The productivity of photosynthesis depends on how much energy is captured by producers and how much energy is transferred onto higher trophic levels. Out of the 100% of solar energy striking the leaf surface, 5% is transmitted straight through the leaf, 5% is reflected off the leaf surface, 40% energy is not absorbed by chlorophyll but is used in the evaporation of water from leaves, 50% energy is of the right wavelength and hence is absorbed by the chlorophyll and other photosynthetic pigments in the chloroplasts. Gross Primary Productivity (GPP) is the rate at which energy is incorporated into organic molecules by an ecosystem.
The units of GPP are kJm-2yr-1
% efficiency of photosynthesis = [GPP/ Amount of energy striking the leaf surface] x 100
Net Primary Productivity is the rate at which energy is transferred into the organic molecules that make up the new biomass of the plant. The energy from carbohydrates, produced as a result of photosynthesis is incorporated into proteins, chromosomes, membranes and other components of the cell.
The units of NPP are kJm-2yr-1
NPP = GPP – Respiration
Succession occurs since ecosystems are constantly changing; they are dynamic. Succession is the process by which communities of plants and animals colonise an area and overtime are replaced by new, more varied communities. This is because the actions of the plants and animals have a modifying effect on the nature of the environment. Succession happens in a series of stages/ seres. Primary succession occurs when bare rock is first colonised. Bare rock/ barren terrain can form when glaciers retreat depositing rocks, volcanoes erupt depositing lava and sand piles into dunes by wind. An inhospitable environment is first colonised by pioneer species. Pioneer species are tolerant to and capable of coping with extremes of temperature, soil pH levels, soil mineral availability and lack of water availability. Pioneer species usually produce vast quantities of wind dispersed seeds/ spores which are capable of reaching isolated situations. They do not require a period of dormancy and hence can germinate quickly. They are photosynthetic and are usually capable of fixing N2 from the atmosphere into the soil as the soil lacks nutrients. They break up the rock surface allowing the build up of organic material. This is the beginning of soil formation. They change the conditions of the habitat to the right amount for other species to cope with the environment. Wind blown moss spores start to grow. Mosses build up more organic matter and the soil can now hold water. As the conditions improve, shallow rooted plant seeds can now grow. As they die, they add more nutrients to the soil. Soon, seeds of larger and more taller plants start to grow. They compete with the plants already present in the habitat and replace the existing communities. As the biodiversity of plants increases, the diversity of animals that can now be supported also increases. Climax community is the different species of plants and animals that make up the final sere of an ecological succession. A climax community is stable, self-sustaining and unchanged, unless the conditions in the habitat change. A particular species usually dominates in the climax community. A dominant species is the one that exerts overriding influence on the rest of the plants and animals present in the habitat. They are usually the largest and most abundant species in the habitat. Sometimes, co-dominance is also seen.
Secondary succession occurs when a land that has already sustained life is suddenly altered (e.g. due to forest fires or land clearance for agriculture). The climax community is usually reached faster than in primary succession. This is because soil is already formed. Seed banks/ spores may already be present in the soil. Animals from surrounding areas may pass by. Succession is associated with increase in biomass. As soil becomes deeper, greater amount of vegetation can be supported and hence larger number and types of animals can also be supported.
