Pollinator effectiveness at the stigma level in a garden
C) Pollinator importance
5.4.6 Future directions
The creation of PE/PI networks is still in its infancy, and more plant-pollinator communities need to be studied in this way to test whether the results in my study are commonplace. Additional research questions raised by this chapter include:
(i) Experimentally manipulating species abundance
Only a small number of studies have manipulated plant-pollinator communities and documented the effect on network structure (e.g. Fontaine et al.,2006; Lopezaraiza-Mikel et al., 2007; Brosi & Briggs, 2013; Goldstein et al. 2016) and the collection of PE data represents an opportunity to expand this. By removing the most abundant Bombus from the flower visitor community, Brosi and Briggs (2013) found a reduction in the levels of floral fidelity (reduced specialisation) of the remaining visitors. This led to a reduction in the proportion of conspecific pollen carried and deposited, and ultimately seed set in the flowers of Delphinium barbeyi. One of the most interesting outcomes of these results is how small changes to the visitor community can have considerable effect on the fitness of individual plants, despite little effect on measures of network robustness. It would be intriguing to expand this to several plants, and test the effect of removing the most abundant pollinator in the garden (Apis) on the pollinator effectiveness of the remaining community.
(ii) Measuring pollen viability
The methods used in this study (fuchsin gel staining) could not distinguish between self and cross conspecific pollen. Consequently, it is possible that visitors which deposited large quantities of self-pollen were given greater importance than visitors that deposited small quantities of cross-pollen. To develop a full picture of the value of visitors as pollinators, future PE networks should attempt to take this into account by considering post-pollination events such as pollen germination, the growth of pollen tubes and ovule fertilisation. For self-
Chapter 5. Pollinator effectiveness of garden flower visitors
159
incompatible species that demonstrate a clear response to self-pollen, e.g. the inhibition of self- pollen in Brassicaceae, this could be performed by counting the number of germinating (cross- ) pollen tubes following the first visit, and using this data to create a more detailed PE network. Patchett & Willmer (in review) demonstrated that the number of pollen grains germinating on the stigma of Brassica rapa was two magnitudes lower than that deposited, which raises intriguing questions about the deposition values in this study. Similarly, Cresswell (1999) found pollen deposition by Bombus in flowers of Brassica napus was three times greater when pollen was not experimentally removed, in accordance with the difference in deposition on emasculated flowers reported by Delmas et al. (2016). However, measuring pollen tube growth is extremely time consuming and not appropriate for all plants especially those that lack self- incompatibility.
However there is a possibility that microsatellite genotyping, or AFLP-PCR, of the pollen collected from stigmas, could be used to clarify the importance of visitors as agents of cross- pollination in urban plant populations (Vamosi et al. 2016). While these methods are still in their infancy, there is potential to use them to determine the identity and distance travelled of pollen in urban environments. Bees are known to vary in their foraging ranges, with a maximum distance of 600m between nesting site and food patch for some solitary species (Gathmann & Tscharntke 2002) and up to 6km in Apis (Hagler et al. 2011). Documenting the genetic identity of pollen deposits along an urban-rural gradient would be fascinating, examining the extent of pollen mixing between urban and rural plant populations, and determining which species of bee (or other visitors) contributed most to the genetic diversity of urban plant gene pools. Furthermore, while the assessment of mixed pollen samples is still problematic (Keller et al. 2014), the low diversity of stigmatic pollen loads makes such data more attractive than processing pollen loads sampled from insect bodies.
Chapter 5. Pollinator effectiveness of garden flower visitors
160
5.5 Conclusion
The purpose of this chapter has been to show the similarities and differences between visitation, PE and PI networks, and evaluate the strengths and limitations of each approach.
Surprisingly, almost all flower visitors deposited pollen and none were found to be consistent ‘cheats’. While the number of grains varied between visitors to each plant, on the whole visitors were more equal in terms of their deposition than predicted by their visitation patterns. Consequently, the specialisation of the PE network was almost identical to the visitation network. However, the combination of both measures (as the PI network) did increase network specialisation slightly, although this did not approach the levels reported in recent pollen transfer networks. Measures of PE confirmed that the most frequent visitor does not always deposit the most pollen, but overall patterns of visitation confirmed that the most abundant visitors often are the most important pollinators.
The statistical methods used to evaluate community-wide pollen deposition were highlighted as an area that requires future attention, as these were shown to greatly influence network structure. The importance of bees as pollinators in gardens was highlighted, and although dipteran visitors had relatively low importance as pollinators, it remains unclear whether pollination in the garden represents a case of functional redundancy or complementarity.
Although the measurement of PE revealed intriguing insights into the interactions in the garden, it is still clear that measures of pollen quality are also needed to truly understand the value of different visitors as pollinators. This represents a considerable sampling effort for future studies at a community level, so it may be more useful to assess the quality of pollen transported along an urban-rural gradient by flower visitors, rather than use a network analysis. While a small collection of PE and PI networks now exist, none have yet been compared to a pollen transport network, and this forms the basis of the next chapter.
161