Coastal and Marine Institute, San Diego State University, 4165 Spruance Road, San Diego, California, 92106, USA.
Bodega Marine Laboratory, University of California, Davis, 2099 Westshore Road, Bodega Bay, California, 94923, USA.
Ecology. 2018 Aug;99(8):1748-1758. doi: 10.1002/ecy.2407. Epub 2018 Jul 6.
Intraspecific variation in primary producer induced defenses may affect how these defenses contribute to population and community regulation. For instance, inducible defenses may play larger roles in primary producer populations that are sensitive to low levels of grazing than populations that only respond to high levels of grazing. Although the incidence of herbivore-induced defenses is well documented, and there are examples of geographic variation in these defenses, we have limited knowledge of the factors that affect sensitivity to grazing (i.e., the minimum grazer density needed to elicit these responses) within and among populations. Filling this gap is necessary to understand the importance of these defenses under different conditions. To address whether the sensitivity of seaweed induced defenses varies geographically, we exposed Northern and Southern California populations of the seaweed Silvetia compressa to five densities of the snail Tegula funebralis, under ambient, regional environmental conditions. Southern seaweeds required high levels of grazing to induce defenses, while all levels of herbivory decreased Northern seaweed palatability. To better understand these different responses to low levels of grazing, we conducted common garden experiments to directly test the roles of experimental environment and herbivore source. Consistent with initial experiments, Northern Silvetia responded to low levels of grazing, regardless of environmental conditions, while Southern Silvetia never responded to low levels of grazing, even after being acclimated to Northern environmental conditions for 24 d. Additionally, Southern Silvetia did not respond to grazing by Northern snails, suggesting that herbivore source did not explain this geographic pattern in inducible seaweed defenses. Together, these observations suggest that the seaweed source explains this geographic pattern in sensitivity. Trade-offs with constitutive defenses did not help explain this pattern, as Southern Silvetia had weaker constitutive defenses than Northern seaweeds. This may be due to a combination of low grazing pressure and low predictability of attack in the South. Thus, population variation in sensitivity may be due to long-term differences in environmental histories, resulting in local adaptation or legacy effects from exposure to local conditions. Overall, our results indicate that these herbivore-induced responses may be more important in Northern Silvetia populations than Southern ones.
种内初级生产者诱导防御的变异可能会影响这些防御作用于种群和群落调节的方式。例如，在容易受到低水平放牧影响的初级生产者种群中，诱导防御可能发挥更大的作用，而在仅对高水平放牧有反应的种群中则不然。虽然食草动物诱导防御的发生已有充分的记录，并且这些防御在地理上也有变化的例子，但我们对影响种群内和种群间对放牧敏感性的因素（即引发这些反应所需的最小食草动物密度）知之甚少。填补这一空白对于了解不同条件下这些防御的重要性是必要的。为了确定海藻诱导防御的敏感性是否存在地理差异，我们在环境条件下，将加利福尼亚北部和南部的 Silvetia compressa 海藻种群暴露于 5 种密度的 Tegula funebralis 蜗牛中，以评估这种敏感性。南部海藻需要高水平的放牧来诱导防御，而所有水平的食草动物都会降低北部海藻的可食性。为了更好地理解这些对低水平放牧的不同反应，我们进行了常规花园实验，直接测试实验环境和食草动物来源的作用。与初始实验一致，无论环境条件如何，北部 Silvetia 都对低水平的放牧做出了反应，而南部 Silvetia 则从未对低水平的放牧做出过反应，即使在经过 24 天适应北部环境条件后也是如此。此外，南部 Silvetia 对北部蜗牛的放牧也没有反应，这表明食草动物来源并不能解释这种海藻诱导防御的地理模式。综上所述，这些观察结果表明，海藻来源解释了这种敏感性的地理模式。与组成防御的权衡关系并不能帮助解释这种模式，因为南部 Silvetia 的组成防御比北部海藻弱。这可能是由于南部放牧压力低和攻击的可预测性低的综合作用。因此，敏感性的种群变化可能是由于长期的环境历史差异导致的，从而导致对当地条件的局部适应或遗留效应。总的来说，我们的研究结果表明，在北方 Silvetia 种群中，这些食草动物诱导的反应可能比南方种群更为重要。
