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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
3.5 discussion 51 abundance 150 100 50 (a) 400 300 200 100 (b) 400 (c) 300 200 100 0 1 4 7 11 16 21 26 0 1 4 7 11 16 21 26 0 1 4 7 11 16 21 26 species rank Figure 3.4: Potential and realized influence of pollinators on the abundance distribution of a plant community at equilibrium. Panel A shows the equilibrium abundance distribution of an exemplary plant community in the absence of pollinators, panel C the same community with plant-pollinator interactions, with a fully connected interaction network and no generalist-specialist trade-off. In panel B, the same abundance distribution as in A is shown, but all abundances have been increased by a constant factor so as to obtain the same total population size as in panel C. The dashed horizontal line marks the extinction threshold of 100 individuals. Species richness was determined as the number of plant species with population sizes above the extinction threshold. The same parameter values as in Fig. 3.2 were used for this figure, and a low rate of vegetative reproduction was chosen (b veg = 5 · 10 −8 ). for fully connected matrices (Fig. 3.5). Regarding the amount of nectar per plant N, a lower limit was defined by the minimum amount needed to sustain the pollinator community with a trade-off in trait matching. Increasing the amount of nectar above this level (N = 4) reduced the difference between the effects of pollinator presence with and without a trade-off (Fig. 3.6). On the other hand, a wider range of degrees of plant niche overlap (values taken from a uniform distribution with boundaries {0.5, 1} instead of {0.65, 0.85}) reduced the positive effect of animal pollination on plant species richness with a high extinction threshold and no trade-off (Fig. 3.6). 3.5 discussion Understanding the factors that explain the variation of biodiversity in space and time is one of the main challenges in ecological research. In the current study, we assess the potential influence of plant-pollinator interactions, one of the most common types of mutualism, on plant diversity. The results of our numerical simulations suggest that the addition of pollinators rarely enhances the diversity of a plant community with alternative means of reproduction. This result is in contrast to the
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L I N K I N G S P E C I A L I S AT
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E R K L Ä R U N G gemäß § 4 Abs
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D A N K S A G U N G Zuallererst mö
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xiv contents 3.3 The Model 43 3.3.1
Page 17 and 18: 1 I N T R O D U C T I O N “Es ist
Page 19 and 20: 1.3 ecological stability 3 vegetabl
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Page 23 and 24: 1.4 specialisation 7 tween the fund
Page 25 and 26: 1.4 specialisation 9 may be promote
Page 27 and 28: 1.4 specialisation 11 ogy through h
Page 29 and 30: 1.4 specialisation 13 tionships bet
Page 31 and 32: 1.5 outline of this thesis 15 1.5.1
Page 33: 1.5 outline of this thesis 17 2007)
Page 36 and 37: 20 plant-pollinator dynamics: stabi
Page 55 and 56: 3 C A N P L A N T- P O L L I N AT O
Page 57 and 58: 3.2 introduction 41 the maximum num
Page 59 and 60: 3.3 the model 43 certain types of r
Page 61 and 62: 3.3 the model 45 time unit, summed
Page 63 and 64: 3.3 the model 47 Figure 3.1: Illust
Page 65: 3.4 results 49 difference in evenne
Page 69 and 70: 3.5 discussion 53 seed numbers betw
Page 71 and 72: 3.6 appendix 55 Despite the existen
Page 73 and 74: 3.6 appendix 57 mean number of visi
Page 75 and 76: 3.6 appendix 59 Table 3.2: Plant di
Page 77: 3.6 appendix 61 proportional differ
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Page 100 and 101: 84 the importance of being synchron
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100 the importance of being synchro
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S Y N T H E S I S 6 The previous fo
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6.1 mechanisms of diversity mainten
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6.2 consequences of climate change
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S U M M A RY For most angiosperm pl
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summary 123 veloped in chapter 2. O
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summary 125 and intensity of extrem
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128 zusammenfassung ten der Erhaltu
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130 zusammenfassung wir den Zusamme
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R E F E R E N C E S Abrams, P.A. (2
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eferences 135 R., Thomas, C.D., Set
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eferences 137 Clavel, J., Julliard,
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eferences 139 role of trade-off str
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eferences 141 Gomez, J.M., Abdelazi
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eferences 143 Ives, A.R. & Carpente
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eferences 145 Leibold, M. & McPeek,
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eferences 147 Moonen, A. & Barberi,
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eferences 149 Pielou, E.C. (1975).
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eferences 151 Schemske, D.W., Mitte
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eferences 153 Visser, M.E. & Hollem
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L I S T O F P U B L I C AT I O N S
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