Ecology practice questions.

A red fox in a woodland hunts at night, dens in a hollow log and feeds mainly on small rodents. This full description of what the fox does and how it interacts with other organisms is its

• habitat, because it describes where the fox is found
• population, because it describes a group of the same species
• niche, because it describes its role and interactions
• community, because it describes interacting species

Answer: niche, because it describes its role and interactions

The niche is the role a species plays, including its feeding, timing and interactions; the habitat is only the physical address where it lives. Students who answer habitat are describing where, not how.

A beetle species is introduced to an island with abundant food and no predators. Its numbers rise exponentially for several generations and then crash. The crash is best explained by

• overshoot of the carrying capacity and resource depletion
• the population reaching a stable plateau at carrying capacity
• density-independent factors acting equally at every density
• an increase in the carrying capacity of the island

Answer: overshoot of the carrying capacity and resource depletion

A J-curve has no braking phase: growth continues past the level the resources can support, so the population overshoots and then dies back. An S-curve, by contrast, levels off at carrying capacity because negative feedback acts before the resources are exhausted.

Which factor is most likely to limit a pond population of water fleas more strongly as their density increases?

• A sudden drop in water temperature after a storm
• The spread of a parasite between individuals
• A pollution spill entering the pond from a road
• The pond drying out during an unusually hot summer

Answer: The spread of a parasite between individuals

Disease and parasitism spread more easily when individuals are crowded, so their effect scales with density; weather events and spills hit the population with the same force whether it is large or small and are therefore density-independent.

A tree species produces a small number of large seeds, grows slowly and lives for more than 200 years. This life strategy is described as

• r-strategist, typical of rapidly changing and disturbed habitats
• r-strategist, typical of stable, climax communities
• K-strategist, typical of early pioneer communities
• K-strategist, typical of stable, late-successional habitats

Answer: K-strategist, typical of stable, late-successional habitats

K-strategists invest heavily in few offspring, mature slowly and live long, which pays off where conditions are predictable and competition matters. The commonest error is crossing the traits over and giving K-strategists many small offspring, which is the r-strategy of pioneer species.

In a capture-mark-recapture study of snails the marks make individuals easier for birds to see, so marked snails are eaten more often. The population estimate given by the Lincoln index will be

• unaffected, because marking does not change the ratio
• too high, because fewer marked snails are recaptured
• too low, because fewer marked snails are recaptured
• too low, because the total sample size has been reduced

Answer: too high, because fewer marked snails are recaptured

In N = (n1 x n2) / n3 the number of marked recaptures sits on the bottom of the fraction, so losing marked individuals inflates the estimate. The method assumes marking does not alter survival or behaviour, and this assumption has been broken.

In a forest the producers fix 15 000 kJ m-2 yr-1 in photosynthesis and lose 6 000 kJ m-2 yr-1 in respiration. The remaining 9 000 kJ m-2 yr-1 is the

• net primary productivity, available to consumers
• gross primary productivity, before respiratory losses
• net secondary productivity of the herbivores
• standing biomass currently held by the producers

Answer: net primary productivity, available to consumers

NPP = GPP - R, so what is left after the producers have respired is the net primary productivity and the only part that can pass to the next trophic level. NPP is a rate of energy fixation per unit area per year, not the stock of biomass standing in the forest at one moment.