This pages gives outline details of the content of option D for HL students. This includes all the SL sections as well as the additional material. There are essential questions and lists of student skills and applications. Helpful for revision. Students can check their understanding and prioritise areas for revision.

C.1 Species and communities

Understanding(s)

• Limiting factors affect the distribution of species.
• Keystone species affect community structure.
• The spatial habitat and interactions with other species gives each species a unique role within a community.
• Interactions between species in a community can be classified according to their effect.

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• Two species cannot survive indefinitely in the same habitat if their niches are identical.
• Application: The symbiotic relationship between Zooxanthellae and reef-building coral reef species.
• Skill: Analysis of a data set that illustrates the distinction between fundamental and realized niche.

C.2 Communities and ecosystems

Understanding(s)

• Food chains show species in Trophic levels and multiple food chains usually exist within a community, shown by a food web.
• Skill: To compare the pyramids of energy from different ecosystems.
• The respiration rate determines the percentage of ingested energy converted to biomass.
• In closed ecosystems energy but not matter is exchanged with the surroundings.
• Application: Conversion ratio is a useful concept in sustainable food production practises.

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• Climate can be used to predict the type of stable ecosystem that will emerge in an area.
• Skill: to be able to analyse a climograph which shows the connection between the type of ecosystem and climatic factors such as rainfall and temperature.

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• Disturbance influences the structure and rate of change within ecosystems.
• Skill: Investigation into the effect of an environmental disturbance on an ecosystem, such as: species diversity, nutrient cycling, water movement, erosion, leaf area index, etc..
• Skill: Analysis of data showing primary succession.

C.3 Impact of humans on ecosystems

Understanding(s)

• Alien species are introduced species which escape into a local ecosystem and become invasive.
• When alien species become invasive there can be a reduction in the numbers of endemic species because of competitive exclusion or the absence of predators.
• Application: Study of the introduction of cane toads in Australia and one other local example of the introduction of an alien species.

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• Biomagnification is the process where pollutants become concentrated in the tissues of organisms at higher trophic levels.
• Application: Discussion of the trade-off between control of the malarial parasite and DDT pollution.

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• Macroplastic and microplastic debris has accumulated in marine environments.
• Application: Case study of the impact of marine plastic debris on Laysan albatrosses and one other named species.

C4 Conservation of biodiversity

Understanding(s)

• The use of indicator species to assess a specific environmental condition.
• The calculation of a Biotic index, using relative numbers of indicator species.

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• Active management of nature reserves or national parks is called In situ conservation.
• Outside their habitat, Ex situ conservation preserves species.

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• Biogeographic factors affect species diversity.
• Richness and evenness are components of biodiversity.

C5 Populations HL

Understanding(s)

• The size of a population can be estimated using sampling. e.g. the Lincoln index.

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• In an unlimited environment exponential growth pattern of a population occurs.
• As a population size reaches the carrying capacity of the environment the rate of growth slows.
• Limiting factors prevent unlimited growth of populations, e.g. Bottom-up control of algal blooms by lack of nutrients and top-down control of algae by herbivory
• Students should understand how rates of natality, mortality, immigration and emigration can be used to explain the phases of sigmoid growth curve and population size. 

C6 Nutrient cycles HL

Understanding(s)

• Atmospheric nitrogen is converted to ammonia by Nitrogen-fixing bacteria.
• Rhizobium bacteria have a mutualistic relationship with some plants and fix nitrogen in roots..
• Bacteria will reduce nitrate in the soil back to atmospheric nitrogen in anaerobic conditions, (Denitrification)
• e.g. in waterlogged soil where insectivorous plants are adapted to survive the low nitrate levels.

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• The application of fertilizer adds Phosphorus to the phosphorus cycle
• The harvesting of agricultural crops removes phosphorous from the cycle.
• The rate of turnover in the phosphorus cycle is much slower than the nitrogen cycle.
• Availability of phosphate may become limiting to agriculture in the future.

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• Leaching of mineral nutrients from agricultural land into rivers causes eutrophication and leads to increased biochemical oxygen demand.