Question

a. Briefly explain ecological pyramids
b. Define primary productivity. Give two factors which affect primary productivity.

Answer 1

a. Ecological pyramids are graphical representation of various ecological parameters at the successive trophic levels of food chains with producers at the base, top carnivores at the apex and intermediate levels in between. Quantity at each level is indicated by length of bar in the graph. The common parameters used in preparing ecological pyramids are number of individual, biomass and energy at different trophic levels. The three ecological pyramids that are usually studies are:

• Pyramid of numbers:

1. Upright: During transfer of food at any trophic level, only 10% of the food present in one trophic level becomes part of the next trophic level. 90% of the food is either lost in wastage or broken down during cellular respiration for providing energy for various life activities. With each successive trophic level, number of individual are decreasing. Therefore the pyramid of number is upright e.g., Grassland ecosystem and Pond ecosystem.
2. Inverted: In some case at each successive trophic level, number of the organisms as higher than in preceding one and the size decrease gradually at successive level. Thus, shape of pyramid may be inverted. For example, a large-sized tree may support and provide nourishment to several frugivorous birds. The number thus increases at each trophic level.
3. Spindle: A tree supports a number of herbivores birds. The herbivores birds are eaten by one or two hawks of the area.

• Pyramid of Biomass

1. Upright: biomass is the amount of living matter at any particular trophic level at a given time. Pyramid of biomass in terrestrial ecosystems is usually upright. For upright pyramid, total biomass of plants in a specific area is more than that of herbivores and it gradually decrease at each successive trophic level. It is least in top carnivores e.g. Tree and grassland ecosystems.
2. Inverted: In aquatic ecosystem, the pyramid of biomass may be inverted. For example, biomass of zooplanktons in higher than that of phytoplankton's as life span of former is longer and the latter multiply much faster through having shortest life span. A number of generation of phytoplankton's may thus be consumed by single generation of zooplanktons. Biomass of fish may still be larger as fishes are larger in size with the longer life span and a number of generation of zooplanktons can be consumed by fishes.

• Pyramid of energy: The pyramid of energy is always upright because the flow of energy is unidirectional from producer to consumer level. The energy content is maximum in producers. The energy decreases at each trophic level of food chain, as part of the energy is lost as heat and major part of energy is liberated during respiration for use in various activities. Only 10% of the energy of previous trophic level is received by next trophic level, as proposed by 10 percent law of Lindeman.

b. Primary productivity is the rate at which biomass or organic matter is produced by plants or producers during photosynthesis per unit area over a time period or it refers to rate at which sunlight is captured by producers for the synthesis of energy-rich organic compounds through photosynthesis. It is expressed in term of weight as ($gm{m}^{-2}$)$y{r}^{-1}$or energy as ($Kcal{m}^{-2})y{r}^{-1}$ to compare the productivity of different ecosystems.

Several biotic and abiotic factors affects magnitude of primary productivity. Following are the two factors:

1. Photosynthetic capacity of producers which means the ability to utilize incident solar radiation to raise gross primary productivity.
2. Solar radiations available