Ecological dynamics

The reference state for this ecological site is the coastal sagebrush community. It is characterized by relatively dense cover of coastal sagebrush (Artemisia californica) with Santa Cruz Island buckwheat (Eriogonum arborescens) and other native shrubs, forbs, and perennial grasses. The historical community has been severely impacted and altered since Anglo-European settlement. Much of the area which historically was coastal sagebrush has been altered to the annual grassland state. Natural states within this community include a coyotebrush (Baccharis pilularis)-dominated state, a native annual state, and a forb state. Several factors have promoted the transition to the annual grassland state, primarily the introduction of non-native Mediterranean species, severe over-grazing of livestock, and a change in the fire regime.

During the mid 1800s and the early 1900s the Channel Islands were heavily impacted by grazing from sheep, goats, cattle, horses, and pigs. Livestock may have been brought to Santa Rosa Island as early as 1805. The first cattle were brought to Santa Cruz Island in 1830 to support 100 exiled Mexican convicts. Ranching began in 1839 with the first private land owner, Andres Castillero. By 1853 the Santa Cruz Island Ranch had a good reputation for its well-bred, healthy Merino sheep. The sheep population steadily increased as they began to roam wild. Their population was estimated to be over 50,000 between 1870 and 1885, and up to 100,000 by 1890. In 1939, as a response to their detrimental effects to the island, 35,000 sheep were rounded up for sale to the mainland and efforts began to eliminate the sheep. An estimated 180,000 sheep were shot during the 1960s and 1970s. In 1987 The Nature Conservancy became the sole owner of the western 90 percent of Santa Cruz Island. They continued to eliminate the sheep, and also began removing cattle from the island. Pigs were reportedly introduced to the island in 1853, and by 1854 were roaming freely. The pigs are currently being eliminated section by section from Santa Cruz Island. (Junak et al., 1995) Many acres were cultivated for various crops, but primarily for hay production. In 1922, 800 acres of hay were cultivated at Christi Ranch, Scorpion Ranch, and near Prisoners Harbor (Junak et al., 1995).

A study of the pollen in soil cores taken from an estuary on Santa Rosa Island reveal dramatic changes since the 1800s (Cole and Liu, 1994). The pollen analysis shows an increase in grass pollen more than double any period recorded in the prior 5000 years. It was suggested this could be due to the introduction of non-native annual grasses. The decline of grass pollen coincides with the introduction of large numbers of grazing livestock in the 1840s. Charcoal fragments also increased at this time, possibly attributed to ranchers burning areas to clear brush or an increased fire potential from the annual grasses. The first stork’s bill (Erodium) pollen was dated to 1850, with a peak in 1894. The non-native stork’s bill thrives on disturbed bare soil. A peak in fungal spores between 1874 and 1894 coincides with the peak in soil erosion and the sheep population. (Cole and Liu, 1994)

The heavy grazing of the livestock on the sagebrush leaves and shoots eventually caused the death of many shrubs, due to a lack of leaf area to support photosynthesis. The livestock also ate the flowers and seeds, reducing the chance of reproduction. The lack of vegetative cover and the trampling of hooves caused severe erosion over most of the island. Much of the nutrient rich topsoil was lost and replaced with shallower soils and harsh subsurface soils. These combined factors reduced the area covered by sagebrush, and enabled the invasion of annual grasses. However, comparison photos from Santa Cruz Island during the years of grazing and the years since show recovery of sagebrush in many areas, particularly on canyon sideslopes (Lyndal L. Laughrin, Ph.D. personal photos).

The natural fire interval for coastal sagebrush on the mainland is about 25 to 30 years (Allen et al,; Keeley and Keeley 1984). Due to the nature of the storms in that area, lightning initiated fires tend to be less frequent on the islands than on the mainland. Some studies suggest that if fire frequency becomes less than 5 to 10 years, the sagebrush shrubs will not be able to regenerate and annual grasses will dominate (Minnich and Scott). The non-native grasses have created a continuous fuel layer that did not exist before, which can quickly ignite and spread. The mixture of annual grasses within the sagebrush community has also increased the flammability and severity of fires within the existing sagebrush patches. Fire frequency has increased due to the intentional setting of fires during the ranching years, and because of accidental fires. Fire may be more frequent than in the past, but fire suppression efforts may counterbalance the increased ignition rate.

The use of fire by Native Americans and its affect on the pattern of sagebrush and grassland communities is unclear (Keeley, 2002). The Chumash Indians lived on, and visited many of the Channel Islands. Records show habitation for more than 6000 years, with an estimate of about 2000 people living on Santa Cruz Island in 1542. It is believed that fire was used in the coastal mountains of California to clear shrublands in favor of grasslands (Keeley, 2002). It is likely this practice was used to some extent on the Channel Islands as well.

The coastal sagebrush community is somewhat adapted to fire. Many of the shrub species in this community have the ability to resprout after moderate fires. After fire, annuals and perennials dominate for a couple of years. Shrubs will resprout within the first year and produce prolific seeds the second year. Through re-sprouting and seeding, the shrubs will begin to re-establish in the area and should regain dominance by the third year.

A study was done concerning the suppression of sagebrush by annual grasses (Eliason and Allen, 1997). It stated that the coastal sagebrush had difficulty competing with the non-native grasses during germination and the first season of growth. The most limiting resource was most likely water. Annual grasses tend to utilize soil water earlier in the season, leaving the soil depleted for the sagebrush. By the second year the grasses had less affect on the sagebrush. The study notes that succession back to sagebrush may be inhibited by the grasses, but restoration by planting young plants or by the removal of the grasses then reseeding, may be successful. The main problem when restoring coastal sagebrush shrubland is the issue of how to regain the biodiversity in the area. (Allen et al.)

Coyotebrush is often the first shrub to re-establish in the non-native annual grasslands. It does best in full sun on bare soil, such as eroded areas or sand dunes. Coyotebrush can develop into large patches and persist for many years. It tends to shade out the annual grasses, creating a suitable habitat for sagebrush seedling development. The sagebrush can eventually dominate because the coyotebrush does not regenerate well under the canopy of sagebrush (Steinburg, 2002).