1).

At coarse scale, six land cover classes were determined:

shrubland of L. cuneifolia; open shrubland (composed of

L. divaricata, bare soil, P. flexuosa and G. decorticans); shrubland

of L. divaricata; Prosopis woodland; ‘‘peladal’’ (bare soil and

L. cuneifolia); and bare soil (roads, trails, and natural patches

bare of vegetation). Both landscapes (Reserve and grazed) pre-

sented different representativity of cover classes. The Prosopis

woodland was the most abundant land cover class (>40%), but

comparatively its cover was higher in the grazed area than in

the Reserve. Next in abundance (>30%) was the shrubland of

L. divaricata. In comparing the Reserve with the grazed area,

the latter was observed to have a higher proportion of open

areas: peladal, bare soil and open shrublands; whereas the Re-

serve showed a higher proportion of shrublands of L. divaricata

and L. cuneifolia (Table 2).

3.2. Small mammal captures

At small scale we captured a total of 916 individuals belonging

to four rodent species: G. griseoflavus, C. musculinus, E. typus,

and A. molinae, in 24,000 trap-nights (4000 trap-nights for

each patch type). The number of individuals captured was

lower in the grazed area than in the Reserve for G. griseoflavus

(grazed ¼ 140, Reserve ¼ 187), A. molinae (grazed ¼ 136,

Reserve ¼ 194) and C. musculinus (grazed ¼ 77, Reserve ¼ 89).

In contrast E. typus showed higher captures in the grazed

area (n ¼ 35) than in the Reserve (n ¼ 58).

At coarse scale we captured a total of 261 individuals, with

3000 trap-nights. All species were captured in the Reserve, al-

though captures of E. typus were very low (n ¼ 6), so this spe-

cies was not considered in the analysis. In the grazed area

only A. molinae and G. griseoflavus were captured.

3.3. Spatial patterns at small scale

Intensity of the point pattern was not constant, either in the

Reserve or the grazed area. Small mammal abundance

exhibited a heterogeneous distribution, and the existence of

a first-order effect was detected for all species. No second-

order effects were detected; the point pattern was random

for all species in both areas (Fig. 2).

Please cite this article in press as: Tabeni, S. et al., Linking small desertand grazed landscape of the Monte, Argentina, Acta Oecolo. (2007), do

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mammal distribution to habitat structure in a protectedi:10.1016/j.actao.2006.11.002

a c t a o e c o l o g i c a x x x ( 2 0 0 7 ) 1 – 1 16

ARTICLE IN PRESS

Table 2 – Representativity of different cover classes at coarse scale between the Reserve and grazed area

Cover classes Area (ha) Area (%) Patch number

Reserve Grazed area Reserve Grazed area Reserve Grazed area

Shrubland of L. cuneifolia 980.46 806.85 8.28 6.81 916 814

Open shrubland of

L. divaricata

1299.33 1818.18 10.97 15.35 1582 2056

Shrubland of L. divaricata 4548.42 3620.16 38.41 30.57 1442 1897

Prosopis woodland 4867.92 5301.72 41.19 44.78 1497 1396

Peladal 16.29 47.25 0.13 0.39 31 44

Bare soil 126.54 244.80 1.06 2.06 248 629

The density of points varied for each species depending on

the environmental heterogeneity present in each area. For

A. molinae, the logistic regression showed correspondence be-

tween the presence of this species and habitat patches with

dense plant cover. Within the Reserve, the Larrea shrubland

and grass and shrub layers were the variables that better

explained the presence of this species. In contrast, in the

Please cite this article in press as: Tabeni, S. et al., Linking smalland grazed landscape of the Monte, Argentina, Acta Oecolo. (20

grazed area clumps of this species occurred in the Prosopis

woodland, habitat in which tree and grass layers were the sig-

nificant variables (Table 3).

C. musculinus was more related to the Larrea shrubland and

grass and shrub layers in either treatment, and for E. typus re-

gressions showed that the overall habitat rather than a partic-

ular layer was the best predictor of this species presence. The

A.molinae

Reserve Grazed area

C. musculinus

Reserve Grazed area

Grazed area G. griseoflavus

Reserve

E. typus

Reserve Grazed area

Fig. 2 – Point pattern analysis at small scale for each species showing the O-ring statistic (bold lines) with confidence

envelopes (solid lines) at small scale (a) and Reserve (b) grazed area.

