High Low High Low High Low Primary Productivity Low High Figure 57.17 A modol of bottom-up offects. At low kvels of primary productivity, herbivore populations cannot obtain enough Food to be maintained;without herbivory, the standing crop biomas of the primary produers such as thee diatoms ince aes as their productivity increases. Abone some threshold, increases in primary productivity ead to increzses in herbivore populations and herbivoe biomus; the biomass of the primury producers then does not increase as primary producta ity increases because the inceasing productivity s cropped by the herbivures. Above another threshold, populations of primary camivores can be sustained. As primary productivity inceaes above this threshold, the carnivores comume the increasing productivity of the herbivores, so the biomas of the herbwore populalions mmains me latively constant whike the biomas of the carnivore populations increases. The biomass af the primary producers s no konger constrained by increases in the herbivure populations and thus also increases wah increasing primary productiv ity. A key to undenstanding the model is to maintain a distinction between the concepts of productivky and standing crop biomass. Hertivore Eonpo ku

High Low High Low High Low Primary Productivity Low High Figure 57.17 A modol of bottom-up offects. At low kvels of primary productivity, herbivore populations cannot obtain enough Food to be maintained;without herbivory, the standing crop biomas of the primary produers such as thee diatoms ince aes as their productivity increases. Abone some threshold, increases in primary productivity ead to increzses in herbivore populations and herbivoe biomus; the biomass of the primury producers then does not increase as primary producta ity increases because the inceasing productivity s cropped by the herbivures. Above another threshold, populations of primary camivores can be sustained. As primary productivity inceaes above this threshold, the carnivores comume the increasing productivity of the herbivores, so the biomas of the herbwore populalions mmains me latively constant whike the biomas of the carnivore populations increases. The biomass af the primary producers s no konger constrained by increases in the herbivure populations and thus also increases wah increasing primary productiv ity. A key to undenstanding the model is to maintain a distinction between the concepts of productivky and standing crop biomass. Hertivore Eonpo ku