Amphibia-Reptilia 30 (2009): 265-271

Effects of forest degradation on abundance and microhabitatselection by ground dwelling Chaco lizards

Nicolás Pelegrin1,*, José María Chani2, Ada Lilian Echevarría3, Enrique Hugo Bucher1

Abstract. Human-induced degradation of Chaco forests has led to a mosaic of habitats with different forest conditions,offering different habitat suitability characteristics to the native fauna. Abundance and microhabitat use of Teius teyou,Liolaemus chacoensis, Cnemidophorus ocellifer, and Tropidurus etheridgei were analyzed in the western Chaco forest ofArgentina. A mature forest that has remained undisturbed for the last 30 years (Los Colorados Biological Station, LC) anda highly disturbed forest (Campo Grande, CG) were compared through diurnal visual encounter surveys along 74 1-kmtransects. Lizards were assigned to the microhabitat category according to the site where they were first observed (bareground, litter/grass/herbs, shrubs or trees). T. teyou preferred bare ground in LC, avoiding litter/grass/herbs and using shrubsaccording to availability, whereas in CG the species preferred shrubs, avoiding bare ground. C. ocellifer used microhabitatsaccording to their availability in LC, whereas in CG, this species preferred shrubs and avoided the remaining microhabitats. L.chacoensis preferred bare ground in LC, using litter/grass/herbs and shrubs according to availability. In CG, the species usedbare ground according to availability, preferred shrubs, and avoided litter/grass/herbs. T. etheridgei preferred bare groundin LC, avoiding litter/grass/herbs, and using shrubs according to availability, whereas in CG both bare ground and shrubswere preferred, and litter/grass/herbs was avoided. Our results suggest that microhabitat selection by lizards in the Chaco is aplastic process influenced by vegetation structure, and probably regulated by lizards’ thermal requirements.

Keywords: Argentina, Chaco, Cnemidophorus ocellifer, forest degradation, habitat preferences, Liolaemus chacoensis, Teiusteyou, Tropidurus etheridgei.

Introduction

Loss of suitable habitat has been considered tobe one of the most important factors affectingherpetofaunal diversity worldwide (Gibbons etal., 2000; Cushman, 2006; Gardner, Barlow andPeres, 2007). Habitat selection by many lizardsdepends primarily on structural characteristicsof the environment (Heatwole, 1977; Martinand López, 1998; Vitt et al., 2007). Conse-quently, assessing lizards’ response to changesin habitat availability, especially when the habi-tats they prefer are unavailable, provides animportant indicator of behavioral and physio-logical plasticity. It has been reported that mi-

1 - Centro de Zoología Aplicada, Facultad de Ciencias Ex-actas, Físicas y Naturales, Universidad Nacional de Cór-doba, CC 122 – X5000AVP Córdoba, Argentina

2 - Ciencias Biológicas, Universidad Nacional de Chilecito,9 de Julio 22 – F5360CKB Chilecito, La Rioja, Ar-gentina

3 - Instituto de Vertebrados, Fundación Miguel Lillo,Miguel Lillo 251 – T4000JFE Tucumán, Argentina*Corresponding author; e-mail:npelegrin@efn.uncor.edu

crohabitat use changes under different habitatconditions (Vega, Bellagamba and Fitzgerald,2000). Therefore, understanding the associa-tions between species abundance and microhab-itat characteristics has a considerable conserva-tion value, since it allows ecologists to makepredictions about a species’ responses to anthro-pogenic and natural changes in habitat (Martinand Salvador, 1995; Law and Dickman, 1998).

The Chaco ecoregion is a vast woodland thatcovers more than 1.2 million km2, includingportions of Argentina, Bolivia and Paraguay(Dinerstein et al., 1995). Before the Europeancolonization, the typical Chaco landscape wasa parkland with patches of forests intermingledwith grasslands. The degree and distribution ofthis patchiness was determined by rainfall, pe-riodic fires and soil type (Bucher, 1982). Sincethe European occupation, the Chaco has beenaffected by a severe process of deforestation andovergrazing, which led to a widespread bush en-croachment and an almost complete eliminationof open grasslands, as well as of grass and herbstrata in forest areas (Bucher and Schofield,

© Koninklijke Brill NV, Leiden, 2009. Also available online - www.brill.nl/amre

266 N. Pelegrin et al.

1981; Bucher, 1982). More recently, the expan-sion of the agricultural frontier has resulted ina rapid process of complete elimination of theoriginal vegetation, to the point that some areasof the Chaco are disappearing at an annual rategreater than that of tropical forests (Zak, Cabidoand Hodgson, 2004; Paruelo, Guerschman andVerón, 2005).

