Human effects on ecosystemsEdexcel A-Level Biology B: Revision notes

Humans affect ecosystems through burning fossil fuels, farming, fishing, deforestation and trade in wild species. You must be able to interpret data about these effects.

• Describe trends with figures from the data, for example the percentage change = (final - initial) / initial x 100.
• A correlation shows that two variables change together. It does not prove that one causes the other.
• Longer records, many sites and data from independent sources make a conclusion more reliable.
• Consider alternative explanations, such as natural variation.

Worked example: carbon dioxide rose from 317 ppm to 418 ppm. Percentage increase = (418 - 317) / 317 x 100 = 31.9%.

The greenhouse effect is natural. Gases such as carbon dioxide and methane absorb infrared radiation re-emitted from the Earth and trap heat in the atmosphere. Burning fossil fuels and deforestation increase atmospheric carbon dioxide, causing the enhanced greenhouse effect and global warming.

Data relating to climate change include rising carbon dioxide concentration, rising mean temperatures, melting ice and rising sea level.

Effects on ecosystems include:

• changes in the distribution of species, moving towards the poles or higher ground
• changes in the timing of life cycle events, for example earlier leafing and breeding
• mismatches between species, for example when food is not available at the time young hatch
• changes in the community composition and loss of species that cannot adapt or move.

Overfishing is removing fish faster than the stock can replace them by reproduction. The stock declines and may collapse.

Data that indicate overfishing:

• a fall in catch per unit effort, which means more effort is needed for the same catch
• a fall in the mean size and age of fish caught, with more immature fish
• falling total catch despite increased effort.

Measures to conserve fish stocks:

• quotas that limit the total catch
• larger mesh sizes so juveniles escape
• closed seasons or areas, such as no-take zones, protecting spawning and nursery areas
• regular monitoring of stock size and enforcement.

The aim is the maximum sustainable yield, the greatest catch that can be taken each year without reducing the stock.

CITES is the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an international treaty (1973) with over 180 member countries. Its aim is to ensure that international trade in wild animals and plants does not threaten their survival.

Species are listed in appendices:

• Appendix I: threatened with extinction, with commercial international trade banned.
• Appendix II: not yet threatened, but trade is controlled by permits.
• Appendix III: protected by one country that asks others for help in controlling trade.

Effects: CITES has reduced legal trade in many listed species and raised awareness, helping some populations to recover.

Limitations: illegal trade continues because of high prices and weak enforcement, countries must enforce it, and it does not protect habitats, which are the main cause of species loss.

Sustainability means using resources in a way that meets present needs without preventing future generations from meeting theirs. This needs management of the conflict between human needs (food, jobs, energy) and conservation.

Fish stocks: quotas and no-take zones reduce catches in the short term, causing lost income. In the long term stocks recover and catches are secured. Retraining, subsidies and aquaculture can help communities.

Reducing causes of climate change: reduce fossil fuel use, use renewable energy, plant trees and improve energy efficiency. These can raise energy costs or cost jobs in the short term.

Successful strategies need evidence, international agreement, enforcement and support for those affected.

The debate about climate change depends on evidence being checked before it is accepted.

• Scientific journals publish methods and data so that others can examine the work.
• Peer review: independent experts in the field assess the methods, data and conclusions before publication, to reduce bias and error.
• Scientific conferences: scientists present findings, which are questioned, challenged and discussed, and collaborations and further studies result.
• Findings are replicated by other scientists, and when different groups obtain consistent results a consensus builds.

Evidence that has not been peer reviewed, for example claims on social media, is less reliable. Peer review is not perfect, but it provides an important check.