12. The Importance of Plant-Bird Mutualisms
257
reported data come mainly from second-growth forests, the same trees
and birds are found in primary forest in the study area (Armesto, unpublished data). Further observations in primary forests are necessary to
understand how bird .behavior and phenology patterns may differ from
those found in secondary forest patches.
The presence of abundant populations of Sephanoides galeritus
throughout the year . and of Elaenia albiceps during summer may be
critical for the reproductive success of many forest species in the temperate
forest of Chiloe . Because of the effectiveness of Sephanoides galeritus as
compared to other. . less . frequent , nonspecific visitors of flowers with
large, red, tubular corollas, this bird species appears as the main pollinator
of a guild of 14 plant species found in the canopy of old-growth and
secondary forest stands. Pollinator dependence of the plant species in ·the
guild is attested byRivero's (1991) data on breeding systems and seed
yield. Six out of the seven plant species for which the breeding system has
been studied are entirely dependent on the pollinator to produce fruit ,
because they are either genetically or morphologically self-incompatible ,
as shown in Table 12.3.
We have also shown that during the period of peak abundance of ripe
fruit of forest species in Chiloe , the migrant passerine Elaenia albiceps
accounts for more than 50% of all fruit-eating birds in the forest and that
this frugivorous species disperses a major proportion of the seeds that fall
into open second-growth areas (Table 12.2). A recent study by Sabag
(1993) shows that Elaenia feeds on fruits of a large proportion of the
fleshy-fruited plants of second-growth and primary forests. Although three
other resident bird species are components of the frugivorous guild in the
forest of Chiloe, their ,abundance is low compared to this migrant species
(see Table 12:1 and Figure 12.2), and they seem to consume fruits of
fewer species (Table 12.2). ·We believe ,however, that the importance of
these latter frugivoresas seed dispersers may increase as further studies
become available (see Willson et al. , this volume). Another member of
the frugivore guild in the forest is the Chiloe fox (Pseudalopex fulvipes) ,
which consumes large numbers of berries, particularly in summer (Armesto
et al. , 1987; Medel et al.,1990) , but it is unknown how effective it may
be as compared to birds in disseminating seeds to suitable germination
sites.
For most tree species in old-growth forest, seed germination or seedling
establishment is ,restricted to tree-fall .gaps (Armesto & Fuentes, 1988;
Veblen , 1985).-Adequate seed dispersal becomes increasingly important
for forest regeneration as open areas become larger and more distant
from seed sources .as a consequence of human disturbance. A vast seed
shadow , such as ,that generated _by the dispersal of seeds by birds , is
important in undisturbed forest stands , because it increases the probability
that a seed will reach a canopy gap or .a site where a canopy gap may soon
form (Hubbell & Foster, 1986).
257
reported data come mainly from second-growth forests, the same trees
and birds are found in primary forest in the study area (Armesto, unpublished data). Further observations in primary forests are necessary to
understand how bird .behavior and phenology patterns may differ from
those found in secondary forest patches.
The presence of abundant populations of Sephanoides galeritus
throughout the year . and of Elaenia albiceps during summer may be
critical for the reproductive success of many forest species in the temperate
forest of Chiloe . Because of the effectiveness of Sephanoides galeritus as
compared to other. . less . frequent , nonspecific visitors of flowers with
large, red, tubular corollas, this bird species appears as the main pollinator
of a guild of 14 plant species found in the canopy of old-growth and
secondary forest stands. Pollinator dependence of the plant species in ·the
guild is attested byRivero's (1991) data on breeding systems and seed
yield. Six out of the seven plant species for which the breeding system has
been studied are entirely dependent on the pollinator to produce fruit ,
because they are either genetically or morphologically self-incompatible ,
as shown in Table 12.3.
We have also shown that during the period of peak abundance of ripe
fruit of forest species in Chiloe , the migrant passerine Elaenia albiceps
accounts for more than 50% of all fruit-eating birds in the forest and that
this frugivorous species disperses a major proportion of the seeds that fall
into open second-growth areas (Table 12.2). A recent study by Sabag
(1993) shows that Elaenia feeds on fruits of a large proportion of the
fleshy-fruited plants of second-growth and primary forests. Although three
other resident bird species are components of the frugivorous guild in the
forest of Chiloe, their ,abundance is low compared to this migrant species
(see Table 12:1 and Figure 12.2), and they seem to consume fruits of
fewer species (Table 12.2). ·We believe ,however, that the importance of
these latter frugivoresas seed dispersers may increase as further studies
become available (see Willson et al. , this volume). Another member of
the frugivore guild in the forest is the Chiloe fox (Pseudalopex fulvipes) ,
which consumes large numbers of berries, particularly in summer (Armesto
et al. , 1987; Medel et al.,1990) , but it is unknown how effective it may
be as compared to birds in disseminating seeds to suitable germination
sites.
For most tree species in old-growth forest, seed germination or seedling
establishment is ,restricted to tree-fall .gaps (Armesto & Fuentes, 1988;
Veblen , 1985).-Adequate seed dispersal becomes increasingly important
for forest regeneration as open areas become larger and more distant
from seed sources .as a consequence of human disturbance. A vast seed
shadow , such as ,that generated _by the dispersal of seeds by birds , is
important in undisturbed forest stands , because it increases the probability
that a seed will reach a canopy gap or .a site where a canopy gap may soon
form (Hubbell & Foster, 1986).