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Island Night Lizard Population Ecology on the Channel Islands

 
Island Night Lizard --Photographer: Gary Fellers

    The island night lizard (Xantusia riversiana) is a medium-sized lizard (adults 70-100 mm body length).  It is endemic to the Channel Islands off the coast of southern California where it occurs on Santa Barbara, San Nicolas and San Clemente Islands.  The island night lizard was listed by the Fish and Wildlife Service as a threatened species in 1977 because of their restricted range, and apparently low population levels on two of the islands.  Population levels were believed to be relatively high on San Clemente Island, but night lizard populations were described as being " . . . critically reduced on San Nicolas and Santa Barbara Islands due to habitat alterations caused by farming, fire, grazing by introduced animals, and invasion by exotic plants which have occurred on one or both islands". 

Extensive research on the Santa Barbara population of island night lizards by Fellers and Drost has shown that this lizard is extremely secretive and sedentary, but not rare within its preferred habitat.  We are now conducting similar studies on San Nicolas Island to evaluate habitat preference, movements, longevity, and population size. 

Project Details

    Island night lizards (Xantusia riversiana) are found only on the Channel Islands, off the coast of southern California.  The lizards occur on three of the eight Channel Islands: Santa Barbara, San Nicolas and San Clemente Islands.  Compared to the island fox, Channel Islands slender salamander, and other Channel Islands endemics, the island night lizard is notably different from its nearest mainland relatives.  In fact, island night lizards are the most morphologically distinct of the endemic vertebrates on the Channel Islands, indicating they have been isolated on the islands for a long time.  In part because they are listed by the Fish and Wildlife Service as a threatened species, there has been interest in the ecology and population status of this secretive lizard.

Work on Santa Barbara Island by Fellers and Drost (1991) showed that island night lizards are not nocturnal as suggested by their name.  They are most active at midday, with little activity in the cool early morning or late afternoon, and none at all at night.  Seasonal activity peaks in the spring when mating takes place, then continues at a lower level through the summer and fall.  Island night lizards breed in April, and 2 - 9 young are born fully developed in September.  Giving birth to live young (as opposed to laying eggs) is not common among reptiles.  Young of the year are active throughout the year, while adults are active primarily during the spring and summer.  The best habitats for island night lizards are boxthorn (Lycium californicum), prickly pear cactus (Opuntia oricola and O. littoralis), and rock with fissures.  Boxthorn is a low-growing shrub with branches that are well armed with sharp, thorn-like tips.  In areas where boxthorn forms extensive thickets, it provides excellent protection from predatory birds.  Lizards commonly eat boxthorn fruits, flowers and leaves, and hence the shrub provides both shelter and food. Prickly pear cactus also grows in dense stands in some areas on the island.  In many prickly pear stands, dead cactus pads fall to the ground and provide abundant cover for both lizards and their invertebrate prey.  The stout branches, pads and numerous spines also maintain a relatively constant temperature and provide formidable protection from predators.  Cracks and crevices in and around rock outcrops and surface boulders apparently provide favorable conditions similar to those afforded by boxthorn and prickly pear.  Suitable rock habitat is present in all of the canyons and on some of the sea cliffs, especially on the south side of Signal Peak. 

Island night lizards are remarkably sedentary and have very small home ranges.  The average size measured on Santa Barbara Island was 17.2 m2 (= 20.5 sq yards).  Similarly sized mainland lizards typically have home ranges of several hundred square meters.  Island night lizards use fissures in the soil, crevices in rocks and dense vegetation [e.g. prickly pear pads and Australian saltbush (Atriplex semibaccata) mats] as nighttime retreats.  Boxthorn and prickly pear are strongly preferred habitats.  If either of these plants is present within a lizard's home range, the lizard will spend most of its time there, to the near-exclusion of other habitats such as grassland.

Island night lizards are slow growing and long-lived with some individuals reaching at least 25 years of age.  Night lizards also have a low reproductive rate compared with other lizards.  Only about half of the adult females bear young in a given year and a female may not have her first brood until she is nearly five years old. 

