Ecological dynamics

The reference state for the ecological site is a native grassland community, which is characterized by open, clumped bunches of native perennial bunchgrasses, including purple needlegrass (Nassella pulchra) and foothill needlegrass (Nassella lepida). Historically, the total canopy cover in the native grasslands was most likely between 50 to 70 percent (Dudek, 2000 p.55), with native annual and perennial forbs occupying the open areas. In the past there was probably a higher diversity of perennial grasses and native forbs than is seen today. In many areas, non-native grasses and forbs have replaced the native species.

This community has been severely impacted and altered since Anglo-European settlement. Undisturbed native grasslands are hard to find, and in their place several different plant communities have established. These communities can be considered transitional states, because in most cases they can convert back to the historical state. The most widespread altered state is the non-native annual grassland state. The other states include the invasive non-native forbs and the coastal sagebrush state. Several factors have promoted these transitions, primarily the introduction of non-native Mediterranean species and the severe long-term over-grazing by livestock and feral animals. This, in turn, has led to a change in the natural fire regime. Currently there is a debate concerning whether the non-native annual grassland should be treated as a state within the native grassland community, or if it has crossed a threshold to become its own new plant community. For this ecological site description it will be treated as a state since the native bunchgrasses are still present and reproducing well, thus showing potential for recovery (Corbin and D'Antonio, 2004).

In the context of this description, the area included as valley and foothill grassland is strongly tied to soils with high clay content, most of which have high shrink-swell potential (vertisols). These types of soils have been repeatedly documented as favoring native bunchgrasses in southern California (Knecht, 1971). The non-native forbs of the site include invasive weeds such as yellow star-thistle (Centaurea solstitialis), black mustard (Brassica nigra), and fennel (Foeniculum spp.). These Mediterranean species are extremely difficult to eliminate. Of these weeds, yellow star-thistle has the greatest ability to alter the soil’s water recharge and depletion pattern within the grasslands (Enloe et al., 2003). Yellow star-thistle has been shown to have a drier soil profile than that of either the non-native annual grasses or the native perennial grasses. It continues to deplete the soil’s water later into the season, and to greater depths than either of the grass communities studied. This can cause a drought-like condition for the grasses even in a normal water year.

In other areas, the non-native annual grasslands are in a state of transition with the coastal sagebrush community. The non-native grassland is composed of slender oat (Avena barbata), wild oat (Avena fatua), ripgut grass (Bromus diandrus), soft brome (Bromus hordeaceus), Spanish brome (Bromus madritensis), stork’s bill (Erodium), common barley (Hordeum vulgare), Darnel ryegrass (Lolium temulentum), and a variety of other native and non-native species.

The coastal sagebrush community can invade native grasslands when the disturbance from grazing and fire is removed. Invasion of coastal sagebrush into the native grasslands is uncommon and different from the recovery of coastal sagebrush in its historical territory. The latter example is described in the coastal sagebrush ecological sites, R020XI113CA and R020XI100CA.

During the mid 1800s and the early 1900s the Channel Islands were heavily impacted by grazing from sheep, goats, cattle, horses and pigs. During this time, many acres of the islands were also cultivated for various crops, with hay as the primary production. In 1922, 800 acres of hay were cultivated at Christi Ranch, Scorpion Ranch, and near Prisoners Harbor (Junak et al., 1995). 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 them. 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)

The heavy grazing by the livestock eventually caused the death of many natural shrubs, grasses and forbs on the island. The livestock also ate the flowers and seeds, reducing the chance of reproduction and causing a lack of leaf area to support photosynthesis. The lack of vegetative cover and the trampling of hooves caused severe erosion over most of the island. With high winds and seasonal rains, much of the nutrient rich topsoil was lost and replaced with shallower soils and harsh subsurface soils. The sheep and goats were perhaps the most devastating of the introduced livestock. These animals grazed everything down to the soil level even eating some of the roots. In hot summers, the remaining grass roots were exposed and quickly died in the heat. This reduced vegetative cover and the ability of the plants to stabilize the soil. Since the removal of the herbivores, there have been many positive signs of recovery in the grasslands. The active erosion has decreased and overall plant cover has increased. Mostly non-native grasses have recovered, but areas of native grassland also seem to be in good health.

While there are areas where vegetation is recovering, the continued up-rooting of the vegetation by the pigs still causes significant damage to large areas across Santa Cruz Island. The pigs dig through the soil looking for roots, acorn seeds, bulbs, and young shoots of plants. This continual disturbance creates bare soil and favors the non-native annual grasses and forbs (Cushman et al., 2004). A pattern has been noticed that seems to suggest that the pigs will avoid areas of dense perennial grasses. Entire areas around native grassland patches tend to be uprooted, but the areas inside the dense perennial grasses are likely to be left alone (Dresser, 2004). It could be that the dense root masses are more work for the pigs to get through.

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 increases in grass pollen of more than double any period recorded in the prior 5,000 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)

Historical data on the natural fire regime for this area is lacking. Natural, lightning-initiated fires are infrequent on the Channel Islands, which is similar to the southern California mainland. The introduction of non-native grasses, grazing, and intentional fires have changed the spatial distribution of the plant communities, in turn altering the natural fire regime. Fires in Southern California naturally occur and spread in summer and fall when the grasses are dormant. The fuels in the native grasslands were discontinuous and patchy. Fire frequencies may have increased since settlement due to the increase in fuel loads, the continuity of the annual grasses, and the addition of human caused fires (Klinger and Messer, 2000).

The use of fire by Native Americans and its affect on the historical landscape mosaic are unclear (Keeley, 2002). The Chumash Indians lived on and visited many of the Channel Islands. Records show habitation for more than 6,000 years, and estimate about 2,000 people living on Santa Cruz Island in 1542. It is believed that fire was used to clear shrublands in favor of grasslands in the coastal mountains of California (Keeley, 2002). It is likely this practice was used to some extent on the Channel Islands as well.

The competition from non-native annual grasses can be reduced with early spring fire, which will kill the seed crops. Both native and annual grasses are affected, but the natives are more likely to resprout. However, spring burning would also affect the desired native annual seeds (Keeley, 2003). Native grassland species are relatively fire adapted, due to the ability to resprout after fire. One study reports increased vigor growth and higher germination in purple needlegrass (Nassella pulchra) seeds after fire (Dyer, 2002). However, the overall effects of using fire for restoration and in changing species composition are not clear, some say it is favorable for native grasslands, while others say that it is detrimental, especially if the fires are too frequent. Native bunchgrasses must be present within the grassland or seedlings need to be planted for the restoration efforts to work.