Department of Biology, University of North Carolina, Chapel Hill, NC, USA, 27599.
Department of Integrative Biology, University of Texas at Austin, Austin, TX, USA, 78712.
Oecologia. 2019 Nov;191(3):609-620. doi: 10.1007/s00442-019-04507-9. Epub 2019 Sep 21.
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.
高资源环境通常有利于快速生长、防御能力差的植物，而资源匮乏的环境通常有利于生长缓慢、防御能力强的植物。解释这种模式的主流假说是，随着资源可用性的增加，防御能力强、生长缓慢的物种被防御能力弱、生长迅速的物种所取代。第二个假说是，资源的可利用性增加会导致更多的资源分配用于生长，而不是防御，这在物种内部是存在的。无论机制如何，如果外来物种相对于本地物种来说，从天敌中释放出来，那么物种内部和物种之间在生长和防御上的分配变化可能因地理起源而不同。为了测试资源可用性是否会改变物种内部和物种之间的生长或防御，以及这种影响是否在本地种和外来种之间存在差异，我们在野外条件下操纵土壤养分供应和地上昆虫食草动物和真菌病原体的接触，以六种同时存在于北卡罗来纳州旧田地的本地和外来草种的个体为研究对象。主流假说的预测——物种层面的天敌影响随着物种对养分的反应性而增加——得到了证实。此外，这种关系在本地种和外来种之间没有差异。第二个假说的预测——在考虑到物种表现的差异后，个体层面的天敌影响随着养分供应的增加而增加——并没有得到支持。总的来说，这些结果支持了这样一种观点，即在本地种和外来种中，植物物种的更替是营养富集对植物群落中生长和防御分配的影响的主要机制。
