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

6.1 mechanisms of diversity maintenance 111
(Eckhart et al., 2006). Yet even if this mechanism should be
widespread, the fact that only relatively inefficient or infrequent
pollinators are expected to exhibit negative frequency-dependence
reinforces the competitive disadvantage of rare plant species.
Thus, it seems highly unlikely that negative frequencydependence
of pollinators can account for diversity maintenance
in animal-pollinated plant communities.
In addition to the potential equalising effect of small-scale
spatial structure of plant communities, spatial structure at a
larger scale may act as a stabilising mechanism. Whereas in
our model of plant-pollinator systems (chapter 2) in the absence
of niche differentiation slight disturbances lead to competitive
exclusion of all but the most abundant plant species in
a given location, different species may be favoured by chance
in different local communities. In this manner, diversity could
be maintained at a regional scale. Moreover, if dispersal between
local populations is sufficiently strong, declining populations
may be rescued by immigration from patches where
the species dominate (“mass effect”: Leibold et al., 2004, and
references therein). Since a mass effect results in a higher percapita
“birth rate” (i.e., births plus immigrants) when a species
becomes rare, it fulfils the criterion for a stabilising mechanism.
How important this mechanism is for diversity maintenance
in real plant communities is currently uncertain, due to a lack
of both data and models. Mutualistic metacommunities have
been modelled by Prakash & de Roos (2004) and Fortuna &
Bascompte (2006), but these studies did not account for competitive
interactions within plant and animal communities. In
general, diversity can be maintained through a mass effect if
local communities are sufficiently separated to have diverging
dynamics, but sufficiently connected by dispersal to prevent
competitive exclusion of declining species. The classical mass
effect is based on spatial heterogeneity of the environment that
favours different species in different locations (Leibold et al.,
2004), but in the case of animal-pollinated plants dominance
of one species in a given location may be the result of incidental
higher initial abundance, which would then be amplified
by its higher pollination success. Further research is needed to
assess the importance of mass effects for coexistence of animalpollinated
plants, but at least for plant species that are generally
rare alternative mechanisms need to be considered.
Another potential stabilising mechanism also involves structured
plant populations, but here the structure is temporal rather
than spatial. Clearly, temporal segregation of flowering phenologies
reduces interspecific competition for pollination. In