Natural History

California State Parks has a great responsibility for preserving and restoring AISP’s natural resources and environment. Initiatives have included restoring the historic profile of the island after the Army removed the top of Mount Caroline Livermore for a Nike Missile launch site, removing non-native, invasive species like eucalyptus groves and reintroducing native plants and continuing habitat restoration for sensitive species.

Natural Resources

An island’s isolation creates a different set of circumstances for plants and animals to adapt and live. Some animals, such as the Angel Island Mole (Scapanus latimanus isularis), have over time and in isolation evolved into a different species than its mainland counterpart. In another example, toyon grows as a shrub on the mainland. Lacking any typical constraints on the island, it sometimes grows to the size of a tree. The isolation that helped to create the island’s unique environmental conditions, however, also limits the ability for these species to adapt in the same manner as on the mainland.

Humans have drastically altered and affected AISP’s environment. The introduction of new animals and plants has dramatically changed the island’s landscape and its habitats. Early on, European settlers stripped the island of its trees for fuel, causing erosion and other problems. To mitigate the erosion and provide a windbreak, the Army planted eucalyptus trees. These trees grew fast, but also created a high fire danger. The introduction of deer for hunting, and the Norwegian rat from passing ships, has caused a multitude of problems for native plants and animals. These unintended consequences have severely affected AISP’s delicate environment.

Geology

The formation of San Francisco Bay and its islands is
a relatively recent event. The Bay Area as we know it today was formed by changing sea levels and the rising of mountains within the last 20 million years. Angel Island was formed around the last Ice Age when the ocean was many miles to the west and much lower than it is today. At first, the island was a part of the Marin Peninsula, but as the ancient Sacramento River cut its way through to the ocean, it sliced off what would become Angel Island from the rest of the hills of Marin.

When the ocean level rose after the Ice Age, seawater filled the valley we now call the San Francisco Bay and Angel Island was created. The riverbed exists today as the deep channel that runs through Raccoon Strait (about 200 ft. deep) to the Golden Gate (about 300 ft. deep) and the Pacific Ocean. The shape of the island is roughly pyramidal, with steep ridges extending downward from central Mount Caroline Livermore. Between the ridges are canyons, which fan out into coves at the water’s edge. The peak is about 788 feet above sea level.

Climate

The island’s climate is characterized by moderate yearly temperatures, due to the marine influence. It has dry summers with morning fog, and wet winters with an average rainfall of 25-30 inches. A prevailing westerly wind blows through the Golden Gate and across the island.

Hydrology

The waters of San Francisco Bay surround Angel Island. Fresh water flowing through the Bay comes from a drainage area of about 63,000 square miles, or about forty percent of the surface area of the state. Daily tidal changes amount to five or six feet. Fresh water on the island is available from natural springs and modern wells. During the Spanish and Mexican periods, these springs were a main attraction to passing ships in need of fresh water. Later, the US military drilled wells and brought fresh water to the island via ship.

Biology

The park has a diversity of flora and fauna living in different ecological zones. Several distinct native plant communities have evolved on the island. Such factors as water (fog, rain, springs, and ocean), solar exposure, temperature, and soil conditions have favored growth of typical California coastal communities: grassland, scrub, mixed evergreen forest, chaparral, coastal strand, and riparian.

Native trees such as coast live oak, madrone, and California bay and native shrubs occur frequently on the island. Poison oak, manzanita, chamise, gooseberry, and currant are also common. Wildflowers, many of which are native, bloom extensively in the spring. Some of the more common ones are soap plant, milk-maid, monkey flower, California poppy, lupine, shooting star, and Fremont star lily.

Several native grasses inhabit the island slopes, including purple needlegrass, pine bluegrass, meadow barley, California fescue, California brome, creeping wild rye, and Torrey melic. Some native species have recovered to various degrees since the 1960s. However, two introduced species, eucalyptus and broom, pose a real threat to other vegetation, because of their ability to grow and spread rapidly; acacia may also pose this threat.

The island is home to a diverse community of marine and terrestrial animal species. Robins, scrub jays, hawks, grebes, cormorants, and pelicans are common birds. Sea lions can be found on rocks off Point Blunt and harbor seals can be seen on the rocky shores near Point Ione. Visitors often encounter raccoons among the trees and hollows.

The island contains one truly endemic animal--the Angel Island Mole (Scapanus latimanus insularis). It is a species of special concern and is on the Department of Fish and Game’s watch list. A government report in 1935 and 1936 noted Angel Island supported a large population of moles. Mole mounds and surface ridges were common across the island, particularly on the north side of the island in moist soil under chaparral (Palmer 1937). There is little recent information regarding the present distribution or status of Angel Island moles. Apparently, there has been no further work on this subspecies since it was described in 1937.

The island has a large mule deer (Odocoileus hemionus) population that descend from those the US Army introduced in 1915 for hunting purposes. In the absence of predators, the mule deer population expanded beyond the island’s carrying capacity.

Native plant communities on the island, along with their associated fauna, have been seriously impacted by past human activities, intensive overgrazing by the introduced deer herd, and by the introduction and expansion of non-native trees and shrubs. Introduced conifers and eucalyptus cover more than 100 acres of the island. Mesic woodland habitats on Angel Island, (e.g., mixed evergreen forest, northern coastal scrub, and chaparral) which provide potentially suitable habitat for moles, have been, and continue to be seriously degraded by these factors (Bolster, 1998). Many other introduced plants have also become well adapted; these include ice plant, century plant, pride of madeira, Monterey pine, and Monterey cypress. There is no Douglas fir on the island, the native forest having been replaced primarily by introduced eucalyptus and Monterey pines.

The Spanish, Mexican, and US military use of Angel Island caused significant changes to the landscape. In the early 19th-century passing ships denuded the native woodland for firewood, ship repairs, and building materials. After the arrival of the Spanish, European annual grasses supplanted the native grasses on the island. Remnants of native oak woodland remained in areas with difficult access, such as steep slopes, cliffs, and north-facing hillsides.

Later, military personnel and civilian workers planted gardens for food; eucalyptus trees for windbreaks and erosion control; and decorative landscaping in residential areas and at the US Immigration Station. Since becoming a State Park in the 1950s, park staff have conducted native plant restoration projects. In 1991 and 1996 a total of 80 acres of eucalyptus were removed from wild land areas of the park; these sites have been largely replaced with coastal scrub habitat, with some invasion by non-native Italian thistle. Cultural landscapes have also been maintained where appropriate.

In October 2008, the Angel Fire burned approximately 303 acres (around forty percent) of wild land in the park. Several existing stands of Monterey pine burned in the fire. These sites will undergo oak restoration. None of the park’s eucalyptus stands burned in the 2008 fire; only scattered small eucalyptus that came up after the eucalyptus logging in 1991 and 1996 burned. The native habitats that burned in the Angel Fire have recovered quickly. The fire burned mostly accumulated fuels and understory plants, and provided an opportunity for native plants to regenerate and replenish the native seed bank.