Linking Specialisation and Stability of Plant ... - OPUS Würzburg

6.1 mechanisms of diversity maintenance 109
plant. However, the difference in reproductive success of the
two plants was lower than in the case of generalist foraging
behaviour. Thus, it seems that flower constancy can alleviate
the fitness difference between rare and common plant species,
but constancy alone is not sufficient to ensure species coexistence.
Flower constancy is particularly well studied in bees, but
it has been reported to occur in all other major groups of pollinators
as well (reviewed by Amaya-Marquez, 2009). Several
hypotheses have been suggested to explain why pollinators exhibit
this behaviour (Chittka et al., 1999). Just like specialisation
at the species level, constancy can be viewed as an adaptation to
maximise resource intake while minimising the costs of learning
to recognise and handle different types of flowers. Thus,
constancy is expected to be common when co-occurring plant
species differ strongly in floral traits, the energetic reward per
flower is large and the costs of travelling are low.
Another mechanism that has been proposed to contribute to
coexistence of plant species sharing pollinators is pollen carryover,
that is, pollen carriage over non-consecutive flower visits.
In the simplest case, carryover is modelled as the maximum
number of flower visits between pollen removal and deposition
(Feldman et al. 2004; chapter 2). An increase in this number
indeed results in slower competitive exclusion of rare plant
species. As the maximum number of flower visits between
pollen removal and deposition approaches infinity, pollen receipt
is completely equalised because every flower visit results
in pollination regardless of a plant species’ relative abundance.
However, in a more detailed model incorporating a trade-off between
the degree of pollen carryover and the amount of pollen
delivered per visit, Montgomery (2009) showed that under certain
conditions pollen carryover may even have a detrimental
effect on the reproductive success of rare plants, particularly
in combination with flower constancy of pollinators and the requirement
of a threshold quantity of pollen for successful pollination.
Such a trade-off probably exists for the majority of plant
species, although some species, for example many orchids, are
able to minimise pollen loss in intervening visits by precisely
positioning packages of pollen on the pollinator’s body (Johnson
& Edwards, 2000). In general, the distribution of pollen of
a donor plant in a sequence of flower visits depends on floral
traits as well as on morphology and behaviour of pollinators
(Harder & Johnson, 2008). Thus, species-specific traits need
to be considered to determine whether pollen carryover contributes
to plant species coexistence in each case.