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Table 3 – Results of logistic regression models at small scale. Models with an area under the curve (ROC), between 0.7 and0.9 indicate higher discriminatory ability. Sensitivity and specificity refer to the percentage of occurrences and absencescorrectly classified. b is the coefficient allocated to each independent variable during the regression, and b0 is the intercept

Species Treatment Habitat n P > c2 Model information Model parameters

Least errorcut-off value

Sensitivity/Specificity

Area underROC curve

Variable b SE P

Akodon molinae Reserve Larrea

shrubland

287 0.014 0.90 0.63 0.73 Shrub 4.16 1.80 0.021

Grass 5.29 2.10 0.012

b0 �0.22 0.90 0.808

Grazed Prosopis

woodland

214 0.000 0.70 0.78 0.88 Tree 6.54 2.67 0.014

Grass 15.04 2.70 0.000

b0 �1.03 0.32 0.001

Calomys

musculinus

671 0.000 0.35 0.68 0.72 Grass 3.20 0.76 0.000

Shrub 2.46 0.80 0.002

Larrea shrubland 1.79 0.20 0.000

b0 �2.86 0.45 0.000

Eligmodontia

typus

Reserve 312 0.000 0.25 0.62 0.64 Sand dunes 1.18 0.32 0.000

b0 �2.21 0.24 0.000

Grazed 319 0.000 0.24 0.62 0.69 Sand dunes 1.16 0.26 0.000

Tree �6.70 2.34 0.004

b0 �1.18 0.19 0.000

Graomys

griseoflavus

Reserve 446 0.006 0.73 0.54 0.59 Herbs 4.91 1.84 0.008

b0 0.77 0.15 0.000

Grazed Prosopis

woodland

156 0.000 0.52 0.78 0.86 Grass 12.77 2.31 0.000

Tree 5.58 2.39 0.020

b0 �1.48 0.34 0.000

most sensitive models included the sand dune habitat in both

the Reserve and grazed area, although in the latter a negative

association with the tree layer was observed (Table 3).

In G. griseoflavus the logistic regression showed no signifi-

cantly related variable in the Reserve, but in the grazed area

it was positively correlated with the Prosopis woodland, and

within this habitat with trees and grasses (Table 3).

3.4. Spatial patterns at coarse scale

All of the species showed a heterogeneous distribution in both

the Reserve and the grazed area, but no second-order effects

were observed. The distribution of A. molinae was correlated

with the Larrea shrubland in the Reserve and with the Prosopis

woodland in the grazed area (Table 4).

The distribution of C. musculinus in the Reserve exhibited

a significant association with the shrubland of L. cuneifolia,

however the discriminatory ability of the model was low

(<0.7). G. griseoflavus was heterogeneously distributed in

both the Reserve and the grazed field The variable retained

by the model in the Reserve was the shrubland of L. cuneifolia,

and in the grazed area the open shrubland of L. divaricata and

bare soil. In all cases association was negative, indicating

a higher probability of absence of this species when these

cover classes occur (Table 4).

4. Discussion

In deserts, where variations in climate, topography, soil and

vegetation create a high interpatch variability (Wiens, 1985;

Please cite this article in press as: Tabeni, S. et al., Linking smaland grazed landscape of the Monte, Argentina, Acta Oecolo. (2

Polis, 1991), population structure is mostly determined by

this spatial and temporal heterogeneity. The role of habitat

structure in desert mammal communities has been widely

documented (Rosenzweig and Winakur, 1969; Holbrook,

1978; Price, 1986). The distribution of patch mosaics provides

animals with different refuges against predators, food compo-

sition and distribution, as well as diverse types of layers that

affect their locomotion (Shenbrot et al., 1999).

Our prediction that species would present different distri-

butions between the Reserve and grazed area is only true for

some species and depends on the scale. Moreover, differing

responses are observed if distribution is considered through-

out the study area (first-order statistics) or if only the distribu-

tion of distances of pairs of points (second-order statistics) is

considered. In the first case, protected and grazed areas, at

the two resolutions studied, are structurally heterogeneous,

which is also perceived by small mammal species. Overall,

species were non-randomly distributed, forming clumps and

using specific habitat patches, layers or cover classes in both

areas according to their ecological requirements.

The few studies reporting distribution patterns of rodents

in grazed and ungrazed areas (Keesing and Crawford, 2001;

Schooley and Wiens, 2001) have found that, at high densities,

spatial patterns tend to be uniform and more equitably dis-

tributed across space, high rodent densities are reached only

in plots where ungulates have been excluded. In the presence

of ungulates, low densities tend to be grouped, reflecting ei-

ther presence of favorable microsites or vegetation contrasts.

