Measurement of Pollination

Pollination efficiency reflects the probability that pollen reaches a conspecific flower. A number of factors influence the efficiency of pollen transport between conspecific reproductive structures. The mechanism of pollen transport, proximity of conspecific plants, pollinator attraction to floral structures, adaptations for carrying pollen, fidelity, and thermodynamic constraints determine the probability that a flower will receive conspecific pollen.

Several methods have been used to measure pollinator activity and pollination efficiency. Observations of the type and frequency of floral visitors can provide a measure of pollinator activity (Aizen and Feinsinger 1994, Ghazoul and McLeish 2001, Sakai et al. 1999, Steffan-Dewenter and Tscharntke 1999, Steffan-Dewenter et al. 2001). Interception traps also can be used to collect insects visiting flowers (S. Johnson et al. 2004). The number of fertilized seeds per flower provides a measure of pollination for self-incompatible species (Steffan-Dewenter et al. 2001, S. Johnson et al. 2004). Kohn and Casper (1992) used electrophoresis to identify seeds containing alleles that did not occur in neighboring plants. G. White et al. (2002) used DNA (deoxyribonucleic acid) marker techniques to measure pollen transfer among trees, Swietenia humilis, in isolated fragments of tropical forest in Honduras.

Wind pollination is highly inefficient.The probability of successful pollen transfer by wind decreases as the cube of distance between plants (Moldenke 1976). However, plant investment in individual pollen grains is negligible so large numbers can be produced, increasing the cumulative probability that some will land on conspecific reproductive structures. Directed transport of pollen by animal pollinators increases efficiency to the extent that the pollinator visits a conspecific flower before the pollen is lost or contaminated with pollen from other plant species. Hence, animal-pollinated plant species may invest energy and nutrients in adaptations to improve the fidelity of the pollinator. These adaptations include nectar rewards to attract pollinators, floral and aromatic advertisements; floral structures that restrict the diversity of pollinators visiting the flowers, synchronized flowering among conspecific individuals, and divergence in time of flowering among plant species to reduce pollen contamination (Heinrich 1979).

Nectar rewards must be sufficient to compensate the pollinator for the foraging effort. For example, a greater nectar return is necessary to attract bees during cooler periods, when energy allocation to thermoregulation is high compared to warmer periods (Heinrich 1979). Heinrich (1979) noted that pollinator fidelity reflects offsetting adaptations. Plants invest the minimum amount of energy necessary to reward pollinators, but pollinators quickly learn to concentrate on flowers offering the greatest rewards. Individual plants in aggregations could attract bees and be pollinated even if they produced no nectar, provided that their neighbors produced nectar. The nonproducers should be able to invest more energy in growth and seed production. However, if these "cheaters" became too common, pollinators would switch to competing plant species that offered greater food rewards (Feinsinger 1983). A. Lewis (1993) suggested that floral characteristics may reflect advantages accruing to the plant when pollinators must make a substantial investment in learning to handle a flower, thereby becoming facultative specialists. Plant investment in attractants and rewards for pollinators represents an evolutionary tradeoff between growth and reproduction (Heinrich 1979) and may affect the ability of light- or resource-limited species to attract pollinators. Bawa (1990) reviewed studies that demonstrated long-distance pollen flow and outcrossing for tropical canopy trees but a high degree of inbreeding for many tropical herbs and shrubs.

Effects of pollination on plant seedling recruitment and ecosystem processes have been measured less frequently. Effects on seed production can be measured as the number of seeds produced when pollinators have access or are excluded from flowers (S. Johnson et al. 2004, Norman and Clayton 1986, Norman et al. 1992, Steffan-Dewenter and Tscharntke 1999, Steffan-Dewenter et al. 2001). Pollinator effects on ecosystem processes should reflect their direct influence on plant reproduction and indirect influence on vegetation dynamics.

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