Ecology Letters, (2010) 13: 803–809 doi: 10.1111/j.1461-0248.2010.01474.x LETTER Negative soil feedbacks accumulate over time for non-native plant species
Abstract Jeffrey M. Diez,1* Ian Dickie,2 The enemy release hypothesis is a common explanation for species invasions, suggesting Grant Edwards,1 Philip E. Hulme,1 that introduced species benefit from leaving behind natural enemies in the native range. Jon J. Sullivan1 and Richard P. However, any such advantage may attenuate over time. In this study, we test a prediction 1 Duncan of this more dynamic enemy release hypothesis: that non-native plant species that 1 Bio-Protection Research became established longer ago exhibit stronger negative feedbacks with the soil. Centre, PO Box 84, Lincoln Consistent with declining enemy release over time, we found increasingly negative soil University, Lincoln 7647, New feedbacks for species established longer ago in New Zealand. Negative soil feedbacks Zealand 2 were also stronger for more widespread species, but weaker for more locally abundant Landcare Research, PO Box 40, Lincoln 7640, New Zealand species, suggesting that species accumulate negative interactions as they spread and can *Correspondence and present be locally regulated by these interactions. We also present data to support the common address: E-mail: assumption that relatives have similar impacts on and responses to soil communities. [email protected]; School of Together, these data highlight the dynamic nature of novel interactions arising from Natural Resources and species introductions. Environment, University of Michigan, 440 Church St, Ann Keywords Arbor, MI 48109 1041, USA Enemy release hypothesis, invasive species, plant–soil feedbacks.
Ecology Letters (2010) 13: 803–809
These mechanisms suggest a more dynamic ERH than INTRODUCTION that usually tested, in which any initial advantage of The enemy release hypothesis (ERH) is widely invoked enemy release may decline over time as enemies and tested as an explanation for the success of non-native accumulate post-establishment (Hawkes 2007). Anecdotal species, suggesting that introduced species benefit from evidence suggests that such processes may be responsible leaving behind natural enemies that regulate their popula- for the boom and bust dynamics of certain non-native tions in the native range (Mitchell & Power 2003; Colautti species (Williamson & Fitter 1996; Simberloff & Gibbons et al. 2004). Nevertheless, any such advantage for intro- 2004), and a recent meta-analysis found support across duced species might be expected to attenuate over time for study systems for a decline in enemy release post- several reasons (Hawkes 2007). First, as introduced species introduction (Hawkes 2007). Other studies examining the become widespread after introduction they have an accumulation of enemies on non-native plant species have increasing probability of encountering potential enemies focused on aboveground invertebrate herbivores and not already resident in the region that could impact their found clear relationships over time (Strong et al. 1977; performance. Second, natural enemies from the home Andow & Imura 1994; Carpenter & Cappuccino 2005). range may be subsequently introduced either intentionally However, none to our knowledge has examined changing or unintentionally, thus Ôcatching-upÕ with their hosts. effects on plant performance mediated by below-ground Third, novel relationships between introduced species and interactions, despite the strong influence of belowground the pathogens, herbivores, and mutualists in the new organisms on plants (Klironomos 2002; van der Putten region may form through exposure and evolutionary et al. 2007a). adaptation (Cox 2004). Recent work has shown that both In this study, we test a prediction of this more dynamic introduced plants (Prentis et al. 2008) and native enemies ERH: that introduced plant species with longer residence (Carroll et al. 2005) may evolve rapidly in response to novel times in a new region should exhibit stronger negative soil biotic associations. feedbacks than species established more recently. We