Grazing, Fire, and the Management of Vegetation on Santa Catalina
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Grazing, Fire, and the Management of Abstract: 19th century ground photos and aerial Vegetation on Santa Catalina Island, photographs reveal that chaparral and coastal sage scrub on Santa Catalina Island resembled California1 modern stands on mainland coastal areas ca. 150 years ago, but were converted into an open herbaceous savanna due to overgrazing by feral goats, pigs, and other exotic fauna. This Richard A. Minnich2 transformation has limited fires due to con- tinuous removal of fuels. Browsing has also modified the floristic composition of brush- lands. Removal of feral herbivores will result in rapid vegetal recovery and increasing fire danger. Santa Catalina Island, located 70 km south of 1944 and 1976 comprehensive vertical aerial Los Angeles, California has experienced a limit- photography. From these data I speculate on ed history of fires during the 20th century. past vegetation change, future fire potential, This is due primarily to continuous grazing and and management of brushland fuels if feral an- denudation of chaparral, coastal sage scrub and imals are removed. herbaceous fuels over the past 150 years by feral goats, pigs, and other exotic fauna. This trend is an extraordinary anomaly in southern GRAZING HISTORY California's mediterranean climate where fire is an annual possibility in most wildlands. Un- Grazing became important on Santa Catalina fortunately, the present "solution" to the fire Island soon after attainment of statehood problem is accompanied by a multitude of other (1850). Commercial sheep and cattle grazing problems. These include island wide erosion, continued on Santa Catalina until the early slope failure, destruction of indigenous wild- 1950's when most of the domestic stock was re- 4 life, and decline or even extinction of numerous moved. The goat is thought to have been plant species, some of which are or were island introduced to Santa Catalina during the late endemics (Thorne 1967). Because of the devas- years of Mexican occupation (1820-1840, Coblentz tation of the island, The Center for Natural 1976). Goats slowly became feral and were left Areas3, under contract with the Los Angeles to multiply at will in the absence of indigenous County Department of Parks and Recreation, has predatory animals. Present populations, number- prepared a Natural Resources Management Plan ing about 5000, are most abundant in three (CNA 1980) on which this study is based. One areas: the Silver-Grand Canyon drainage system, recommendation of the report is the removal of Gilbralter Rock on the north coast, and on the feral herbivores, especially goats and pigs. West End. These were excluded by fencing from Such a plan if implemented will result in interior drainages east of the isthmus in the increasing fire danger due to the accumulation 1950's. Goats prefer most herbaceous plants. of fuels in recovering vegetation. Most chaparral and coastal sage species except for Rhus integrifolia, Malosma laurina, and This study documents a vegetation history of Salvia spp. are browsed during the summer and Santa Catalina Island by investigating the na- fall when herbaceous cover is desiccated ture of pregrazing vegetation on 19th century (Coblentz 1980). Pigs were introduced from ground photographs; present vegetation as mapped Santa Rosa Island in the 1930's. They most com- from 1976 color infrared aerial photography; monly occur in deep soiled areas with dense long-term vegetation change as seen from retakes chaparral cover and which are free of goats of old photographs, and comparative analysis of (Coblentz 1980). Although the pig is a grazer and browser, its greatest impact on vegetation results from its fruit—eating habits, the thoroughness of which inhibits the reproduction 1Presented at the Symposium on Dynamics and of many woody species. Management of Mediterranean-type Ecosystems, June 22-26, 1981, San Diego, California. FIRE HISTORY 2Assistant Professor of Geography, Uni- versity of California, Riverside, Calif. Although Santa Catalina Island may be more exposed to maritime influences than mountain 3South Gardiner, Maine ranges on the southern California mainland, it is still subject to summer drought, desiccation 4Personal communication from A. Douglas of vegetation, and high fire potential. In Probst, Santa Catalina Island Conservarey spite of these conditions, only a few fires, September, 1979. mostly less than 10 hectares, have occurred in Gen. Tech. Rep. PSW-58. Berkeley, CA: Pacific Southwest Forest and Range 444 Experiment Station, Forest Service, U.S. Department of Agriculture; 1982. recorded times. This trend contrasts sharply European grasses form the major portion of with the fire history of the mainland where the total plant cover. They thrive best in deep thousands of hectares are burned each year, and loamy soils on alluvium, marine