Biological Conservation 97 (2001) 229±237
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Lizard species distributions and habitat occupation along an urban gradient in Tucson, Arizona, USA S.S. Germaine *, B.F. Wakeling Arizona Game and Fish Department, Research Branch, 2221 West Greenway Road, Phoenix, AZ 85023, USA Received 1 March 2000; received in revised form 22 May 2000; accepted 17 July 2000
Abstract Despite expanding human populations in metropolitan areas world-wide, habitat relationships of lizards have seldom been examined along urban gradients. We tested three hypotheses addressing lizard species distributions, site occupation, and assemblage organization along the urban gradient in Tucson, Arizona, USA. One hundred twenty-nine random sampling plots were distributed from undisturbed-natural land beyond the boundary of development to highly developed land within the city. We recorded lizard species abundances and 19 variables describing habitat physiognomy, ¯oristics, and spatial relationships of native remnant and developed land parcels in each plot. We ordinated lizard species and habitat data in a canonical correspondence analysis, and tested signi®cance of relationships in a Monte Carlo randomization test. We then tested eects of individual habitat features on site occupation by lizards using a presence±absence transformation of lizard abundance data and multiple logistic regression. Both species distributions and site occupation were in¯uenced by factors describing habitat physiognomy, ¯oristics, and landscape spatial relationships. Finally, we examined total abundance, number of species, and evenness across ®ve categories describing increasing residential density. All three measures of assemblage organization peaked at low±moderate levels of development, and declined markedly as residential density increased beyond moderate levels. Distinct attributes of urban development in¯uence lizard species distributions, site occupation, and assemblage organization. Attention to these attributes will allow land developers to mitigate the eects of future urban development on lizard populations. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Lizards; Urban development; Habitat relationships; Species richness; Species diversity
1. Introduction Expanding human populations are altering natural environments world-wide at an accelerating rate (Wilson, 1986). Anthropogenic disturbances often result in habitat fragmentation, degradation, and loss, and these phenomena threaten biological diversity (Primack, 1993). Nowhere are anthropogenic eects on environments more apparent than in urban areas. As urbanization intensi®es locally, the physical structure and species composition of native habitat becomes increasingly altered from a natural state (Whitney and Adams, 1980; Theobald et al., 1997). As development progresses, native vegetation is replaced by man-made features, building density increases, soils become compacted, and micro-climates becomes less stable (Bradley, * Corresponding author. Tel.: +1-602-789-3662. E-mail address:
[email protected] (S.S. Germaine).
1995). Non-native vegetation increasingly replaces native plants in developed areas, and undeveloped native habitat remnants become increasingly insularized (Soule et al., 1988; Germaine, 1995). Land within the matrix surrounding native remnants may become increasingly impenetrable as developmental alterations continue (Goszczynski, 1979). Isolated native habitat remnants support fewer species than similar-sized plots located in undeveloped areas (Bolger et al., 1997a). Consequently, developed areas become increasingly less able to support populations of many native wildlife species (Gavareski, 1976; Dickman and Doncaster, 1987; Sears and Anderson, 1991). Reptile population responses to urban development are perhaps the least understood of all classes of vertebrates in urban environments. Although losses of lizards from urban areas have been documented in recent literature, cause is seldom determined, and in some cases reports of losses are purely retrospective and anecdotal
0006-3207/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0006-3207(00)00115-4
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S.S. Germaine, B.F. Wakeling / Biological Conservation 97 (2001) 229±237