During the course of their long lives, island night lizards accumulate a lot of injuries including regenerated tails, scale injuries (including missing, scarred or otherwise damaged scales), and miscellaneous cuts, missing toes, eye injuries, infections and the presence of cactus spines.  All larger lizards have some injuries, while only about 1/3 of the smaller lizards do.  Several factors contribute to the high incidence of injuries.  Because night lizards are long-lived and slow growing, large individuals will have accumulated injuries for 20 years or more.  In addition, recovery from some injuries can be slow, perhaps related to the lizards' exceedingly low metabolic and growth rates.  The most common injuries apparently result from fighting with other lizards. 

Both the color and pattern on the back of adult island night lizards are highly variable.  Though these patterns grade into one another, there are several distinct types of which the mottled pattern is the most common (44 % of the lizards).  Less frequent patterns include plain, blotched, striped, broken stripes, ocellated, and finely mottled.  The colors seen in island night lizards match the color of the shrubs, lichen-covered rocks and ground which make up their habitat and appear to provide good camouflage.  Such cryptic coloration is often associated with the sort of sedentary lifestyle that the lizards have.  However, the variety of colors and patterns present among the lizards is unusual.  Few other terrestrial vertebrates show the marked color and pattern differences seen in island night lizards.  In those that do, it is generally seen as geographic variation over a comparatively large range. 

Island night lizards feed on a wide variety of terrestrial spiders and insects, but they also eat an unusually large amount of plant material for a small lizard.  Common animal food items include ground-dwelling spiders, pillbugs, a ground beetle (Amara sp.), moth larvae, and ants.  Common plant food items include fruits of Australian saltbush, flowers, fruits and leaves of boxthorn, fruits of iceplant, and composite (Asteraceae) flower parts.  In turn, island night lizards are eaten by several predatory birds.  On Santa Barbara Island, we found remains of night lizards in pellets of barn owls and burrowing owls and we saw kestrels catching and eating lizards on several occasions.  Barn owls may be the most important lizard predators on the island: a sample of 422 Barn owl pellets contained remains of 113 night lizards. 

The densities of island night lizards in the boxthorn habitat on Santa Barbara Island are greater than those of any other ground-dwelling lizard that has been studied.  We found island night lizards in densities of 1,300 lizards/acre in boxthorn and 1,000 lizards/acre in prickly pear.  Most mainland lizards have densities of 10 - 100 per acre.  The high density of island night lizards is probably due to several interrelated factors.  With their unusually low metabolic rate, the lizards do not have high energy demands and can live on about half the food that other similar-sized lizards require.  In addition, with their varied diet, they can satisfy this demand with a wide range of both plant and animal foods.  Finally, night lizards are very sedentary.  Their sedentary nature combined with small, overlapping home ranges permits more individuals to pack into a given area.  This unique combination of low metabolism, diverse diet and small home range allows the extraordinary densities achieved by island night lizards. 

In 1977, the U.S. Fish and Wildlife Service listed the island night lizard as a threatened species because of its restricted range and apparently low population levels on two of the islands (SBI and San Nicolas Island).  Favorable night lizard habitat has been reduced because of past farming and grazing, however the wide distribution of the lizards on the island, their exceptionally high densities, and their protection within a National Park all clearly indicate that they are not threatened with extinction, as was previously thought.

More than 900 individually marked island night lizards were captured on San Nicolas Island, California, between 1984 and 2007.  Our data suggest that at least a few lizards are probably more than 20 years old, and one lizard would be 31.5 years old if it grew at an average rate for the population.  Ages of 20 and 30 years seem reasonable given the remarkably slow growth during capture intervals of more than a decade for five of the lizards which we estimated to be 20 or more years old. Like other lizards, island night lizard growth rates vary by size, with larger lizards growing more slowly.  In general, growth rates were somewhat greater on San Nicolas Island (compared with Santa Barbara Island), and this increase was sustained through all of the intermediate size classes.  The higher growth rate may account for the somewhat larger lizards present on San Nicolas Island, although we cannot discount the possibility that night lizards on San Nicolas are merely living longer. 

Objectives

Our objectives are to evaluate habitat preference, movements, longevity, and population size of island night lizards on San Nicolas Island.  Of particular importance to the management of island night lizards were their habitat relationships, recruitment and population age distribution, important food resources, predation, and other sources of mortality.

USGS Contact For This Project
Gary Fellers - Emeritus
gary_fellers@usgs.gov
(415) 464-5185
Pt. Reyes Field Station
Pt. Reyes National Seashore
Pt. Reyes, CA 94956-9799
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