Department of Biology and Biochemistry, University of Houston, Houston, Texas 77204, USA.
Ecology. 2011 Feb;92(2):276-81. doi: 10.1890/10-0760.1.
The shrub Iva frutescens, which occupies the terrestrial border of U.S. Atlantic Coast salt marshes, supports a food web that varies strongly across latitude. We tested whether latitudinal variation in plant quality (higher at high latitudes), consumption by omnivores (a crab, present only at low latitudes), consumption by mesopredators (ladybugs, present at all latitudes), or the life history stage of an herbivorous beetle could explain continental-scale field patterns of herbivore density. In a mesocosm experiment, crabs exerted strong top-down control on herbivorous beetles, ladybugs exerted strong top-down control on aphids, and both predators benefited plants through trophic cascades. Latitude of plant origin had no effect on consumers. Herbivorous beetle density was greater if mesocosms were stocked with beetle adults rather than larvae, and aphid densities were reduced in the "adult beetle" treatment. Treatment combinations representing high and low latitudes produced patterns of herbivore density similar to those in the field. We conclude that latitudinal variation in plant quality is less important than latitudinal variation in top consumers and competition in mediating food web structure. Climate may also play a strong role in structuring high-latitude salt marshes by limiting the number of herbivore generations per growing season and causing high overwintering mortality.
灌木 Iva frutescens 占据了美国大西洋沿岸盐沼的陆地边界，支持着一个食物网，这个食物网在纬度上有很大的变化。我们测试了植物质量（高纬度较高）、杂食性动物（仅在低纬度存在的一种螃蟹）的消耗、中型捕食者（存在于所有纬度的瓢虫）的消耗或草食性甲虫的生活史阶段是否可以解释大陆尺度上食草动物密度的野外模式。在一个中观实验中，螃蟹对草食性甲虫施加了强烈的自上而下的控制，瓢虫对蚜虫施加了强烈的自上而下的控制，两种捕食者都通过营养级联使植物受益。植物起源的纬度对消费者没有影响。如果中观实验中储存的是成虫而不是幼虫，那么食草性甲虫的密度就会更大，而在“成虫甲虫”处理中，蚜虫的密度会降低。代表高纬度和低纬度的处理组合产生的食草动物密度模式与野外的模式相似。我们的结论是，植物质量的纬度变化不如顶级消费者和竞争在调节食物网结构中的纬度变化重要。气候也可能通过限制每个生长季节的食草动物世代数量和导致高越冬死亡率，在塑造高纬度盐沼方面发挥重要作用。
