crossed the of water eastward to Brooks Island and were reproducing there. Within two years, they had spread across the entire island from west to east and had apparently exterminated the entire population of house mice. The biologists believed this occurred because the stress of competition caused the house mice to stop reproducing. Today, voles are present throughout Brooks Island, but most notably in the grassland habitat, where the ground is riddled with burrows. The population has a very high frequency of buff fur color, thought to be a founder effect due to the small original population. Brooks Island is a habitat for harbor seals that haul out at the island en route from their nearby rookery on the Castro Rocks. Reptiles, amphibians and fish When the sea level rose after the Ice Age, populations of salamanders and garter snakes were marooned on Brooks Island, where they persist today. There are three species of amphibians on the island. The slender salamander (Batrachoseps attenuatus) is found in the north and east of the island, where the soil is damp most of the year. The arboreal salamander (Aneides lugubris) is represented by a small population around the pond in the former quarry on the southern shore. The Sierran chorus frog (Pseudacris sierra) has also been reported from Brooks Island, but it is unclear whether there is a breeding population. Two color phases of the western terrestrial garter snake (Thamnophis elegans), one with a red background the other with a green background, are present on Brooks Island. They're believed to prey on the northern alligator lizard (Elgaria coerulea) which lives around the island's ponds, as well as tree frogs and the ponds' population of introduced mosquitofish (Gambusia affinis). On one occasion a western pond turtle (Actinemys marmorata) was recorded. Birds More than 100 species of birds have been recorded on Brooks Island, and 18 species nest there. Caspian terns The island hosts the San Francisco Bay’s largest nesting colony of Caspian terns. The terns are relatively recent arrivals, first recorded in the south bay in 1922 and nesting on Brooks Island since around 1980. They now occupy much of the man-made sandspit that stretches west from the north side of the island. The terns feed on fish from areas around the Golden Gate and San Pablo Bay. However, the tern population has been decreasing, apparently due to reduced nesting area. The sandspit is