Proximity to source populations and untidy gardens predict occurrence of a small lizard in an urban area
Highlights
► Generalist common skinks were absent from most private urban gardens. ► Dispersal opportunities across the city limited occurrence. ► Grassy undeveloped paddocks may provide habitat for source populations. ► Skink occurrence was positively associated with garden messiness.
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.
Section snippets
Study animal and area
Common skinks are small lizards (maximum snout-to-vent length = 77 mm; Gill & Whitaker, 1996) that typically occupy dry, open grassy sites or low scrubby vegetation, with basking surfaces and refuges from predators (Patterson and Daugherty, 1990, Whitaker et al., 2002). They reach sexual maturity when approximately 20–21 months old, producing 3–5 live young (Barwick, 1959). They are opportunistic in their diet, eating a wide range of insects and spiders, as well as berries (Barwick, 1959, Gill,
Characteristics of variables at occupied and unoccupied sites
Nearly half (n = 75; 46%) of all properties had a paddock next door: of these 85% (n = 64) had skinks, while only 30% (n = 27) of properties with no adjacent paddock had skinks. A large proportion of properties had no or less than 20% native vegetation (62% of all properties), and only 20 properties (11.3%) ranked above 2. Cats were present in nearly all properties (83%) with neighbours’ cats passing through regularly even when there was a resident cat. Mean values for variables in occupied and
Factors affecting landscape-level distribution of common skinks
The occurrence of common skinks in gardens across the Dunedin urban area was influenced by a combination of factors at different spatial scales. Unlike lizards in woodland landscapes, where variations in habitat quality sometimes overrode effects of habitat fragmentation (Santos et al., 2008), in this urban area variables relating to habitat fragmentation and the potential role of meta-populations in maintaining urban populations (“distance to city edge”, “adjacent paddock”) were more important
Conclusions
Despite the small size of the city and high proportion of green space (gardens cover 36% of the total area, Mathieu et al., 2007), and the small space requirements of skinks, they were largely absent from the central suburbs and were patchily distributed in less highly ordered gardens located mainly around the edges of the city. Small generalist and adaptable species that are very abundant in highly modified rural landscapes might be expected to thrive in garden habitat in urban environments,
Acknowledgements
Thomas Mattern, Amber Smyth, Jolene Oldman and Rosie Palmer assisted in data collection, Deanna Clements provided random locations, and Brian Niven statistical advice. We thank the Dunedin Botanic Gardens for allowing access, Marieke Lettink for advice on lizard behaviour and ACO construction, Amy Adams for providing the map, Philip Seddon for critically reading the manuscript, and the householders who provided access to their gardens. The study was supported by PBRF funding to the first author.
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