covered with sticky mucus that help them to hold onto branches. Below the tree layers the forest floor is covered with decaying vegetation, which decomposes rapidly due to the hot and humid climate. Savanna vegetation Savanna vegetation includes a mosaic of grasses, trees and shrubs that have adapted to survive the dry season. Species are xerophytic (adapted to drought) with adaptations that include deep tap roots to reach the water table and sunken stomata on the leaves to reduce moisture loss. Species are also pyrophytic (adapted to fire) meaning that many trees have a thick bark, a large amount of biomass (stored energy) below the ground and the ability to regrow rapidly after burning. Savanna grasses have their growth tissue located at the base of the shoot close to the soil surface so that burning or grazing actually encourages its growth. Examples of plant species in the savanna include the acacia, palm and baobab trees and elephant grass, which can grow over 5 m tall.
figure 7.21 Acacia savanna, Taita Hills, Kenya
Nutrient cycling: Gersmehl diagrams, soil fertility, energy flows and trophic levels Within ecosystems, nutrients are circulated and reused frequently. Macronutrients (those used in large quantities) such as oxygen, carbon, hydrogen and nitrogen and micronutrients (trace elements used in small quantities) such as magnesium, sulfur and phosphorus can all be absorbed by plants. Animals eat the plants and take on Precipitation the nutrients. The nutrients are eventually returned back to the Biomass soil when the plants and animals die and are broken down by Fallout Pathway decomposers. All nutrient cycles involve stores and flows between Litter the soil, litter and biomass, the proportions of which can be shown in the Gersmehl diagram (Figure 7.22). In the tropical rainforest the warm humid climate results in a high input of nutrients from Uptake weathering and precipitation. The continual growing season means Pathway that most of the nutrients are held in biomass storage; however, Runoff Decay Pathway there are few nutrients held in the litter layer as the breakdown of nutrients is rapid. Where trees have been removed, precipitation is Soil not intercepted and nutrients are leached out of the soil store; as a Circles - compartments size proportional to amounts of nutrient stored result, rainforest soils are relatively infertile despite appearances. In Leaching Arrows - width proportional to amounts contrast, the biomass and litter stores in the savanna are relatively Weathering of nutrient flow small as a result of the shorter growing season and fire respectively. figure 7.22 Gersmehl diagram to show nutrient cycling in the tropical rainforest The soils are quite fertile as most of the nutrients are found in this store so they are not burnt or leached out of the system. The main driving force in terms of energy within an ecosystem is the Sun – it provides the energy required for photosynthesis and drives the hydrological cycle. The flow of energy can be shown in food chains and webs, which are the natural systems in which organisms feed on Precipitation each other to survive. An organism’s place within that web is the trophic level. There are four trophic levels; primary producers, primary consumers, secondary consumers and tertiary consumers. Fallout Pathway Biomass Between each trophic level it is estimated that there is a 90 per cent loss of energy, which is a result of respiration and excreta. In Litter the tropical rainforest the primary producer trophic level consists of ferns, bamboo and moss. Insects, spiders, fish and parrots are Uptake Pathway the herbivores that eat the producers; they are known as primary consumers. The secondary consumers, which eat the primary Runoff Decay Pathway consumers, are animals such as bats, amphibians and reptiles whilst the tertiary consumers, which are at the top of the food Soil chain, include snakes and carnivorous mammals such as jaguars. In contrast, savannas are dominated by tall grasses, which are Circles - compartments size proportional to amounts of nutrient stored the main primary producers, however shrubs and sparse trees Leaching Arrows - width proportional to amounts such as the acacia are also found within this trophic level. Primary Weathering of nutrient flow consumers include giraffes, antelopes, wildebeests, elephants, figure 7.23 Gersmehl diagram to show nutrient cycling in the tropical rhinos and, in Australia, kangaroos. These herbivores frequently savanna Tropical environments 159
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