Institute of Evolution and Ecology, Plant Ecology Group, University of Tübingen, Germany.
Ann Bot. 2012 Nov;110(7):1403-10. doi: 10.1093/aob/mcs076. Epub 2012 Apr 6.
The enemy release hypothesis assumes that invasive plants lose their co-evolved natural enemies during introduction into the new range. This study tested, as proposed by the evolution of increased competitive ability (EICA) hypothesis, whether escape from enemies results in a decrease in defence ability in plants from the invaded range. Two straightforward aspects of the EICA are examined: (1) if invasives have lost their enemies and their defence, they should be more negatively affected by their full natural pre-invasion herbivore spectrum than their native conspecifics; and (2) the genetic basis of evolutionary change in response to enemy release in the invasive range has not been taken sufficiently into account.
Lythrum salicaria (purple loosestrife) from several populations in its native (Europe) and invasive range (North America) was exposed to all above-ground herbivores in replicated natural populations in the native range. The experiment was performed both with plants raised from field-collected seeds as well as with offspring of these where maternal effects were removed.
Absolute and relative leaf damage was higher for introduced than for native plants. Despite having smaller height growth rate, invasive plants attained a much larger final size than natives irrespective of damage, indicating large tolerance rather than effective defence. Origin effects on response to herbivory and growth were stronger in second-generation plants, suggesting that invasive potential through enemy release has a genetic basis.
The findings support two predictions of the EICA hypothesis - a genetically determined difference between native and invasive plants in plant vigour and response to enemies - and point to the importance of experiments that control for maternal effects and include the entire spectrum of native range enemies.
入侵植物假设认为，在引入新范围时，侵入性植物会失去与其共同进化的天敌。本研究按照进化增强竞争力假说（EICA）的假设进行了测试，即是否逃避天敌会导致入侵范围的植物防御能力下降。本研究检验了 EICA 的两个直接方面：（1）如果外来物种已经失去了它们的天敌和防御能力，那么它们应该比本地同种植物更容易受到其完整的自然前入侵食草动物谱的负面影响；（2）尚未充分考虑入侵范围中因天敌释放而导致的遗传基础上的进化变化。
从几种原产（欧洲）和入侵（北美）种群中采集的千屈菜（Lythrum salicaria）暴露于其原生范围的自然种群中所有地上食草动物的重复影响下。实验在从野外采集的种子中培养的植物以及这些植物的后代中进行，这些植物的后代去除了母体效应。
与本地植物相比，引入植物的绝对和相对叶片损伤更高。尽管入侵植物的生长速度较慢，但无论受到何种伤害，它们最终的体型都比本地植物大得多，这表明它们具有较大的耐受性，而不是有效的防御能力。尽管第二代植物的生长速度较慢，但对食草动物和生长的反应的起源效应更强，这表明通过天敌释放获得的入侵潜力具有遗传基础。
这些发现支持 EICA 假说的两个预测——本地和入侵植物在植物活力和对天敌的反应方面存在遗传差异——并指出了控制母体效应并包括整个原生范围天敌的实验的重要性。
