Ecosystems and Biodiversity Questions and Answers

Answer: D

The extinction caused mainly by human activity is the Holocene extinction.

It is the present, ongoing loss of species during the Holocene epoch, driven by habitat destruction, overharvesting, invasive species, pollution and climate change. The Permian-Triassic and Cretaceous-Paleogene extinctions were natural events caused by volcanism, climatic change or an asteroid impact, and the collapse of the Carboniferous rain forest was a climate-driven event as well. None of those three had a human cause.

Answer: A

A stream ecosystem is a lotic ecosystem.

Lotic (from the Latin for washed) means flowing water: streams, rivers and springs. Ponds, bogs and wetlands hold still or very slow water and are lentic (standing-water) systems or, in the case of bogs and wetlands, waterlogged land, so none of them is lotic.

Answer: C

The pyramid that is always upright is the pyramid of energy.

At every trophic level some energy is lost as heat in respiration, so the energy that reaches each level is less than that of the level below, and the pyramid is never inverted. The pyramid of numbers can be inverted (one tree supporting many insects) and the pyramid of biomass can be inverted too (in a pond, small phytoplankton support a larger standing mass of zooplankton). Hence the combined option of numbers and biomass is wrong.

Answer: D

Azolla pinnata is a fern.

Azolla is a small, free-floating aquatic fern that holds the nitrogen-fixing cyanobacterium Anabaena azollae in its leaves. It is therefore used as a bio-fertiliser in rice fields. The cyanobacterium inside it is the blue-green alga, not the plant itself, and the plant is neither a green nor a red alga.

Answer: B

The growth realisation factor is (K - X)/K.

In the logistic equation the growth rate is the product of the intrinsic rate, the population size and the fraction of the carrying capacity that is still unused.

\[ \dfrac{dX}{dt} = rX\left(\dfrac{K - X}{K}\right) \]

That bracketed fraction is the realisation factor, which falls to zero when X reaches K. The option with X in the denominator would grow without limit as X falls, K squared is not part of the equation, and (K - X) alone lacks the scaling by K.

Answer: A

Both statements are true and the reason explains the assertion. Energy flow in an ecosystem obeys the laws of thermodynamics: energy is conserved, and at each transfer much of it is degraded to heat, so only about 10 to 20 percent passes to the next trophic level while 80 to 90 percent is lost. This is why flow is one-way, from the sun through producers to consumers.

That unidirectional flow with large losses at each step is itself a consequence of the second law, so (R) is the correct explanation of (A) and the other three options are ruled out.

Answer: A

Both statements are true and the reason explains the assertion. Marine biodiversity is reported to peak in the mid-latitudes in all oceans and along the coasts of the western Pacific, and sea surface temperature is the factor that best explains this pattern; the warm western Pacific coasts have the highest temperatures.

As (R) gives the temperature factor behind (A), option A is correct. Options B, C and D deny either the truth of a statement or its explanatory link.

Answer: B

The rate of replacement of species along a gradient of habitats is beta diversity. It measures the change in species composition between habitats or along an environmental gradient.

Alpha diversity is the species richness within a single habitat, and gamma diversity is the total diversity of a large region made up of many habitats. Species diversity is the general term and does not name the between-habitat turnover.

Testing each of the remaining options against the same fact shows that every one of them fails on the point the question is asking about, which confirms the keyed answer.

Answer: A

The correct matching is a-ii, b-i, c-iv, d-iii, which is code A. Succession on bare rock is a lithosere, on sand it is a psammosere, in an aquatic habitat it is a hydrosere, and in a dry habitat it is a xerosere.

Options B, C and D pair at least one habitat with a wrong name; for example B gives psammosere to rock and lithosere to sand.

Testing each of the remaining options against the same fact shows that every one of them fails on the point the question is asking about, which confirms the keyed answer.

Answer: C

A species whose individuals survive only in captivity or other human-controlled conditions is said to be wild extinct (extinct in the wild).

Ecologically extinct means so few individuals remain that the species no longer affects its ecosystem, mass extinct refers to the loss of very many species at once, and anthropogenic extinct is extinction caused by humans in general. None of these names the captivity-only condition.

Testing each of the remaining options against the same fact shows that every one of them fails on the point the question is asking about, which confirms the keyed answer.