Human impact on biodiversity practice questions.

Which combination of characteristics makes an animal species most vulnerable to extinction?

• Small range, specialist diet, low reproductive rate
• Small range, generalist diet, high reproductive rate
• Wide range, specialist diet, high reproductive rate
• Wide range, generalist diet, low reproductive rate

Answer: Small range, specialist diet, low reproductive rate

Vulnerability rises when a species cannot escape a disturbance in space (small range), cannot switch food source (specialist), and cannot replace losses quickly (low reproductive rate). A generalist with a wide range can shift its niche, which is why such species often survive the same land-use change.

The Nile perch, a large predatory fish, was introduced into Lake Victoria in the 1950s. Which mechanism best explains the sharp fall in the number of endemic cichlid species that followed?

• The perch out-competed the cichlids for shallow nesting sites
• Predation by the perch removed cichlids faster than they bred
• The perch carried a disease that killed only adult cichlids
• The perch raised the lake's oxygen demand and suffocated the cichlids

Answer: Predation by the perch removed cichlids faster than they bred

An introduced top predator that the native prey has never encountered meets no evolved defence, so predation losses exceed the cichlids' rate of replacement. Invasive species harm native biodiversity mainly through predation, competition or disease, and here the documented route is direct predation.

Landings from a cod fishery rose for two decades and then collapsed, as happened on the Grand Banks off Newfoundland. In systems terms, the collapse happened because

• the cod population had reached its carrying capacity
• fishing effort stayed below the maximum sustainable yield
• predation by other fish species increased as the catch grew
• harvesting exceeded the stock's rate of replacement

Answer: harvesting exceeded the stock's rate of replacement

A fish stock is a renewable store only while the outflow (catch) stays at or below the rate at which the store is replenished by recruitment. Maximum sustainable yield is the largest catch that still allows replacement, not the largest catch that the fleet is technically able to take.

After clear-felling, a tropical rainforest site loses its fertility within a few growing seasons, while a cleared temperate grassland keeps its fertility for far longer. The main reason is that in the rainforest

• most nutrients are stored in the living biomass, not the soil
• the soil store of nutrients is far larger than the biomass store
• decomposition is too slow for nutrients to be released at all
• rainfall is too low to support regrowth once the trees are gone

Answer: most nutrients are stored in the living biomass, not the soil

In a rainforest the nutrient store sits in the vegetation and cycles rapidly through fast decomposition, so removing the biomass removes the store itself and heavy rain leaches what little remains. This is why rainforest biodiversity has low resilience to clearance despite the system's high productivity.

Which combination of characteristics makes an introduced species most likely to become invasive in its new range?

• Rapid reproduction, broad diet, no local predators
• Rapid reproduction, narrow diet, many local predators
• Slow reproduction, broad diet, many local predators
• Slow reproduction, narrow diet, no local predators

Answer: Rapid reproduction, broad diet, no local predators

An invader needs to build numbers faster than anything can remove them, and a broad niche means the new range is unlikely to lack what it eats. Escaping the predators, parasites and diseases of its home range is often the decisive factor, which is why a species can be unremarkable at home and destructive abroad.

A reserve holds the last 40 individuals of a forest mammal. Their habitat is now secure, poaching has stopped and food is plentiful, yet the population keeps declining. The most likely explanation is

• the reserve has exceeded its carrying capacity for this particular species
• inbreeding and lost genetic variation in a very small population
• the species has become an r-strategist in these conditions
• competition with a species occupying an identical niche

Answer: inbreeding and lost genetic variation in a very small population

Below a minimum viable population, chance alone becomes dangerous: skewed sex ratios, a run of bad years and inbreeding depression reduce fertility and survival even when the habitat is perfect. This is why conservation targets a population size rather than merely the survival of the last individuals.