Covering Lessons 13–18: predation and herbivory, trophic cascades, keystone species, introduced species, ecological succession, biodiversity, human impacts, and conservation strategies.
Removing dingoes from Australian grazing land causes kangaroo and rabbit populations to increase. This is best described as:
Which statement best describes a keystone species?
According to the enemy release hypothesis, why do introduced species often thrive?
Which of the following distinguishes primary succession from secondary succession?
What is meant by “functional redundancy” in an ecosystem?
Which IUCN Red List category indicates the highest risk of extinction while a species still survives in the wild?
What is “extinction debt”?
In eutrophication, what directly causes fish death after an algal bloom?
What is the primary purpose of wildlife corridors in fragmented landscapes?
Which of the following is an example of ex-situ conservation?
Explain the trophic cascade that occurs when an apex predator is removed from an ecosystem. In your answer:
(a) Describe the direct effect of predator removal on the prey population. 2 MARKS
(b) Explain two indirect effects on lower trophic levels or ecosystem processes, with a clear chain of causation for each. 4 MARKS
(c) Use the dingo as your Australian example. 1 MARK
(a) When an apex predator is removed, predation pressure on its prey is released. The prey population increases because mortality from predation is reduced and survival/reproduction improve (2 marks).
(b) Indirect effect 1: Increased prey (herbivores) consume more vegetation, reducing plant biomass and ground cover. With less vegetation, soil becomes exposed to erosion, altering hydrology and nutrient cycling (2 marks).
Indirect effect 2: Reduced ground cover eliminates habitat for ground-nesting birds, reptiles, and small mammals. Vegetation composition shifts from palatable grasses to unpalatable weeds as grazing pressure changes competitive balances (2 marks).
(c) In Australia, dingo removal from grazing land caused kangaroo and rabbit populations to surge. Overgrazing degraded vegetation, increased soil erosion, and contributed to dryland salinity — demonstrating the four-level cascade from predator loss to landscape degradation (1 mark).
Distinguish between primary and secondary ecological succession. In your answer:
(a) Define each type of succession. 2 MARKS
(b) Provide one named Australian example for each type. 2 MARKS
(c) Explain why primary succession is generally slower than secondary succession. 1 MARK
(a) Primary succession occurs on bare substrate where no soil exists. Pioneer species must colonise raw rock or sand and begin soil formation from scratch (1 mark). Secondary succession occurs on previously vegetated land where soil remains intact after disturbance such as fire, flood, or logging (1 mark).
(b) Australian example of primary succession: coastal sand dune succession, where spinifex grass colonises bare sand and gradually builds soil for shrub and tree establishment (1 mark). Australian example of secondary succession: post-fire recovery in eucalyptus woodland, where epicormic shoots and soil seed banks allow rapid regeneration after bushfire (1 mark).
(c) Primary succession is slower because soil must form from scratch through organic matter accumulation, weathering, and nutrient cycling — processes that take decades to centuries. Secondary succession is faster because seed banks persist, nutrients remain in the soil, and roots can resprout (1 mark).
Evaluate whether Australia should prioritise creating new national parks (in-situ conservation) or establishing more captive breeding programs (ex-situ conservation) for its threatened mammals. In your answer:
(a) Identify two advantages of national parks for conservation. 2 MARKS
(b) Identify two disadvantages of captive breeding programs. 2 MARKS
(c) Make a justified recommendation that considers ecological, economic, and ethical dimensions. 2 MARKS
(a) National parks protect entire ecosystems and hundreds of species simultaneously at relatively low cost per species. They preserve evolutionary processes, ecological relationships, and ecosystem services such as carbon storage and water purification (2 marks).
(b) Captive breeding is extremely expensive per individual. Animals may lose wild survival skills through adaptation to captivity, and small captive populations suffer genetic drift and inbreeding depression. Ex-situ programs do not preserve the ecological context of the species (2 marks).
(c) Australia should prioritise in-situ conservation (national parks and threat abatement) as the foundation because it is more cost-effective and protects ecosystems. However, ex-situ captive breeding should be maintained as a critical safety net for species on the brink of extinction, such as the corroboree frog and northern hairy-nosed wombat (1 mark). The two approaches are complementary — the eastern barred bandicoot recovered precisely because captive breeding provided insurance while predator-proof fences prepared safe habitat for reintroduction (1 mark).
Mark checkpoint as complete