Pollination is a biological process in plants that involves transfer of pollen grains from the anther (male part) of a flower to the stigma (female part) either of the same flower or of a different flower of the same plant species. This movement of pollen grains from anthers to stigmas is influenced by vectors which may either be biotic for example; variety of animals such as insects, birds and mammals or abiotic for example; water and wind (Kearns & Inouye, 1993). Pollination, however, is a prerequisite for fertilization in flowering plants which involve fusion of male nuclei with female nuclei in the ovule leading in seed development. Ecology on the other hand, is the study of the relationships between living organisms including man and their physical environment. It involves the interaction of living organisms with each other and with their physical environment which is made up of matter and energy (Hall, 2014). Therefore, pollination ecology may be defined as the interaction between plants and animal in their process to grow, develop, reproduce and thrive within an ecosystem.
Alpine zone is a high-altitude ecosystem with elevation above 3000 meters above sea level. This biome is found mainly in high elevation mountains in the world. It is a treeless area beyond timberline characterized by extremely cool or cold summers, low biotic diversity, that is, low herbaceous or shrubby vegetation with broad mats of mosses or lichen and short life-cycles (Billings & Mooney, 1968). The common flora here is angiosperms with few species of bryophytes, lichens, and ferns. In addition, all plants are adapted to sweeping winds and disturbances of the soil (K??rner, 1999). However, there is acute reduction in species number with escalating elevation attributed to abrasive weather conditions. Following this rigorous environmental circumstances, alpine plant acquire adaptive strategies in order to thrive within alpine habitats. Some of these adaptations may be observable while others may be unperceivable to a researcher.
Pollination in most alpine plants is facilitated by pollen vectors such as insects (e.g. bees, flies, moths, and butterflies) and birds (e.g. sunbirds) which visit the plant to forage for rewards such as nectar, pollen, wax, oil among others. However, most of the pollen vectors are specialized pollinators among alpine plants. This specialization may be attributed to diverse adaptive strategies by individual alpine plant species across the alpine landscape. In these habitats, an individual pollen vector may be the exclusive pollinator of a plant species (Primack, 1978).
Pollination is the vital phase that generates seeds which are cardinal for reproductive success in plants. Alpine habitats characterized by very high temperature in daytime and extremely low temperatures at night and short, cold, and unpredictable summers, limit sexual reproduction at all phases of plant development hence retarded or halted reproductive success (Hedhly, 2011) therefore resulting in their decline, hence, extinction. Different plant species have varying temperature limits for reproductive processes with regards to their adaptation to temperature of a particular habitat (McKee, 1998). For example, alpine plants experience a broad range of wavering temperatures in daytime and at night during their growth period. These characteristic alpine conditions coupled with strong winds and uncertain storms have drastically affected pollen vectors. Consequently, low pollen (Utelli & Roy, 2000).
The concern on how alpine plants cope with extreme weather conditions and unpredictable pollinator visitation lies in the plant’s adaptive mechanisms. Specialized growth and reproductive adaptations are crucial to flowering plants in alpine environments, which experience short summers and low temperatures that limit plant metabolic and photosynthetic rates (K??rner, 1999). However, in order for alpine plants to adjust to exhaustive alpine weather conditions as well as maintain their reproductive cycle, they develop both vegetative and reproductive adaptive strategies. Some alpine plants have embraced autonomous self-fertilization (a vital evolutionary strategy) as their means of reproduction to counter the limited opportunities for outcrossing (Arroyo et al., 2006). This is because self-compatible plants are less dependent on pollinators’ activity.
However, there still thrive some species which are strict out-crossers. For these plant species, reproduction is achieved by pollinators; hence considerable resources are invested in pollinator attractants and rewards to sustain reproductive success among outcrossing species (K??rner, 1999). The flower color, flower size (Galen, 1999), odor (Shuttleworth & Johnson, 2009), nectar volume and pollen production (Irwin et al., 2004) are fundamental traits for successful cross-pollination. Some self-incompatible alpine plants also exhibit prolonged floral longevity which is associated with habitat, pollinator type, taxonomy class and breeding system (Primack, 1985) as an adaptive mechanism to compensate for unpredictable weather that is unfavorable to pollinator visitation thereby maintaining outcrossing. Cooler atmospheric temperatures in alpine habitats results in slower development of flowers thereby, prolonged flower life-span.
Moreover, alpine plants may also use a range of strategies that are profitable to their pollinators and themselves as well in order to alleviate effective pollen transfer. For example, due to cold weather, some alpine plants may provide refuge to some visitors or protection from predators. Heat and shelter may be regarded as secondary rewards for flower visitors in cold weather (X. Zhu et al., 2013). In addition, apomixes, late-acting self-pollination and desertion of zoophily for anemophily or a mixture of both (i.e. ambophily) among high-mountains plants are an adaptive strategy for reproductive assurance (Gul??as & Traveset, 2012; Totland & Sottocornola, 2001).
The limited number of pollinators in alpine ecosystems has a tremendous influence in both mating system and evolutionary process among alpine flora. Furthermore, reliable pollinator services may be also marred by fluctuating weather distinctive to alpine ecosystem and short blooming period (K??rner, 2003). Scarcity of pollinators restrains outcrossing process hence reproductive output (Garc??a-Camacho & Totland, 2009) promoting autogamous selfing which favor reproductive success.
Alpine ecosystems experience pollen limitation or reduced reproductive success due to inadequate supply of pollen attributed to low pollinator abundance and unreliable pollinator services (Ashman et al., 2004; Garc??a-Camacho & Totland, 2009). Increased competition among alpine outbreeding plants is also evident due to diminished pollinators’ abundance. However, some species have adapted selfing to counteract low pollinator visitation and promote reproductive assurance (Ashman et al., 2004). It has been reported that the frequency of selfing mechanism and asexual reproduction have influenced pollen limitation among alpine taxa (Crawford, 1989).
Fruit set and seed set are also hampered by unfavorable alpine conditions and therefore extremely low output is achieved. In other cases the seeds produced may be inviable. However, this may not be the case in all alpine taxa; some plant species may exhibit successful fruit and seed set during short summer season. In the contrary, some developing fruits may fail to ripen during the short summer period; however, they become dormant until the subsequent summer when they resume maturity (Bliss 1958; Amen, 1966). Dormancy is a protective mechanism among some alpine plans, though not all since some species germinate if introduced in favorable growth conditions. Dormancy is more common among dominant and abundant species and may contribute to their success.
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