Biology Reference
In-Depth Information
Table 2.9
Pollinators of
Spiranthes tuberosa
and
S. vernalis
Species
Pollinator/state or province
References
S. tuberosa
Apidae
Bombus
sp. /?
Catling (
1980b
)
a
Halictidae
Augochlorella pura
(Say)/NC
Stevenson (
1973
)
Unknown bee, possibly Halictidae /?
Catling (
1980b
)
S. vernalis
Apidae
Apis melifera
L./GA
Catling (
1980b, 1983c
)
B. impatiens
/NC
Stevenson (
1973
)
B. pennsylvanicus
/NC
Stevenson (
1973
)
B.
sp. /?
Catling (
1980b
)
a
Unobserved; likely based on floral morphology and/or flowering pattern
?
Study sites were not given, and therefore no data are available
The available evidence indicates a lower level of pollinator specificity among
most northeastern species of
Spiranthes
(Catling
1983c
). Sheviak (
1982
), for exam-
ple, noted that in greenhouse-grown specimens,
B. impatiens
moved between
S.
cernua
and
S. odorata
and between
S. cernua
and
S. ochroleuca
effecting pollina-
tion. A similar lack of specificity in natural populations is clearly evident from the
tabulation of
Spiranthes
species and their known pollinators (Table
2.3
-
2.9
). Thus,
for example, Catling (
1980b, 1983c
) identified
B. perplexus
Cresson,
B. terricola
,
and
B. vagans
as pollinators of
S. lacera
var.
lacera
(Table
2.5
) and
S. romanzoffi-
ana
(Table
2.6
) even though these orchids differ in the size and structure of their
flowers (Catling
1983c
). Hybrids are known and the resulting nothospecies is rec-
ognized as
S.
×
simpsonii
Catling and Sheviak (Simpson and Catling
1978
; Catling
1980b
; Catling and Sheviak
1993
). In general, the granular pollinia in
Spiranthes
permit the deposition of pollen from a single pollinium on several successively
visited flowers, augmenting any potential for hybridization (Catling
1983c
).
Larson and Larson (
1987, 1990
) studied the foraging behavior of bumblebees on
S. romamzoffiana
at three sites on Vancouver Island. Although they also found low
levels of pollinator specificity, visitation rates were usually high, possibly in
response to a continuous and/or abundant nectar reward. Within a 5 × 12-m study
plot of a dozen sparsely distributed plants, the pollinators, principally
B. bifarious
,
zigzagged along a clearly directional flight path and were attracted to conspicuous,
tall plants with long inflorescences more frequently than to short plants with smaller
inflorescences, which they often bypassed. An association between larger inflores-
cences and higher pollinator visitation rates is not uncommon (e.g., Schmid-Hempel
and Speiser
1988
). In the case of
S. romanzoffiana
, a foray by a single bee did not
usually involve repeat visits to any one plant; however, in a series of forays, bees
revisited the four most “conspicuous” plants at frequencies that may have exceeded
the optimal foraging behavior wanted to maximize nectar rewards (Larson and
Larson
1990
).
Visits were also correlated at a lower level with nearest neighbor distance and
the number of open flowers per inflorescence (Larson and Larson
1990
). Other
studies have also shown increased pollinator visits in populations with closely
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