Accumulation of local pathogens: a new hypothesis to explain exotic plant invasions

Abstract

Recent studies have concluded that release from native soil pathogens may explain invasion of exotic plant species. However, release from soil enemies does not explain all plant invasions. The invasion of Ammophila arenaria (marram grass or European beach grass) in California provides an illustrative example for which the enemy release hypothesis has been refuted. To explore the possible role of plant Á soil community interactions in this invasion, we developed a mathematical model. First, we analyzed the role of plant Á soil community interactions in the succession of A. arenaria in its native range (north-western Europe). Then, we used our model to explore for California how alternative plant Á soil community interactions may generate the same effect as if A. arenaria were released from soil enemies. This analysis was carried out by construction of a 'recovery plane' that discriminates between plant competition and plant Á soil community interactions. Our model shows that in California, the accumulation of local pathogens by A. arenaria could result in exclusion of native plant species. Moreover, this mechanism could trigger the rate and spatial pattern of invasive spread generally observed in nature. We propose that our 'accumulation of local pathogens' hypothesis could serve as an alternative explanation for the enemy release hypothesis to be considered in further experimental studies on invasive plant species.Dynamics of plants and soil communities are closely linked. Dead plant material provides organic carbon to the decomposing soil organisms that subsequently supply nutrients to the plants ( Wardle et al. 2004). The living plant biomass also amplifies a characteristic subset of the soil community, consisting of root-associated herbivores, pathogens, and symbiotic mutualists (Kowalchuk et al. 2002). The specific effect of a plant species on the soil community, together with the return effect on plant growth, results in a net plant Ásoil feedback effect (Bever et al. 1997 (Klironomos 2002, Reinhart et al. 2003, Callaway et al. 2004a). This observation is in compliance with the commonly acknowledged enemy release hypothesis (Maron and Vila 2001, Keane andCrawley 2002). However, not all exotic plant invasions can be explained by enemy release (Colautti et al. 2004). Other explanations focus on a negative impact of exotic plant species by producing toxic compounds (Bais et al. 2003, Vivanco et al. 2004.However, the soil communities that are amplified in the root zone of one plant species may also affect the performance of other plant species, which influences the outcome of plant competition (Van der Putten and Peters 1997). Consequently, exotic plant species could indirectly influence the performance of plant species that are native in the invaded habitat. In order to examine FORUM FORUM FORUMFORUM is intended for new ideas or new ways of interpreting existing information. It provides a chance for suggesting hypotheses and for challenging current thinking on ecological issues. A lighter prose, designed to ...

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