Covering Lessons 06–12: abiotic factors, population growth, competition, symbiotic relationships, ecological sampling, ecosystem comparison, and multi-factor prediction.
Which abiotic factor is the primary determinant of the alpine treeline for snow gums in the Australian Alps?
A population of European rabbits is introduced to an island with abundant food and no predators. For the first three years, the population doubles every six months. Which growth curve does this represent?
Two species of honeyeater feed on nectar from the same eucalypt trees. One species feeds in the morning, the other in the afternoon. What ecological principle does this demonstrate?
The relationship between mycorrhizal fungi and eucalyptus tree roots is best described as:
A wildlife biologist needs to estimate the population of feral pigs in a national park. The pigs are nocturnal, wide-ranging, and difficult to observe directly. Which sampling method is most appropriate?
In a mark-recapture study, researchers tag and release 30 agile wallabies. In the second capture of 50 wallabies, 10 are tagged. What is the estimated population size (N)?
Which marine ecosystem has the highest biodiversity per unit area?
Why are tropical open ocean surface waters typically nutrient-poor despite high sunlight?
A student states: “The carrying capacity of a grassland is fixed at 500 kangaroos because that is the maximum number the land can support.” Which statement best evaluates this claim?
Climate models predict a 2°C warming in the Australian Alps. Which additional factor could prevent the alpine treeline from shifting upward as predicted?
A researcher places twelve 1 m×1 m quadrats randomly in a 3,000 m² grassland to estimate the population of kangaroo grass (Themeda triandra). The counts are: 5, 8, 12, 6, 9, 7, 11, 4, 10, 8, 6, 14.
(a) Calculate the mean density of kangaroo grass per square metre. Show your working. 2 MARKS
(b) Estimate the total kangaroo grass population in the grassland. 1 MARK
(c) Explain two sources of error that could make this estimate inaccurate, and describe how the researcher could reduce each error. 4 MARKS
(a) Total individuals = 5 + 8 + 12 + 6 + 9 + 7 + 11 + 4 + 10 + 8 + 6 + 14 = 100 [1 mark]. Total quadrat area = 12 × 1 = 12 m². Mean density = 100 / 12 = 8.33 plants per m² (accept 8.3 or 8) [1 mark].
(b) Population estimate = 8.33 × 3,000 = 25,000 plants (accept 24,900–25,000) [1 mark].
(c) Any two of the following, with matching solutions [2 marks each]:
Compare line transects and belt transects as methods for investigating species distribution along an environmental gradient.
(a) Describe the data produced by each method. 2 MARKS
(b) State one situation where a line transect would be preferred and one where a belt transect would be preferred. Justify your choices. 3 MARKS
(a) A line transect produces qualitative or semi-quantitative data: presence/absence and relative abundance of species along the gradient, recorded where each species touches the line [1 mark]. A belt transect produces quantitative data: actual counts, density, or percentage cover of all individuals within a defined strip on either side of the line [1 mark].
(b) A line transect is preferred when time is limited and the research question only requires knowing which species occur where along the gradient — for example, a rapid survey of rocky shore zonation to document species presence with tide height [1 mark]. A belt transect is preferred when the research question requires comparing population density or biomass between zones — for example, measuring how barnacle density changes along an intertidal gradient, where actual counts are needed for statistical analysis [1 mark]. The belt transect is more time-consuming but produces data suitable for quantitative comparison [1 mark]. Total: 5 marks.
Use the multi-factor framework to predict what would happen to mangrove distribution in a NSW estuary if sea level rises by 0.5 m over the next 50 years and existing seawalls prevent landward migration. In your answer, integrate at least one abiotic factor, one biotic factor, and one population dynamic concept.
6 MARKSAbiotic factor: Mangroves require a specific salinity range (approximately 20–40 ppt) and regular tidal inundation. A 0.5 m sea level rise increases the frequency and duration of tidal inundation at current mangrove sites [1 mark]. At the seaward edge, prolonged submersion may exceed the tolerance limit of some mangrove species, causing stress and reduced growth [1 mark].
Biotic factor: Where mangroves are prevented from shifting landward by seawalls, they are squeezed into a narrower band. This increases intraspecific competition for space and light among mangrove seedlings [1 mark]. At the landward edge, salt marsh species that previously competed with mangroves will be outcompeted as salinity increases, but the seawall prevents the mangroves from occupying that space, leaving it as degraded habitat [1 mark].
Population dynamic: The effective carrying capacity for mangroves in the estuary decreases because the total suitable habitat area shrinks [1 mark]. With reduced recruitment space, the mangrove population may stabilise at a lower density or decline if adult mortality exceeds seedling establishment. This population decline would cascade to species that depend on mangroves for habitat, such as juvenile fish and crab species [1 mark]. Total: 6 marks.
Mark checkpoint as complete