Looks can be deceiving: ecologically similar exotics have different impacts on a native competitor
Exotic species are often predicted to successfully invade when their functional traits differ from species in recipient communities. Many studies have related trait differences among natives and invaders to competitive outcomes. Few studies, however, have tested whether functionally similar invaders have similar competitive impacts on natives. We investigated interactions in communities of a native annual forb Waitzia acuminata (Asteraceae) and two invasive annual grasses that are ecologically similar and co-occur in southwestern Australia. Using a combination of field and laboratory experiments and several performance measures, we assessed impacts of these grasses on W. acuminata. We also examined differences among species in their responses to intraspecific versus interspecific competition, including their frequency dependence. The two similar exotic grasses differed in interaction impacts, with one facilitating and the other suppressing the native. In general, intraspecific competition was stronger than interspecific competition for the native, while evidence of competition was weak for the exotics. These patterns may reflect that W. acuminata does well in these communities due to the combined impacts of stabilization and facilitation, whereas the exotics benefit from limited stabilization (mediated by their weak intraspecific competition) or weak interspecific competition with W. acuminata. We found divergent impacts of the exotic species despite their similar functional traits. We demonstrate that a native species may benefit from interactions with an exotic “benefactor” species, highlighting the potential importance of positive interactions in invaded communities. Our findings underscore the necessity of considering neutral and positive interactions in addition to competition in understanding invasion dynamics in real plant communities.
KeywordsAnnual plants Coexistence Competition Facilitation Functional traits
Richard J. Hobbs, John M. Dwyer, and Myron Zalucki provided advice on experimental design, data analysis, and editorial input on earlier versions of this manuscript. Hao Ran Lai, Janneke HilleRisLambers, Xingwen Loy, Angela Gardner, Lalita Fitrianti, and the Schmidt Lab provided assistance with data collection. This research was funded by a grant from the Australian Research Council (DP1094413) awarded to Richard J. Hobbs, Margaret M. Mayfield, and Robert D. Holt. The Western Australian Department of Parks and Wildlife generously provided access to research sites.
Author contribution statement
All authors conceived the ideas underlying the study. The experiments were designed primarily by CW and MM with input from RH. CW collected the data with input from RH and analysed the data with input from MM. CW wrote the manuscript with substantial input from RH and MM.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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