Department of Biology, Coastal and Marine Institute, San Diego State University, San Diego, California, 92182, USA.
Department of Environmental Science and Policy, University of California Davis, Davis, California, 95616, USA.
Ecology. 2019 Mar;100(3):e02625. doi: 10.1002/ecy.2625.
Predator responses to gradients in prey density have important implications for population regulation and are a potential structuring force for subtidal marine communities, particularly on rocky reefs where herbivorous sea urchins can drive community state shifts. On rocky reefs in southern California where predatory sea otters have been extirpated, top-down control of sea urchins by alternative predators has been hypothesized but rarely tested experimentally. In laboratory feeding assays, predatory spiny lobsters (Panulirus interruptus) demonstrated a saturating functional response to urchin prey, whereby urchin proportional mortality was inversely density-dependent. In field experiments on rocky reefs near San Diego, California, predators (primarily the labrid fish California sheephead, Semicossyphus pulcher) inflicted highly variable mortality on purple urchin (Strongylocentrotus purpuratus) prey across all density levels. However, at low to moderate densities commonly observed within kelp forests, purple urchin mortality increased to a peak at a density of ~11 urchins/m . Above that level, at densities typical of urchin barrens, purple urchin mortality was density-independent. When larger red urchins (Mesocentrotus franciscanus) were offered to predators simultaneously with purple urchins, mortality was density-independent. Underwater videography revealed a positive relationship between purple urchin density and both the number and richness of fish predators, but these correlations were not observed when red urchins were present. Our results demonstrate highly variable mortality rates across prey densities in this system and suggest that top-down control of urchins can occur only under limited circumstances. Our findings provide insight into the dynamics of alternate community states observed on rocky reefs.
捕食者对猎物密度梯度的反应对种群调节具有重要意义，是亚潮间带海洋群落的潜在结构力量，特别是在石质珊瑚礁上，那里的草食性海胆可以驱动群落状态的转变。在加利福尼亚南部的石质珊瑚礁上，由于掠食性海獭已经灭绝，替代捕食者对海胆的自上而下的控制作用已被假设，但很少进行实验验证。在实验室喂食实验中，掠食性刺龙虾（Panulirus interruptus）对海胆猎物表现出饱和的功能反应，即海胆的比例死亡率与密度呈负相关。在加利福尼亚圣地亚哥附近的石质珊瑚礁上进行的野外实验中，捕食者（主要是 labrid 鱼加州羊头鱼，Semicossyphus pulcher）在所有密度水平上对紫色海胆（Strongylocentrotus purpuratus）猎物造成高度可变的死亡率。然而，在通常在海带林中观察到的低至中等密度下，紫色海胆的死亡率在密度约为 11 只/时达到峰值。在那个水平以上，在典型的海胆荒地密度下，紫色海胆的死亡率与密度无关。当同时向捕食者提供较大的红色海胆（Mesocentrotus franciscanus）和紫色海胆时，死亡率与密度无关。水下录像显示，紫色海胆的密度与鱼类捕食者的数量和丰富度呈正相关，但当红色海胆存在时，这些相关性就不存在了。我们的研究结果表明，在该系统中，猎物密度的死亡率存在高度的可变性，并表明只有在有限的情况下才能对海胆进行自上而下的控制。我们的发现为了解石质珊瑚礁上观察到的替代群落状态的动态提供了线索。
