Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa.
Ecology. 2023 Dec;104(12):e4180. doi: 10.1002/ecy.4180. Epub 2023 Nov 1.
Community structure is driven by the interaction of physical processes and biological interactions that can vary across environmental gradients and the strength of top-down control is expected to vary along gradients of primary productivity. In coastal marine systems, upwelling drives regional resource availability through the bottom-up effect of nutrient subsidies. This alters rates of primary production and is expected to alter algae-herbivore interactions in rocky intertidal habitats. Despite the potential for upwelling to alter these interactions, the interaction of upwelling and grazing pressure is poorly understood, particularly for warm-temperate systems. Using in situ herbivore exclusion experiments replicated across multiple upwelling regimes, we investigated the effects of both grazing pressure, upwelling, and their interactions on the sessile invertebrate community and biomass of macroalgal communities in a warm-temperate system. The sessile invertebrate cover showed indirect effects of grazing, being consistently low where algal biomass was high at upwelling sites and at nonupwelling sites when grazers were excluded. The macroalgal cover was greater at upwelling sites when grazers were excluded and there was a strong effect of succession throughout the experimental period. Grazing effects were greater at upwelling sites, particularly during winter months. There was a nonsignificant trend toward greater grazing pressure on early than later successional stages. Our results show that the positive bottom-up effects of nutrient supply on algal production do not overwhelm top-down control in this warm-temperate system but do have knock-on consequences for invertebrates that compete with macroalgae for space. We speculate that global increases in air and sea surface temperatures in warm-temperate systems will promote top-down effects in upwelling regions by increasing herbivore metabolic and growth rates.
群落结构是由物理过程和生物相互作用驱动的，这些过程和相互作用在环境梯度上可能会发生变化，并且自上而下的控制强度预计会沿着初级生产力梯度发生变化。在沿海海洋系统中，上升流通过营养物质补贴的底向上效应驱动区域资源的可利用性。这会改变初级生产力的速率，并预计会改变岩石潮间带栖息地中藻类-食草动物的相互作用。尽管上升流有可能改变这些相互作用，但上升流和放牧压力的相互作用知之甚少，特别是对于暖温带系统。我们通过在多个上升流制度下重复进行的现场食草动物排除实验，研究了放牧压力、上升流及其相互作用对暖温带系统中固着无脊椎动物群落和大型藻类群落生物量的影响。固着无脊椎动物的覆盖度表现出放牧的间接影响，在上升流点藻类生物量高的地方和食草动物被排除的非上升流点，藻类生物量一直很低。在排除食草动物的上升流点，大型藻类的覆盖度更大，并且在整个实验期间存在强烈的演替效应。在上升流点，放牧的影响更大，尤其是在冬季月份。在早期演替阶段，食草动物的压力比后期更大，但没有显著趋势。我们的研究结果表明，在这个暖温带系统中，营养物质供应的正底向上效应并没有压倒自上而下的控制，但对与大型藻类争夺空间的无脊椎动物有连锁反应。我们推测，暖温带系统中空气和海表面温度的全球升高将通过增加食草动物的新陈代谢和生长速度，在上升流区域促进自上而下的效应。
