HSC Exam Practice

Sample response. Symbiosis is any close, long-term interaction between two different species that live in direct association. The three types are: mutualism (+/+), in which both species benefit; commensalism (+/0), in which one species benefits and the other is neither helped nor harmed; and parasitism (+/−), in which one species (the parasite) benefits at the expense of the other (the host), which is harmed.

Marking notes. 1 mark for a correct definition of symbiosis (close, long-term interaction between different species). 1 mark for correctly naming all three relationship types. 1 mark for providing the correct sign notation for all three types. No mark awarded if sign notation is wrong or missing.

Sample response. Ectoparasites live on the surface of the host's body, whereas endoparasites live inside the host's body. An Australian example of an ectoparasite is the Australian paralysis tick (Ixodes holocyclus), which attaches to the skin of dogs, cats or humans. An Australian example of an endoparasite is the tapeworm (Spirometra or similar species) that lives in the intestine of dingoes and other carnivores.

Marking notes. 1 mark for correctly distinguishing ectoparasite (on surface) from endoparasite (inside body). 1 mark for a correctly named and identified Australian ectoparasite example. 1 mark for a correctly named and identified Australian endoparasite example. Organism names must include indication of where the parasite lives (on/in host).

Sample response. Rhizobium bacteria living in the root nodules of wattles (Acacia) fix atmospheric nitrogen (N₂), converting it into ammonium (NH₄⁺), which the plant can absorb and use for growth and protein synthesis. In return, the wattle provides the bacteria with carbohydrates (sugars produced through photosynthesis) as an energy source and a protected nodule environment that excludes oxygen, allowing nitrogen fixation to occur.

Marking notes. 1 mark for identifying that Rhizobium provides fixed nitrogen to the plant (accept: ammonium, ammonia, usable nitrogen). 1 mark for identifying that the wattle provides carbohydrates/sugars to the bacteria. 1 mark for identifying any additional detail such as the protected (anaerobic) nodule environment or the form of nitrogen fixed (N₂ → NH₄⁺).

Sample response. Commensalism is defined by one species benefiting and the other being truly unaffected — neither helped nor harmed. Demonstrating this requires proving that the second species experiences zero net cost or benefit, which demands controlled experiments. In practice, many relationships that appear commensal are found, on closer examination, to involve a subtle benefit or harm to the supposedly neutral partner, causing the interaction to be reclassified as weak mutualism or weak parasitism.

Marking notes. 1 mark for identifying that the requirement to prove true neutrality (zero effect on the second species) is the central difficulty. 1 mark for explaining that most apparently commensal relationships reveal subtle costs or benefits upon closer study, blurring the boundary.

Sample response. A dead host is a destroyed habitat and a lost food source for the parasite. Parasites that kill their hosts immediately eliminate their own means of survival and reproduction. Natural selection therefore favours parasites that keep their hosts alive and mobile long enough to spread to new hosts, because mobile hosts encounter more potential new hosts and vectors. Over many generations, highly virulent (host-killing) parasite strains leave fewer descendants than less virulent strains, causing parasite populations to evolve toward reduced virulence.

Marking notes. 1 mark for identifying that killing the host destroys the parasite's habitat/food source and ends the current infection. 1 mark for identifying that keeping the host mobile maximises the parasite's transmission opportunities (contact with new hosts or vectors). 1 mark for framing this as a natural selection pressure: virulent strains have lower reproductive success, so less virulent strains are favoured over time.

Sample response. A host is the organism that harbours the parasite and is directly harmed by it; the parasite feeds on or within the host's body. A vector is an organism that transmits the parasite from one host to another but does not itself serve as the parasite's primary habitat and is not necessarily harmed. In the case of malaria, the human is the host: Plasmodium lives within human red blood cells and causes disease. The mosquito (Anopheles species) is the vector: it picks up Plasmodium when it takes a blood meal from an infected human and transmits it to a new human host during a subsequent bite, but the mosquito is not the primary host of the parasite and is not significantly harmed by the infection.

Marking notes. 1 mark for correctly defining host (organism harmed by and harbouring the parasite). 1 mark for correctly defining vector (organism transmitting the parasite between hosts, not itself the primary host). 1 mark for correctly applying both terms to Plasmodium/malaria, identifying human as host and mosquito as vector.

Sample response (a). As tick burden increases from 0 to 80 ticks per wallaby, host daily movement declines steadily. At zero ticks, wallabies move approximately 800–1000 m per day; at 80 ticks, movement falls to near zero. The relationship is negative and appears to be roughly linear across the range shown, with no period of stable or increasing movement at any point on the x-axis.

Marking notes (a). 1 mark for correctly identifying the negative (inverse) relationship between tick burden and host movement. 1 mark for supporting this with at least one specific value from the graph (e.g. approximately 800 m at 0 ticks, near 0 at 80 ticks).

Sample response (b). Tick reproductive output peaks at approximately 25–30 ticks per wallaby. At higher tick burdens, the host's movement is severely reduced (the wallaby becomes immobile or very weak). A host that cannot move encounters fewer other wallabies, so ticks on that host have fewer opportunities to drop off and find new hosts in which to complete their life cycle. Additionally, a heavily debilitated or dying host may have reduced blood flow and tissue quality, directly limiting the resources available to female ticks for egg production. Both factors cause reproductive output per female to fall sharply at high burdens.

Marking notes (b). 1 mark for correctly reading the peak from the graph (accept 20–35 ticks). 1 mark for linking declining host mobility to reduced tick transmission opportunities (ticks cannot reach new hosts). 1 mark for additional reasoning such as declining host resource quality or host death reducing conditions for egg production.

