Recall that a tipping point is the minimum threshold point that if a system reaches it, it will move away from its original equilibrium state to another (new) equilibrium state.

Tipping points lead to the collapse of the original ecosystem and development of a new equilibrium. For example, the deforestation of the Amazon rainforest reduces generation of water vapour through transpiration and consequently reduces cooling and precipitation necessary for the maintenance of the remaining forest.

Measuring changes in an ecosystem due to human activity

Human impacts on ecosystems include toxins from mining activity, landfills, eutrophication

Changes in the ecosystem depend on the human activity involved. Methods used for measuring abiotic and biotic components of an ecosystem must be appropriate to the human activity being studied. In your local area there will be locations where you can investigate the effect that human disturbance has had on natural ecosystems. These may be areas of forest that have been harvested for timber or grassland habitats that are regularly trampled by walkers, for example. 

Various methods you can use to study the effect of human activities, including:

• Carrying out capture–mark–release–recapture methods on invertebrate species in disturbed and undisturbed sites

• Measuring species diversity using Simpson’s reciprocal index

• Using indicator species (HL).

• Measuring variables such as light levels, temperature and wind speed. You could also calculate the average diameter of tree stems at breast height (DBH) and the degree of canopy openness (the amount of sky can you see through the canopy of the forest), which would give you measures of tree biomass (organic matter) and leaf cover.

• Measuring soil erosion – in areas with high precipitation this can be calculated by measuring the depth of soil remaining under free-standing rocks and stones, where soil around these solid objects has been eroded away.

• Measuring soil variables such as soil structure, nutrient content, pH, compaction levels and soil moisture

RECALL: Keystone species are species that are vital for the continuing function of the ecosystem: without them the ecosystem may collapse. Keystone species therefore have a role in the sustainability of ecosystems. An example is the agouti of tropical South and Central America, which feeds on the nuts of the Brazil nut tree.

• The Brazil nut tree (Bertholletia excelsa) is a hardwood species that is found from eastern Peru, eastern Colombia and eastern Bolivia through Venezuela and northern Brazil. 

• They are the tallest trees in the Amazon (they grow up to 50 m). The agouti is a large forest rodent and the only animal with teeth strong enough to open the Brazil nut tree’s tough seed pods. 

• The agouti buries many of the seeds around the forest floor, so it has access to food when the Brazil nuts are less abundant. 

• Some of these seeds germinate and grow into adult plants. Without the agouti, the Brazil nut tree would not be able to distribute its seeds and the species would eventually die out. 

• Without the Brazil nut tree, other animals and plants that depend on it would be affected; for example, harpy eagles use the trees for nesting sites. 

• Brazil nuts are one of the most valuable non-timber products found in the Amazon as they are a protein-rich food source and their extracted oils are a popular ingredient in many cosmetic products. 

• The sale of Brazil nuts provides an important source of income for many local communities. Given the complexity of ecosystems, keystone species may be difficult to identify. 

• In addition, many keystone species may be species that are yet unidentified. By conserving whole ecosystems (i.e. establishing protected areas), rather than attempting to conserve individual species, the complex interrelationships that exist there will be preserved, including the keystone species.

Activity 1

• Describe the role of a keystone species in a named ecosystem

The integrity and functioning of ecosystems, necessary for the ongoing maintenance of biodiversity and to minimize extinction, is measured using the biodiversity intactness index (BII). Biodiversity intactness can be defined as the average abundance of originally present species relative to an intact ecosystem, assessed geographically by biomes and major marine ecosystems (e.g. coral reefs).

Changes to biosphere integrity have passed a critical threshold

The planetary boundaries model is used to indicate the environmental ceiling beyond which lie unacceptable environmental degradation and potential tipping points in Earth systems. One of the planetary boundaries relates to biodiversity loss. The planetary boundaries model indicates that changes to biosphere integrity have passed a critical threshold.

The unit E/MSY (extinction per million species years) means that if there were a million species on Earth, one would go extinct every year, while if there was only one species it would go extinct in one million years. The average duration of mammalian species, for example, is estimated to be 10 000 000  years and so their background extinction rate is therefore 1 E/MSY, i.e. one extinction per million species per year or one extinction per 1000 species per century. The rate of natural extinctions is known as the ‘background extinction rate’. The background extinction range for most animal groups is between 0.1 and 1 E/MSY. Research indicates that the upper limit for species extinction rate (the rate at which species disappear) is less than 10 extinctions per million species-years (E/MSY).

The integrity and functioning of ecosystems, necessary for the ongoing maintenance of biodiversity and to minimise extinction, is measured using the biodiversity intactness index (BII). Biodiversity intactness can be defined as the average abundance of originally present species relative to an intact ecosystem, assessed geographically by biomes and major marine ecosystems (e.g. coral reefs).

• If the BII is 90% or more, the area has enough biodiversity to be a resilient and functioning ecosystem.

• If the BII is under 90%, biodiversity loss means ecosystems become less resilient.

• If the BII is 30% or less, the area’s biodiversity has been depleted and the ecosystem could be at risk of collapse

Studies suggest that the current rate of extinction is 100–1000 E/MSY and so between 100 and 1000 times higher than the background rate of extinction. There is an interrelationship between ecosystems and species diversity. Disturbance of ecosystems due to human activity has led to loss of biosphere integrity. Extinction rates provide evidence that the planetary boundary for biosphere integrity has been crossed