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Fragmentation in ecological systems has been a subject of interest for the last 20 years. This interest has arisen from concerns about the recent consequences of human activities such as deforestation or changes in agricultural practices, as well as the recognition that natural systems are structured in space and time. Urban environments represent newly fragmented habitats, and thus provide the opportunity to study the impact of fragmentation in ecological systems. They may also be of particular interest for analysing recent evolutionary processes, as shown in recent studies (Shocat et al., 2006). Compared with a single large population, fragmented populations exhibit reduced population size and reduced gene flow among remnant demes. One of the main changes accompanying the reduction in population size is the increased importance of stochastic processes (Saunder et al., 1991; Aizen & Feinsinger, 1994; Young et al., 1996) resulting in a higher probability of a local population becoming extinct (Shaffer, 1981; Ellstrand & Elam, 1993).
Low population densities are also expected to threaten population demography in a deterministic way. At low density, the difficulties of gametes in encountering one another may cause a decrease in the per capita growth rate, a phenomenon named the ‘Allee effect’ (Allee et al., 1949). This is defined as a positive relationship between density or population size and any components of fitness (Stephens, 1999) sometimes referred to as ‘inverse density dependence’ (Courchamp et al., 1999). Low population sizes and/or low population density (the number of individuals per unit area) can result in an Allee effect, but the two factors often interact in empirical studies (Ghazoul, 2005). Depending on the strength of the Allee effect, population dynamics can be disrupted and populations may eventually go to extinction deterministically, as shown in theoretical (Schreiber, 2003) and empirical studies (Groom, 1998).
Because plants are sessile, sexual reproduction depends on pollen vectors and low densities lead to a lower probability of pollen transfer, resulting in a lower seed set (Ashman et al., 2004). Moreover, in animal-pollinated plants, a small population size may decrease the attractiveness to pollinators (e.g. insects), resulting in a smaller quantity of pollen available for outcrossing. Also, pollen quality may decrease at low density because of higher relatedness among individuals or because of similar self-incompatibility alleles shared by pollen donors and pollen recipients in the case of self-incompatible species (Kearns & Inouye, 1997). Pollinator behaviour (foraging) can change too. By disrupting plant pollinator interactions, low densities resulting from fragmentation could therefore create conditions necessary for the Allee effect to occur.
Plants have evolved mating strategies to cope with pollen limitation and particularly with the Allee effect. When faced with recurrent low population sizes such as expected in fragmented systems, populations may evolve towards selfing in order to ensure seed production (Darwin, 1876; Baker, 1955). Reproductive assurance has been considered to be one of the major forces for the frequent transition from outcrossing to selfing in hermaphrodites (Holsinger, 2000). Thus, analysing the Allee effect in fragmented populations provides a relevant ecological context for analysing selection pressures on mating systems in hermaphroditic plants (Cheptou, 2004; Moeller & Geber, 2005).
In this study, we used the urban environment to investigate the pollination process and reproduction in fragmented populations in the mostly outcrossing species Crepis sancta (Asteraceae). Urban environments consist of a matrix that is mostly unsuitable for plants because of constructions, asphalt and buildings. Within this mainly unsuitable area, small and sparse suitable patches constitute highly fragmented habitats where plants can persist (mostly ruderal species). Interestingly, ecological studies on urban systems have sometimes reported higher variety diversity than in surrounding agricultural landscapes (Wania et al., 2006). Because of their artificiality, urban environments provide a useful system for investigating population dynamics by taking advantage of the regular geometry of habitats and their well-known history. The study species C. sancta is widespread in the city of Montpellier (France) where it occurs in very small and isolated populations (from 1 to 40 plants maximum in our study system). Interestingly, this species has been intensively studied in the near countryside where it occurs in very large populations, sometimes more than a hundred thousand individuals per hectare (Cheptou et al., 2002). This study aimed to investigate the impact of population density as a consequence of fragmentation on the pollination process and its demographic consequences (the Allee effect). Specifically, we addressed the following questions:
Is pollinator activity positively related to density in urban populations?
Is seed set positively related to density (the Allee effect) in urban populations?
Do urban populations exhibit higher selfing rates than rural populations from the nearby countryside?
Is there evidence for a greater ability to self-fertilize in urban populations?