IB Biology D4.2: Stability and change worked solutions

Light enters and heat leaves, so energy crosses the glass. The vessel is sealed, so no matter crosses; the bacteria recycle nutrients inside it.

A keystone species has a disproportionate effect on its community. Abundance and trophic position do not define it.

Extra nutrients make algae multiply. When they die, bacteria decompose them using oxygen, so BOD rises.

Re-establishing habitat connectivity is a rewilding method. The other options add inputs, remove a keystone role or create a monoculture.

A pollutant biomagnifies if it persists, is not excreted and is stored in tissue, usually fat.

There is no soil and no previous community, so the succession is primary. Pioneers such as lichens and cyanobacteria need no soil.

Species diversity, plant size, production, food-web complexity and nutrient cycling all increase as the community develops.

The predator no longer controls the mussels, which spread and exclude other species, so diversity falls.

(a) \frac{6300-8400}{8400}\times100=-25.0\%, a decrease of 25.0% (1). (b) Trees return water to the air by transpiration; with fewer trees there is less water vapour, less cooling and less rainfall (1).

(a) A floating bag resembles a jellyfish, so the turtle mistakes it for food (1). (b) Plastic cannot be digested; it blocks the gut or fills the stomach so the turtle feels full and starves (1).

(a) The harvest must stay below the rate of replacement (1). (b) At 20 and 80 replacement (6 and 7) is below 8, so the catch is unsustainable (1). At 50 replacement is 11, greater than 8, so it is sustainable there (1). (c) Any two: estimate stock size by survey (1); age structure, a falling proportion of young fish signals too few breeders (1); catch per unit effort over time (1).

(a) \frac{6.6-2.4}{2.4}\times100=+175\% (1). (b) Yield rises as fertiliser increases (1), steeply up to about 100 kg per ha and then levelling off at about 7.3 to 7.4 t per ha (1). (c) 200 kg gives only 0.1 t per ha more, \frac{7.4-7.3}{7.3}\times100=1.4\% (1); the extra cost and the greater leaching risk are not justified, and data from one crop and site cannot show effects on soil or water (1).

(a) Independent variable: temperature (1); dependent variable: final mass of pond weed (1). (b) Any one of light intensity, volume of water, starting mass of weed, source of sediment (1). (c) The jar lacks the size, variety of species and changing conditions of a real pond, so conclusions may not carry over (1).

(a) The fish-eating bird, which has the highest concentration, 480 (1). (b) \frac{54.0}{1.8}=30 times (1). (c) The insecticide is persistent, so it is not broken down or excreted, and it is stored in fat (1). Phytoplankton take it up (1). Each consumer eats many organisms from the level below and so adds their stored doses, so the concentration rises at each higher trophic level (1).

Nitrate and phosphate are leached into the river (1). The nutrients cause rapid growth of algae, a bloom (1). The bloom shades the water, so submerged plants die (1). Aerobic bacteria decompose the dead algae and plants and use dissolved oxygen, so the BOD rises (1). Dissolved oxygen falls too low for fish and other aerobic animals, which die (1).

Any six points, with at least two benefits and two risks. Benefits: nitrogen supplies the nutrient that limits growth, so yield per hectare rises (1); more food from the same area (1); higher yield can spare land for natural habitat (1). Risks: surplus nitrate leaches into water and causes eutrophication (1); manufacture uses much energy, raising the carbon footprint (1); fertiliser can harm soil organisms and nearby ecosystems (1). A closing judgement, such as applying the amount crops need at the right time, can replace one point (1).

(a) Any one of: removal in harvested crops, leaching, run-off, erosion (1). (b) Phosphate rock is replaced only over geological time but is mined quickly, so reserves fall (1); farms need a constant input to replace what is lost, so a shortage would reduce yields and food supply (1).

(a) Primary succession (1). (b) Lichens release acids that weather the rock into mineral particles (1); dead lichens decompose and add organic matter, forming a thin soil in which mosses and small plants can root (1). (c) Any two: larger plants (1); higher primary production (1); more species (1); more complex food webs (1); faster nutrient cycling (1).

(a) The dominant trees were always pine, spruce or birch (1) and richness stayed between 14 and 17 taxa over 8 000 years, so the community persisted, with only small fluctuation (1). (b) Any one: animal numbers, fungi, or species that leave no pollen (1). (c) Any two: more light reaches the ground, so ground plants increase (1); loss of nesting and shelter for canopy animals (1); fallen timber changes decomposition and nutrient cycling (1); more soil erosion on the cleared patch (1). Naming the disturbance in the first sentence is expected. (d) Genetic diversity gave survivors and seed sources; nutrient recycling kept the soil fertile; or climate stayed within tolerance (1).

(a) 0.10\times12\,000=1\,200 canes (1). (b) Year 1: 12\,000+1\,200-1\,400=11\,800 (1). Year 2 regrowth is 1\,180, so 11\,800+1\,180-1\,400=11\,580 at the start of year 3 (1). Harvest exceeds regrowth in both years, so the stock falls and the harvest is not sustainable (1). (c) 1\,100<1\,200, so the stock rises in year 1 and the harvest is sustainable while regrowth stays above 1 100 (1). (d) Any two: stock size by sample plots (1); regrowth or recruitment of young palms (1); harvested amount each year (1).

(a) In cyclical succession the community passes through the same sequence of phases repeatedly, with no single climax (1); in arrested succession an outside factor, usually human, holds the community at an early stage (1). (b) Sheep graze and browse seedlings of birch and pine (1); the seedlings cannot grow into trees, so woodland does not develop (1); the fenced plot, where sheep are excluded, shows that grazing is the cause (1). (c) Mature heather degenerates and dies, leaving gaps (1); seedlings colonise the gaps and the building phase starts again (1).

(a) \frac{40-2000}{2000}\times100=-98.0\% (1). (b) \frac{0.45-0.60}{0.60}\times100=-25.0\% (1). (c) Benefit: it killed the mosquitoes that carry malaria, so cases fell by 98% (1). Harm: the insecticide is persistent and not biodegradable, so it is stored in fat and biomagnifies to top predators (1); the eggshells of the birds thinned by 25%, reducing breeding success (1). A balanced conclusion, for example that the benefit to health must be weighed against long-term harm to food chains (1).

(a) \frac{180}{2000}\times100=9.0\% (1). (b) Energy is lost as heat in respiration and in waste at each transfer, so less is available at higher levels (1). The insecticide is not broken down or excreted but stored in fat (1), and each consumer eats many organisms from the level below, so it collects the stored doses of all of them and the concentration rises (1). (c) Only about 10% of energy passes on (1), so after three or four transfers too little remains to support a viable population of another level (1).

(a) During the day algae photosynthesise and release oxygen (1), faster than all organisms use it in respiration (1). (b) At night photosynthesis stops while respiration continues, so oxygen is used with none made (1). (c) Dead algae are decomposed by aerobic bacteria (1); their respiration uses dissolved oxygen, so the BOD rises (1); oxygen falls below the level fish need, so they die (1).

(a) Fewer trees mean less transpiration, so less water vapour, cooling and rainfall (1); below some uncertain minimum area the forest can no longer sustain its own rainfall and begins to change towards savanna (1). (b) Drier forest dies and burns more easily (1), which removes more forest, reducing rainfall further, so the original change is amplified (1). (c) Re-establish connected native habitat between fragments, or reintroduce keystone seed-dispersing species (1); this restores movement of organisms and gene flow, so natural regeneration can proceed (1).