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110 synthesis In one of the earliest theoretical papers on plant competition for pollination, Levin & Anderson (1970) already pointed to the important role of the spatial distribution of plant species for plant and pollinator fitness, but to my knowledge only Campbell (1986) explicitly incorporated small-scale spatial structure into her model of plant reproduction. Nonspatial models such as the one presented in chapter 2 usually assume a homogeneous landscape with random distribution of plant species and "blindly searching" pollinators. Deviations from these patterns can contribute to species coexistence in several ways. Spatial autocorrelation of conspecific plants may minimise pollen loss to heterospecific flowers and shorten travelling times of specialised pollinators, assuming that pollinators move primarily between neighbouring flowers. Thus, the destabilising effects of both generalised and specialised plant-pollinator interactions can be mitigated. Since most natural plant communities show some degree of spatial autocorrelation of conspecifics due to dispersal limitations (Crawley, 2009), this mechanism is likely to be important. Whether it is sufficient to completely equalise the reproductive success of plant species with different relative abundances merits further investigation. In addition, since competition for water, light and nutrients is strongest among direct neighbours, spatial aggregation of conspecifics can reduce the relative strength of interspecific competition for abiotic resources in plant communities (Stoll & Prati, 2001; Bolker et al., 2003). 6.1.2 Stabilising mechanisms Given the destabilising properties of plant-pollinator interactions, are there intrinsic mechanisms that actually contribute to stable coexistence of animal-pollinated plant species? Behavioural flexibility of foraging pollinators could in principle have a stabilising effect if pollinators preferentially target rare plant species. Whereas good theoretical support and empirical evidence for negative frequency-dependent pollination of rewardless flowers exist (Smithson & MacNair, 1997; Gigord et al., 2001), pollinators visiting rewarding flowers are generally expected to exhibit positive frequency-dependence to maximise their foraging success (Eckhart et al., 2006, and references therein). However, if the most efficient or common pollinator species preferentially visits the most abundant plant, other pollinators may benefit from focusing on less common plant species (Possingham, 1992). This phenomenon has been reported for visitors to two colour morphs of Clarkia xantiana
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
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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
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1 I N T R O D U C T I O N “Es ist
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1.3 ecological stability 3 vegetabl
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1.3 ecological stability 5 tions an
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1.4 specialisation 7 tween the fund
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1.4 specialisation 9 may be promote
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1.4 specialisation 11 ogy through h
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1.4 specialisation 13 tionships bet
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1.5 outline of this thesis 15 1.5.1
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1.5 outline of this thesis 17 2007)
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20 plant-pollinator dynamics: stabi
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3 C A N P L A N T- P O L L I N AT O
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3.2 introduction 41 the maximum num
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3.3 the model 43 certain types of r
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3.3 the model 45 time unit, summed
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3.3 the model 47 Figure 3.1: Illust
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3.4 results 49 difference in evenne
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3.5 discussion 51 abundance 150 100
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3.5 discussion 53 seed numbers betw
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3.6 appendix 55 Despite the existen
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3.6 appendix 57 mean number of visi
Page 75 and 76: 3.6 appendix 59 Table 3.2: Plant di
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Page 116 and 117: 100 the importance of being synchro
Page 123 and 124: S Y N T H E S I S 6 The previous fo
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Page 131 and 132: 6.2 consequences of climate change
Page 137 and 138: S U M M A RY For most angiosperm pl
Page 139 and 140: summary 123 veloped in chapter 2. O
Page 141: summary 125 and intensity of extrem
Page 144 and 145: 128 zusammenfassung ten der Erhaltu
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Page 149 and 150: R E F E R E N C E S Abrams, P.A. (2
Page 151 and 152: eferences 135 R., Thomas, C.D., Set
Page 153 and 154: eferences 137 Clavel, J., Julliard,
Page 155 and 156: eferences 139 role of trade-off str
Page 157 and 158: eferences 141 Gomez, J.M., Abdelazi
Page 159 and 160: eferences 143 Ives, A.R. & Carpente
Page 161 and 162: eferences 145 Leibold, M. & McPeek,
Page 163 and 164: eferences 147 Moonen, A. & Barberi,
Page 165 and 166: eferences 149 Pielou, E.C. (1975).
Page 167 and 168: eferences 151 Schemske, D.W., Mitte
Page 169 and 170: eferences 153 Visser, M.E. & Hollem
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