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

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(facilitative) interactions between plant species is considered
separately. For each mechanism, I try to answer the following
questions: Is it theoretically plausible? What is the empirical
evidence that the mechanism is important for diversity maintenance
in real plant communities? I conclude this section with a
discussion of open questions.
6.1.1 Equalising mechanisms
As mentioned in chapter 1, equalising mechanisms contribute
to species coexistence by reducing fitness differences between
community members. Numerous trade-offs may equalise the
fitness of flowering plants with different strategies, for example,
a trade-off between allocation of resources to pollinator
attraction and production of ovules (Haig & Westoby, 1988),
but these do not affect the fitness disadvantage of plant species
with low relative abundance. In the following, I discuss the
effects of behavioural traits of pollinators and the spatial structure
of plant communities that may decrease the difference in
the quality and/or quantity of pollinator visits received by rare
and abundant plant species.
Flower constancy, the specialisation of individual pollinators
on particular flowering plants, reduces loss of pollen to heterospecific
flowers and clogging of stigmas with pollen of other
plant species (Levin & Anderson, 1970; Straw, 1972; Goulson,
1994; Montgomery, 2009). The effect on plant reproduction is
essentially the same as for specialisation at the species level.
However, whether pollinator constancy really reduces the difference
in reproductive success between rare and common plant
species depends on the relative numbers of pollinators specialising
on each plant species and their visitation rates. Obviously,
specialisation on a rare plant species can only be profitable for
a pollinator if the reward per time it receives is at least as high
as for specialisation on a more common plant species. Since
pollinators foraging on rare plant species have higher costs of
travelling than those foraging on more abundant plants, the
only way to achieve equal rewards per time is if rare plant
specialists receive a higher reward per flower visited. In a
game-theoretic model of pollinator foraging behaviour, Kunin
& Iwasa (1996) explored the conditions for equal profitability
of specialisation on a rare and a common plant species, respectively,
and the consequences for plant reproductive success. Assuming
that both plant species produce nectar at identical rates,
they found that the rarer plant species always received disproportionately
fewer pollinator visits compared to the common