Plant growth provides food for herbivores that in turn canimpact the productivity of the ecosystems that they graze.Herbivores remove green leaves and therefore lower potentialwhole-ecosystem photosynthesis by reducing the leaf area index.However herbivores may also reduce the amount of shading in thelower plant canopy and fertilize soils with their feces and urine,increasing the amount of light and nutrients for the remaininguneaten plants. Removal of leaves by herbivores may also warm upsoils, leading to higher respiration by the plants and soilheterotrophs. Thus the relationship between long-term productivityand grazing is difficult to predict. Further complicating therelationship is the influence of weather, particularlyprecipitation and its impact on soil water availability. DavidAugustine and Sam McNaughton (2006) studied the influence ofgrazing on aboveground net primary productivity (NPP) in Kenyanrangeland ecosystems differing in nutrient availability.Theinfluence of herbivore grazing on aboveground NPP was evaluatedusing treatment plots where herbivores were excluded (no grazing)and control plots where herbivores grazed freely (grazing).Rainfall at the research site is characteristic of seasonaltropical savannas, with pronounced dry and wet seasons.
Question 3. What do the results shown inTable 3 indicate about the influence of grazing onaboveground NPP in a year with above average rainfall? (Note thatthe experimental site was shifted this year, contributing to somebetween year differences in NPP.
Table 3 Above average rainfall |
| Aboveground NPP (g/m2) ± SEM |
Site fertility | Grazing | No grazing |
Low nutrient | 60 ± 15 | 120 ± 25 |
High nutrient | 310 ± 55 | 255 ± 45   |
Question 4. After viewing the above tables,what factors might explain the influence of variation in rainfalland nutrients on the effect of herbivores on NPP?
