Data from: Plant traits and plant biogeography control the biotic resistance provided by generalist herbivores

1.Globalization and climate change trigger species invasions and range shifts, which reshuffle communities at an exceptional rate and expose plant migrants to unfamiliar herbivores. Dominant hypotheses to predict plant success are based on evolutionary novelty: either herbivores are maladapted to consume novel plants (enemy release hypothesis), or novel plants are maladapted to deter herbivores (biotic resistance hypothesis). Since novelty can work both ways, it fails to consistently predict when herbivores will consume novel over non-novel plants. Surprisingly, the value of using plant traits to predict herbivore consumption of novel plants remains largely unexplored. We hypothesized that (1) plant traits explain generalist herbivore consumption rates of novel and non-novel plants, and (2) any effect of novelty will be grounded in consistent trait differences between native and novel plants. Lastly, we expected to find (3) differences in plant traits and plant consumption rates across latitude.

2.To test these hypotheses, we measured the consumption rate of plant species for a tropical and a temperate generalist herbivore in controlled feeding trials by offering them a large variety of 40 plant species from different geographical origins. Therefore, whether a plant was novel depended on the herbivore used, allowing us to disentangle plant identity from plant novelty. We also measured plant chemical traits and determined whether traits, geographic origin or novelty best explained herbivore consumption rates.

3.Both generalist herbivores consumed more of plants with a high nitrogen-to-phenolic compounds ratio, irrespective of the plant's novelty to the herbivore. A pattern of increasing plant's nitrogen-to-phenolics ratio with latitude could explain why both the tropical and temperate herbivore consumed more of plants from temperate regions. Plant novelty and its geographic origin no longer explained consumption rates once differences in nitrogen-to-phenolic compounds ratio were taken into account.

4.We show that differences in plant traits along a latitudinal cline determine herbivore consumption rates, irrespective of whether plants are novel or familiar. Therefore, we propose that integrating evolutionary novelty theory with plant traits and biogeography will increase our understanding of the consequences of plant species migration beyond biogeographical barriers.

1. 全球化与气候变化引发物种入侵与分布范围转移，以极快的速度重塑群落结构，并使入侵植物暴露于此前未曾接触的植食性动物（herbivore）面前。当前预测植物入侵成功的主流假说均基于进化新奇性（evolutionary novelty）理论：要么认为植食性动物无法适应取食外来植物（天敌释放假说，enemy release hypothesis），要么认为外来植物难以有效抵御植食性动物（生物抵抗假说，biotic resistance hypothesis）。由于新奇性效应存在双向性，该理论无法稳定预测植食性动物为何会偏好取食外来植物而非本土植物。令人意外的是，利用植物功能性状预测植食性动物对外来植物取食偏好的研究仍未得到充分探索。我们提出三项假说：（1）植物功能性状可以解释广食性植食性动物对外来与本土植物的取食率；（2）任何由新奇性带来的效应，均源于本土植物与外来植物之间稳定的性状差异；最后，我们预期（3）植物功能性状与植物取食率会随纬度梯度产生显著差异。
2. 为验证上述假说，我们开展受控取食实验：选取40种来自不同地理起源的植物，分别投喂热带与温带广食性植食性动物，测定其对各植物物种的取食率。由此，某一植物是否为外来植物，取决于所使用的植食性动物类群，这使得我们能够将植物本身的物种特性与植物的外来性效应分离开来。我们同时测定了植物的化学功能性状，以明确性状、地理起源或外来性三者中，哪一项能最佳解释植食性动物的取食率差异。
3. 两种广食性植食性动物均更偏好取食氮酚比（nitrogen-to-phenolic compounds ratio）较高的植物，且这一偏好不受植物对该类植食性动物是否为外来植物的影响。植物氮酚比随纬度升高而升高的格局，可解释为何热带与温带广食性植食性动物均更偏好取食来自温带地区的植物。当纳入氮酚比差异的影响后，植物外来性与地理起源便不再能解释植食性动物的取食率差异。
4. 本研究表明，沿纬度梯度分布的植物功能性状差异决定了植食性动物的取食率，与植物对该类植食性动物是外来还是本土物种无关。据此，我们提出应将进化新奇性理论与植物功能性状及生物地理学研究相结合，以加深我们对植物物种突破生物地理屏障发生迁移后所产生的生态效应的理解。