Hanley M E, Sykes R J
School of Biological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK.
Ann Bot. 2009 Jun;103(8):1347-53. doi: 10.1093/aob/mcp081. Epub 2009 Apr 7.
Although the causes and consequences of seedling herbivory for plant community composition are well understood, the mechanisms by which herbivores influence plant species recruitment to the established phase remain less clear. The aim was to examine how variation in the intensity of seedling herbivory interacts with growth-defence trade-offs and herbivore feeding preferences to affect plant community development.
Using 14-d-old seedlings of Trifolium pratense and T. repens, relative growth and susceptibility to herbivory by the snail Helix aspersa was quantified to elucidate putative growth-defence trade-offs for these species. Then mixed assemblages of 14-d-old Trifolium seedlings were exposed to herbivory by zero, two, five or ten snails and determined how variation in the intensity of herbivory affected competitive interactions into the mature phase (as measured by total plant biomass at 120 d old).
In the absence of herbivory, communities were dominated by T. pratense; a result expected on the basis that it yielded larger and presumably more competitive seedlings. However, when seedlings were exposed to herbivory, the balance of competition shifted. At low levels of herbivory (two snails), both Trifolium species contributed equally to total plant biomass. More intense herbivory (five snails) resulted in almost total mortality of T. pratense and dominance of the mature community by T. repens. The most intense herbivory (ten snails) effectively removed all seedlings from the experimental community.
The study illustrates a mechanism whereby spatio-temporal fluctuations in seedling herbivory, when coupled with species-specific variation in competitive ability and sensitivity to herbivore attack, can differentially influence plant recruitment into the mature phase. This mechanism may be a key element in our attempts to understand plant species coexistence, since fluctuations in plant recruitment are fundamental to the many theories that view coexistence as a consequence of a spatio-temporal lottery for dominance over regeneration micro-sites.
虽然幼苗食草作用对植物群落组成的原因和后果已得到充分理解，但食草动物影响植物物种从幼苗期到成株期补充过程的机制仍不太清楚。本研究旨在探讨幼苗食草作用强度的变化如何与生长-防御权衡以及食草动物的取食偏好相互作用，从而影响植物群落的发展。
以14日龄的红三叶草和白三叶草幼苗为材料，对其相对生长速率以及被蜗牛（即散大蜗牛）取食的易感性进行量化，以阐明这些物种假定的生长-防御权衡。然后，将14日龄的三叶草幼苗混合组合暴露于0只、2只、5只或10只蜗牛的取食环境中，测定食草作用强度的变化如何影响其到成熟阶段的竞争相互作用（以120日龄时的总植物生物量衡量）。
在没有食草作用的情况下，群落以红三叶草为主导，这一结果基于其产生更大且可能更具竞争力的幼苗而预期得到。然而，当幼苗遭受食草作用时，竞争平衡发生了变化。在低强度食草作用（2只蜗牛）下，两种三叶草物种对总植物生物量的贡献相同。更强的食草作用（5只蜗牛）导致红三叶草几乎全部死亡，成熟群落以白三叶草为主导。最强的食草作用（10只蜗牛）有效地从实验群落中清除了所有幼苗。
该研究阐明了一种机制，即幼苗食草作用的时空波动，与物种特异性的竞争能力差异以及对食草动物攻击的敏感性相结合时，可不同程度地影响植物向成熟阶段的补充。这种机制可能是我们理解植物物种共存的关键要素，因为植物补充的波动是许多将共存视为在再生微位点上进行时空优势抽签结果的理论的基础。
