Island Night Lizard

Xantusia riversiana

Description

The island night lizard (Xantusia riversiana ) is a medium-sized lizard, but it is one of the largest members of its family. Adults range in length from 2.5-4.5 in (6.4-11.4 cm) from snout to vent (anal opening). Coloration is spotted gray to brown above with sooty stripes. The closest relatives of this species on the California mainland are X. henshawi and X. vigilis. The island night lizard has been described in literature by some herpetologists as Klauberina riversiana.

Behavior

The island night lizard is a live-bearer. When compared with other lizards, the reproductive potential for this species is quite low. Mating begins in March, and gestation proceeds through the summer months, with the appearance of young in September. As of the mid-1990s the sex ratio of the island night lizard population was 50:50, but only about half of the adult female population was reproductively active in a given year. Furthermore, X. riversiana does not attain sexual maturity until the third or fourth year of life, an extraordinary delay for a lizard of its size. With such a low capacity for reproductive output, the island night lizard cannot support high rates of predation.

The species represents an extreme pattern of lizard life-history characteristics, including slow growth, low reproductive effort, late maturation, long life span, and low predation. These life-history features magnify the importance of the potential of predation on lizard populations. Secretive habits afford a measure of protection; however, the potential for exotic predators to overwhelm the reproductive capacity of the island night lizard poses a very real threat to the species. This reclusive lizard seeks cover beneath rocks or in burrows to protect itself from predators and to regulate its body temperature. It limits its activity to times and places of optimal air temperature. The island night lizard cannot tolerate temperatures above 90°F (32.2°C) and retreats from extreme ground surface temperatures on hot days, usually to areas with dense groundcover.

Habitat

The preferred habitat for the night lizard appears to be areas of thick, low-lying vegetation growing on rocky soil. They seem especially abundant in older patches of prickly-pear where dead pads have accumulated on the ground. North of Seal Cove, the habitat provides a continuous mixture of low, dense vegetation and rock outcrops. South of Seal Cove, maritime desert scrub likely supports a large population. Where shafts of sunlight penetrate to the ground through the vegetation, a checkerboard of small areas with differing temperatures is created. Lizards move about to select a proper thermal environment and find food with minimal risk of exposing themselves to predators. Dominant plants in the habitat include thick patches of cacti, matted thickets of box thorn, and thickets of non-native Australian saltbush.

Distribution

Endemic to the California Channel Islands, the island night lizard is thought to have diverged from its mainland ancestors during the Miocene epoch. This lizard is found in isolated portions of three of the Channel Islands—San Clemente, San Nicolas, and Santa Barbara. When the species was listed in 1987, it was estimated that the total lizard population on San Clemente Island was 800-1,300 individuals per hectare of prime habitat. The San Nicolas Island population was estimated at 14,800. The number of individuals on Santa Barbara Island was estimated to be 550-700 individuals.

Threats

The Channel Island habitats provide a special set of ecological circumstances for the island night lizard. It is one of three lizard species on San Nicolas Island and the only one on Santa Barbara Island. Predators are few, and maritime climatic conditions relatively stable. These aspects have all been emphasized as contributing to the survival of the island night lizard on the Channel Islands and to the extinction of mainland stock. Under these circumstances, the island night lizard has evolved a life-history strategy emphasizing low reproductive potential and long life span—a pattern that is extremely sensitive to disturbance by habitat destruction and introduction of exotic species.

There is no information on the status of the island night lizard prior to ranching activities and the introduction of feral animals on San Clemente Island. However, with habitat structure as the predominant influence on present distribution, it is possible to deduce the change from past habitat modification on the island. The optimum habitat, maritime desert scrub, is largely the result of climate and soil conditions along the western coast of the island and probably has not been altered to the detriment of the lizards by grazing mammals. Key habitat changes most likely occurred in upland areas on the southern half of San Clemente Island, where grazing and soil erosion have replaced shrub and herbaceous vegetation with grassland, cholla cactus, and bare ground. Rocky areas are now well exposed by the loss of original vegetation and are a deteriorated habitat for the island night lizard. Upland regions lacking rock outcrops or scattered boulders (such as the flat grassland northwest of the mid-island) probably did not support large island night lizard populations even when covered by shrubs. However, rocky areas shaded by shrubs would be a superior structural habitat for the lizards, and a diversity of shrub and herbaceous species may also serve as a source of food. A reasonable hypothesis is that the most extensive deterioration of the island night lizard habitat has occurred with the vegetation changes on rocky upland areas of the southern half of the island. Because its habitat is widely dispersed on the island, and because the lizard occurs in small numbers in many isolated areas, the entire island of San Clemente has been designated as critical habitat for the species.

A variety of mammals and birds actually or potentially prey on the island night lizard. Native predators include the common raven, the American kestrel, the burrowing owl, the San Clemente loggerhead shrike, and the island fox; exotic predators are feral cats and possibly rats.

Conservation and Recovery

Removing feral cats has been an important step in this species' recovery.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
http://pacific.fws.gov/

Reference

Goldberg, S.R., and R.L. Bezy. 1974. "Reproduction in the Island Night Lizard, Xantusia riversiana. " Herpetologica 30: 350-360.