Use of the exotic tree Myrica faya by native and exotic birds in Hawai'i Volcanoes National Park.

The exotic nitrogen-fixing tree Myrica faya is invading Hawai'i Volcanoes National Park (HA Va). Observations of avian use ofM.faya demonstrated that although four species of native birds visited the trees, they rarely fed on the fruits. Seven species of exotic birds were seen visiting M.faya, and five of these were observed ingesting the fruit. The most frequent visitor and consumer was the Japanese White-eye (Zosterops japonicus). Over one-third of the captured Z. japonicus produced fecal samples containing M. faya seeds, and those seeds were as viable as those picked from M. faya trees and beneath their canopies. canopy of other trees and shrubs (Smathers and Gardner 1979). This pattern of establishment reflects M.faya seed rain; from 2.5 to 4.8 seeds per month have been captured in seed traps (1 m") under the native tree Metrosideros polymorpha, while none was captured in the open (Vitousek and Walker 1989). This pattern of seed rain is characteristic of plants in which seeds are dispersed by perching birds (Glyphis et al. 1981, McDonnell and Stiles 1983, McDonnell 1986). The fruit of M. faya is a fleshy drupe (Gardner 1985) believed to be bird-dispersed in its native habitat. Smathers and Gardner (1979) speculated that the exotic Japanese White-eye (Zost erops japonicus) is the major agent of dispersal in HAva because of its abundance and its broad diet , which includes fruit, foliage insects, and M .polymorphanectar. However, LaRosa et al. (1985) combined experiments on caged birds with 86 hr of observation on avian use of M.faya and concluded that although Z.japonicus is a frequent visitor to M .faya, it rarely consumes the fruit. They suggested that the exotic House Finch (Carpodacus mexicanus) and the native 'Oma'o (Myadestes obscurus) -are-moreimportant agents of M.faya dispersal in HAva. We observed avian visits to M. faya, seed consumption, and presence and germination potential of seeds in the feces ofcaptured birds across a range of sites colonized by M. faya. 88 1 Manu script accepted I May 1989. 2 Department of Biological Sciences, Stanford University, Stanford, California 94305. 3 Ecosystem Science and Technology Branch, NASAAmes Research Center, Moffett Field, California 94035. THE SYMBIOTIC nitrogen-fixing tree Myrica fa ya Ait. (Myricaceae) was introduced to Hawai'i from the Canary Islands or Azores late in the nineteenth century. Colonization of M.faya is favored by human disturbance, as is true of most exotic species in Hawai'i and elsewhere (Allan 1936, Egler 1942, Orians 1986, Crawley 1987). However, M.faya also colonizes and dominates otherwise undisturbed primary successional sites and seasonal submontane forests in Hawai'i Volcanoes National Park (HAVa) (Whiteaker and Gardner 1985). Within HAvo, M . faya is invading strongly nitrogen-deficient sites that previously lacked a symbiotic nitrogen-fixing plant. Colonization by M.faya significantly increases ecosystem-level inputs and biological availability of nitrogen (Vitousek et al. 1987, Vitousek and Walker 1989); it therefore represents a more serious threat to native biota and natural successional patterns than invasions that do not alter ecosystem-level properties. In HAVO , M.faya establishes primarily in open-canopied areas in microsites under the Use of Myricafaya by Native and Exotic Birds-WooDWARD ET AL. 89