Proximity to source populations and untidy gardens predict occurrence of a small lizard in an urban area

Abstract

Very little is known about how lizards respond to urbanization. Given their small space requirements, opportunist lizard species can potentially exist in small patches of suitable urban habitat. This study identified the relative importance of landscape and habitat-level factors that influenced the occurrence of an opportunist and generally abundant species, the common skink Oligosoma nigriplantare polychrome in suburban private gardens in New Zealand. Private gardens are potentially important in sustaining urban wildlife as they make up a significant proportion of the total area of many cities. Common skinks were largely absent from the central suburbs and distributed in less highly ordered gardens around the city edge. Skink occurrence was influenced mainly by landscape-level features that reflect opportunities for dispersal into gardens from source populations (distance to city edge; presence of an adjacent grassy paddock) and habitat quality (messiness). To a lesser extent skink occurrence was associated with lower cat densities, less native vegetation and south-facing sites. Within private gardens common skinks were associated with areas with a high proportion of basking surfaces, lower plant species diversity, shorter distances to potential cover, and vegetation that was low in height and made up mainly of long exotic grasses. Despite common skinks’ ability to thrive in exotic vegetation and their small spatial requirements, it is unlikely that they will thrive in urban areas unless gardening practices change to increase resource availability and provide refuges from predators.

Introduction

The responses of lizard populations to urbanization are probably the least understood of all vertebrates (Germaine & Wakeling, 2001). Urban landscapes are highly fragmented, composed of a heterogeneous mix of habitats ranging from intensely developed built-over areas, to suburban gardens and remnants of natural vegetation. While a number of studies report negative effects of habitat fragmentation on lizards in non-urban landscapes (MacNally and Brown, 2001, Smith et al., 1996), others suggest that vegetation type and structure as well as features such as geology and aspect may have a greater influence than habitat fragmentation on lizard community structure (Jellinek et al., 2004, Kitchener et al., 1980, Santos et al., 2008). Lizards are ectotherms, and consequently have smaller space and energy requirements than birds or mammals (Pough, 1980), characteristics that may enable them to sustain populations in small patches of suitable habitat. Generalist lizard species might thrive in highly fragmented urban landscapes if there are patches of sufficiently good habitat that provide opportunities for thermoregulation and refuges from predators. If dispersal can occur across disturbed habitats a widespread distribution can be maintained by forming a meta-population (Hoehn et al., 2007, Sarre et al., 1995, Smith et al., 1996).

Private gardens cumulatively comprise a large proportion of the total area in many cities (21.8–26.8% in Belfast, Edinburgh, Leicester, Oxford and Cardiff; 20% in Greater London; 23% in Sheffield (Gaston et al., 2004, London Biodiversity Partnership, 2005, Loram et al., 2007). The greater the degree of urbanization, the more important gardens become as a resource (Loram et al., 2007). People maintain high diversity of plant species in gardens, which can remove resource limitations for some species. However, variations in vegetation characteristics, garden management practices and in large-scale landscape-level spatial relationships have the potential to influence the occurrence and abundance of lizards. Species richness and abundance peaked at low-moderate housing densities in Tucson, Arizona, but most species dropped out with increasing housing density, increasing proportions of exotic vegetation and longer distances to remnant native patches (Germaine & Wakeling, 2001). Lizards show three response groups to urbanization: species adept at exploiting the urban environment, species adapted to moderate levels of urbanization, and species sensitive to low levels of urban modification and habitat fragmentation (Germaine & Wakeling, 2001).

A good example of a small, abundant, generalist species that should thrive in an urban environment is the common skink (Oligosoma nigriplantare polychrome), which is the most widespread lizard in New Zealand, found at high densities in native and modified habitat throughout the South Island and the lower North Island (Barwick, 1959, Freeman, 1997, Patterson, 1985, Towns and Elliott, 1996), and the only skink found in two of the major urban areas (Whitaker, 1999). A diurnal heliotherm, common skinks eat a wide range of invertebrates, most commonly spiders, and berries and fruits (Barwick, 1959, Gill, 1976, Patterson, 1985). Common skinks appear to be at least adapted to moderate levels of urbanization, and less vulnerable to extinction in modified landscapes than more specialised lizard species. However, resource availability may not be the only determinant of population persistence for skinks. High densities of introduced predators known to eat skinks, such as cats (Felis catus; Fitzgerald, 1988, Morgan et al., 2009, van Heezik et al., 2010) and European hedgehogs (Erinaceus europaeus; Moss & Saunders, 2001) may reduce the viability of small sub-populations and limit dispersal across the urban matrix, reducing connectivity between populations. In New Zealand, where there are no native mammalian predators, just over half of lizard species are confined to offshore islands where introduced predators are absent (Towns & Daugherty, 1994).

The factors limiting the occurrence of common skinks in New Zealand urban areas are not known, but may operate at several scales. With gardens potentially providing the largest resource in residential areas, we aimed to identify whether the occurrence of this generalist species in gardens was limited primarily by landscape connectivity and dispersal opportunities, predation pressure, or habitat features. Because lizards occupy small areas within gardens, we also aimed to identify features at the scale of lizard territory size that influence their occurrence.