12. The Importance of Plant-Bird Mutualisms
249
impact of biodiversity losses on ecosystem function (Chapin, Schulze, &
Mooney, 1992).
In temperate forests , the geographical ' distribution and overall importance of plant -avian mutualistic interactions are still poorly understood
(e.g. , Armesto, Rozzi, Miranda , & Sabag, 1987; Herrera, 1984; Willson,
1986, for seed dispersal). In temperate forests of southern Chile, about
70% of the forest flora, including more ' than 90% of subcanopy and
successional trees , bear fleshy fruits (Armesto & Rozzi, 1989); which are
consumed by birds and other frugivorous vertebrates (Armesto et al.,
1987; Willson, 1991). The proportion of fleshy-fruited plant species in
southern Chile is higher than that reported for many North American
temperate forests (Willson, 1986) and similar to some tropical .forests
(Gentry, 1982). Recent work in southern Chile (Smith-Ramfrez, 1993)
revealed that at least 14 plant species are pollinated by hummingbirds. In
contrast, nectar-eating birds (Jaksic & Feinsinger, 1991) and hummingbird
flowers seem to be rare in conifer or temperate deciduous forests of
North America .
Because of the critical role of plant-animal mutualistic associations in
the maintenance of local and regional diversity, we ask whether pivotal
species occur among avian seed dispersers and pollinators in South
American temperate rainforests. To answer this question , we review the
information on bird population densities and diets and examine the
flowering and fruiting patterns of their food plants . We examine the guild
structure of these mutualistic assemblages and discuss how Chilean temperate forests differ from or resemble North American temperate and
Neotropical forests with respect to keystone plant-avian relationships.
Finally, we speculate on the susceptibility of temperate forest-assemblages
to habitat disruption.
Study Area
Evergreen temperate rain forests occur in southern Chile between 38°S
and 55°S (Armestoet al., 1987; Arroyo et al. , this volume). Our studies
of plant-frugivore interactions have been conducted primarily in a 40year-old , second-growth temperate rainforest, at 150m elevation , in
central ' Chiloe Island , Chile (42°30' S). Phenological records and observations of hummingbird visits to plants have been conducted in a broader
range of habitats , including second-growth forest sites as well as primary
forests at higher elevations . ,(150-400m) . Lowland forests (below 200m)
have been cleared to a large extent to open land for pastures ' and for
timber production , leaving isolated patches of old-growth and secondary
forest. Dominant trees in these secondary forests are Drimys winteri
(Winteraceae) , Ovidia pillo-pillo (Thymeliaceae) , Amomyrtus luma
249
impact of biodiversity losses on ecosystem function (Chapin, Schulze, &
Mooney, 1992).
In temperate forests , the geographical ' distribution and overall importance of plant -avian mutualistic interactions are still poorly understood
(e.g. , Armesto, Rozzi, Miranda , & Sabag, 1987; Herrera, 1984; Willson,
1986, for seed dispersal). In temperate forests of southern Chile, about
70% of the forest flora, including more ' than 90% of subcanopy and
successional trees , bear fleshy fruits (Armesto & Rozzi, 1989); which are
consumed by birds and other frugivorous vertebrates (Armesto et al.,
1987; Willson, 1991). The proportion of fleshy-fruited plant species in
southern Chile is higher than that reported for many North American
temperate forests (Willson, 1986) and similar to some tropical .forests
(Gentry, 1982). Recent work in southern Chile (Smith-Ramfrez, 1993)
revealed that at least 14 plant species are pollinated by hummingbirds. In
contrast, nectar-eating birds (Jaksic & Feinsinger, 1991) and hummingbird
flowers seem to be rare in conifer or temperate deciduous forests of
North America .
Because of the critical role of plant-animal mutualistic associations in
the maintenance of local and regional diversity, we ask whether pivotal
species occur among avian seed dispersers and pollinators in South
American temperate rainforests. To answer this question , we review the
information on bird population densities and diets and examine the
flowering and fruiting patterns of their food plants . We examine the guild
structure of these mutualistic assemblages and discuss how Chilean temperate forests differ from or resemble North American temperate and
Neotropical forests with respect to keystone plant-avian relationships.
Finally, we speculate on the susceptibility of temperate forest-assemblages
to habitat disruption.
Study Area
Evergreen temperate rain forests occur in southern Chile between 38°S
and 55°S (Armestoet al., 1987; Arroyo et al. , this volume). Our studies
of plant-frugivore interactions have been conducted primarily in a 40year-old , second-growth temperate rainforest, at 150m elevation , in
central ' Chiloe Island , Chile (42°30' S). Phenological records and observations of hummingbird visits to plants have been conducted in a broader
range of habitats , including second-growth forest sites as well as primary
forests at higher elevations . ,(150-400m) . Lowland forests (below 200m)
have been cleared to a large extent to open land for pastures ' and for
timber production , leaving isolated patches of old-growth and secondary
forest. Dominant trees in these secondary forests are Drimys winteri
(Winteraceae) , Ovidia pillo-pillo (Thymeliaceae) , Amomyrtus luma