Long Jeremy D, Porturas Laura D
Biology Department and Coastal & Marine Institute Laboratory, San Diego State University, San Diego, California, United States of America.
PLoS One. 2014 Oct 13;9(10):e110419. doi: 10.1371/journal.pone.0110419. eCollection 2014.
Plant communities are disturbed by several stressors and they are expected to be further impacted by increasing anthropogenic stress. The consequences of these stressors will depend, in part, upon the ability of plants to compensate for herbivory. Previous studies found that herbivore impacts on plants can vary from negative to positive because of environmental control of plant compensatory responses, a.k.a. the Compensatory Continuum Hypothesis. While these influential studies enhanced our appreciation of the dynamic nature of plant-herbivore interactions, they largely focused on the impact of resource limitation. This bias limits our ability to predict how other environmental factors will shape the impact of herbivory. We examined the role of salinity stress on herbivory of salt marsh cordgrass, Spartina foliosa, by an herbivore previously hypothesized to influence the success of restoration projects (the scale insect, Haliaspis spartinae). Using a combination of field and mesocosm manipulations of scales and salinity, we measured how these factors affected Spartina growth and timing of senescence. In mesocosm studies, Spartina overcompensated for herbivory by growing taller shoots at low salinities but the impact of scales on plants switched from positive to neutral with increasing salinity stress. In field studies of intermediate salinities, scales reduced Spartina growth and increased the rate of senescence. Experimental salinity additions at this field site returned the impact of scales to neutral. Because salinity decreased scale densities, the switch in impact of scales on Spartina with increasing salinity was not simply a linear function of scale abundance. Thus, the impact of scales on primary production depended strongly upon environmental context because intermediate salinity stress prevented plant compensatory responses to herbivory. Understanding this context-dependency will be required if we are going to successfully predict the success of restoration efforts and the ecological consequences of anthropogenic disturbances.
植物群落受到多种压力源的干扰，预计还会受到日益增加的人为压力的进一步影响。这些压力源的后果将部分取决于植物补偿食草作用的能力。先前的研究发现，由于植物补偿反应的环境控制，即所谓的补偿连续体假说，食草动物对植物的影响可能从负面到正面不等。虽然这些有影响力的研究增强了我们对植物 - 食草动物相互作用动态性质的认识，但它们主要关注资源限制的影响。这种偏差限制了我们预测其他环境因素将如何塑造食草作用影响的能力。我们研究了盐度胁迫对盐沼大米草（Spartina foliosa）食草作用的影响，食草动物是一种先前假设会影响恢复项目成功的昆虫（蚧壳虫，Haliaspis spartinae）。通过结合对蚧壳虫数量和盐度的田间和中宇宙操纵，我们测量了这些因素如何影响大米草的生长和衰老时间。在中宇宙研究中，大米草在低盐度下通过长出更高的茎来过度补偿食草作用，但随着盐度胁迫增加，蚧壳虫对植物的影响从正面转变为中性。在中等盐度的田间研究中，蚧壳虫降低了大米草的生长并增加了衰老速度。在该田间地点进行的实验性盐度添加使蚧壳虫的影响恢复为中性。由于盐度降低了蚧壳虫的密度，随着盐度增加，蚧壳虫对大米草影响的转变不仅仅是蚧壳虫数量的线性函数。因此，蚧壳虫对初级生产的影响在很大程度上取决于环境背景，因为中等盐度胁迫阻止了植物对食草作用的补偿反应。如果我们要成功预测恢复努力的成功以及人为干扰的生态后果，就需要了解这种背景依赖性。
