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Floral Quality Signals Lure Pollinators and Their Predators

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Floral Quality Signals Lure Pollinators and Their Predators Ann. Zool. Fennici 41: 421–428 ISSN 0003-455X Helsinki 27 April 2004 © Finnish Zoological and Botanical Publishing Board 2004 Floral quality signals lure pollinators and their predators Astrid M. Heiling1 & Marie E. Herberstein2 1) Institute of Zoology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria; current address: Department of Biological Sciences, Macquarie University, North Ryde, 2109 NSW, Australia (e-mail: [email protected]) 2) Department of Biological Sciences, Macquarie University, North Ryde 2109 NSW, Australia Received 23 July 2003, revised version received 21 Oct. 2003, accepted 24 Oct. 2003 Heiling, A. M. & Herberstein, M. E. 2004: Floral quality signals lure pollinators and their predators. — Ann. Zool. Fennici 41: 421–428. Flowers exploit the sensory capabilities of pollinators to lure them. The crab spider Thomisus spectabilis benefits from this system by responding to floral signals that are intended to lure honeybees, Apis mellifera, which they prey upon. We investigated whether honeybees and crab spiders discriminate between flowers of Jasminum mesnyi plants that are either fully rewarding, with two reproductive stamens, or not. We found that both honeybees and crab spiders preferred fully rewarding flowers in the field. Moreover, honeybees preferred small flowers over larger ones, while the height above the ground and the number of petals did not influence flower choice of honeybees and crab spiders. Under experimental conditions, crab spiders showed the same choice pat- tern as observed in the field. This indicates that they actively search for fully rewarding flowers, which are also preferred by their potential prey. Fully rewarding J. mesnyi lure honeybees through visual, olfactory or a combination of these floral signals and T. spectabilis exploits this communication to enhance their chance of prey encounter. Introduction In fact, pollinators are attracted to pollen and anthers through gustatory (Galen & Kevan 1983, In animal communication, signals often match Hansen et al. 1998), tactile (Gack 1981), olfac- the sensory system of the intended receiver tory (Lunau 1992), and visual signals (Chittka & (Guilford & Dawkins 1991). In the interac- Menzel 1992, Lunau 2000). tion between plants and pollinating animals, Bees, which are probably the most exten- sensory biases are being exploited, a process sively studied group of animals interacting with called ‘sensory exploitation’ (Basolo 1995, plants, are responsive to various floral character- Johnstone 1996). The floral signals produced istics that indicate the presence of food resources. by plants match the inherent sensory abilities For example, they discriminate amongst flowers of pollinating insects they attract (e.g., Chittka with different sizes, preferring larger flowers et al. 1994, Chittka et al. 1999). Due to the (Ohara & Higashi 1994). Similarly, between energetic demands of pollinators, their prefer- flowers of the same species, they prefer flow- ences should result in a directional selection for ers containing wider (Ashman et al. 2000) and floral cues associated with predictable pollen longer (Ashman 2000) petals. Moreover, bees and nectar resources (Heinrich & Raven 1972). are attracted to symmetrical flowers (Møller & 422 Heiling & Herberstein • ANN. ZOOL. FENNICI Vol. 41 a b Petaloid crab spider species matches the flower colour Position of stamen 2 stamens making them well camouflaged (e.g., Chittka 2001, Théry & Casas 2002), the Australian T. spectabilis is highly conspicuous against a flower background. These spiders manipulate flower Fully rewarding flower Not fully rewarding signals through UV-reflecting body colouration, a novel example of deceptive signalling (Heiling Fig. 1. Schematic representation of the two morphs of et al. 2003). flowers in J. mesnyi plants. (a) Flower containing two In the present study, we extend our investiga- stamens, i.e. these flowers are fully pollen productive, (b) represents the less pollen productive morph of flow- tions on this predator–prey system by combining ers in which one stamen is converted into a petaloid. the data from field observations and experi- ments. Focusing our investigations on a single plant species, Jasminum mesnyi (Oleaceae) that Eriksson 1995, Møller & Scorci 1998) that also shows variable pollen availability: some flow- offer more food resources to pollinators than less ers contain two productive stamens, while other symmetrical flowers within a species (Møller flowers on the same plant have converted one & Scorci 1998). Furthermore, odour (Galen & stamen into a non pollen producing petaloid Kevan 1983, Wells & Wells 1985), and flower stamen. We predict that honeybees preferentially colour (Real 