Two contradictory hypotheses have been put forth to forecast alien invasiveness: being either functionally similar, or dissimilar, to resident natives along environmental gradients. The ‘try-harder’ hypothesis predicts that alien plants will be functionally dissimilar to natives and should thus exhibit exaggerated trait values when compared to natives in respect to resource extraction or stress tolerance. In contrast, the ‘join-the-locals’ hypothesis, which is compatible with “environmental filtering”, predicts functional similarities among alien and native species in richer, but not in resource-limited environments. Here, we propose a framework that links the successful alien plant strategy, i.e. being functionally similar or dissimilar to natives, to the harshness of the environment and the availability of resources. We tested these two hypotheses using a trait-environment dataset of 33 alien and 130 native plants in 96 sites covering a gradient of soil resources (organic matter, nitrogen, soil moisture) in Saint-Katherine, an arid protected area in Egypt. We estimated 18 interaction coefficients between three candidate traits (specific leaf area, above-ground biomass, height) of alien and native plants as well as soil resources using linear mixed-effects models. Additionally, we calculated the mean and the hierarchical functional distance among aliens and natives along soil gradients. Our results revealed that in extreme resource-limited environments, aliens and natives were functionally similar and had relatively equal trait values consistent with environmental filtering, thus supporting the ‘join-the-locals’ hypothesis. However, in environments richer in resources, aliens and natives were functionally dissimilar with aliens exhibiting more exploitative trait values (taller, higher SLA and biomass) than natives, providing support for the ‘try-harder’ hypothesis. While demonstrated in only one arid system, results suggest that linking soil resource availability with functional divergence and convergence among native and alien plants could be used as an informative strategy to predict alien invasiveness. Hence, future studies should investigate the functional response of alien and native plants in different environments against different resource gradients to test for the generality of the patterns we found.

已经提出了两个相互矛盾的假设来预测外来入侵性：沿环境梯度在功能上与居民相似或不同。“更努力”的假设预测外来植物在功能上将与本地人不同，因此在资源提取或胁迫耐受性方面与本地人相比，应该表现出夸张的性状值。相反，与“环境过滤”兼容的“本地参与”假说预测了外来物种和本地物种在更丰富的环境中（但在资源有限的环境中）的功能相似性。在这里，我们提出了一个框架，该框架将成功的外来植物策略（即功能上与本地人相似或不同）与环境的恶劣性和资源的可用性联系起来。我们在埃及干旱保护区圣凯瑟琳省的96个站点中，使用了33种外来植物和130种本地植物的性状-环境数据集，对这两个假设进行了检验，这些土壤覆盖了土壤资源（有机物，氮，土壤水分）的梯度。我们使用线性混合效应模型估算了外来植物和本土植物的三个候选性状（比叶面积，地上生物量，高度）与土壤资源之间的18个相互作用系数。此外，我们计算了沿土壤梯度的外星人和当地人之间的平均值和等级功能距离。我们的研究结果表明，在极端的资源有限的环境中，外星人和当地人在功能上相似，并且具有与环境过滤一致的相对相等的特征值，从而支持“加入当地人”假设。然而，在资源丰富的环境中，外星人和原住民在功能上有所不同，与外来人相比，其表现出更多的剥削性状价值（塔勒，更高的SLA和生物量），为“更难尝试”的假设提供了支持。虽然仅在一个干旱系统中得到证明，但结果表明，将土壤资源的可用性与本机和外来植物之间的功能差异和趋同联系起来，可以用作预测外来入侵性的信息策略。因此，未来的研究应该调查不同环境中外来植物和本土植物对不同资源梯度的功能响应，以检验我们发现的模式的普遍性。为“更努力”的假设提供支持。虽然仅在一个干旱系统中得到证明，但结果表明，将土壤资源的可用性与本机和外来植物之间的功能差异和趋同联系起来，可以用作预测外来入侵性的信息策略。因此，未来的研究应该调查不同环境中外来植物和本土植物对不同资源梯度的功能响应，以检验我们发现的模式的普遍性。为“更努力”的假设提供支持。虽然仅在一个干旱系统中得到证明，但结果表明，将土壤资源的可获得性与本地植物和外来植物之间的功能差异和趋同联系起来，可以用作预测外来入侵性的信息策略。因此，未来的研究应该调查不同环境中外来植物和本土植物对不同资源梯度的功能响应，以检验我们发现的模式的普遍性。