Tribe Calypsoeae - The Pollination Biology of North American Orchids: North of Florida and Mexico - page 111

Some investigators have hypothesized that the number of pollinator visits to
unrewarding plant species should be negatively correlated with population size
(Stoutamire 1971 ) and positively correlated with the local frequency of rewarding
plant species (Laverty and Plowright 1988 ; Laverty 1992 ). Thus, for example, the
frequency of encounters between bees and flowers might be relatively high in large,
dense populations resulting in a reduction in the time required for the bees to learn
that the flowers offer no reward. At the same time, a local abundance of food plants
might serve to attract pollinators, some of which might visit unrewarding flowers by
mistake, the so-called magnet species effect (e.g., Dafni 1983 ).
Data on the influence of population parameters and the magnet species effect in
Calypso are inconsistent and often contradictory. Alexandersson and Agren ( 1996 ),
for example, found the hypothetical relationship to be true only in 1 year of a 3-year
study on variety bulbosa in northern Sweden. In the other 2 years, pollen removal
was neither significantly related to population size nor the local frequency of simul-
taneously flowering plants with richly rewarding flowers. In addition, the proportion
of plants setting fruit was not significantly correlated with either variable in any year
of the study. Similarly, Ackerman ( 1981 ) found that percent fruit set was largely
independent of population size in northern California populations of variety occi-
dentalis , even though larger populations produced more fruits, and some reduction
of percent fruit set was observed in very small or very large populations.
Boyden ( 1982 ) found no relationship between population density and pollination
rates in variety americana near Banff, where large, dense clones and thousands of
flowers were spread over a wide area (Mosquin 1970 ). Krell ( 1977 ), on the other
hand, observed that flowers of variety occidentalis were pollinated less frequently
in northwestern Idaho when they occurred in dense patches than when they were
more widespread.
Mosquin ( 1971 ) reported that flowering of variety americana in the Banff area
extended from late May through the third week of June. Relatively few entomophil-
ous plant species were in bloom in May, and insects were forced to compete for the
relatively scarce nectar and pollen resources available. Later, plants, such as Salix
and Taraxicum officinale L. that offered a nearly unlimited amount of nectar and
pollen, came into bloom, and plants were then competing for pollinators. Under
these circumstances, any magnet species effect resulting from the attraction of addi-
tional pollinators would almost surely be outweighed by the increased competition
for their services.
Ackerman ( 1981 ) reported one to four pollinia present on each pollinated stigma
of variety occidentalis . Thus, the four pollinia removed as a unit (Fig. 6.1b ) are
capable of separating. Although capsule set was unaffected, experimental crosses
revealed a significant difference in seed production in flowers pollinated by one-half
of a pollinium compared to those that received two pollinia. Proctor and Harder
( 1994 ) believe that two pollinia of variety americana are sufficient to fertilize most
of the ovules contained in a single flower. They suggest, therefore, that the size of
the pollinia rather than the size of the pollinarium was selected to correspond to the
number of ovules present in each ovary and that the full complement of pollinia in
each pollinarium is utilized in the complete pollination of several flowers.