A growth–defense trade-off is general across native and exotic grasses
High-resource environments typically favor quick-growing, poorly defended plants, while resource-poor environments typically favor slow-growing, well-defended plants. The prevailing hypothesis explaining this pattern states that, as resource availability increases, well-defended, slow-growing species are replaced by poorly defended, fast-growing species. A second hypothesis states that greater resource availability increases allocation to growth at the expense of defense, within species. Regardless of mechanism, if exotic species are released from enemies relative to natives, shifts in allocation to growth and defense both within and among species could differ by geographic provenance. To test whether resource availability alters growth or defense, within and among species, and whether any such effects differ between natives and exotics, we manipulated soil nutrient supply and access of aboveground insect herbivores and fungal pathogens under field conditions to individuals of six native and six exotic grass species that co-occurred in a North Carolina old field. The prevailing hypothesis’ prediction—that species-level enemy impact increases with species’ nutrient responsiveness—was confirmed. Moreover, this relationship did not differ between native and exotic species. The second hypothesis’ prediction—that individual-level enemy impact increases with nutrient supply, after accounting for species-level variation in performance—was not supported. Together, these results support the idea, across native and exotic species, that plant species turnover is the primary mechanism underlying effects of nutrient enrichment on allocation to growth and defense in plant communities.
KeywordsBiological invasions Old fields Poaceae Resource-enemy release hypothesis Top-down bottom-up
We thank J. Bruno, J. Cronin, E. Mordecai, R. Peet, M. Rua, M. Welsh, J. Umbanhowar, P. Wilfahrt, and J. Wright for advice on the design and analysis of this study. L. Fridie, T. Hodges, A. Kinniburgh, J. Napier, E.A. Reece, M. Sandoval, and S. Tolar assisted with field and lab work. This study was funded by a Doctoral Dissertation Improvement Grant to CEM and RWH (NSF-DEB-1311289). UNC’s Alma Holland Beers Scholarship and WC Coker Fellowship provided summer funding to RWH. FWH was supported by the NSF GRFP.
Author contribution statement
RWH and CEM designed the study; RWH performed the study, analyzed the data, and wrote the first draft of the manuscript; FWH enemy identified and quantified damage, and analyzed damage data; all authors revised the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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