Abstract
Propagule pressure is considered a major driver of plant invasion success. Great propagule pressure would enable invasive species to colonize new areas overcoming the resistance of native species. Many highly invasive aquatic macrophytes regenerate from vegetative propagules, but few studies have experimentally investigated the importance of propagule pressure and biotic resistance, and their interaction, in determining invasion success. By manipulating both recipient habitat and the input of vegetative propagules of the invasive seaweed Caulerpa cylindracea in mesocosm, we examined whether higher propagule pressure would overcome the resistance of a native congeneric (Caulerpa prolifera) and influence its performance. With the native, C. cylindracea population frond number decreased irrespectively of pressure level. High propagule pressure did not increase stolon length and single plant size decreased due to the effects of intra- and interspecific competition. Native biomass decreased with increasing C. cylindracea propagule pressure. These results indicate that higher propagule pressure may fail in enhancing C. cylindracea invasion success in habitats colonized by the native species, and they suggest that biotic resistance and propagule pressure co-regulate the invasion process. These findings emphasize the need to preserve/restore native seaweed populations and may help to design effective management actions to prevent further C. cylindracea spread.
Highlights
The invasion of environments by non-native plant species (NNS) is an increasing threat to biodiversity and functioning of ecosystems globally[1,2]
On the other hand, increasing number of studies indicates that in stressful habitats native species may facilitate, for example by physically trapping propagules within the leaf canopy, stabilizing substrates or ameliorating abiotic stresses, rather than compete with NNS23,25, which is in direct opposition of predictions of biotic resistance[26]
The number of fronds was reduced with high pressure, and on average there was a higher number of fronds per plant in the absence of the native species than in its presence
Summary
The invasion of environments by non-native plant species (NNS) is an increasing threat to biodiversity and functioning of ecosystems globally[1,2]. Numerous factors may individually affect the establishment of a NNS outside its native range[3,4] Understanding how these factors interact each other to determine the outcome of the invasion process is one of the main goals of ecology, and it is highly relevant to improving our ability to manage invasive species. Understanding the role of vegetative propagule pressure and biotic resistance is crucial to improve our knowledge on invasion processes and to formulate effective strategies to reduce the risk of new marine habitat invasions and the chance of spread of already introduced species, i.e., expend more efforts in restoring and maintaining resident native vegetation if invasion resistance is more important or limiting the release of more propagules if propagule pressure is determinant
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