2.4. Determinants of flowering period length
2.4.2. Pollinator availability
The success of reproduction in plants is not entirely the result of genetic background and abiotic environmental factors, but also the interactions between plants and pollinators.
Pollination occurs during the flowering period of a plant, when pollen is transferred from the anther of one individual to the stigma of another (or the same) individual of the same species (Öpik and Rolfe, 2005). There are a numerous modes for pollen to be transferred. These include the wind and living vectors, such as vertebrates and insects, known as pollinators (Öpik and Rolfe, 2005). The floral traits of a plant species are important in attracting the right pollinator or pollinators. The relationship between plants and pollinators has been studied extensively from population to community level (Kearns and Inouye, 1997).The process of pollination plays an important role in flowering patterns, as the activity of pollinating agents, such as bees, may have seasonal variations, much like flowering (Stevenson et al., 2008).
However, the effect of pollinator availability on the duration of flowering is somewhat poorly understood.
Ramírez (2006) notes that factors such as climate; life form; flowering time and habitat structure, determine the distribution of both flowering plant species and pollinators. This suggests that flowering phenology may potentially determine the abundance of pollinators, particularly in areas where seasonality is easily distinguishable. However, according to
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Elzinga et al. (2007), pollinators are only attracted to flowering plants once certain densities of flowers are in bloom. Elzinga et al. (2007) suggests that the onset of flowering is likely to depend on the synchronicity of an individual plant and its neighbours, when the pollination process is dependent on flowering density. In plant populations of lower flowering densities, greater variation in flowering phenology may be seen (Elzinga et al., 2007). Similarly, many studies have identified the increase and decrease of pollinators according to the abundance of flowers in a given population that has varied plant species (Stevenson et al., 2008). Elzinga et al. (2007) therefore suggest that the variation of the onset of flowering or peak flowering times does not only depend on a phenotypic variation, but also on the length of the flowering period.
Waser (1979) specifically examined the correlation between pollinator (hummingbirds) availability and the initiation of flowering of a perennial shrub Fouquieria splendens in Arizona. Waser (1979) stated that if through evolutionary history, plants are able to detect changes in temperature and photoperiods to induce flowering, then surely natural selection has allowed for plants which are animal-pollinated, to flower during a period when the respective pollinators are abundantly available. Furthermore, Waser (1979) notes that this type of occurrence is more noticeable when specific pollinators appear at a certain time during a season in order to visit an individual plant species which experiences little or no competition, as this can alter the flowering time. The hummingbird migration to southern Arizona is relatively short, lasting no more than a few weeks. During this time the hummingbird population forage predominantly across the entire range of F. splendens.
Hummingbirds, together with bees are the primary pollinators of F. splendens in southern Arizona. However, in some cases flowering did not coincide with the hummingbird migration. Despite this, Waser (1979) believes that there it is possible for flowering periods and pollinator abundance hummingbirds to synchronize.
In most plant communities, a relationship between flowering period and pollinator abundance may not be evident, due to the variation of species occurring in one area. In this circumstance, different plant species may be flowering at the same time during the season, therefore competing for available pollinators, or may flower asynchronously to avoid the competition (Brown et al., 2002).
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Levin and Anderson (1970) considered how plants compete for the same pollinators when flowering simultaneously. In most cases when plants compete for resources such as light, moisture or nutrients, the weaker species is excluded. The competition for pollinators amongst plants determines the reproductive success of the species. Many plants can have the same flowering period and be pollinated by the same animals or insects, resulting in competition, therefore making floral signals and the availability of pollen and nectar significant in the success of the species (Levin and Anderson, 1970). It is advantageous for plants to meet the preferences of the pollinator, as this allows the pollinator to reduce the energy spent on searching for food sources (Levin and Anderson, 1970). However, these preferences are subject to change according to the abundance of the pollen or nectar reward, as well as the abundance of the flowering species. On a similar basis, O’Neil (1999) suggested that pollinator visits may increase in a specific area where the number of flowering plants increases, resulting in pollinator competition and possibly fewer visits by pollinators to individual plants. Levin and Anderson (1970) suggested that flowering plant species that compete for pollinators may be closely related or taxonomically remote, and may have similar floral structure or very distinct and different structures.
Competition for pollinators may also be seen between invasive and native plant species and this has frequently been studied. Brown et al. (2002) found that the invasive species Lythrum salicaria reduced both pollinator visitation and seed set of the native species Lythrum alatum.
The possible explanation for the shift in pollinator visitation was that, Lythrum salicaria had larger floral structures with more flowers on displays. The pollinators were also found to move regularly between the invasive and native species, which may have also reduced the quantity and quality of the pollen in the native species (Brown et al., 2002).
The correlation between pollinator availability and competition, in influencing the onset and duration of flowering is not frequently studied and hence is poorly understood. Kochmer and Handel (1986) make brief reference to the influence that phylogenetics may have in determining flowering phenology at a community level in comparison to competition between pollinators. In addition, Kochmer and Handel (1986) proposed that if pollinator competition occurs in flowering plant communities, it may assist in fine-tuning the flowering time and length within the phylogenetic constraints of various plant species.
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