Pollinator Functional Groups

Functional groups of pollinators may be more or less restricted to groups of plants based on floral or habitat characteristics (Bawa 1990). A large number of pollinators are generalists with respect to plant species. This functional group includes many beetles, flies, thrips, etc. that forage on any floral resources available. Specialist pollinators often exploit particular floral characteristics that may exclude other pollinators. For example, nocturnally flowering plants with large flowers attract primarily bats, whereas plants with small flowers attract primarily moths. Long, bright-red flowers attract birds but are largely unattractive to insects (S. Johnson and Bond 1994). Such flowers often are narrow to hinder entry by bees and other insect pollinators (Heinrich 1979) but may nonetheless be pollinated by some insects (Roubik 1989). Pollen feeders feed primary on pollen (e.g., beetles and thrips) and are likely to transport pollen acquired during feeding, whereas others are primarily nectar-feeders (e.g., beetles, butterflies, moths, and flies) and transport pollen more coincidentally. In fact, many nectar feeders avoid

Scarab Beetle Blintzt Bird Base

I Examples of pollinators. A: Honey bee, Apis mellifera, Louisiana, United States. B: Scarab beetle, Fushan Experimental Forest, Taiwan.

the reproductive organs, often by perforating the base of the flower to reach the nectar (Dedej and Delaplane 2004) or, in the case of ants, may reduce pollen viability (Peakall et al. 1987). Bees, especially Apis spp., primarily feed on pollen and nectar. Functional groupings also reflect attraction to floral odors. For example, dung-, fungus-, and carrion-feeding flies and beetles are the primary pollinators of plants that emit dung or carrion odors (Appanah 1990, Norman and Clayton 1986, Norman et al. 1992).

Ants frequently exploit floral resources but have little importance as pollinators. Peakall et al. (1987) suggested that antibiotic secretions produced by most ants, to inhibit infection by entomophagous fungi in a subterranean habitat, also inhibit germination of pollen. Ants lacking these secretions are known to function as pollinators.

Pollinator functional groups also have been distinguished on the basis of habitat preferences, such as vegetation stratum (Fig. 13.2). Appanah (1990) distinguished four groups of plant-pollinator associations in a tropical lowland dipterocarp forest in Malaysia. The forest floor stratum was characterized by low visibility and limited airflow. Floral rewards were small, reflecting low productivity of light-limited plants and low energy requirements of associated pollinators, and flowering times were extended, increasing the probability of pollination by infrequent visitors. The plant-pollinator association of this stratum was dominated largely by nonselective, low-energetic beetles, midges, and other flies. These pollinators were attracted over short distances by strong olfactory cues,

Centris fusci-

ventris

Epicharis albofasciata

Centris fusci-

ventris

Epicharis albofasciata

Eulaema polychroma

Euglossa hemichlora

Pollination Bees Hopea

| Vertical stratification of pollinator species in a tropical rainforest. The two bee species above pollinate flowers in the upper canopy and the two species below pollinate flowers in the subcanopy. From Perry (1984) © George V. Kelvin/Scientific American.

Eulaema polychroma

Euglossa hemichlora r50

| Vertical stratification of pollinator species in a tropical rainforest. The two bee species above pollinate flowers in the upper canopy and the two species below pollinate flowers in the subcanopy. From Perry (1984) © George V. Kelvin/Scientific American.

often resembling dung or carrion, which have limited effective range. The under-story stratum shared many of the environmental features of the forest floor. Plants in this stratum also offered limited visual cues and floral rewards and were pollinated by nonspecific trapliners (i.e., species that revisit particular plants along an established circuit; e.g., trigonid bees, solitary wasps, and butterflies).The overstory stratum generally was characterized by brightly colored flowers, held above the canopy to attract pollinators over a wide area, and brief, highly synchronized flowering within plant species. Dominant pollinators were Apis dorsata and trapliners such as carpenter bees, birds, and bats. Dipterocarps in the genera Shorea, Hopea, and Dipterocarpus formed a separate association based on tiny flowers with limited nectar rewards and nocturnal flowering. Thrips and other tiny, flower-feeding insects were the primary pollinators. By contrast, Sakai et al. (1999) observed that beetles (chrysomelids and curculionids), rather than thrips, were the primary pollinators of these tree species in Sarawak. Finally, some plant species representing various canopy positions were cauliflorous (i.e., they produced flowers along the trunk or main branches).These flowers usually were large, or small and clumped; pale colored; odiferous; and produced during a brief, highly synchronized period. Pollinators included under-story and overstory insects, birds, and bats. Momose et al. (1998b) noted that longdistance pollinators tended to be less common in Malaysian forests than in Neotropical forests.

Roubik (1993) experimentally manipulated availability of floral resources from different canopy strata in tropical forests in Panama. Results indicated that the apparent fidelity of pollinator species to particular canopy strata reflected pollinator preferences for particular floral resources. Most pollinator species were attracted to their preferred floral resources regardless of their location in the canopy.

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