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52 can plant-pollinator interactions promote plant diversity? predictions of a recent more general model of plant-animal mutualistic systems (Bastolla et al., 2009). In our simulations, the presence of pollinators most strongly affected plant community composition when the plants’ intrinsic rate of reproduction in the absence of pollinators was too low to occupy all available habitat. This seems plausible, given that unlike some other types of mutualistic services (e.g. corals offering shelter for microalgae: Stachowicz, 2001), pollination does not create new habitat, nor does it allow plants to colonize habitat types where they could not grow in the absence of pollinators. In reality, underutilized habitat usually occurs in early successional stages of community dynamics, but rarely at equilibrium. However, in environments subject to frequent disturbances accompanied by high mortality, the balance between reproduction and mortality may well result in equilibrium densities far below the habitat capacity. For such situations, our model predicts that the addition of animal pollinators will increase the total population size of the plant community at equilibrium simply because the added fertility (seed production) induced by pollination increases population growth. Plant-pollinator systems without a trade-off penalizing generalist flower visitors produce the highest increase, due to the fact that both the size of the overall animal population and their rate of successful flower visitation are highest in the absence of such a trade-off (data not shown). Although the establishment of a larger overall population would principally allow for the coexistence of more species above the extinction threshold, in most cases species richness declined in the presence of pollinators. This is due to the fact that typically only a small number of plant species benefit from the pollinators’ services in terms of increased abundance at equilibrium, at the expense of other plant species. The fact that this negative effect of animal pollination diminishes as pollen carryover increases indicates that its cause lies in the reduced quantity of pollen that rare plants receive under conditions of limited pollen carryover (Benadi et al., 2012b). Introducing pollination thus amplifies existing differences in plant species abundance. Interestingly, interaction matrices with a trade-off had a much stronger negative effect on species richness compared to matrices without such a trade-off. This difference can be understood by considering the dual effect of animal pollination on plant birth rates. First, pollination generally leads to an increase in the mean number of seeds per plant. Yet secondly, pollination also increases the variation in per-capita seed production between species. The effect of a certain absolute difference in
3.5 discussion 53 seed numbers between species on the relative difference in seed numbers is largest when the mean number of seeds per plant is low. Consequently, the negative effect of variation in seed numbers on plant coexistence is most pronounced when plant birth rates are generally low, as is true in the case of a tradeoff limiting pollinator effectiveness. For the same reason, pollinator presence has a stronger negative effect on plant species richness with a lower rate of vegetative reproduction. Only for specific parameter combinations, namely a low rate of vegetative reproduction, a narrow range of plant competition coefficients, the absence of a generalist-specialist trade-off and when assuming a high extinction threshold of 100 individuals, does the addition of pollinators have a positive effect on species richness of the plant community. For real ecosystems, assuming a higher extinction threshold in relation to the habitat capacity is justified if communities underlie larger fluctuations of environmental conditions (Pimm et al., 1988). However, even though according to our model under such conditions interactions with pollinators indeed promote plant diversity, the positive effect on plant diversity is still less pronounced than it could be if pollinators increased the equilibrium population sizes of all plant species in equal proportion. Instead, the presence of pollinators enhances the growth of some populations, while inhibiting that of rarer ones due to the above mentioned disadvantage in pollination rates. As a consequence, at equilibrium medium-sized populations are largely absent, and the evenness of the community of surviving plant species is close to its theoretical maximum. The three types of interaction network structures examined in this study differ in their degrees of specialisation: While full and diagonal matrices represent extreme cases of uniformly generalised and specialised interactions, respectively, a nested matrix contains a mixture of specialist and generalist species interacting in a way that maximizes the number of shared interactions between members of the same guild (Bastolla et al., 2009). In this study, we used nested and diagonal matrices of the same connectance, and thus also the same mean number of interactions per species. These two matrix types differ only with regard to the distribution of links among species. Depending on the structure of the interaction network, limited pollen carryover and a trade-off in trait matching influence population growth rates to different degrees. The effect of limited pollen carryover is most pronounced for full interaction matrices, because here the probability that a flower visitor has previously visited flowers of other species is greatest. Likewise, systems
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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
Page 21 and 22: 1.3 ecological stability 5 tions an
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 and 66: 3.4 results 49 difference in evenne
Page 67: 3.5 discussion 51 abundance 150 100
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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102 the importance of being synchro
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104 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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