terraces, or the accumulative fire frequency over several gentle surfaces, but may even prevail on steep million hectares of coastal sage scrub and chap- rocky slopes. Density and cover varies signifi- arral averages once every 30 to 40 years (Hanes cantly depending on grazing patterns but occurs 1977; Minnich 1978). The most compelling evi- in gradients having little correlation with dence for the fire potential on the Channel fencing, owing perhaps to the territorial be- Islands comes from the summer of 1979 when 7,000 havior of goats (Coblentz 1976). ha were burned on nearby San Clemente Island. All but 1500 goats had been removed from San Coastal sage scrub is relatively uncommon Clemente during the middle 1970's. (20 percent of island cover) compared with its abundance in the coastal mountains of mainland southern California (Mooney, 1977). This com- PRESENT VEGETATION munity occurs on steep south and east facing slopes with little or no soil. Contiguous A vegetation map of the island was inter- stands reminiscent of the mainland (0.5-1.5 m preted from 1:22,000 color infrared photography tall; greater than 70 percent cover) are wide- taken by overflight in February, 1976. The map spread in the vicinity of Avalon. In more was digitized according to physiognomic class. heavily grazed areas coastal sage scrub becomes Polygons were then correlated with other digi- increasingly degraded, fragmented, and admixed tized data including elevation, slope, aspect, with Opuntia littoralis. Field observations soil depth, and erosion (Table 1). indicate that Artemisia californica and Erio- gonum giganteum thrive best in areas protected from goats. Salvia apiana and S. mellifera are General Vegetation Features dominant elsewhere. The vegetation resembles most overgrazed Evergreen sclerophyllous scrub or chaparral landscapes of the world (see Mooney and Conrad rarely forms extensive stands except on north 1977). Indigenous woody cover is predominantly facing slopes and in canyons. Only one third a small tree savanna with continuous herbaceous form contiguous cover. The remainder exist in understory, strikingly reminiscent of the oak parklands of mostly tall shrubs with sharp woodlands found in coastal central California browse lines, underlain by annual grassland, (Minnich 1980). The prevailing plant commu- coastal sage scrub or Opuntia. Although chap- nities include annual grassland, coastal sage arral tends to cover steep slopes, it is assoc- scrub, chaparral, and woodlands (see Barbour and iated with deeper soils, less erosion, and more Major, 1977). rapid soil permeability than other communities Table 1--Summary of Vegetation of Santa Catalina Island (total hectares and percent of total island area for each variable) Vegetation Total Elevation (ml) Slope Class (degrees) Slope Aspect Type Area 0-183 184-366 >366 0-10 10.1-30 20.1-30 31.1-40 >40 NN,N NE,E SE,E SW,W Grassland 11912 54 63 72 71 69 61 56 54 63 53 59 73 Coastal Sage Scrub 3383 20 19 12 10 14 21 21 18 6 23 32 14 Chaparral 2756 15 16 10 8 12 15 20 13 27 21 2 6 Urban, Cult, Bare 1183 11 2 6 11 5 3 3 15 4 3 7 7 Erosion Vegetation Depth to Bedrock (m) None- Slope Permeability Type 0-0.6 0.6-1.0 >1.0 Moderate Severe Failure Flooding No Soil Slow Rapid Grassland 58 67 48 59 65 43 43 48 66 58 Coastal Sage Scrub 21 11 7 16 20 7 8 23 15 16 Chaparral 13 17 11 20 8 5 14 9 15 17 Urban, Cult, Bare 8 5 34 5 17 45 35 19 4 9 445 (Table 1). This trend is inconsistent with Pre-Grazing Vegetation mainland vegetation where chaparral tends to occupy the steepest and rockiest slope (Wells, Although goats were introduced perhaps no 1962). This difference may reflect a long term earlier than the 1840's and sheep a decade feedback from overgrazing inasmuch as present later, most old photographs dating to the 1880's deep rooted chaparral stands that have survived show severe overgrazing and even more widespread continuous browse pressure are responsible for vegetation damage than is seen at present (Min- the maintenance of deep soils. nich 1980). Grass cover is clipped or stripped, Opuntia is widespread, coastal sage scrub is The floristic and age composition of chapar- nearly absent, and browse-lined chaparral shrubs ral varies with slope exposure. Rhus integri- are widely scattered. A few very early photos folia and Malosma laurina, tend to form open may give a glimpse of the nature of pregrazing stands on south facing slopes. Juveniles are vegetation. A photo of Mt. Black Jack taken often seen side by side with old individuals cir. 1885, for example, shows a continuous stand within a single stand. On north facing slopes, of Adenostoma fasciculatum, with embedded chaparral is dominated primarily by Quercus Quercus dumosa, Rhus integrifolia, and Malosma dumosa and Rhus integrifolia. Individuals are laurina (Minnich 1980). Photographs of the same typically old, robust, shrubs 5-10 m tall with site in 1900 and 1980 show that Adenostoma had d.b.h. of 30-50 cm.