A previous research in our study area reports that only two

species showed significant differences in abundance between

protected and grazed areas, the grass mouse with higher

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Table 4 – Results of logistic regression models at coarse scale. Models with an area under the curve (ROC), between 0.7 and0.9 indicate higher discriminatory ability. Sensitivity and specificity refer to the percentage of occurrences and absencescorrectly classified. b is the coefficient allocated to each independent variable during the regression, and b0 is the intercept

Species Area n P > c2 Model information Model parameters

Least errorcut-off value

Sensitivity/Specificity

Area underROC curve

Variable b SE P

Akodon molinae Reserve 103 0.0000 0.28 0.63 0.71 Shrubland of

L. cuneifolia

3.87 0.96 0.000

b0 �0.92 0.26 0.000

Grazed 81 0.0001 0.14 0.80 0.87 Prosopis woodland 8.39 2.76 0

b0 �6.15 1.67 0.000

Calomys

musculinus

Reserve 75 0.0207 0.06 0.55 0.62 Shrubland of

L. cuneifolia

3.55 1.47 0.016

b0 �2.87 0.55 0.000

Graomys

griseoflavus

Reserve 168 0.0887 0.90 0.60 0.63 Shrubland of

L. cuneifolia

�1.97 1.08 0.068

b0 2.25 0.28 0.000

Grazed 114 0.0388 0.74 0.60 0.63 Open shrubland

of L. divaricata

�1.21 0.68 0.076

Bare soil �6.88 3.95 0.082

b0 1.48 0.37 0.000

abundance inside the Reserve, and the silky mouse in the

grazed area (Tabeni and Ojeda, 2005). However we found

that density was not constant for all species in either treat-

ment, which showed a contrast between suitable and unsuit-

able habitats.

Patterns of spatial use by these small mammals have been

addressed at macrohabitat and microhabitat levels (Gonnet

and Ojeda, 1998; Corbalan and Ojeda, 2004; Corbalan, 2006;

Corbalan and Debandi, 2006). It has been proven that species

can distinguish among the three major habitat types. Habitat

preferences at macrohabitat level reported for A. molinae in

this area indicate a close association with densely vegetated

patches, specifically Larrea shrublands with high grass cover

(Corbalan and Ojeda, 2004). Besides, across its geographic

range, this species has been trapped in grass clumps near wa-

tercourses Contreras (1968). The coarse scale used in our study

is coincident with former results that indicate that this

species perceives contrasts in the landscape, and is closely

related to patches with high plant cover. However habitat

alterations cause changes in its use of space, while in the

Reserve high density areas correspond to Larrea shrublands,

in the grazed area these clumps are found in some parts of

the Prosopis woodland. In this woodland, vegetation patches

not visited by cattle contain a sufficiently high grass density

that would act as a refuge for A. molinae, probably reducing

its vulnerability to predation.

For G. griseoflavus, the explanatory ability of the model at

coarse scale was low, but it was possible to observe that this

species shows a tendency to avoid complex landscape units

(Shrubland of L. cuneifolia) in the Reserve, and open sites in

the grazed area. This result is not in agreement with earlier

studies reporting a higher association with complex macroha-

bitats inside the Reserve (Corbalan and Ojeda, 2004). Such

a difference can likely be attributed to the grain used, since

in those studies the same fine grain was used to elucidate

the use of space at different habitat levels.

The absence of data for some species such as C. musculinus

or E. typus at coarse scale can be attributed to the fact that as

Please cite this article in press as: Tabeni, S. et al., Linking smaland grazed landscape of the Monte, Argentina, Acta Oecolo. (2

grain size increases, species widely scattered in small patches

are less likely to be recorded than if they are highly aggregated

(Wiens, 1989). Although it is advisable to define grain and ex-

tent from an organism-centered perspective (Kotliar and

Wiens, 1990) this is not very pragmatic because of technical

limitations, and because in many cases we ignore the resolu-

tions at which organisms perceive the habitat (McGarigal and

Marks, 1995). In fact, in our study these results might be

regarded as a first approach to the grain and extent perceived

by these species.

At small resolution, records on the area indicate that hab-

itat selection by small mammals is weak, suggesting that

a combination of multiple factors (morphology, escape behav-

ior and diet) allows them to use different microhabitats in

a opportunistic fashion (Corbalan, 2006). As Jorgensen (2004)

pointed out, the appropriate spatial scale for microhabitats

is worth further discussion, and other habitat components,

not only habitat structure (i.e. food amount and quality, ther-

moregulation), can be important at this selection level (Gon-

net and Ojeda, 1998; Corbalan, 2006).

At a sampling scale of 15 m, A. molinae shows an associa-

tion with litter cover (Corbalan, 2006). However, at this scale

we found important predictors of the presence of this species,

such as grasses, trees and shrubs. Preference for vegetated

sites, from coarse to fine grain, suggest that this species se-

lects habitats equally at any scale, even under different eco-

logical conditions (ungrazed/grazed).

