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Fragmentation and Seed Dispersal in Freshwater Wetlands

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Fragmentation and Seed Dispersal in Freshwater Wetlands Fragmentation and seed dispersal in freshwater wetlands Fragmentatie en zaadverspreiding in zoetwaterwetlands ISBN: 978-94-6108-290-9 Cover: Hester Soomers Grafische vormgeving: Gildeprint Enschede Figuren: Geomedia, Faculteit Geowetenschappen, Universiteit Utrecht Foto’s: Hester Soomers Printed by: Gildeprint, Enschede Printed on FSC certified paper © 2012 Alle rechten voorbehouden. Niets uit deze uitgave mag worden verveelvoudigd, opgeslagen in een geautomatiseerd gegevensbestand, of openbaar gemaakt, in enige vorm of op enig wijze, hetzij elektronisch, mechanisch, door fotokopieën, opnamen, of op enig andere manier, zonder voorafgaande schriftelijke toestemming van de rechthebbende. Fragmentation and seed dispersal in freshwater wetlands Fragmentatie en zaadverspreiding in zoetwaterwetlands (met een samenvatting in het Nederlands) Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de rector magnificus, prof.dr. G.J. van der Zwaan, ingevolge het besluit van het college voor promoties in het openbaar te verdedigen op woensdag 30 mei 2012 des middags te 4.15 uur door Hester Soomers geboren op 5 augustus 1977 te Heerlen Promotoren: Prof.dr. M.J. Wassen Prof.dr. J.T.A. Verhoeven Co-promotor: Dr. P.A. Verweij CONTENTS Chapter 1 Introduction 7 Chapter 2 The effect of habitat fragmentation and abiotic factors on fen plant occurrence 29 Chapter 3 Factors influencing the seed source and sink functions of a floodplain nature reserve in the Netherlands 51 Chapter 4 The dispersal and deposition of hydrochorous plant seeds in drainage ditches 75 Chapter 5 Linking habitat suitability and seed dispersal models in order to analyse the effectiveness of hydrological fen restoration strategies 99 Chapter 6 Wind and water dispersal of wetland plants across fragmented landscapes 127 Chapter 7 Synthesis 163 Abstract 181 Samenvatting 187 Dankwoord 195 Curriculum Vitae 201 Chapter 1 Chapter 1 Introduction 8 | Chapter 1 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 R38 R39 Introduction | 9 1.1 SCOPE R1 R2 The focus of this thesis is on plant seed dispersal via surface water in fragmented freshwater R3 wetlands. R4 In the next sections, I argue why habitat fragmentation is a threat to plant biodiversity R5 and why it is relevant to study seed dispersal in the context of habitat fragmentation. Also, R6 I explain why freshwater wetlands were chosen as a study ecosystem. This chapter ends with R7 the aims and outline of this thesis, where a preview of the executed research is given and the R8 research approach is explained. R9 R10 1.2 HUMAN INDUCED ENVIRONMENTAL THREATS TO PLANT BIODIVERSITY R11 R12 Human impact on nature has been severe in the past centuries. The rate of biodiversity loss R13 massively increased since the beginning of the industrial period, and is now more than hundred R14 times higher than in the pre-industrial period, for well studied taxonomic groups (Vitousek R15 et al., 1997; Rockström et al., 2009). The Royal Botanic Gardens in Kew estimated more than R16 20% of all plant species to be threatened with extinction (Royal Botanic Gardens 2010) and R17 May (2011) states that approximately 70% of the evaluated angiosperm plants are estimated R18 to be threatened. There is evidence that a rich biodiversity contributes to the stability of R19 ecosystems (e.g. Folke et al. 2004, Tilman et al. 2006)) and that reduced plant species diversity R20 may negatively affect ecosystem functioning (Naeem et al. 1994, Isbell et al. 2011). R21 Land use change is seen as the main driver of the loss of biodiversity worldwide (Vitousek R22 et al. 1997, Sala et al. 2000, Millennium Ecosystem Assessment 2005, Rockström et al. 2009). A schematisation of the effects of land use change on plant habitat is given in Figure 1.1. R23 Conversion of natural ecosystems into agricultural land or urban areas has led to habitat loss R24 for many plant species; croplands and pastures now occupy ca 40% of the land surface (Foley R25 et al. 2005). R26 Additionally, habitat quality of the remaining natural ecosystems is often negatively affected R27 by land use change. Fertilizers used in modern agriculture are a major source of excess nitrogen R28 and phosphorus in surface water and also cause leaching of nutrients to groundwater and R29 increased atmospheric nitrogen deposition (Foley et al. 2005). Consequently, the resulting R30 eutrophication of terrestrial and aquatic natural habitats has led to higher plant productivity R31 and thereby often to reduced plant diversity (Grime 1979, Moore and Keddy 1988, Moore R32 et al. 1989). Apart from eutrophication, human land use may lead to desiccation of natural R33 habitats. Drainage of wetlands to enable agricultural activities and groundwater extraction for R34 drinking water negatively affect groundwater dependent ecosystems (Grootjans et al. 1988, R35 Wassen et al. 1990). In addition, river