THE IMPORTANCE OF DIFFERENT BIRD SPECIES AS SEED D1SPERSERS
1C Greg Murrayn index that weights the contribution of each
bird species relative to that of the alternative
dispersers of each of its food plants (Equa-
tion 1) is a more useful estimator of "importance" of
each bird to the plant community overall than is a
simple count of the number of fruit species eaten by
each bird. The relative importance of bird species /
in the community is
where Tf is the total number of bird species feeding
on plant species i, and S is the total number of plant
species included in the community (in this case, the
total number of plant species included in the study).
Citj = 1 if bird species j consumes fruits of plant spe-
cies i or 0 if it does not. The index can vary from a
minimum of 0 for species that feed on no plants in-
cluded in the study to a maximum of 1.0 for a bird
species that is the only consumer of all plants in-
cluded. This index ignores the relative intensity of use
of each plant species by different animals. It equates
all plant species, so a small rare forest floor herb or
epiphyte contributes as much to the index as does a
dominant canopy tree. Because this index is a func-
tion of the total number of plant species included in
the diet, intensively studied species tend to have
higher importance values than those studied less
systematically. Despite these biases, the index pro-
vides greater insights than simple counts of fruits
consumed.
We computed the importance index for frugivorous
birds in the Monteverde area (Wheelwright et al. 1984,
Bronstein and Hoffman 1987, Sargent 1994, K. G.
Murray, unpubl. data). Data on 75 bird species and
179 plant species were included. Importance values
of fruit-eating birds at Monteverde vary over two
orders of magnitude. A few species are disproportion-
ately important (Fig. 8.25). The results of the analy-
sis parallel those of the simple counts of food plants
reported as marginal totals in Wheelwright et al.
(1984): species that feed on many kinds of fruits have
high index values. Surprisingly, some small frugi-
vores (e.g., Black-faced Solitaires, Long-tailed Mana
kins, Prong-billed Barbets, and Common Bush-
Tanagers) have values as high as or higher than many
of the large species (e.g., Resplendent Quetzals, Three-
wattled Bellbirds, and Black Guans) on which large-
seeded plants depend. The small frugivores appear to
be disproportionately important to a large number of
plants, many of which are less conspicuous. These
small birds could be considered "keystone mutual-
ists" (sensu Gilbert 1980) just as quetzals, bellbirds,
toucanets, and guans; the loss of any of these species
could have profound effects on the biodiversity of
Monteverde.
We used the same approach to estimate the rela-
tive importance of different plant species from the
point of view of Monteverde birds. We used the same
equation (1), but J;- is the importance of plant species
/, Tf is the total number of plants fed on by bird spe-
cies i, and S is the total number of bird species in-
cluded in the sample. CitJ- = 1 if plant species / is in-
cluded in the diet of bird species i or 0 if it is not.
Using the same data matrix, we found that a smallFigure 8.25. Importance values (equation 1) for the 15
frugivorous bird species with the highest indices at
Monteverde. Common names of bird species, and numbers
of plant species whose fruits are included in the diets of
each, are (from left to right) Emerald Toucanet (95), Black-
faced Solitaire (51), Long-tailed Manakin (37), Resplendent
Quetzal (38), Prong-billed Barbet (30), Common Bush-
Tanager (29), Mountain Robin (44), Golden-browed
Chlorophonia (14), Three-wattled Bellbird (29), Black Guan
(26), Yellow-throated Euphonia (7), Orange-bellied Trogon
(14), Olive-striped Flycatcher (9), Keel-billed Toucan (16),
and Dusky-capped Flycatcher (8), See text for sources.294 Plant-Animal InteractionsA