Stability and change — IB Diploma Biology HL

Revision notes

10 short notes, in the order of the specification. Each one in short:

1. Stability of natural ecosystems

   Stability is a property of natural ecosystems: they can keep the same structure and functions over long periods, sometimes millions of years. Stability requires a supply of energy, recycling of nutrients, genetic diversity, and climatic variables remaining within tolerance levels. If one requirement fails, the ecosystem may change to a different state.

2. Amazon deforestation as a possible tipping point

   Deforestation of the Amazon rainforest is an example of a possible tipping point in ecosystem stability. A large area of forest is needed to generate atmospheric water vapour by transpiration, which causes cooling, air flows and rainfall. Below some minimum forest area, rainfall could fail and the forest could become savannah. That minimum area is uncertain.

3. Mesocosms and keystone species

   Practical

   A mesocosm is a small experimental ecosystem used as a model to investigate how variables affect ecosystem stability. Sealed glass vessels are preferable because matter cannot enter or leave but energy can still be transferred. A keystone species has a disproportionate impact on community structure, and removing it risks ecosystem collapse.

4. Sustainable harvesting and agriculture

   Harvesting a renewable resource from a natural ecosystem is sustainable only when the rate of harvesting is lower than the rate of replacement. This is assessed by monitoring populations, as with Brazil nut trees and Atlantic cod. The sustainability of agriculture depends on soil erosion, leaching of nutrients, supply of fertilizers, agrochemical pollution and carbon footprint.

5. Eutrophication

   Eutrophication is the enrichment of aquatic and marine ecosystems with nutrients, here from leaching of nitrogen and phosphate fertilizers. Algae grow rapidly, block light and die. Decomposers breaking down the dead matter respire aerobically, increasing biochemical oxygen demand (BOD). Oxygen levels fall and fish and other animals die.

6. Biomagnification of pollutants

   Biomagnification is the increase in concentration of a toxin in the tissues of consumers at each higher trophic level. Persistent, fat-soluble pollutants such as DDT and mercury are not broken down or excreted, so they accumulate. Each consumer eats many prey, so top predators carry the highest concentrations and suffer the most harm.

7. Microplastic and macroplastic pollution of the oceans

   Plastics are persistent in the natural environment because they are non-biodegradable. Macroplastics, such as bags and fishing nets, entangle marine animals and are swallowed, blocking guts. They break into microplastics, under 5 mm, which are eaten by plankton and filter feeders and pass along food chains. Media coverage of these effects changed public behaviour.

8. Rewilding

   Rewilding is the restoration of natural processes in ecosystems so that they become self-sustaining. Methods include reintroduction of apex predators and other keystone species, re-establishment of connectivity of habitats over large areas, and minimisation of human impact, including by ecological management. At Hinewai Reserve in New Zealand, native forest is regenerating through gorse.

9. Ecological succession and primary succession

   HL only

   Ecological succession is the sequence of changes in a community over time. It can be triggered by changes in both the abiotic environment and biotic factors. During primary succession on new land, such as sand dunes, the size of plants, primary production, species diversity, complexity of food webs and nutrient cycling all increase.

10. Cyclical succession, climax communities and arrested succession

    HL only

    In some ecosystems there is a cycle of communities rather than a single unchanging climax community, as in heather heathland. Given specific environmental conditions, succession tends to lead to a particular climax community. Human influences can prevent it from developing: grazing by farm livestock and drainage of wetlands cause arrested succession.