NEET Biology Ecosystem MCQs Set 07

Answer: (d) Agroecosystem

Explanation:
1. An agroecosystem (cropland) is a human-managed system of farming and is the largest anthropogenic ecosystem.
2. Zoo, garden and aquarium are much smaller man-made systems.

Teacher's Note:
1. Cropland is the largest man-made ecosystem.
2. It has low genetic diversity but high productivity because humans supply inputs.

Answer: (b) 2–10%

Explanation:
1. Only a small part of incident solar radiation is photosynthetically active (PAR).
2. Plants capture just 2-10 percent of PAR (about 1-5 percent of incident radiation) in making organic matter.

Teacher's Note:
1. PAR is about 50 percent of incident solar radiation.
2. Plants use only 2-10 percent of this PAR.

Answer: (a) GPP

Explanation:
1. Productivity is the rate of biomass production per unit area per unit time.
2. When it is measured as the total organic matter made in photosynthesis, it is gross primary productivity (GPP).

Teacher's Note:
1. GPP is the total photosynthetic production.
2. NPP is what remains after the producers' own respiration: NPP = GPP minus R.

Answer: (b) 10 J

Explanation:
1. Producers trap about 1 percent of the 10,000 J, which is 100 J.
2. Only 10 percent passes to the next level, so T2 receives 10 J.

Teacher's Note:
1. Apply the 10 percent law at each step.
2. The first step (sunlight to producers) is not a 10 percent step; here it is given as 1 percent.

Answer: (c) Species composition

Explanation:
1. Energy flow, decomposition and nutrient cycling are functions of an ecosystem.
2. Species composition describes who lives there, so it is a structural feature.

Teacher's Note:
1. Structure: species composition, stratification, abiotic components.
2. Function: productivity, decomposition, energy flow, nutrient cycling.

Answer: (c) DFC, GFC

Explanation:
1. On land most energy passes through the detritus food chain (DFC).
2. In aquatic ecosystems the grazing food chain (GFC) is the main channel of energy flow.

Teacher's Note:
1. Remember: land is DFC dominant, water is GFC dominant.
2. This is because aquatic producers are mostly small and quickly eaten.

Answer: (b) Standing state, functional

Explanation:
1. The amount of inorganic nutrients (carbon, nitrogen, calcium and so on) in the soil at a given time is the standing state.
2. Energy flow is a function, so it is a functional feature.

Teacher's Note:
1. Standing crop is the living biomass at a trophic level.
2. Standing state is the amount of nutrients present at a time.

Answer: (b) Photosynthesis

Explanation:
1. Carbon is taken from the atmosphere as carbon dioxide mainly by photosynthesis.
2. Decomposition and respiration return carbon to the air instead.

Teacher's Note:
1. About 4 x 10^13 kg of carbon is fixed in photosynthesis every year.
2. Respiration, decomposition and burning return it to the atmosphere.

Answer: (d) Upright

Explanation:
1. In most ecosystems producers are more numerous than herbivores, and herbivores more than carnivores.
2. So the usual pyramid of numbers is upright.

Teacher's Note:
1. Exceptions are a tree ecosystem (spindle shaped) and a parasitic food chain (inverted).
2. The pyramid of energy is always upright.

Answer: (b) Lichens

Explanation:
1. Lichens are the pioneer community on bare rock.
2. They secrete acids that dissolve rock, helping weathering and soil formation.

Teacher's Note:
1. Primary succession on rock starts with crustose lichens, followed by foliose lichens and mosses.
2. This is xerarch succession.

Answer: (a) 49%, 71%

Explanation:
1. Carbon is about 49 percent of the dry weight of organisms.
2. About 71 percent of the global carbon is dissolved in the oceans.

Teacher's Note:
1. Carbon is second only to water in living tissue by amount.
2. Forests hold a large share of the rest of the carbon.

Answer: (d) Phosphorus

Explanation:
1. Carbon, hydrogen and nitrogen have a gaseous reservoir (the atmosphere).
2. Phosphorus has its reservoir in rocks, so it follows a sedimentary cycle.

Teacher's Note:
1. Gaseous cycles: carbon, nitrogen, oxygen.
2. Sedimentary cycles: phosphorus, sulphur, calcium.

Answer: (b) Nutrient cycling – > 10%

Explanation:
1. Soil formation accounts for about 50 percent of the cost of ecosystem services.
2. Nutrient cycling is below 10 percent, so the statement 'more than 10 percent' is incorrect.

Teacher's Note:
1. Other shares: climate regulation 6 percent and wildlife habitat 6 percent.
2. Robert Constanza valued ecosystem services at about 33 trillion US dollars per year.

Answer: (b) 33 trillion US dollars

Explanation:
1. Costanza and his team estimated the value of ecosystem services at about 33 trillion US dollars a year.
2. This is about twice the global GNP.

Teacher's Note:
1. The services include air and water purification, pollination and flood control.
2. The point is that natural systems have an enormous economic value.

Answer: (a) Tropical rain forests

Explanation:
1. Stratification is the vertical layering of plants.
2. It is best developed in the tropical rain forest, which has five layers from ground vegetation to tall emergent trees.

Teacher's Note:
1. Stratification increases the space used and supports more species.
2. Grasslands have few layers.

Answer: (c) Tropical rain forest

Explanation:
1. Tropical rain forests receive heavy rain and warm sun all year.
2. They have the highest net primary productivity among the given options.

Teacher's Note:
1. Deserts and tundra have the lowest productivity.
2. Among the ecosystems of the world, coral reefs and estuaries are also highly productive.

Answer: (d) Tundra

Explanation:
1. Tundra has very low temperatures and a short growing season.
2. Its productivity is the lowest among the options given.

Teacher's Note:
1. Low temperature and low light limit photosynthesis.
2. Productivity rises from tundra to temperate to tropical forests.

Answer: (a) Starts with solar energy

Explanation:
1. The detritus food chain (DFC) begins with dead organic matter, not with solar energy.
2. It ends in the grazing food chain where detritivores are eaten by predators.

Teacher's Note:
1. GFC starts with producers using solar energy.
2. DFC is the major pathway on land.

Answer: (d) They are degradation product of protein and fats and are produced by the process of mineralisation

Explanation:
1. Humus is a dark, amorphous substance that resists microbial action.
2. It is a nutrient reservoir and raises soil water-holding capacity.
3. It is made by humification of lignin- and cellulose-rich detritus, not by mineralisation of proteins and fats, so (4) is wrong.

Teacher's Note:
1. Humification forms humus; mineralisation releases inorganic nutrients.
2. Humus is broken down very slowly.

Answer: (c) Energy flow is cyclic

Explanation:
1. Energy flow in an ecosystem is unidirectional: it enters as light and leaves as heat.
2. So the statement that it is cyclic is incorrect.

Teacher's Note:
1. Matter cycles but energy does not.
2. Pyramid of energy is always upright, and ecological efficiency is about 10 percent.