regulation for energy supply and flood protection has led to loss or degradation of wetlands (Kareiva et al. 2007). Also, pollution and acidification R36 due to human activity leads to habitat degradation (Rockström et al. 2009). R37 Furthermore, land use change typically results in a fragmented distribution of the remaining R38 habitat for many plant and animal species (Stockwell et al., 2003) (Figure 1.1). Habitat R39 10 | Chapter 1 R1 fragmentation is defined as a process during which “a large expanse of habitat is transformed R2 into a number of smaller patches of smaller area, isolated from each other by a matrix of R3 habitats unlike the original” (Wilcove et al., 1986). In practice, habitat loss always accompanies R4 habitat fragmentation, because the ‘removal of habitat’ causes a fragmented distribution of R5 the remaining habitat patches situated around the converted area (see: Figure 1.1). R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 Figure 1.1. Simplified visualisation of the process of land use change (A) and the resulting habitat R33 fragmentation (B). Land use change might lead to habitat loss, habitat degradation and habitat R34 fragmentation. Habitat fragmentation is reflected by decreased total habitat area, decreased average patch size, increased habitat edge/area ratio and increased patch isolation. R35 R36 In the ecological literature, habitat fragmentation is sometimes referred to as the change in R37 landscape configuration (increase in number of patches, decrease in patch sizes, and increase R38 in isolation of patches) regardless of habitat loss. In this thesis, we will refer to this concept R39 Introduction | 11 as ‘habitat fragmentation per se’ (Fahrig, 2003). It is valuable to investigate the additional R1 effects of habitat fragmentation per se on biodiversity or species viability. In the next section, R2 the different processes and factors associated with habitat fragmentation, and its relation R3 with seed dispersal are explained. R4 R5 R6 1.3 HABITAT FRAGMENTATION R7 R8 1.3.1 Population size R9 Habitat fragmentation leads to a decreased total habitat area and decreased average habitat patch size (Figure 1.1). A decreasing patch size is very likely to lead to a decrease in population R10 size of the plant species occupying the patch. With decreasing population size, random effects R11 become increasingly important for local population dynamics (Hanski and Gaggiotti 2004). R12 Population dynamics of plant populations are influenced by demographic and environmental R13 stochasticity. Demographic stochasticity implies the random variation in ‘birth’ and death of R14 individuals. In very small populations, demographic stochasticity leads to higher extinction R15 risks than in larger populations (Hanski and Gaggiotti 2004). R16 Furthermore, every sub-population is subject to environmental stochasticity, meaning the R17 yearly random variation in the environment due to, for instance, variation in temperature R18 and precipitation. Environmental stochasticity is a major cause of local extinction and the local R19 extinction risk due to environmental stochasticity increases with decreasing population size R20 (Hanski and Gaggiotti 2004). R21 Additionally, diminished population size caused by habitat fragmentation results in reduced R22 genetic variation at (sub-)population level, because the remaining individuals represent only a fraction of the original gene pool (Young et al., 1996). Subsequently, in the longer term, R23 reduced population size and isolation can lead to genetic drift (i.e. a change in allele frequency R24 from one generation to the next, caused by random sampling of alleles) and increased R25 inbreeding, which again cause a reduced genetic variation within sub-populations or within R26 individuals (i.e. reduced heterozygosity) respectively (Saunders et al. 1991, Ellstrand and Elam R27 1993, Matthies et al. 2004, Leimu et al., 2010). Reduced heterozygosity within individuals of R28 a sub-population can reduce individual fitness (e.g. Oostermeijer et al. 1995, Gaggiotti and R29 Hanski 2004) by an increased expression of deleterious recessive alleles, and thereby reduce R30 sub-population viability. In the long term, reduced genetic variation within sub-populations R31 may limit species’ ability to respond to changing abiotic conditions. Therefore, reduced R32 genetic variation (both within sub-populations and within individuals) can lead to increased R33 probability of local extinction. Increased local extinction, again, leads to a reduced genetic R34 variation within the population as a whole (Young et al., 1996). All these processes may R35 increase the extinction risk of small sub-populations of plant species (see: Joshi et al. 2006, Collins et al. 2009). R36 R37 R38 R39 12 | Chapter 1 R1 1.3.2 Edge effects R2 The breaking apart of a large population into several smaller ones not only leads to smaller R3 habitat patches but also to a larger habitat edge/-area ratio, compared to that in the original R4 population.
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