Importance of invasion mechanisms varies with abiotic context and plant invader growth form

1. Many invasion hypotheses propose biotic interactions as the main mechanism to explain non-native species' success.  Despite the evidence that the strength of biotic interactions varies with abiotic context, it remains unclear whether the importance of the different mechanisms proposed to explain invasion predictably varies with abiotic context and whether this variation is consistent across different growth forms.

2. We reviewed studies at a global scale to evaluate whether evapotranspiration, latitude, precipitation, and temperature influence the importance of disturbance, enemy release, facilitation, and novel weapons mechanisms to explain non-native plant invasions.  In total, we calculated 171 effect sizes for ~300 non-native plant species covering a wide range of environmental conditions and growth forms.

3. Environmental context and plant growth form influenced the role played by each invasion mechanism.  The importance of disturbance in facilitating invasion exhibited a quadratic relationship with latitude and temperature and decreased with increasing precipitation.  In mixed communities and trees, disturbance was mediated by either evapotranspiration, latitude, precipitation, or temperature.  Enemy release exhibited a quadratic relationship with evapotranspiration, latitude, and precipitation, and was positively related to temperature.  The importance of enemy release was also contingent on growth form and was highly context-dependent, enemy release responses for grasses, and trees were modulated by either evapotranspiration, latitude, precipitation, or temperature.  The importance of facilitation decreased with increasing temperature.  In forbs, facilitation decreased with evapotranspiration and temperature.  The importance of novel weapons was more strongly confirmed for studies conducted at lower evapotranspiration, precipitation, and higher latitudes, and exhibited a quadratic relationship with temperature.

4. Synthesis. Our results show that environmental conditions not only filter non-native species depending on physiological tolerances but may also influence the importance of invasion mechanisms.

1. 众多入侵假说（invasion hypotheses）均提出，生物相互作用（biotic interactions）是解释外来物种成功入侵的核心机制。尽管已有研究表明生物相互作用的强度随非生物环境（abiotic context）而异，但目前仍尚不明确：不同入侵解释机制的重要性是否会随非生物环境呈现可预测的变化，且该变化在不同植物生长型（growth form）间是否保持一致。
2. 本研究在全球尺度上综述已发表研究，以评估蒸散发（evapotranspiration）、纬度（latitude）、降水（precipitation）与温度（temperature）是否会影响干扰（disturbance）、天敌释放（enemy release）、促进作用（facilitation）及新武器假说（novel weapons）等入侵解释机制的重要性。最终，我们针对约300种覆盖多样环境条件与生长型的外来植物物种，共计算得到171个效应量（effect sizes）。
3. 环境条件与植物生长型会调控各入侵机制的作用效果。干扰促进入侵的重要性随纬度与温度呈二次函数关系，且随降水增加而降低。在混生群落（mixed communities）与木本植物中，干扰的作用受蒸散发、纬度、降水或温度介导。天敌释放的重要性与蒸散发、纬度及降水呈二次函数关系，且与温度呈显著正相关。天敌释放的重要性同时依赖于植物生长型，且具有极强的环境依赖性：禾本科植物与木本植物的天敌释放响应会受蒸散发、纬度、降水或温度调控。促进作用的重要性随温度升高而降低。在杂类草（forbs）中，促进作用随蒸散发与温度的增加而减弱。新武器假说的重要性在蒸散发与降水较低、纬度较高的研究中得到了更为有力的验证，且其重要性与温度呈二次函数关系。
4. 综合分析。本研究结果表明，环境条件不仅会通过生理耐受性筛选外来物种，同时还会影响各入侵机制的相对重要性。