trapped by this species [7]. It has also been shown that Bombus spp. are the main
pollinators of S. flava [43] and S. alata [44], and ants are the main prey of S. alata
[45, 46] and S. purpurea in some habitats [47, 48], with Diptera being taken by S.
purpurea growing in a different habitat [49]. Therefore, pollinator trapping is
unlikely to occur in Sarracenia spp. However, Bombus spp. are frequently trapped
by an S. purpurea population that has been introduced from North America to
Britain, despite these insects being major pollinators of this population, with one
study showing that 49/170 pitchers trapped more than one Bombus individual [50].
This difference in pollinator trapping between native and introduced populations
may reflect the coevolutionary history between Sarracenia and Bombus (see Sect. 5
for a detailed discussion).
2.3
Snap Traps
The degree of pollinator trapping in Dionaea muscipula was investigated by
Youngsteadt et al. [51], who showed that the major pollinators of this carnivorous
plant that have high visiting frequencies and pollen loads are the sweat bee
Augochlorella gratiosa, other bees (e.g., Lasioglossum creberrimum), and beetles
(Typocerus sinuatus and Trichodes apivorus), while the major prey species are
terrestrial invertebrates, including spiders (40%), ants (26%), and beetles (11%).
Thus, there is very little overlap between the prey and pollinator fauna, and none of
the major pollinator species are trapped. This low level of pollinator trapping may be
due to the spatial separation of the flowers and rosette-shaped traps [51].
3
Carnivorous Plant Signals and Pollinator Trapping
3.1
Visual Signals
Some species of Drosera, Dionaea, Nepenthes, and Sarracenia produce red trap
leaves through the expression of the secondary metabolite anthocyanin [52, 53].
These red trap leaves are conspicuous to the human eye due to their strong contrast
against the green background [2]. It has been hypothesized that the red coloration of
the trap leaves functions as a signal to attract insects effectively because, in some
carnivorous plants, the color of the trap leaves changes from green to red under
nutrient-deficient conditions (see [54] for experimental studies on Drosera spatulata
and Dionaea muscipula). In a pioneer study on this hypothesis, Schaefer and Ruxton
[16] showed that the prey capture rate of Nepenthes ventricosa was higher in pitchers
that were artificially colored red than in those that were artificially colored green.
However, while this study clearly demonstrated the attractiveness of red traps, its
ecological validity was questioned because the experiment was conducted in Germany, which is outside the natural range of N. ventricosa, and the composition of
prey that were trapped in the experiment (mainly Diptera) was not consistent with
the compositions in the native habitats of this species (mainly ants) [55].
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K. Tagawa
pollinators of S. flava [43] and S. alata [44], and ants are the main prey of S. alata
[45, 46] and S. purpurea in some habitats [47, 48], with Diptera being taken by S.
purpurea growing in a different habitat [49]. Therefore, pollinator trapping is
unlikely to occur in Sarracenia spp. However, Bombus spp. are frequently trapped
by an S. purpurea population that has been introduced from North America to
Britain, despite these insects being major pollinators of this population, with one
study showing that 49/170 pitchers trapped more than one Bombus individual [50].
This difference in pollinator trapping between native and introduced populations
may reflect the coevolutionary history between Sarracenia and Bombus (see Sect. 5
for a detailed discussion).
2.3
Snap Traps
The degree of pollinator trapping in Dionaea muscipula was investigated by
Youngsteadt et al. [51], who showed that the major pollinators of this carnivorous
plant that have high visiting frequencies and pollen loads are the sweat bee
Augochlorella gratiosa, other bees (e.g., Lasioglossum creberrimum), and beetles
(Typocerus sinuatus and Trichodes apivorus), while the major prey species are
terrestrial invertebrates, including spiders (40%), ants (26%), and beetles (11%).
Thus, there is very little overlap between the prey and pollinator fauna, and none of
the major pollinator species are trapped. This low level of pollinator trapping may be
due to the spatial separation of the flowers and rosette-shaped traps [51].
3
Carnivorous Plant Signals and Pollinator Trapping
3.1
Visual Signals
Some species of Drosera, Dionaea, Nepenthes, and Sarracenia produce red trap
leaves through the expression of the secondary metabolite anthocyanin [52, 53].
These red trap leaves are conspicuous to the human eye due to their strong contrast
against the green background [2]. It has been hypothesized that the red coloration of
the trap leaves functions as a signal to attract insects effectively because, in some
carnivorous plants, the color of the trap leaves changes from green to red under
nutrient-deficient conditions (see [54] for experimental studies on Drosera spatulata
and Dionaea muscipula). In a pioneer study on this hypothesis, Schaefer and Ruxton
[16] showed that the prey capture rate of Nepenthes ventricosa was higher in pitchers
that were artificially colored red than in those that were artificially colored green.
However, while this study clearly demonstrated the attractiveness of red traps, its
ecological validity was questioned because the experiment was conducted in Germany, which is outside the natural range of N. ventricosa, and the composition of
prey that were trapped in the experiment (mainly Diptera) was not consistent with
the compositions in the native habitats of this species (mainly ants) [55].
782
K. Tagawa