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| Invasive exotic plant species often have fewer
natural enemies and suffer less damage from herbivores in
their new range than genetically or functionally related species
that are native to that area. Although we might expect
that having fewer enemies would promote the invasiveness
of the introduced exotic plant species due to reduced enemy
exposure, few studies have actually analyzed the ecological
consequences of this situation in the field. Here, we examined
how exposure to aboveground herbivores influences shifts in dominance among exotic and phylogenetically related native
plant species in a riparian ecosystem during early establishment
of invaded communities. We planted ten plant communities
each consisting of three individuals of each of six exotic
plant species as well as six phylogenetically related natives.
Exotic plant species were selected based on a rapid recent
increase in regional abundance, the presence of a congeneric
native species, and their co-occurrence in the riparian ecosystem.
All plant communities were covered by tents with insect
mesh. Five tents were open on the leeward side to allow herbivory.
The other five tents were completely closed in order
to exclude insects and vertebrates. Herbivory reduced aboveground
biomass by half and influenced which of the plant species
dominated the establishing communities. Exposure to herbivory
did not reduce the total biomass of natives more than
that of exotics, so aboveground herbivory did not selectively
enhance exotics during this early stage of plant community
development. Effects of herbivores on plant biomass depended
on plant species or genus but not on plant status (i.e., exotic
vs native). Thus, aboveground herbivory did not promote the
dominance of exotic plant species during early establishment
of the phylogenetically balanced plant communities. | |
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