Ecosystem Test - 59

Net primary productivity is the rate at which all the plants in an ecosystem produce net useful chemical energy.

An invasive species is a species that is not native to a specific location (an introduced species), and that has a tendency to spread to a degree believed to cause damage to the environment, human economy or human health. The criteria for invasive species has been controversial, as widely divergent perceptions exist among researchers as well as concerns with the subjectivity of the term "invasive". Several alternate usages of the term have been proposed. The term as most often used applies to introduced species (also called "non-indigenous" or "non-native") that adversely affect the habitats and bioregions they invade economically, environmentally, or ecologically. Such invasive species may be either plants or animals and may disrupt by dominating a region, wilderness areas, particular habitats, or wildland–urban interface land from loss of natural controls (such as predators or herbivores).

Natural or background extinction is a slow process of replacement of existing species with the better adapted species due to alternate evolution, changes in environmental conditions, predators, and disease.

A stable community is self-perpetuating and final biotic community that is in perfect harmony with the physical environment. It does not show much of changes and is usually resilient to natural or man made disturbances. 

In oceans, there are lots of nutrients but no sufficient sunlight is available in deep areas hence, oceans have the least productivity. Grasslands are less productive ecosystem with annual productivity of 200−1000 gm/m2. Maximum productivity can be seen in coral reefs followed by estuaries. Salt marshes are estuaries where brackish and salt water mix. Salt marsh plants(halophytes) are salt tolerant and adapted to water levels that fluctuate with tides. Tides carry in the nutrients that stimulate plant growth in marshes.

The given pyramid represent pyramid of biomass in an aquatic ecosystem. In this pyramid, the biomass of phytoplankton, i.e., producers (represented by A) is smaller than that of zooplanktons, i.e., primary consumers(represented by B). Phytoplanktons support zooplanktons as the latter feed upon them.

Secondary succession is one of the two types ecological succession of a plants life. As opposed to the first, primary succession, secondary succession is a process started by an event (e.g. forest fire, harvesting, hurricane, etc.) that reduces an already established ecosystem (e.g. a forest or a wheat field) to a smaller population of species, and as such secondary succession occurs on preexisting soil whereas primary succession usually occurs in a place lacking soil. Many factors can affect secondary succession, such as trophic interaction, initial composition, and competition-colonization trade-offs. The factors that control the increase in abundance of a species during succession may be determined mainly by seed production and dispersal, micro climate; landscape structure (habitat patch size and distance to outside seed sources); bulk density, pH, and soil texture (sand and clay). Forest fire can result into deforested site and further succession is carried out.

In a grassland ecosystem, the pyramid of numbers is upright. If primary producers are approximately $$6$$ million plants, then number of top carnivores must be less than $$6$$ million. Hence, according to the options given, there would be $$3$$ million top carnivores.

Correct option: A