Ecosystems and Biodiversity Questions and Answers

Answer: C

An organism that feeds on dead organic matter is a saprotroph.

Saprotrophs, such as most fungi and many bacteria, secrete enzymes onto dead plant and animal material and absorb the dissolved products. They are the decomposers that return nutrients to the soil.

Autotrophs make their own food from inorganic substances, by photosynthesis or chemosynthesis. Chemotrophs obtain energy from chemical reactions and include chemoautotrophs that feed on nothing organic. Heterotrophs is a broader term covering every organism that relies on organic food, including herbivores and predators that eat living organisms; saprotrophs are the particular heterotrophs the question describes.

Answer: C

The organism most commonly used as a biofertilizer is Nostoc.

Nostoc is a filamentous cyanobacterium with heterocysts, specialised cells in which nitrogen fixation takes place. Along with Anabaena, Aulosira and Tolypothrix, it is a mainstay of blue-green algal biofertilizer for rice paddies, where it can add some 20 to 30 kg of nitrogen per hectare.

Chroococcus (printed 'Choococcus') and Aphanocapsa are unicellular colonial cyanobacteria without heterocysts and are not used as biofertilizers. Pediastrum is a colonial green alga, which cannot fix nitrogen at all.

Answer: D

Viruses are not used for bioconversion.

Bioconversion uses living cells or their enzymes to turn one substance into another: yeasts ferment sugars to ethanol, moulds such as Aspergillus produce citric acid and enzymes and transform steroids, and bacteria make vinegar, amino acids and biogas. Each has its own metabolism to carry out the conversion.

Viruses have no metabolism of their own. They can replicate only inside a host cell and do not convert substrates into products, so they are the one option not used for bioconversion.

Answer: B

Azolla is a biofertilizer because it harbours Anabaena, which fixes nitrogen.

The small floating fern Azolla holds the cyanobacterium Anabaena azollae in cavities of its leaves. The cyanobacterium fixes atmospheric nitrogen in its heterocysts and supplies it to the fern, so Azolla grown in rice fields and then incorporated into the soil adds nitrogen for the crop.

Rhizobium fixes nitrogen in the root nodules of legumes, not in Azolla. Azotobacter is a free-living nitrogen fixer in soil. Mycorrhiza is a fungal association with roots that helps plants take up phosphorus; it does not fix nitrogen.

Answer: A

The odd one out is Agrobacterium - horseradish.

The other three pairs are nitrogen-fixing associations between a microbe and a plant: Anabaena fixes nitrogen inside the leaves of the fern Azolla, the actinomycete Frankia forms nitrogen-fixing nodules on the roots of alder, and Rhizobium forms nodules on the roots of peas and other legumes.

Agrobacterium does not fix nitrogen for its host. Agrobacterium tumefaciens is a plant pathogen that causes crown gall and A. rhizogenes causes hairy root, which is used to grow hairy root cultures of plants such as horseradish. That pairing is a pathogen or tool, not a nitrogen-fixing symbiosis.

Answer: C

Plot size is decided from the species-area curve.

To find the minimum area that represents a plant community, quadrats of increasing size are laid out and the number of species recorded in each is plotted against area. The curve rises steeply at first and then levels off. The area at which it flattens, where further enlargement adds few new species, is taken as the minimal area and fixes the quadrat size for the study.

The number of species alone, or the area covered by vegetation, does not tell you how large a plot must be; only the relationship between the two does. Random placement decides where quadrats go, not how big they are.

Answer: B

Species richness is the number of species in an area.

Richness is the simplest measure of diversity: a count of the different species present in a given area or sample, without regard to how many individuals each has. A forest plot with 40 tree species has a richness of 40.

The number of genera is a higher taxonomic count, not species richness. Species composition is which species are present, the identity rather than the count. Relative abundance is how individuals are shared among species, which is evenness; measures such as the Shannon index combine richness and evenness.

Answer: C

Such newly formed reservoirs are oligotrophic.

Oligotrophic water bodies are poor in nutrients such as nitrogen and phosphorus. As a result they have low primary productivity, little plankton and sparse aquatic life, though the water is usually clear and rich in oxygen. Over time, as nutrients and sediment flow in, such a water body tends to move towards the mesotrophic and eutrophic states.

Epiphytic describes plants that grow on other plants, parasitic describes organisms that live at the expense of a host, and saprophytic describes organisms that feed on dead matter. None of them is a term for the nutrient status of a water body.

Answer: B

The dodo is extinct.

The dodo was a large flightless pigeon found only on Mauritius. It had evolved without predators, so it nested on the ground. After sailors arrived in the late sixteenth century it was hunted, and the rats, pigs and monkeys they brought destroyed its eggs. The last widely accepted sighting dates from 1662, and it is the standard example of extinction caused by humans.

Rare, endangered and critically endangered all describe species that still survive, in increasingly threatened IUCN categories. No dodo survives, so none of them applies.

Answer: A

Net primary productivity refers to apparent photosynthesis.

Gross primary productivity is the total rate of photosynthesis, the whole amount of organic matter fixed by plants. Plants use part of it at once for their own respiration. What is left, the rate at which organic matter is stored and made available to consumers, is net primary productivity:

\[ \text{NPP} = \text{GPP} - R \]

Because it is the net gain measured after respiration, it is also called apparent photosynthesis or net assimilation. 'Total photosynthesis' and 'total assimilation' are gross primary productivity, which includes the part lost to respiration, and 'assimilation' alone does not make the net distinction.