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Habitat fragmentation reduces the size and increases the isolation of habitats and populations (Saunders et al. 1991). Stochasticity may affect small, isolated populations more strongly than larger ones (Shaffer 1987). Environmental stochasticity can drive small populations to extinction, while genetic stochasticity randomly reduces heterozygosity and increases the accumulation of deleterious mutations (Lande 1995; Lynch et al. 1995). Demographic stochasticity, i.e. random deviations in survival and reproduction of individuals from expected population means, may change sex ratios or proportions of style morphs in populations. Inbreeding, which can be pronounced in small, isolated populations, again reduces genetic variability and increases the accumulation of deleterious mutations. Furthermore, inbreeding depression can lower individual fitness and population viability (Ellstrand & Elam 1993; Young et al. 1996). Both biotic interactions (Olesen & Jain 1994), such as pollination, plant–herbivore, plant–pathogen and intraspecific interactions, and abiotic effects (Saunders et al. 1991) can be changed by habitat fragmentation in ways that may affect plant performance or population fitness, respectively. Differences in abiotic conditions between the edge and the centre of habitat remnants, affect larger proportions of smaller populations.
We studied several of these issues in the common wetland plant Primula farinosa L. (Primulaceae) in the centres and at the edges of 27 Swiss fen habitats of different size and degree of isolation. This clonal species is characteristic of calcareous fens (Caricion davallianae alliance; Ellenberg 1996). These are wetlands of high plant species diversity (c. 30 higher plant species per 2 m2; Pauli 1998) and among the few remaining seminatural ecosystems of Central Europe. In Switzerland, a 90% reduction of the wetland area since 1850 has caused high levels of destruction and fragmentation of habitats such as calcareous fens (Broggi & Schlegel 1989; Hintermann 1992). To date, habitat fragmentation has appeared to be more detrimental for rare, non-clonal, short-lived habitat specialists, than for perennial generalists with clonal reproduction (Fischer & Stöcklin 1997), such as P. farinosa. However, as the latter are major contributors to ecosystem productivity, any detrimental effects of fragmentation on such apparently well-buffered species would be cause for concern.
Demographic stochasticity causes the morph ratios of small and isolated populations of several heterostylous plants to deviate strongly from expected values (e.g. Barrett et al. 1989; Eckert & Barrett 1992; Ågren & Ericson 1996; Baker et al. 2000). Because plants of each morph are only compatible with plants of the other morph in distylous species, habitat fragmentation may limit mating opportunities and thus decrease reproduction (Barrett 1992). We studied whether deviations from the expected 1 : 1 morph ratio were more pronounced in smaller, more isolated populations of the distylous P. farinosa.
Whereas edge effects have long been recognized for animals and forest plants (e.g. Cadenasso et al. 1997; Lahti 2001), they have received little attention in studies on other plant species. We hypothesized that nutrient influx from surrounding agricultural land and altered pollination would result in larger plant size, but lower reproductive output and plant density, and more pronounced herbivory at habitat edges, but that differences between edges and centres would be less in smaller habitats.
Finally, we studied whether changes in plant size and density, reproductive output and pathogen incidence were reflected in lower fitness in more isolated, smaller fragments and at habitat edges. As adult plants are often less sensitive to changes than seedlings or juveniles (Oostermeijer et al. 1994; Jules 1998; Rose et al. 1998) and the effects of fragmentation may not therefore become apparent immediately (Eriksson 1996), it was necessary to consider the stage structure of P. farinosa (Saunders et al. 1991; Bühler & Schmid 2001).
We hypothesized that P. farinosa populations in smaller and more isolated habitats will show lower plant densities, altered morph and plant size structures, lower plant size, reproductive output and seed germination and lower leaf herbivory, frugivory and smut infection. We also predicted that edge effects would differ between large and small, and between more or less isolated populations.