Spatially variable propagule pressure and herbivory influence invasion of chaparral shrubland by an exotic grass
Spatially variable propagule pressure and herbivory influence invasion of chaparral shrubland by an exotic grass
复制标题
空间变化的繁殖压力和食草影响外来草对丛林灌木丛的入侵
DOI：
10.1007/s00442-005-0259-1
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发表时间：
2006
期刊：
影响因子：
2.7
通讯作者：
J. Lambrinos
中科院分区：
文献类型：
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作者：
J. Lambrinos
尽管许多研究已经确定了抵抗或促进生物入侵的机制，但很少有研究明确测试抵抗和促进机制如何相互作用来推动入侵成功。在美国加利福尼亚州，未受干扰的地中海型灌木丛抵抗了多年生草本植物Cortaderia Jubata的入侵。然而，在某些情况下，即使在没有大规模干扰的情况下，这种抵抗也被惊人地打破了。我测试了这样一种假设，即这些入侵是由于局部生物抗性强度的降低而促进的。我通过在不同的微生境中添加朱巴塔种子和苗木来评估入侵成功，这些微生境包括：茶树林的边缘、离林缘不同距离的灌木树冠下，以及林分内的树冠间隙。当暴露在哺乳动物食草动物的环境中时，幼苗存活率从林分边缘的近40%急剧下降到林分中仅10米处的零。然而，当移植物受到草食性疾病的保护时，离边缘的距离对移植物的存活率没有显著影响。林缘和林隙的出苗率也高于林下，但这种差异不是由于食草性的差异造成的。入侵繁殖植物到达土壤表面的通量是巨大的，沿边缘和林隙内的通量大于林分冠层下的通量。与这些模式相对应的是，随着距离林分边缘的距离，自然发生的苗木数量急剧下降，林隙内的苗木远比预期的更常见，因为林分内的林隙频率给定。尽管林内对入侵的生物抵抗力很强，但朱巴塔的盖度在过去9年里增加了20%。这些结果表明，相对数量的敏感边缘栖息地和入侵繁殖的供应可以促进入侵，即使面对强大的局部生物抗性。
Although numerous studies have identified mechanisms that either resist or facilitate biological invasions, few studies have explicitly tested how resisting and facilitating mechanisms interact to drive invasion success. In California, USA, undisturbed Mediterranean-type shrublands have resisted invasion by the perennial tussock grass Cortaderia jubata. In some cases, however, this resistance has been spectacularly breached even in the absence of large-scale disturbance. I tested the hypothesis that these invasions are facilitated by local reductions in the strength of biotic resistance. I evaluated invasive success using C. jubata seed and seedling additions at different microhabitats: the edge of a chaparral stand, under shrub canopy at different distances from the stand edge, and in canopy gaps within the stand. When left exposed to mammalian herbivores, seedling survivorship decreased sharply from nearly 40% on the stand edge to zero just 10 m into the stand. When transplants were protected from herbivory, however, distance from the edge had no significant influence on transplant survivorship. Seedling emergence was also greater on the edge and in canopy gaps than under the canopy, but these differences were not caused by differences in herbivory. The flux of invasive propagules reaching the soil surface was immense and greater along the edge and within gaps than under the stand canopy. Mirroring these patterns, naturally occurring seedling abundance declined dramatically with distance from the stand edge, and seedlings were far more common within stand gaps than would be expected given gap frequency within the stand. Despite strong biotic resistance to invasion within the stand, the cover of C. jubata has increased 20% over the last 9 years. These results suggest that the relative amount of susceptible edge habitat and the supply of invasive propagules can facilitate invasion even in the face of strong local biotic resistance.