Recovery Plan for the Northern Spotted Owl - DRAFT

Biology of Owl Prey, Older Forest Associates,and Riparian EcosystemsUnique Food Webs of Older Forest EcosystemsTemperate coniferous forests of the Pacific Northwest are unique among theforests of the world for a number of reasons (Waring and Franklin 1979), andthe structural and functional diversity of these forests provides habitat for adiverse array of plants and animals. Of particular interest is the high speciesrichness of birds, mammals, amphibians, molluscs, arthropods, and plantsthat characterize the complex food webs in these forests. Among the vertebrateand invertebrate fauna, there are numerous species of granivores, detritivores,folivores, herbivores, fungivores, and a diverse group of aquatic species includingfishes, amphibians, and arthropods (Harris 1984:52). The mammalianorder of insectivores (shrews and moles) is represented in the Pacific Northwestforests by more species than in any other temperate or boreal forest in NorthAmerica. Two of the most important food webs in these coniferous forestsinclude foliage and fungi eating mammals (referred to as folivores andfungivores, respectively) that are prey for the northern spotted owl.Two folivores that inhabit coniferous forests in the Pacific Northwest belong tothe genus Phenacomys (Arborimus), which is restricted to western Oregon andnorthwest California (Figure D. la). The red tree voles (Phenacomyslongicaudus, P. porno) are particularly unusual among rodents in that they livealmost their entire life in the canopy of Douglas-fir trees. These species areassociated with old-growth Douglas-fir forests (Aubry et al. 1991, Ruggiero etal. 199 lb), although they do occur in lower numbers in forests of other treespecies and younger successional stages. The diet of the P. longicaudus isalmost entirely needles of Douglas-fir (Maser et al. 1981). In turn, the red treevole is one of the primary prey species of the northern spotted owl in theCascade Mountains of Oregon (Forsman et al. 1984). The third species in thisgenus is the white-footed vole (Phenacomys albipes), an extremely rare speciesthat is not as strongly restricted to the forest canopy (Figure D.l.b). Because ofits rarity, much less is known about this species. For example, of approximately10,000 individual small mammals and amphibians trapped with pitfalltraps in the Oregon Coast Range, only 60 were white-footed voles (Gomez 1992).He found this species to be more abundant in deciduous forests than elsewhere,and it was associated with riparian areas where deciduous vegetationwas more abundant. Voth et al. (1983) suggested that the species' diet isrestricted to foliage of deciduous plants.A second important food web in coniferous forests in the Pacific Northwestinvolves fungivores. Although a number of species eat fungi, the western redbackedvole and the northern flying squirrel are most notable within thisgroup. The northern flying squirrel is the primary prey species for the northernspotted owl in Washington and most of Oregon. The red-backed vole isamong the top five prey species (Forsman et al. 1984, Thomas et al. 1990) andis endemic to the Pacific Northwest (Figure D.5.a). Old-growth forests areoptimal habitat for the flying squirrel and the red-backed vole. The red-backedvole spends most of its time in underground burrows and feeds on the fruitingbodies of hypogeous fungi (Maser et al. 1978a). Many mammals consumefungi, but few depend on this food source to the extent that the red-backedvole does. Gashwiler (1959, 1970) and Goertz (1964) reported a decrease orelimination of this species following clear-cutting. The decrease is believed tohave resulted from the disappearance of hypogeous fungi after this form of366