Optimal number of flowers probed per plant in relation to floral display size

Possible factors causing patch depression during foraging on a plant

How many flowers does a pollinator probe after landing on a plant or inflorescence.? As shown in Fig. 14.1B and Table 14.1, previous authors have found that pollinators tend to probe only a few flowers on a plant or inflorescence before leaving even when far more flowers are present. Furthermore, the proportion of flowers probed generally declines considerably with increasing display size. Reasons suggested by plant biologists for short visitation sequences include: (1) satiation of pollinators; (2) draining all floral rewards on a plant; (3) the need to avoid predator or (aggressive) competitors; and (4) the need to find mates or other types of food (Stephenson 1982; Snow et al. 1996). However, none of these ideas seems applicable to most plant-pollinator systems studied so far, especially to bumble bees and their flowers.

Instead, here we address this problem from the standpoint of optimal foraging. We use "plant" as a general term that may indicate either an individual plant or an inflorescence, which would in practice be regarded by pollinators as one flower patch. Because plants are distributed as discrete patches, moving between plants is more costly for a pollinator than moving within a plant in terms of time and energy expended (Heinrich 1975). Therefore, the pollinator should probe all available flowers on a plant unless the rate of energy gain declines as it stays longer on it. Thus, a pollinator's decision largely depends on the presence or absence of a gradual decrease in the rate of energy gain within a plant, i.e., "patch depression" (originally termed "depression" in Charnov et al. 1976).

Two major mechanisms could cause patch depression on a plant. One possibility is variation in nectar productivity coupled with pollinators' non-random flower choice. For example, plants that have flowers on simple vertical inflorescences often have a pattern of decreasing rate of

Table 14.1. A tabulation of reports concerning how parameters of pollinator visitation (visitation rate per plants^ number of flowers probed on a plant proportion of flowers probed per plant tc/F, and visitation rate perflowerNj) correlate with floral display size (F); the notations +, -, and ± indicate positive, negative, and statistically non-significant relationships found between these parameters and floral display size

Table 14.1. A tabulation of reports concerning how parameters of pollinator visitation (visitation rate per plants^ number of flowers probed on a plant proportion of flowers probed per plant tc/F, and visitation rate perflowerNj) correlate with floral display size (F); the notations +, -, and ± indicate positive, negative, and statistically non-significant relationships found between these parameters and floral display size

Plant density and plant species

VP

tc

tJF

Vt

Pollinator

References

High (>0.1 plants m~2)

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