(e.g. McCoid et al., 1994; Walker et al., 1996). Herpetofauna populations in urban areas have been impacted by destruction of protective ground cover (Minton, 1968; Orser and Shure, 1972), entrapment in window wells (Heck, 1971), increased predation (Schaaf and Garton, 1970), collecting (Beebe, 1973), and pollution and pesticides (Anderson, 1965; Scott, 1973). Roads act as barriers to dispersal and increase mortality for both herpetofauna and small mammals (Gibbs et al., 1971; Mader, 1984; Fahrig et al., 1994). In addition, human disturbance to critical habitat components can result in the loss of microhabitats needed for predator avoidance and reproductive activities (Hecnar and M'Closkey, 1998). Finally, the cumulative eects of urban development of landscapes over time and space may constitute a major impact on wildlife in aected areas (Theobald et al., 1997). Using multivariate analyses, we examined habitat relationships of lizards along the rural±urban gradient in Tucson, Arizona. Germaine et al. (1998) documented three patterns of habitat disturbance (changes in habitat physiognomy, ¯oristics, and spatial relationships of habitat patches) associated with urban development that aected avian communities. We hypothesized that these patterns of habitat disturbance resulting from urban residential development also aect lizard communities. Speci®cally, we tested the following hypotheses concerning lizard±habitat relationships along the urbanization gradient: (1) lizard species distributions (abundance) are not related to habitat features describing the urbanized gradient; (2) habitat composition does not aect site occupation (presence) by lizards; and (3) habitat changes along the urban gradient do not aect total abundance, species richness, or evenness of the lizard assemblage. 2. Methods 2.1. Study area We conducted this study in the greater Tucson metropolitan area, in southeastern Arizona. The study area included land within 19.2 km of the center of Tucson (a point mid-way between the geographic and population centers), and comprised 1158 km2, most of the Tucson Basin. Tucson contains Lower and Upland Sonoran vegetative types, as well as riparian and xeroriparian vegetation (mixed riparian desert scrub series; Brown, 1994). While relicts of these cover types are found within the urbanized boundary, most of the urban landscape now contains man-made structures, exotic landscaping, or has otherwise been modi®ed away from its natural state. Elevation in the Tucson vicinity ranged from 640 m along the Santa Cruz River to over 2770 m at Mount
Lemmon. In 1994 Tucson received 37.0 cm of precipitation, divided almost equally between the summer and winter rainy seasons. Mean daily maximum temperature in June was 40.2 C, 1.7 C above average (NOAA, 1994). The metro-Tucson population was estimated at >450,000 in 1993 (Arizona Department Economic Security, 1996). Population is expected to grow in Tucson to 830,000 in 2000, and 1.25 million by 2025 (Arizona Department Economic Security). 2.2. Plot selection and location We sampled lizards and habitat features across the range of residential development present in Tucson. We generated 129 100-m radius random sampling plots along roads and trails throughout the study area using a Geographical Information System (GIS). All plots were 2 m wide at channeled sandy bottom, with riparian vegetation along banks), or urbanized (no land left in a native vegetative state, all vegetative cover consisting of landscaping, ornamental plantings, or weeds). Urbanized land was further classi®ed as treed (>60% canopy closure), savannah (30±60% canopy closure), or open (60% native trees and cacti), mixed (30±60% native trees/cacti) or
S.S. Germaine, B.F. Wakeling / Biological Conservation 97 (2001) 229±237 Table 1 Habitat descriptors, variable codes, means, and ranges for land cover types in 129 sample plots in Tucson, 1994 Variablea
Code
Mean
S.E.
Range
House density/ha House (%) Apartments/businesses (%) Paved or graded (%) Urban treed (%)b Urban savannah(%) c Urban-open native (%)d,e Urban-open mixed (%)d,f Urban-open exotic (%)d,g Lower Sonoran (%): Upland Sonoran (%): Native (LSV+USV; %) Disturbed riparian (%)h Undisturbed riparian (%)i Riparian (RIPD+RIPU; %) Plot heterogeneity Distance to mainland (m) Distance to patch 5 1 ha (m) Distance to riparian (m)
HDEN PCTH PCTA PCTP URBT URBS URON UROM UROE LSV USV NATV RIPD RIPU RIP HET DISM DISP DISR
3.26 8.31 1.38 18.40 0.33 1.46 3.37 3.68 16.07 13.20 27.52 41.81 0.50 3.66 4.16 3.12 3304 293 820
0.32 0.77 0.37 0.85 0.17 0.50 0.83 1.05 1.96 2.45 2.94 2.98 0.21 0.65 0.66 0.08 276 45 97
0±14.8 0±37.7 0±27.0 3.2±62.2 0±19.1 0±54.2 0±62.5 0±79.7 0±80.0 0±95.0 0±92.2 0±95.0 0±17.9 0±40.7 0±40.7 2±6 0±10296 0±2822 0±4752
a
% indicates proportion of ground surface covered within plots. Residentially developed, with>60% tree canopy closure. c Residentially developed, with 30±60% tree canopy closure. d Residentially developed, with 60% native trees and shrubs. f Containing 30±60% native trees and shrubs. g Containing >60% exotic trees and shrubs. h >2 m wide at sandy bottom, 51 bank devoid of wash vegetation. i >2 m wide at sandy bottom, wash vegetation undisturbed. See Section 2 for complete descriptions. b
exotic (