For the other species, our results are partially coincident

with previous studies. G. griseoflavus utilizes closed, shrubby

microhabitats in the Andean foothills (Gonnet and Ojeda,

1998), and subshrubs, shrubs, and trees in the Nacunan Re-

serve (Corbalan, 2006). Use of these vegetation layers is asso-

ciated with its ability to climb (Mares, 1977). Our results

show that it perceives the Reserve as homogeneous but the

model yielded a low discrimination. The weak association be-

tween some small mammal species and the microhabitat sup-

ports interpretations that these species might have very

generalized habitat requirements (Coppeto et al., 2006).

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ARTICLE IN PRESS

Although G. griseoflavus has been characterized as a generalist

with great plasticity to occupy different types of environment,

such as xeric habitats, cultivated fields, sandy areas with

rocks, and riparian forests (Redford and Eisenberg, 1992; Gon-

net and Ojeda, 1998), it strongly relates to patches having tree

and grass layers in the grazed area.

Distribution at small scale in E. typus indicates that this

species is associated with sand dune habitats in both areas,

but no variable within these clumps was more explanatory

than the entire habitat. Its morphological characteristics al-

low it to exploit open and sandy habitats (Mares, 1975; Ojeda,

1989; Corbalan, 2004), and its abundance is significantly higher

in grazed areas (Tabeni and Ojeda, 2005). It occurs in areas that

on other continents are occupied by kangaroo rats and gerbils

(Redford and Eisenberg, 1992; Mares, 1975; Shenbrot et al.,

1999) where open microhabitats may allow more unimpeded

mobility, increasing their efficiency in gathering food or re-

ducing their vulnerability to raptors (Waser and Ayers, 2003).

E. typus displays behavioral strategies like foraging close to

cover where predation risk would be lower rather than at

open sites. Because of its lack of cheek pouches, it runs

a greater risk of predation when carrying food; this risk de-

creases when they recover the harvested seeds they have

stored under shrubs (Giannoni et al., 2001).

On comparing the pattern of habitat use in both areas

(ungrazed/grazed), we found that E. typus has similar ecomor-

phological attributes to species occurring in grazed areas in

other deserts, such as Gerbillus paeba (Eccar et al., 2000), Gerbil-

lus gleadowi (Wada et al., 1995), Dipodomys merriami and D. ordii

(Jones and Longland, 1999; Jones et al., 2003; Valone and

Sauter, 2005). In contrast, small mammals associated with

dense cover like A. molinae, and like Perognathus spp. and Chae-

todipus spp. in North America (Jones and Longland, 1999), oc-

cupy undisturbed areas.

Population structure in many vertebrates can respond to

habitat heterogeneity or be independent of it, i.e. be patchily

distributed but not occupy discrete habitat patches (Amarase-

kare, 1994). Our results show that distribution of the studied

species can, to a great extent, be accounted for by environ-

mental heterogeneity. Klaas et al. (2000) found that topo-

graphic variability may be related to the patterns of

distribution of gopher mounds. Other studies, instead, suggest

that part of the demographic variability occurs independently

of habitat structure (Bowman et al., 2000), and that spatial

structure, e.g. in Dipodomys spectabilis, seems not to result

from habitat heterogeneity per se, but from the interaction be-

tween habitat and capacity of dispersal (Amarasekare, 1994).

In our study, part of the variability not explained in the pat-

terns found requires further assessment of factors affecting

the population structure of these species, such as predation,

dispersal and behavior.

In sum, environmental heterogeneity was significant for all

species, and distribution patterns seem to be restricted to hab-

itat availability. Even though population abundance affords

important information on species responses to disturbed

environments, analyses of point patterns reveal a different

type of information that can be used to predict the impact of

disturbances on the native fauna. For example, even those

species that tend to avoid grazed areas find in them suitable

patches available, and although the Reserve provides effective

Please cite this article in press as: Tabeni, S. et al., Linking smaand grazed landscape of the Monte, Argentina, Acta Oecolo. (2

protection to some species, others find their ecological re-

quirements outside.

The two scales analyzed allowed us to detect critical levels

for assessing distribution patterns. We found that, depending

on the species considered, both coarse and fine grain variables

account for the relationship of small mammals with the

environments.

Despite the increased documentation on spatial patterns,

which contributes to the understanding of organism re-

sponses to landscape features, numerous aspects of the natu-

ral history of these species remain poorly studied. These

issues are essential to elucidate the processes producing the

patterns observed, specifically in disturbed environments.

Acknowledgments

Special thanks to Benjamin Bender for helping us do the field-

work and drawing the map, to Nelly Horak for the English ver-

sion of the manuscript, and to Thorsten Wiegand for his

valuable suggestions and critical reading of the manuscript.

This study was partially supported by the SETCyP and CONI-

CET Research Grants PICT 03281 and PIP 02884.

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