Herbivory is a significant driver of algal community dynamics on coral reefs. However, abiotic factors such as the complexity and orientation of the benthos often mediate the impact of herbivores on benthic communities. We experimentally evaluated the independent and interactive effects of substrate orientation and herbivorous fishes on algal community dynamics on a coral reef in the Florida Keys, USA. We created horizontal and vertical substrates, mimicking the trend in the reduction of vertical surfaces of coral reefs, to assess how algal communities developed either with herbivory (open areas) or without herbivory (herbivore exclosures). We found that substrate orientation was the dominant influence on macroalgal community composition. Herbivores had little impact on community development of vertical substrates as crustose algae dominated these substrates regardless of being in exclosures or open areas. In contrast, herbivores strongly impacted communities on horizontal substrates, with upright macroalgae (e.g., Dictyota spp., articulated coralline algae) dominating herbivore exclosures, while filamentous turf algae and sediment dominated open areas. Outside of exclosures, differences between vertical and horizontal substrates exposed to herbivores persisted despite similar intensity of herbivory. Our results suggest that the orientation of the reef benthos has an important impact on benthic communities. On vertical surfaces, abiotic factors may be more important for structuring algal communities while herbivory may be more important for controlling algal dynamics in flatter areas. Thus, the decline in structural complexity of Caribbean coral reefs and the flattening of reef substrates may fundamentally alter the impact that herbivores have on benthic community dynamics.

中文翻译：

---

食草和底物取向对珊瑚礁藻类群落动态的交互作用

食草动物是珊瑚礁藻类群落动态的重要驱动因素。然而，底栖动物的复杂性和定向等非生物因素通常会调节食草动物对底栖群落的影响。我们通过实验评估了底物方向和食草鱼类对美国佛罗里达群岛珊瑚礁藻类群落动态的独立和交互影响。我们创建了水平和垂直基质，模仿珊瑚礁垂直表面减少的趋势，以评估藻类群落如何在食草动物（开放区域）或没有食草动物（食草动物围栏）的情况下发展。我们发现底物方向是对大型藻类群落组成的主要影响。食草动物对垂直基质的群落发展几乎没有影响，因为无论是在围场还是开阔区域，甲壳藻都控制着这些基质。相比之下，食草动物强烈影响水平基质上的群落，直立的大型藻类（例如，Dictyota spp.，铰接珊瑚藻）在食草动物的围栏内占主导地位，而丝状草皮藻类和沉积物则占主导地位的开放区域。在围栏之外，尽管食草动物的强度相似，但暴露于食草动物的垂直和水平基质之间的差异仍然存在。我们的研究结果表明，珊瑚礁底栖生物的方向对底栖生物群落有重要影响。在垂直表面上，非生物因素对于构建藻类群落可能更为重要，而食草动物对于控制平坦地区的藻类动态可能更为重要。因此，加勒比珊瑚礁结构复杂性的下降和珊瑚礁基质的变平可能从根本上改变食草动物对底栖群落动态的影响。