In: Biology
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 3 Above average rainfall |
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Aboveground NPP (g/m2) ± SEM |
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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?
Ans 3) As per the table, it shows that when the site has low nutrient and is less fertile, the NPP or net primary production is different as compared to soil that has high fertility. The NPP is the total amount of carbon and energy which enters in the ecosystem. It is found that when there is grazing in presence of above average rainfall, in case of those areas that have low nutrient, the nutrition condition does not increase much but when there is high nutrient area, with the increased rainfall, the NPP or net primary productivity tends to increase and hence the nutrition concentration increases a lot. However, in the areas where grazing does not take place, the increase in the nutrient concentration does not increase by a great amount.
Ans 4) The factors that would influence the variation in rainfall and nutrients on effect of herbivores on NPP is that in the areas where the grazing is more and there is more rainfall, it lead to reduction in the NPP. Hence when there is more grazing, in some areas, it tends to increase the soil fertility due to the waste products of the animals and hence the NPP is higher. Hence, in the areas where there is no grazing, the nutrition condition of the soil does not vary much and in those areas where there is grazing, in presence of high rainfall, the fertility increases very high in existing fertile soil.