Daleo Pedro, Iribarne Oscar
Laboratorio de Ecologia, FCEyN, Universidad Nacional de Mar del Plata, CC 573 Correo Central B7600WAG, Mar del Plata, Argentina.
Ecology. 2009 Sep;90(9):2368-74. doi: 10.1890/08-2330.1.
The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from facilitation to competition as environmental stress decreases. Although restricted to facilitation/competition, the mechanistic model behind the hypothesis is easily modified to include other negative interactions that are as important as competition in structuring natural communities, e.g., herbivory. To evaluate this hypothesis we experimentally tested if the balance between the facilitative and trophic effect of an intertidal, burrowing, herbivorous crab in marsh plants is context dependent and shifts from positive to negative as stress decreases. By sampling salt marshes differing in sediment size characteristics, we show that sites with larger sediment particle size had less stressful oxygen levels than sites with fine sediment particles, and that the level of stress was reduced by the presence of crab burrows. We then conducted a factorial experiment manipulating sediment size and crab presence. Results show that, by decreasing soil anoxic stress, crabs increase plant growth in stressful zones, but their ecological importance as herbivores increases in more benign zones. Our findings suggest that the balance between positive and negative interactions along stress gradients is more important than previously perceived and also applies to facilitation and herbivory between animals and plants.
胁迫梯度假说预测，植物之间的相互作用取决于环境背景，会随着环境胁迫的减轻从促进作用转变为竞争作用。尽管该假说仅限于促进作用/竞争作用，但背后的机制模型很容易修改，以纳入其他在构建自然群落中与竞争同样重要的负面相互作用，例如食草作用。为了评估这一假说，我们通过实验测试了潮间带穴居食草蟹对沼泽植物的促进作用和营养效应之间的平衡是否取决于环境背景，以及是否会随着胁迫的减轻从正向转变为负向。通过对沉积物粒度特征不同的盐沼进行采样，我们发现沉积物颗粒较大的地点的缺氧水平比细颗粒沉积物地点的压力小，并且蟹洞的存在降低了胁迫水平。然后我们进行了一项析因实验，控制沉积物粒度和蟹的存在情况。结果表明，通过降低土壤缺氧胁迫，蟹类促进了胁迫区域植物的生长，但它们作为食草动物在更适宜区域的生态重要性增加。我们的研究结果表明，沿着胁迫梯度的正负相互作用之间的平衡比之前认为的更为重要，并且也适用于动植物之间的促进作用和食草作用。
