Variation in defensive traits against herbivores of native and invasive populations of <i>Carpobrotus edulis</i>

The Enemy Release Hypothesis (ERH) predicts that the escape from natural enemies, such as specialist herbivores, may explain the invasiveness of some invasive alien plants, maximizing their investment in growth and reproduction. This release from natural enemies might decrease the investment in expensive defense mechanisms (i.e., digestibility reducers) against the attack of specialist enemies, whilst increasing the investment in defenses (i.e., cheap toxins) and tolerance against the attack of generalist herbivores, as exposed by the Shifting Defense Hypothesis (SDH). To test this, we conducted a greenhouse experiment to compare morphological and physiological traits of <em>Carpobrotus edulis</em>, collected in its native (South Africa) and introduced (Chile and Spain) ranges, attacked by the generalist spittlebug <em>Philaenus spumarius</em> and the specialist scale insect <em>Pulvinariella mesembryanthemi</em>. Our results do not support the ER and SD hypotheses. We found that <em>C. edulis</em> plants collected from native and introduced ranges showed no significant differences in growth and defensive compounds studied for both controls and those plants under attack by the generalist <em>P. spumarius</em> (i.e., showing no increase in biomass or changes in biochemical defenses). In contrast, the specialist herbivore <em>P. mesembryanthemi</em> induced the production of total phenols and tannins and reduced the growth and survival of <em>C. edulis</em> plants. Overall, we found strong evidence that <em>C. edulis</em> is negatively affected by the attack of its specialist herbivore, but not by that of generalist herbivore, regardless of origin. The observed tolerance to generalist herbivores suggests the intriguing possibility of trait selection, allowing <em>C. edulis</em> plants to tolerate generalist herbivores more than specialists.

天敌释放假说（Enemy Release Hypothesis, ERH）预测，逃离专食性植食动物等自然天敌的调控，或可解释部分外来入侵植物的入侵性，使其在生长与繁殖上的资源投入达到最大化。这种从自然天敌压力中的解脱，可能会降低植物对专食性天敌攻击的昂贵防御策略（如消化抑制剂类防御）的资源投入，同时正如防御策略转移假说（Shifting Defense Hypothesis, SDH）所揭示的，增加对广食性植食动物攻击的防御（如廉价毒素类防御）以及耐受性的资源投入。
为验证上述假说，我们开展了温室盆栽实验，比较了从原生分布区（南非）以及引入分布区（智利与西班牙）采集的食用日中花（*Carpobrotus edulis*）的形态与生理性状，这些植株分别受到广食性尖胸沫蝉（*Philaenus spumarius*）与专食性介壳虫（*Pulvinariella mesembryanthemi*）的取食为害。
我们的研究结果并不支持ERH与SDH假说。研究发现，无论是对照组还是受到广食性尖胸沫蝉攻击的植株，来自原生与引入分布区的食用日中花在生长性状与所检测的防御化合物含量上均无显著差异，即未表现出生物量增加或生化防御的变化。与之相反，专食性植食动物*Pulvinariella mesembryanthemi*的取食会诱导植株合成总酚类与单宁物质，并抑制食用日中花的生长与存活。
总体而言，我们发现有力证据表明：食用日中花会受到其专食性植食动物攻击的负面影响，但不会受到广食性植食动物攻击的影响，且这一规律不受种群起源地的制约。本次观测到的对广食性植食动物的耐受性，引出了一个值得深入探索的性状选择可能性：即食用日中花植株对广食性植食动物的耐受性要强于对专食性植食动物的耐受性。