Maron John L
University of California, Bodega Marine Laboratory, Box 247, Bodega Bay, CA 94923, USA fax: 707-875-2089; e-mail:
Oecologia. 1997 Apr;110(2):284-290. doi: 10.1007/s004420050161.
Seedlings suffer high mortality in most plant populations, with both competition and herbivory proposed as being important mechanisms causing seedling death. The relative strength of these factors, however, is often unknown. Here I ask how interspecific competition for light and insect herbivory jointly affect seedling survival of bush lupine (Lupinus arboreus), a native shrub common to coastal California. Bush lupine seedlings germinate in grasslands during winter, and throughout spring potentially compete for light with surrounding fast-growing annual grasses. By early summer, after grasses have died, seedlings can be defoliated by a locally abundant caterpillar, the western tussock moth (Orgyia vetusta). I examined the relative importance of competition and insect herbivory on seedling survival in two separate experiments. First, I compared seedling mortality in plots either exposed to or protected from tussock moth larvae. Plants were protected from herbivory by the judicious use of insecticide; control plants were sprayed with water. Tussock moth herbivory resulted in significantly greater (31%) seedling mortality. To determine the effects of interspecific competition for light on seedling survival, I manipulated the density of grass surrounding lupine seedlings. I removed all vegetation surrounding some individuals, and left intact vegetation surrounding others. Reducing competition resulted in a 32% increase in seedling survival from February to June, as well as changes in seedling growth. To determine whether there are interactive effects of competition and herbivory on seedling survival, I enclosed tussock moth larvae on half of all surviving seedlings within each of the two prior competition treatments, comparing growth and survival of defoliated and undefoliated seedlings. Defoliation in June led to an additional 50% mortality for individuals that had grown with competitors through spring, and a 53% additional mortality for seedlings that grew without competitors through spring. Thus, although competition and herbivory both caused substantial seedling mortality, there was no statistical interaction between these factors. Competition-free plants were not less vulnerable to herbivory than plants that previously grew with competitors. Taken together, these experiments indicate that competition and herbivory are both important sources of mortality for bush lupine seedlings.
在大多数植物种群中，幼苗死亡率很高，竞争和食草作用被认为是导致幼苗死亡的重要机制。然而，这些因素的相对强度往往未知。在此，我探究了对光照的种间竞争和昆虫食草作用如何共同影响加州沿海常见的本土灌木——羽扇豆（Lupinus arboreus）幼苗的存活情况。羽扇豆幼苗在冬季于草地中萌发，在整个春季可能会与周围快速生长的一年生草本植物竞争光照。到初夏，草本植物死亡后，幼苗可能会被当地常见的毛虫——西部毒蛾（Orgyia vetusta）吃光叶子。我在两个独立实验中研究了竞争和昆虫食草作用对幼苗存活的相对重要性。首先，我比较了暴露于或免受毒蛾幼虫侵害的地块中的幼苗死亡率。通过合理使用杀虫剂来保护植物免受食草动物侵害；对照植物喷洒水。毒蛾的食草作用导致幼苗死亡率显著更高（31%）。为了确定对光照的种间竞争对幼苗存活的影响，我操纵了羽扇豆幼苗周围草本植物的密度。我移除了一些个体周围的所有植被，而让其他个体周围的植被保持完整。减少竞争导致2月至6月期间幼苗存活率提高了32%，以及幼苗生长发生变化。为了确定竞争和食草作用对幼苗存活是否存在交互作用，我在之前两种竞争处理的每一种中，将毒蛾幼虫圈养在所有存活幼苗的一半上，比较了被吃光叶子和未被吃光叶子的幼苗的生长和存活情况。6月的叶片被吃光导致在春季与竞争者一起生长的个体额外死亡率为50%，而在春季没有竞争者生长的幼苗额外死亡率为53%。因此，尽管竞争和食草作用都导致了大量的幼苗死亡，但这些因素之间没有统计学上的交互作用。没有竞争的植物并不比之前与竞争者一起生长的植物更不容易受到食草动物的侵害。综上所述，这些实验表明竞争和食草作用都是羽扇豆幼苗死亡的重要原因。