Evidence shows that the Chaco vertebratefauna has been affected by human-inducedchanges in the natural environment (Parera,2003). In the specific case of reptiles, Leynaudand Bucher (2005) found changes in abundanceof reptiles in response to overgrazing and the re-sulting changes in vegetation conditions in theWestern Chaco region.

An experimental management system for thesustainable use of Chaco resources has beendeveloped in Los Colorados, a 10 000 ha ex-perimental station, since 1976. Based on graz-ing suppression as the key management toolthrough the use of fence enclosures (Bucher andHuszar, 1999), this multi-species ranching sys-tem was implemented with the aim of restoringthe original Chaco conditions, while providinghigher economic returns in a sustainable man-ner. Thus, Los Colorados Biological Station of-fers a unique opportunity to compare a vari-ety of ecological parameters between a restoredChaco forest and the surrounding degraded, un-managed forests. In this overarching context, weevaluate changes in abundance and microhabi-tat use by sympatric diurnal, ground dwelling

Chaco lizard species, by comparing Los Col-orados Biological Station and a neighboringhighly degraded ranch.

Materials and methods

Study area

The study was conducted in Salta province, northern Ar-gentina, which is located in the Chaco ecoregion (NT0210,Olson et al., 2001). According to Köppen-Geiger’s clas-sification, the local climate type is Cwa (Peel, Finlaysonand McMahon, 2007). Mean temperature is 28.8◦C in Jan-uary (warmest month) and 16.6◦C in July (coldest month).Annual rainfall averages 550 mm (summer rains, October-March) (Bianchi and Yáñez, 1992). The lizard fauna in thestudy area includes 13 species (Lavilla, Cruz and Scrocchi,1995; Leynaud and Bucher, 2005), from which two are ar-boreal species (Tropidurus spinulosus and Urostrophus gal-lardoi).

Two forest fragments with contrasting vegetation con-ditions were selected: 1) degraded forest: Campo Grande(CG) cattle ranch (7500 ha) (24◦43′S, 63◦17′W), and 2)forest in good conditions: Los Colorados Biological Sta-tion (LC) (10 000 ha) (24◦39′S, 63◦17′W) (fig. 1). CampoGrande is a cattle and goat ranch that has been highly de-graded by overgrazing and deforestation. Vegetation is char-acterized by isolated clumps of trees and low, unpalatableshrubs intermingled with large, open areas of bare soil.Los Colorados preserves a typical, well restored semi-aridChaco woodland. Schinopsis quebracho-colorado and As-pidosperma quebracho-blanco (10-14 m high) are the dom-inant tree species. A dense shrub stratum (3-4 m high) isintermingled with grasses and cacti (Varela, 2001). The areawas wire-fenced in 1976 and all domestic cattle were re-moved. As a result, the present vegetation cover is veryclose to the primeval forest conditions (Varela, 2001). Themain differences in vegetation between sites are detailed intable 1. It is important to emphasize that since vegetationstrata may overlap vertically, total cover is fairly higher than100% at each site.

Figure 1. Location of the study sites.

Effects of forest degradation on abundance and microhabitat selection 267

Lizard sampling

Sampling was carried out in the summer of 1993, in themonths of January and February. Diurnal visual encountersurveys were conducted by two observers along 74 1-kmlong transects (43 in LC and 31 in CG) along internal pathsof both sites. Lizards observed along each transect wereidentified and assigned to one of the following microhabitatcategories: 1) bare ground, 2) litter/grass/herbs 3) shrubs,and 4) next to a tree trunk (immediately around the trunkor up to 1 meter high). This study is restricted to grounddwelling species. Arboreal lizards were not included in thisresearch due to their cryptic coloration and habits that makethem difficult to detect.