Sample response (c). The graph shows that tick reproductive output peaks at moderate burden but falls at both very low and very high burdens. A single tick on an otherwise healthy, mobile host benefits from the host's wide-ranging movement, which increases the chance of the tick encountering new hosts and dispersing its offspring. However, very high burdens suppress host mobility so severely that the host can no longer move to new locations, reducing tick dispersal and access to additional hosts. Natural selection therefore favours ticks that collectively maintain a burden in the range that keeps the host alive and mobile, maximising transmission. Any individual tick lineage that contributes to excessively high burden risks destroying the shared resource of host mobility and reducing its own reproductive success.

Marking notes (c). 1 mark for identifying that moderate burden maximises both host mobility and tick reproductive output. 1 mark for identifying the trade-off: higher burden increases tick numbers per host but reduces host mobility and therefore transmission opportunity. 1 mark for framing this as a natural selection argument — individual tick lineages that over-exploit the host have lower fitness because they reduce the collective mobility of the host.

Sample response. Symbiosis is any close, long-term interaction between two different species living in direct association. The three types are mutualism (+/+), in which both species benefit; commensalism (+/0), in which one benefits and the other is unaffected; and parasitism (+/−), in which one species (the parasite) benefits at the expense of the other (the host), which is harmed.

In Australian ecosystems, mutualism is exemplified by mycorrhizal fungi and eucalypt roots: the fungus extends hyphae 10–100 times further into the soil than roots alone, absorbing phosphorus and water and transferring them to the tree, while the tree provides photosynthetic sugars to fuel the fungus. Commensalism is illustrated by cattle egrets following grazing sheep or kangaroos across Australian farmland: the egrets capture insects disturbed by hooves, while the grazers are unaffected. Parasitism is illustrated by the Australian paralysis tick (Ixodes holocyclus) on dogs, cats or marsupials: the tick gains a blood meal while the host suffers blood loss and neurotoxin exposure.

The mycorrhizal mutualism is especially critical in Australia because Australian soils are among the most ancient and nutrient-poor on Earth, severely depleted in phosphorus through millions of years of weathering. Without fungal hyphae extending the root's absorptive surface, eucalypts cannot access enough phosphorus for photosynthesis or growth. Experimental removal of mycorrhizal fungi reduces eucalypt seedling biomass by 50–80% and more than halves post-fire seedling survival. This makes mycorrhizal networks a foundational ecological infrastructure, not merely a beneficial addition.

Comparing the consequences of losing mutualistic versus parasitic interactions: loss of key mutualisms such as mycorrhizal networks or pollinator relationships would cause immediate declines in plant productivity, seedling survival, and forest regeneration. Because eucalypt forest productivity underpins the entire food web, loss of mycorrhizal mutualism cascades upward, reducing leaf and seed availability for herbivores, hollow habitat for arboreal marsupials and insectivorous birds, and soil invertebrate diversity through reduced root exudate. Parasite loss from an ecosystem would, by contrast, tend to increase host population sizes in the short term. However, parasites also play important ecological roles: they regulate host populations, preventing competitive exclusion, and drive evolutionary arms races that maintain host genetic diversity. Complete removal of parasitism could therefore destabilise communities through unchecked population growth of formerly controlled host species.

Evaluation: while parasitism and commensalism both contribute to ecosystem function, mutualism is arguably the most ecologically important type in Australian ecosystems because it underpins primary productivity and ecosystem structure at the most fundamental level. Without mycorrhizal and nitrogen-fixing mutualisms, the plants that support all higher trophic levels cannot establish or grow adequately in Australia's nutrient-poor soils. Commensalism generates local benefits for individual organisms but rarely determines ecosystem-level productivity. Parasitism regulates populations and maintains diversity, but its removal tends to cause short-term increase rather than immediate collapse. The ecological consequences of mutualism loss are therefore more immediate and more structurally catastrophic than the loss of either commensalism or parasitism. The claim that mutualism is the most ecologically important type is strongly supported, but with the caveat that all three relationship types contribute to ecosystem resilience, and none is ecologically dispensable.

Marking criteria.

• 1 mark — Defines all three relationship types correctly with their sign notation (+/+, +/0, +/−).
• 1 mark — Names at least one specific Australian example for each of the three types, with both organisms identified.
• 1 mark — Correctly describes the resource exchange in the mycorrhizal mutualism (fungal hyphae provide phosphorus/water; eucalypt provides sugars).
• 1 mark — Explains why Australian soils make the mycorrhizal mutualism especially critical (ancient, heavily weathered, phosphorus-poor soils).
• 1 mark — Describes the ecological consequences of losing key mutualisms (plant productivity collapse, food-web cascade).
• 1 mark — Describes the ecological consequences of losing parasitic interactions (population imbalance, loss of diversity pressure), showing parasites are not purely harmful to ecosystems.
• 1 mark — Makes a direct comparison between the ecosystem-level consequences of mutualism loss versus parasitism loss.
• 1 mark — Reaches an explicit, justified evaluative conclusion about the relative ecological importance of the three relationship types, with a nuanced qualification (all types contribute; mutualism underpins primary productivity).
• 1 mark — Response is logically structured, uses precise lesson terminology throughout (symbiosis, mutualism, commensalism, parasitism, host, ectoparasite/endoparasite, mycorrhizal, hyphae, nitrogen fixation, food web, sign notation), and maintains scientific register.