. Bulletin. Science. •• ••• 1 1 1 1 1 1 1 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year Fig. 2. Common Ravens observed on the "Joshua Tree" BBS Route between 1970 and 2000, and on the "Cottonwood" BBS Route between 1973 and 1997. dardized survey methods. The increases in numbers observed may be correlated with the fact that the route is adjacent to, and travels through, the town of Joshua Tree, which has increased in human population size in the last 30 years; between 1970 and 1990, Joshua Tree's population more than tripled from 1,211 to 3,898 persons (California State
. Bulletin. Science. •• ••• 1 1 1 1 1 1 1 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year Fig. 2. Common Ravens observed on the "Joshua Tree" BBS Route between 1970 and 2000, and on the "Cottonwood" BBS Route between 1973 and 1997. dardized survey methods. The increases in numbers observed may be correlated with the fact that the route is adjacent to, and travels through, the town of Joshua Tree, which has increased in human population size in the last 30 years; between 1970 and 1990, Joshua Tree's population more than tripled from 1,211 to 3,898 persons (California State Department of Finance 2000). A second BBS route ("Cottonwood") runs from the park's south entrance to approximately its center. Relatively few ravens have been observed on this route (range 0-7) despite its having been surveyed in 20 different years, and in contrast to the "Joshua Tree" route, the small numbers have decreased since the early 1990's (Fig. 2). On the "Cholla Garden" route, which runs roughly east-west in the central portion of the park, the number of ravens observed has ranged from seven in 1996 to 21 in 1995; no increase has been detected. BBS surveys occur strictly along paved roads, and because ravens tend to be attracted to roads (Knight and Kawashima 1993, Boarman and Heinrich 1999), BBS routes potentially overestimate the abundance of ravens in an area. Further- more, the results of surveys like the BBS that are performed only once each year should be interpreted conservatively. Despite these caveats, increases in the num- bers of ravens in the 1990's on the "Joshua Tree" route well above numbers from the 1970's and 1980's likely reflect actual increases in raven density in this area. Like the BBS, the Adopt-a-Raven Transects were vehicular surveys. Counts were made at roughly km intervals on five different routes in JTNP and one adjacent to it, ranging from twice to 35 times each. The route lengths are
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