As with other lizard sampling methods, the possibilitythat the visual encounter technique might be influenced bydifferences in detectability related with the surrounding en-vironment cannot be discarded (i.e., bare ground versusdense cover). However, we assume that if differences ex-isted, the possible error introduced would be small and sim-ilar in both sites, therefore not influencing comparisons be-tween them.

Data analysis

Lizard’s relative abundance was estimated as the averagenumber of lizards recorded per transect. Differences be-tween sites were tested with the Mann-Whitney U test (P �

Table 1. Cover (%) of bare ground, litter/grass/herbs,shrubs, and tree canopy cover in Los Colorados and CampoGrande (taken from Leynaud and Bucher, 2005).

Cover type Los Colorados Campo Grande(% cover) (% cover)

Bare ground 7 46Litter/grass/herbs 93 54Shrubs 34 7Tree canopy cover 80 76

0.05). Differences in species abundance within sites wereevaluated using the Kruskal-Wallis test (P � 0.05).

The proportion of available microhabitat was measuredbetween 1994 and 1998 (Leynaud and Bucher, 2005). Lit-ter/grass/herbs cover was estimated using sampling frames,shrub cover using the point intercept-pin frame technique,and tree cover using a spherical densitometer (see detailsin Leynaud and Bucher, 2005). Preference was evaluatedas the relationship between microhabitat use and its avail-ability using Bonferroni’s simultaneous confidence intervals(Neu, Byers and Peek, 1974), with a 95% confidence level.Changes in habitat use between sites were estimated us-ing Fisher’s exact test (P < 0.05). Bonferroni’s simulta-neous confidence intervals were calculated using a calcu-lation worksheet application; the remaining statistical testswere performed using Infostat© Version 2008 (Grupo Info-stat, FCA, Universidad Nacional de Córdoba).

Results

The list of all the species observed and theirabundance is presented in table 2. A total ofseven species were observed in both sites, ofwhich only T. teyou, C. ocellifer, L. chacoensisand T. etheridgei were abundant enough to makestatistical inferences regarding habitat use.

Total lizard abundance (all lizard individualsrecorded per transect) was higher in CG thanin LC (LC mean ± SE = 3.42 ± 0.62, CGmean ± SE = 7.23 ± 1.02, Mann-Whitney Utest, W = 1415, P = 0.005), mainly due to thehigh abundance of T. teyou and L. chacoensisin CG. Of the four dominant species, all but T.etheridgei showed differences in abundance be-

Table 2. List of lizard species observed in LC and CG and their abundance (number of lizards per transect ± SE).P values � 0.05 indicate significant differences in abundance between sites (Mann-Witney U test).

Family/species Los Colorados (N = 43) Campo Grande (N = 31) W P

Lizards per transect ± SE Lizards per transect ± SE

TeiidaeTeius teyou 1.30 ± 0.38 4.74 ± 1.04 1319.0 0.067Cnemidophorus ocellifer 1.21 ± 0.27 0.32 ± 0.10 958.0 0.012Ameiva ameiva 0.12 ± 0.05 0.26 ± 0.10 1219.0 0.316

TropiduridaeStenocercus doellojuradoi 0.12 ± 0.06 0.00 ± 0.00 1100.5 0.083Liolaemus chacoensis 0.28 ± 0.09 1.65 ± 0.47 1424.5 0.001Tropidurus etheridgei 0.37 ± 0.14 0.26 ± 0.12 1117.5 0.482

GymnophtalmidaeVanzosaura rubricauda 0.02 ± 0.02 0.00 ± 0.00 1147.0 0.396

Total 3.42 ± 0.62 7.23 ± 1.02 1415.0 0.005

268 N. Pelegrin et al.

tween sites. Regarding within-site relative abun-dances, C. ocellifer and T. teyou were domi-nant in LC (Kruskal-Wallis test, H = 32.77,P < 0.0001), whereas T. teyou and L. chacoen-sis were dominant in CG (Kruskal-Wallis test,H = 33.61, P < 0.0001).

Differences in abundance were correlatedwith shifts in microhabitat use between sites.T. teyou was more abundant in CG (marginallysignificant difference, table 2), along with asignificant shift in microhabitat use. In LC,T. teyou preferred bare ground, avoiding lit-ter/grass/herbs and using shrubs according toavailability, whereas in CG the species preferredshrub cover, avoiding bare ground (table 3).

