Plant growth provides food for herbivores that in turn can impact the productivi

ID: 184456 • Letter: P

Question

Plant growth provides food for herbivores that in turn can impact the productivity of the ecosystems that they graze. Herbivores remove green leaves and therefore lower potential whole-ecosystem photosynthesis by reducing the leaf area index. However herbivores may also reduce the amount of shading in the lower plant canopy and fertilize soils with their feces and urine, increasing the amount of light and nutrients for the remaining uneaten plants. Removal of leaves by herbivores may also warm up soils, leading to higher respiration by the plants and soil heterotrophs. Thus the relationship between long-term productivity and grazing is difficult to predict. Further complicating the relationship is the influence of weather, particularly precipitation and its impact on soil water availability. David Augustine and Sam McNaughton (2006) studied the influence of grazing on aboveground net primary productivity (NPP) in Kenyan rangeland ecosystems differing in nutrient availability.The influence of herbivore grazing on aboveground NPP was evaluated using treatment plots where herbivores were excluded (no grazing) and control plots where herbivores grazed freely (grazing). Rainfall at the research site is characteristic of seasonal tropical savannas, with pronounced dry and wet seasons.

Question 3. What do the results shown in Table 3 indicate about the influence of grazing on aboveground NPP in a year with above average rainfall? (Note that the experimental site was shifted this year, contributing to some between year differences in NPP.

Table 3Above 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 rainfall and nutrients on the effect of herbivores on NPP?