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

50 can plant-pollinator interactions promote plant diversity?
proportional difference
in species richness
1.0
0.5
0.0
−0.5
−1.0
low
full matrix nested matrix diagonal matrix
no trade−off
trade−off
high
low high low high
vegetative reproduction
Figure 3.3: Effect of plant-pollinator interactions on plant species richness with a high extinction
threshold (100 individuals). Boxplots show the proportional difference between values
with and without pollinators for 100 simulation runs. All parameter values are
identical to those of Fig. 3.2.
flower visitors resulted in markedly decreased species richness,
particularly for full and nested networks.
Evenness of the plant community remained largely unaffected
by the presence of pollinators (Fig. 3.2, lower panel). However,
in most simulation runs plant evenness increased slightly under
the influence of pollinators. A further increase of evenness
would not have been possible, since the evenness of the community
of surviving plant species was already close to its maximum
in the absence of pollinators (Table 3.2).
Direct examination of the distribution of plant abundances
at equilibrium showed that even in those cases with a positive
effect of pollinators on plant species richness the observed increase
in species richness fell short of the expected increase
based on the size of the total plant population (Fig. 3.4). This
discrepancy was due to the fact that not all plant species benefited
from the presence of pollinators. Whereas the distribution
of plant abundances ordered by rank showed a gradual
decrease from most- to least-abundant in the absence of pollinators,
the addition of pollinators resulted in a sharp transition
between a group of highly abundant plant species and a second,
larger group of species with population sizes close to zero.
Deviations from the default parameter set did not result in
major changes concerning the effects of the presence of pollinators
on plant community composition (Figs. 3.5-3.7). While
the results for moderately higher pollen carryover (B = 5) were
nearly identical to those of B = 1, at B = 50 the negative effect
of pollinators on species richness was reduced, particularly