Relative abundance of C. ocellifer decreasedin CG (table 2). In LC this species used micro-habitats according to their availability, whereasin CG the species showed a strong preferencefor shrubs, avoiding the remaining microhabi-tats (table 3).

L. chacoensis was more abundant in CG (ta-ble 2). This species preferred bare ground inLC, using the remaining microhabitats accord-

ing to availability. In CG, the use of bare groundwas proportional to availability, whereas shrubswere strongly preferred and litter/grass/herbswere avoided (table 3).

The abundance of T. etheridgei did not differbetween sites (table 2). The species preferredbare ground in LC, avoiding litter/grass/herbs,and using shrubs according to availability. InCG T. etheridgei preferred bare ground andshrubs, and avoided litter/grass/ herbs (table 3).None of the species were observed next to treetrunks in either site.

Less abundant species observed in visual sur-veys showed no differences in abundance be-tween sites, although S. doellojuradoi and V.

rubricauda were absent in CG (table 2). S. doel-

lojuradoi was only observed in litter/grass/herbs(60%) and shrubs (40%) in LC. A. ameiva waspredominantly observed under shrubs (80%) inLC, and in bare ground (38%) and under shrubs(50%) in CG. The only V. rubricauda speci-men detected was observed in litter/grass/herbsin LC.

Table 3. Microhabitat preferences of lizard species in Los Colorados and Campo Grande. Bonferroni’s confidence intervalswere estimated according to Neu, Byers and Peek (1974). P values � 0.05 indicate significant differences in observedproportions between sites (Fisher’s exact test). Op: proportion of lizards observed in each microhabitat, Ep: proportionof lizards expected (according to microhabitat availability). Habitat use: (+) selected, (−) avoided, (=) proportional toavailability.

Species/microhabitat

Los Colorados Campo Grande P

Op Ep Bonferroni’s Op Ep Bonferroni’sconfidence interval confidence interval

Teius teyou (n: LC = 56, CG = 147)Bare ground 0.5 0.05 0.30-0.76 + 0.2 0.43 0.09-0.26 − <0.0001Litter/grass/herbs 0.4 0.7 0.23-0.67 − 0.0 0.5 0.04-0.06 = <0.0001Shrubs 0.1 0.25 −0.01-0.35 = 0.8 0.06 0.09-0.89 + <0.0001

Cnemidophorus ocellifer (n: LC = 55, CG = 10)Bare ground 0.1 0.05 0.01-0.40 = 0.0 0.43 0.12-0.12 − 0.339Litter/grass/herbs 0.8 0.7 0.61-1.03 = 0.1 0.5 0.35-0.45 − 0.0001Shrubs 0.1 0.25 −0.02-0.34 = 0.9 0.06 0.35-1.25 + <0.0001

Liolaemus chacoensis (n: LC = 12, CG = 51)Bare ground 0.8 0.05 0.48-1.39 + 0.6 0.43 0.18-0.83 = 0.309Litter/grass/herbs 0.2 0.7 −0.19-0.72 = 0.1 0.5 0.10-0.16 − 0.239Shrubs 0.0 0.25 −0.10-0.49 = 0.3 0.06 0.18-0.47 + 0.053

Tropidurus etheridgei (n: LC = 16, CG = 8)Bare ground 0.6 0.05 0.26-1.14 + 0.5 0.43 0.57-1.07 + 0.673Litter/grass/herbs 0.1 0.7 −0.15-0.59 − 0.0 0.5 0.15-0.15 − 0.536Shrubs 0.3 0.25 0.08-0.75 = 0.5 0.06 0.57-1.07 + 0.363

Effects of forest degradation on abundance and microhabitat selection 269

Discussion

Abundance of T. teyou, C. ocellifer, L. cha-coensis, and T. etheridgei changed between sitesaccording to the habitat preferences of eachspecies. T. teyou increased in abundance in CG,which is consistent with its association withopen and disturbed habitats (Cei, 1993; Videlaand Puig, 1994). C. ocellifer was less abundantin CG, despite its preference for bare groundand rocks in the Caatinga and Cerrado ecore-gions in central and northeastern Brazil (Vitt,1995; Mesquita and Colli, 2003).

