Recovery Plan for the Northern Spotted Owl - DRAFT

forests. Alternatively, habitat selection by each sex may be the result ofreversed sexual dimorphism, which may have evolved for other reasons besidesfood competition (Muller 1986).Competition for resources can occur between different species, this is commonlycalled interspecific competition. The use of any finite resource in onearea by more than one species can result in competition, if the depletion of theresource by one species negatively affects another species. Competition iscommonly invoked as a selective mechanism for the evolution of niche partitioning(Cody 1974). For example, the relative differences in body size ofmembers of the Pacific Northwest owl community may be an expression of pastcompetition that led to the evolution of differences in body size and foragingstrategies that minimize diet or habitat overlap. Alternatively, the owl communitystructure simply may be an expression of adaptive radiation (adapting toregional environmental conditions) at some time in the past (Wiens 1989).Nevertheless, competition can be a serious problem for a species when anexotic (nonnative) animal of similar body size and ecological requirementsinvades its habitat. The recent invasion of the barred owl into the range of thespotted owl (Taylor and Forsman 1976) is an example of potential competitionbetween closely related species. Barred owls are larger and more aggressivethan spotted owls in interspecific territorial interactions. They also feed on abroader range of prey, occupy a wider range of habitats, and have smallerannual home ranges than do spotted owls (Hamer 1988). Further, they areknown to have to have displaced spotted owls from their territories (Allen pers.comm.). Thus, barred owls are a competitive threat to spotted owls.Hybridization: At least three spotted owl/barred owl hybrids have beenobserved in the wild (Forsman pers. comm.). It is common in nature for closelyrelated species to hybridize, especially where habitat disruption has occurred(Short 1965, Johnsgard 1970, Mayr and Short 1970, Short 1972). Vincent(1990) expressed concern about the recent invasion of barred owls and thepotential effect of hybridization on the integrity of the spotted owl as a species.Several biological outcomes are possible given the rapid expansion of barredowls into the range of the spotted owl. First, the barred owl could, throughextensive hybridization, genetically "swamp" the spotted owl. Second, a"hybrid swarm" could develop in specific areas of contact. Third, selectioncould act against hybrids, thus favoring development of effective isolatingmechanisms. Fourth, low levels of hybridization could occur continuouslywithout loss of the identity of either species. Fifth, hybridization could be arandom event. In only the first case is the genetic integrity of the spotted owlseriously challenged. However, in declining populations any loss of spotted owlreproductive capacity to hybridization must be considered a real threat, butprimarily because of its effect on the short-term demography of the species.Predation: Another form of interspecific interaction is predation (the killing ofone organism by another for food). As a medium-sized owl, the spotted owlkills and eats smaller owls. Therefore, it is not surprising that the larger greathomed owl (Bubo virginianus) kills and eats spotted owls. This is called a foodchain. Predation by great horned owls on spotted owls is a potential hypothesisto explain spotted owl use of old-growth forests, or to explain spotted owlavoidance of open habitats (Forsman et al. 1984). These two species commonlyshare the same habitats, but great horned owls tend to occupy sites thatare more fragmented and open than those used by spotted owls (Johnson pers.comm.), perhaps because their large size makes them less maneuverable indense forest. There is no current test of this hypothesis (i.e., relative predationrates by great homed owls on spotted owls using habitats with differentstructure). However, great homed owls probably prey on spotted owls opportunisticallyrather than seeking spotted owls as prey (Forsman pers. comm.).