Stability & Change (DP IB Biology)

True.

There is uncertainty around the area of rainforest that would need to be lost before the ecosystem becomes unstable. This means that we do not know how close the Amazon rainforest is to losing stability.

The formula for calculating percentage change in forest cover is:

percentage change = (change in forest coverinitial forest cover) × 100

Keystone species are species that have a disproportionate effect on the structure and function of their ecosystem.

The removal of a keystone species can cause loss of ecosystem stability, which may involve local species extinctions and eventual ecosystem collapse.

A sustainably harvested resource is one which is replaced as rapidly as it is harvested, meaning that it does not run out.

The sustainability of resource harvesting can be assessed by, e.g.

• monitoring population size

• monitoring harvest volumes alongside reproduction rates

• monitoring age structure of a population, i.e. checking that enough mature individuals are present for reproduction

Factors that affect the sustainability of agriculture include:

• soil erosion

• leaching of nutrients

• supply of fertilisers and other inputs

• pollution due to agrochemicals

• carbon footprint

Soil erosion can impact sustainability of agriculture as follows:

• removal of ground cover plants means that the roots that hold the soil together are lost

• soil can easily be washed or blown away, leading to soil erosion

• there is a reduction in the availability of soil needed for crop/grass growth

Leaching is the process by which rainwater dissolves and washes away soluble nutrients from the soil, potentially causing nutrient runoff into nearby water bodies.

The carbon footprint of agriculture refers to the carbon dioxide released into the atmosphere due to agricultural practices, including the use of fossil fuels for transportation, machinery, and fertiliser production.

Eutrophication is the process that occurs when water bodies receive artificially large inputs of nutrients, resulting in:

• excess growth of plants at the water surface

• death of plants below the surface

• decomposition of dead plant matter

• depletion of dissolved oxygen due to the activity of decomposers

• death of other aquatic organisms due to reduced oxygen

The nutrients that cause eutrophication when leached into water bodies are:

• nitrates

• phosphates

The primary cause of oxygen depletion in eutrophic waters is the increased respiration of bacterial decomposers as they break down dead organic matter.

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen needed by aerobic microorganisms to break down organic material in water.

Biomagnification is the increase in concentration of non-biodegradable pollutants with ascending trophic level through a food chain.

Pollutants become more concentrated at higher trophic levels during biomagnification because organisms at the bottom of the food chain each consume a small volume of pollutant, and then the organisms higher up the food chain consume many smaller organisms and receive a larger dose of pollutant.

Examples of pollutants that can cause biomagnification include:

• DDT

• mercury

DDT (dichlorodiphenyltrichloroethane) is a persistent pollutant that was widely used as an insecticide in the mid-20th century before being banned due to its harmful environmental effects.

The organisms that are affected most by DDT in food chains are the top predators, e.g. birds of prey that consume fish containing DDT lay eggs with thinner shells.

Macroplastics are large plastic items that are more than 5 mm in length, including plastic bags, bottles, food packaging, and fishing nets.

Microplastics are small pieces of plastic that are less than 5 mm in length, often resulting from the breakdown of larger plastic items.

Plastic caused problems when it enters the natural environment because it is non-biodegradable and so remains in the environment for many years. Plastic in the environment can be eaten by and trap animals, causing serious harm.

False.

Plastic pollution has a large negative impact on both terrestrial and marine habitats.

Scientists have changed public behaviour around plastic use by clearly communicating the problems that plastic can cause in popular media.

Rewilding aims to restore ecosystems by re-establishing natural ecosystem processes; this allows ecosystems to become self-sustaining with little human management.

The purpose of reintroducing apex predators is to control populations of herbivores, which allows the restoration of habitat vegetation and increases biodiversity.

True.

The reintroduction of keystone species can alter the structure of an ecosystem.

Wildlife corridors connect small pockets of habitat, allowing organisms to roam over larger areas and access more resources.

Human influence can be limited by, e.g.

• preventing the harvesting of resources, such as logging, fishing, or agriculture

• reducing disturbance by humans

• implementing sustainable management

An ecosystem that has undergone ecosystem restoration is the Hinewai Reserve in New Zealand.

There are many other examples, but this example is given in the syllabus.

Ecological succession is the progressive change in the species that make up an ecological community over time.

False.

Succession can be triggered by changes in both abiotic and biotic factors.

Primary succession is succession that occurs when newly formed or newly exposed land is inhabited by an increasing number of species.

The main changes that occur in an ecosystem during the process of primary succession are increases in:

• size of plants

• primary production

• species diversity

• complexity of food webs

• nutrient cycling.

False.

Succession does not always result in a climax community. Succession can be cyclical, or it may be interrupted by human activities.

Cyclical succession is when succession results in a changing cycle of communities rather than a single, unchanging climax community.

True.

Given any specific environmental conditions, ecological succession tends to lead to a particular type of climax community, e.g. in a temperate climate we would expect succession to eventually result in temperate forest.

The grazing activity of livestock, such as sheep and cattle, prevent tree seedlings from establishing. This means that a forest community may not develop.

Drainage of wetlands will prevent the formation of peat bogs.