S. doellojuradoi and V. rubricauda were notdetected in CG. These species may have beenpresent at low numbers; therefore they mighthave been recorded if sampling effort had beenmore intense. S. doellojuradoi was previouslyrecorded in the CG site by Leynaud and Bucher(2005), who found that it was significantly moreabundant in LC, in association with mature for-est vegetation.

The fact that L. chacoensis was abundant inCG and preferred open areas inside the forestin LC somewhat disagrees with the category offorest dwelling lizard provided by Cei (1993).However, L. chacoensis’ preference for denudedand disturbed areas in the Chaco is consistentwith the general characteristics of the genus Li-olaemus, which is well adapted to and broadlydiversified in the arid regions of Argentina, par-ticularly Patagonia.

The similarity in density values of T. etherid-gei observed in both sites may be related to thehigh plasticity in habitat use that characterizesthis species. Indeed, it has been reported usinga wide range of habitats: sandy substrates in theBrazilian Cerrado (Vitt, 1991) and rock cracks,loose rocks, thorny vegetation (shrubs, cacti),and the trunk of Prosopis trees in the Chaco(Cei, 1993; Cruz, 1997).

The observed human-induced changes in theChaco vegetation may affect habitat selectionby lizards in several ways: 1) by increasing soiland air temperature (Vitt et al., 2007), whichmay influence lizards’ thermoregulatory behav-ior and therefore selection of preferred micro-

habitats (Adolph, 1990); 2) by increasing theexposure of lizards to predators (Attum, Easonand Cobbs, 2007); 3) by changing microhabi-tat availability (Vega, Bellagamba and Fitzger-ald, 2000); and 4) by changing availability anddistribution of lizards’ prey (Dennis, Young andBentley, 2001; Burrow et al., 2002; Fabricius,Burger and Hockey, 2003).

It is of worth noting the importance of shrubsas habitat option in the degraded forest of CG,were other cover types are unavailable. The ob-served shift in habitat preference towards shrubcover by most of the studied species in the de-graded site (CG) could be explained in termsof anti-predator behavior and thermoregulatoryconstraints of lizards. From an anti-predatorypoint of view, lizards’ habitat selection is mainlyinfluenced by refuge availability and the de-gree of exposure to predators (Downes andShine, 1998). Although tree density was simi-lar in both sites, no lizards were observed as-sociated with trees either in LC or in CG. InLC, the higher density of grass/herbs and shrubscould have provided lizards with more refugeoptions than in CG, where cattle overgrazingand trampling eliminates vegetation cover ex-tensively. In other words, the observed increaseof bare ground in response to decreased shruband litter/grass/herbs cover in CG may forcelizards to seek refuge under shrubs, simply be-cause they are the only alternative left. Froma thermoregulation perspective, habitat selec-tion may depend on the thermoregulatory con-straints of each species and the availability ofadequate thermal environments (Adolph, 1990).In forests like LC, canopy and shrubs filter sunradiation, leading to a lower-temperature habi-tat with less direct radiation. Here, heliothermiclizards (like L. chacoensis and T. teyou) usu-ally prefer open gaps for basking (Vitt et al.,1998); whereas in CG the higher substrate tem-peratures due to the reduced vegetation covercould have determined the selection of shrubsby lizards in search of shadow to avoid over-heating (Videla and Puig, 1994). For example,T. teyou preferred bare ground in LC but not

270 N. Pelegrin et al.

in CG, which may be due to the fact that bareground under no shrub or tree cover may over-heat beyond a critical threshold in terms of thelizard’s preference. Clearly, this is a very inter-esting line of research that should be more thor-oughly explored, because of its significant im-plications in terms of lizard conservation andmanagement in highly disturbed regions like theChaco.

Acknowledgements. We are thankful to FUDECHA (Fun-dación para el Desarrollo del Chaco) for permission to workin Los Colorados Biological Station. Financial support wasprovided to EHB by CONICET, project PIP 6286; NP hasa fellowship from CONICET, and is a doctoral candidateof Carrera del Doctorado en Ciencias Biológicas, UNC. Wethank J. Brasca for improving the English style, and MALZuffi and an anonymous reviewer for their useful commentsabout the manuscript.

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Received: August 8, 2008. Accepted: January 20, 2009.