12. The Importance oLPlant-Bird Mutualisms
Susceptibility to Disturbance
261
Although its geographical range extends into the subtropical woodland of
Paraguay and northern Argentina, it is likely that the population numbers
of the migrant frugivore Elaenia albiceps .are regulated in the temperate
forests of southern South America, where this species breeds (Vuilleumier,
1985). Summer populations in Chiloe consist largely of juveniles (Sabag,
1993). Recent expansion of the timber and wood-chip industry in Chile
has resulted in a rapid decrease in the area of natural forest and an
increase in the area planted with exotic species, particularly Pinus radiata
and Eucalyptus spp. (INFOR, 1991). Forest fragmentation and reduction
in area have a negative i mpact on the species richness and population
density of breeding birds 'in North America (Bolger, Alberts , "& Soule,
1991; Lynch, 1987). A similar decline is occurring in the temperate forest
of southern Chile (Willson et al., 1994). Long-term censuses of bird
populations in forest fragments are necessary to assess these trends.
We predict that anegative-feedback dynamics driven by human disturbance to forests may result in a serious dropIn bird abundances.
Habitat destruction will lead to a rapid decline in' bird populations due to
the diminishing food resources, which iii'turn may cause further reduction
in plant populations. . The reductions in bird populations are ' likely
to affect entire guilds of plants, eventually leading to a loss of those
species that fail to reproduce or disperse their seed to suitable sites for
establishment .
Fruit-eating birds, particularly Elaeniaalbiceps , are likely to be rare or
absent from areas planted with exotic conifers, which are entirely devoid
of fleshy fruits. In these areas ; which 'currently occupy more than 1.5
million hectares between 35°Sand 39°S.(Armesto, Smith-Ramirez , Leon ,
& Arroyo , 1992),'the recovery of nativeforest will be severely impaired
in the future by low seed inputs to the soil seed bank,
Open areas 'covered by grassland and lacking shrubs or trees are also
barren territory for frugivores. Guevara, Purata, and Maarel (1986) have
shown that isolated fruiting trees left standing in open areas may help
initiate the recolonization ,processvbecause they attract birds carrying
seeds. from surrounding forests. W~ suggest that leaving some fleshyfruited trees standing should be a policy in clearing processes where
eventual restoration is desired. Also patches of intact forest must remain
in the regional landscape in order to provide suitable habitat for frugivorous birds and sources of seed.
On the other hand , plantations of introduced Eucalyptus species,
abundant in south-central Chile, attract large numbers of hummingbirds
during their 'flowering ' season ,(Smith-Ramirez .petsonal observation) .
Although this phenomenon IIl:ay compensate for the lack of suitable
native habitat in ' some areas, these introduced trees mayattract the birds
away from their native food plants, negatively affecting the seed crop of
Susceptibility to Disturbance
261
Although its geographical range extends into the subtropical woodland of
Paraguay and northern Argentina, it is likely that the population numbers
of the migrant frugivore Elaenia albiceps .are regulated in the temperate
forests of southern South America, where this species breeds (Vuilleumier,
1985). Summer populations in Chiloe consist largely of juveniles (Sabag,
1993). Recent expansion of the timber and wood-chip industry in Chile
has resulted in a rapid decrease in the area of natural forest and an
increase in the area planted with exotic species, particularly Pinus radiata
and Eucalyptus spp. (INFOR, 1991). Forest fragmentation and reduction
in area have a negative i mpact on the species richness and population
density of breeding birds 'in North America (Bolger, Alberts , "& Soule,
1991; Lynch, 1987). A similar decline is occurring in the temperate forest
of southern Chile (Willson et al., 1994). Long-term censuses of bird
populations in forest fragments are necessary to assess these trends.
We predict that anegative-feedback dynamics driven by human disturbance to forests may result in a serious dropIn bird abundances.
Habitat destruction will lead to a rapid decline in' bird populations due to
the diminishing food resources, which iii'turn may cause further reduction
in plant populations. . The reductions in bird populations are ' likely
to affect entire guilds of plants, eventually leading to a loss of those
species that fail to reproduce or disperse their seed to suitable sites for
establishment .
Fruit-eating birds, particularly Elaeniaalbiceps , are likely to be rare or
absent from areas planted with exotic conifers, which are entirely devoid
of fleshy fruits. In these areas ; which 'currently occupy more than 1.5
million hectares between 35°Sand 39°S.(Armesto, Smith-Ramirez , Leon ,
& Arroyo , 1992),'the recovery of nativeforest will be severely impaired
in the future by low seed inputs to the soil seed bank,
Open areas 'covered by grassland and lacking shrubs or trees are also
barren territory for frugivores. Guevara, Purata, and Maarel (1986) have
shown that isolated fruiting trees left standing in open areas may help
initiate the recolonization ,processvbecause they attract birds carrying
seeds. from surrounding forests. W~ suggest that leaving some fleshyfruited trees standing should be a policy in clearing processes where
eventual restoration is desired. Also patches of intact forest must remain
in the regional landscape in order to provide suitable habitat for frugivorous birds and sources of seed.
On the other hand , plantations of introduced Eucalyptus species,
abundant in south-central Chile, attract large numbers of hummingbirds
during their 'flowering ' season ,(Smith-Ramirez .petsonal observation) .
Although this phenomenon IIl:ay compensate for the lack of suitable
native habitat in ' some areas, these introduced trees mayattract the birds
away from their native food plants, negatively affecting the seed crop of