1981, Waser & Price 1981, Kevan visit flowers that offer a higher quantity of pollen & Baker 1983, Chittka & Menzel 1992) also resources and that crab spiders show the same affect flower choice in bees, whereas the bright- preferences. ness of objects does not seem to have any influ- ence (Giurfa et al. 1995, Lunau et al. 1996). Plants benefit from frequent visits of animals Methods by reproductive service, while pollinators gain nectar or pollen resources (e.g., Harder et al. Study area and study organism 2001). However, signalling systems are often manipulated or exploited, resulting in a disad- Field observations were carried out over 4 con- vantage for either the signaller or the receiver. secutive days in October 2001, in the urban area This situation often occurs in the communication of Brisbane, Australia. Our choice experiment, system between predator and prey, where the using spiders collected in Brisbane and flow- predator exploits the biases of potential prey to ers from the Sydney area, was performed on enhance foraging success. For example, fireflies the Campus grounds of Macquarie University, of the genus Photuris mimic signals of courting Sydney, in October 2002. heterospecific fireflies (Photinus sp.) to attract During early summer, one of the dominant males of this genus as prey (Lloyd & Wing flowering species in the Brisbane region is Jas- 1983). minum mesnyi (Oleaceae), the common primrose Crab spiders are predators that occupy jasmine, an evergreen shrub native to south west- flowers where they hunt for pollinating insects ern China. Cultivated clones mostly contain semi (Schmalhofer 1999). Previous studies revealed double, yellow flowers with 6 to 10 petals. The that various signalling modalities, such as olfac- corolla is actinomorphic. The androecium either tory (Aldrich & Barros 1995, Krell & Krämer consists of two stamens born on the corolla 1998), visual and tactile ones (Morse 1988, tube, or one of the stamens is converted to a Greco & Kevan 1994) guide them in their choice petal, forming the petaloid stamen (P. Hind pers. of hunting sites. Recent experimental studies on comm.; Fig. 1). In Brisbane, J. mesnyi is one of Thomisus spectabilis revealed that these crab the hunting substrates of Thomisus spectabilis, spiders exploit the behaviour of their insect prey the model predator in our study. by responding to olfactory cues produced by T. spectabilis (Araneae: Thomisidae) are daisies that also attracted honeybees (Heiling et diurnally active, anthophilous ambush predators. al. 2004). Moreover, while body colour of some These spiders use their raptorial forelimbs, rather ANN. ZOOL. FENNICI Vol. 41 • Exploitation of flower quality signals 423 than a web, to restrain prey that land on the flow- their corollas enclosing an angle of 120°. Each ers. They either sit on or beneath the petals of spider was placed in front and in between the jasmine flowers when hunting prey (pers. obs.). two flowers at a distance of 5 cm from the centre We found them preying on honeybees (Apis mel- of the spider carapace. The spider was facing lifera, Apidae), the model prey animal in our the two flowers with the two morphs assigned study. randomly to the left or the right side. The experi- ment was performed under natural daylight con- ditions and each spider and flower was used only Floral characteristics and visitation rates once. by crab spiders and honeybees To investigate whether T. spectabilis and A. Reflectance properties of flowers mellifera prefer jasmine flowers as feeding grounds that show certain characteristics, we To examine whether the morphological differ- concentrated our observations on a large shrub ences between flowers with or without petaloid of J. mesnyi covering approximately 15 square stamens also translate into differences in the metres. All the inflorescences on the shrub were reflectance properties as perceived by a honey- checked for the presence of female T. spectabilis bee, we measured the spectral reflectance (300 to and the spiders were removed. We measured 700 nm) of flowers. In both morphs, we meas- several parameters from these flowers (N = 23) ured the reflectance of petals in the periphery of including the height of the flower above the the flowers (3 mm from the tip of the petals). To ground, the diameter of the flower, the number measure the centre of fully reproductive flowers, of petals, and whether the flower contained two we considered an area within a radius of 3 mm reproductive stamens or petaloid stamens. The from the flower centre. In flowers with converted same parameters were taken from flowers that stamens we measured the lower side of the peta- were visited by honeybees (N = 23). The rela- loid stamens that are visible to oncoming hon- tive frequency of inflorescences containing two eybees and cover the flower centre (see Fig. 1). reproductive stamens was ascertained from
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