Darcy-Hall Tara L
W. K. Kellogg Biological Station and Department of Zoology, Michigan State University, Hickory Corners, MI 49060, USA.
Oecologia. 2006 Jul;148(4):660-71. doi: 10.1007/s00442-006-0405-4. Epub 2006 Mar 23.
The relative effects of nutrients and herbivores on primary producers are rarely compared across ecosystems that vary in potential primary productivity. Furthermore, proposed mechanisms to explain such patterns remain understudied. Here, I examine the strength of nutrient and grazer (herbivore) limitation (i.e., the extent to which producers' growth is limited by insufficient nutrient supply or herbivory) of benthic algae across 13 southwest Michigan lakes that vary widely in productivity (i.e., resource supply). I compare the observed patterns of algal limitation and species composition to those predicted by two simple models: one that includes multiple species and species' traits (the food-web model) and one that includes no variation in species or traits (the food-chain model). Species in the food-web model are assumed to display a tradeoff between resource competitive ability and resistance to herbivory (the "keystone predator" tradeoff). Among these lakes, benthic algal nutrient limitation was positive (x=0.083 day-1) and declined significantly along a lake N:P gradient. In contrast, grazer limitation was negative (x=-0.019 day-1) and was not significantly related to any of the measured lake productivity variables. Negative grazer limitation indicated that the removal of grazers caused unexpected declines in algal biomass, which were potentially due to indirect, positive effects (e.g., nutrient recycling) of grazers. Nutrient limitation was significantly stronger than grazer limitation across lakes, which was more consistent with the food-web versus food-chain model. Changes in algal composition were also broadly consistent with predictions of the food-web model in that vulnerable, superior nutrient competitors dominated in low productivity lakes and more grazer-resistant species were observed in high productivity lakes. In general, these results point to the importance of examining limiting factors across systems and the consideration of key species' traits when predicting and interpreting patterns.
在潜在初级生产力各异的生态系统中，很少有人比较养分和食草动物对初级生产者的相对影响。此外，用于解释此类模式的机制仍未得到充分研究。在此，我研究了密歇根州西南部13个生产力差异很大（即资源供应情况不同）的湖泊中底栖藻类的养分和食草动物（食草者）限制强度（即生产者生长受养分供应不足或食草作用限制的程度）。我将观察到的藻类限制模式和物种组成与两个简单模型预测的结果进行比较：一个模型包含多个物种及其特征（食物网模型），另一个模型不考虑物种或特征的变化（食物链模型）。食物网模型中的物种被假定在资源竞争能力和对食草动物的抵抗力之间表现出权衡（“关键捕食者”权衡）。在这些湖泊中，底栖藻类的养分限制呈正向（x = 0.083天⁻¹），并沿湖泊氮磷梯度显著下降。相比之下，食草动物限制呈负向（x = -0.019天⁻¹），且与任何测量的湖泊生产力变量均无显著关联。负的食草动物限制表明，去除食草动物会导致藻类生物量意外下降，这可能是由于食草动物的间接正向影响（如养分循环）所致。在各个湖泊中，养分限制明显强于食草动物限制，这与食物网模型而非食物链模型更为一致。藻类组成的变化也大致符合食物网模型的预测，即易受影响的、养分竞争能力强的物种在低生产力湖泊中占主导地位，而在高生产力湖泊中观察到更多抗食草动物的物种。总体而言，这些结果表明在预测和解释模式时，研究不同系统中的限制因素以及考虑关键物种特征的重要性。
