Ecosystems and Biodiversity Questions and Answers

Answer: B

The phytoplankton stage consists of blue green algae, green algae and diatoms.

A hydrosere is the succession that turns open water into land. It begins in the bare water with microscopic floating algae, the phytoplankton stage: blue-green algae, green algae and diatoms. Their remains start to build up organic matter on the bottom.

The later stages are named for their plants. Vallisneria, Utricularia and Hydrilla are submerged plants, the second stage. Nelumbo, Nymphaea and Limnanthemum are floating-leaved plants, the third stage. Typha, Sagittaria and Phragmites are reed-swamp plants, a later amphibious stage on the way to marsh meadow and woodland.

Answer: D

Emergent woody plants (printed 'Energent') that tolerate sea water are mangroves.

Mangroves are trees and shrubs of tropical and subtropical tidal coasts that grow in salt water. They cope with salinity by excluding salt at the roots or excreting it from their leaves, and with waterlogged mud by breathing roots (pneumatophores) and stilt roots. The Sundarbans is the largest mangrove forest in the world.

Grasses and herbs are not woody. Shrubs is a growth form, not the name for the salt-tolerant woody vegetation of the sea coast; most shrubs cannot tolerate sea water at all.

Answer: A

The mangrove species is Rhizophora.

Rhizophora, the red mangrove, is a typical true mangrove. It stands on arching stilt roots in tidal mud and shows vivipary: its seeds germinate while still on the tree, and the long seedling drops into the mud ready to root. Species such as Rhizophora mucronata are common in the Sundarbans and along India's coasts.

Azadirachta is the neem tree and Mangifera the mango, both of dry land. Casuarina (printed 'Casurina') is planted on coastal sand as a windbreak but is not a mangrove and does not grow in tidal salt water.

Answer: B

The largest population an environment can support is its carrying capacity.

Carrying capacity, K, is the maximum population of a species that the resources of an environment (food, water, space, shelter) can sustain indefinitely. In the logistic growth model, a population's growth slows as it approaches K and stops when it reaches it.

Population structure is the age and sex composition of a population. Optimum population is the size at which welfare or output per head is greatest, usually less than the maximum. Minimum population is the smallest size at which a population can survive, the minimum viable population. None of them is the maximum the resources can support.

Answer: D

The zone is the littoral zone.

The littoral (intertidal) zone is the shore strip of a sea or lake that is covered by water at high tide or high water and exposed to air at low tide. Its organisms, such as barnacles, mussels and seaweeds, must tolerate drying, wave action and changing temperature.

The pelagic zone is the open water away from the bottom and shore, and the benthic zone is the bottom. 'Lentic' describes still-water habitats such as lakes and ponds as a whole, not a zone at their edge.

Answer: C

The key gives the ultimate function of an ecosystem as the flow of energy.

An ecosystem is defined functionally by two processes: a one-way flow of energy, captured from sunlight by producers and passed through consumers and decomposers until it is lost as heat, and the cycling of nutrients. Energy flow drives everything else, including the cycling of matter, so the standard ecology texts treat it as the fundamental function of every ecosystem.

Survival of species, balance of natural systems and biological control are outcomes that depend on energy flow rather than the basic function itself.

Answer: C

It indicates the start of primary succession.

Primary succession begins on bare ground where no soil or life existed before, such as fresh lava or exposed rock. Lichens are the pioneers: they tolerate drying, and their acids and trapped dust begin forming soil. Mosses follow, then grasses, shrubs and eventually a climax community.

Intraspecific competition and resource partitioning describe interactions within established communities, and environmental resistance is the set of factors that limits population growth. None of them describes life colonising a lifeless surface.

Answer: A

The lowest net primary productivity per unit area is in an open ocean.

The open ocean is nutrient-poor: nitrogen, phosphorus and iron are scarce in its sunlit surface layer, so phytoplankton grow slowly and average net primary productivity is only about 125 g of dry matter per square metre per year, similar to a desert. Its enormous area makes it important globally, but per unit area it is among the least productive ecosystems.

Estuaries and coral reefs are among the most productive ecosystems, at around 1,500-2,500 g per square metre per year, and freshwater lakes lie in between.

Answer: C

The phosphorus cycle is entirely sedimentary.

Phosphorus has no significant gaseous form in the atmosphere. Its reservoir is phosphate rock and sediments; it enters ecosystems by weathering, passes through organisms, and returns to the soil and finally to marine sediments, rejoining land only over geological time when sediments are uplifted.

Carbon and nitrogen cycles have large atmospheric reservoirs and are called gaseous cycles. The phosphorus cycle passes through both aquatic and terrestrial ecosystems, so it is not confined to either.

Answer: C

The conversion to nitrates that plants can absorb is nitrification.

In the nitrogen cycle, organic nitrogen is first broken down to ammonia (ammonification). Nitrifying bacteria then oxidise it, Nitrosomonas turning ammonium into nitrite and Nitrobacter turning nitrite into nitrate. Nitrate is the form most plants take up from the soil, so it is the step that produces plant-available nitrate that the question describes.

Ammonification produces only ammonia, not nitrate. Denitrification does the reverse, reducing nitrate to nitrogen gas and losing it to the air. 'Deeutrophication' is not a step of the nitrogen cycle.