Ecological Organisation and Factors
This topic covers the hierarchical organisation of life, from individual organisms to entire ecosystems. It explains how populations interact with each other and their non-living environment, and the factors that cause their numbers to change over time.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Levels of organisation are nested: Individual organisms of one species form a Population. Different populations interacting in the same area form a Community. A Community interacting with its non-living environment forms an Ecosystem.
- Abiotic factors are non-living influences on a community, such as temperature, light intensity, water availability, and soil pH. Biotic factors are living influences, like predation, disease, and competition.
- Population size is determined by the balance of four factors: births, deaths, immigration (moving in), and emigration (moving out). Population growth is limited by factors like food supply, predation, and disease.
- Interdependence is crucial. Key interactions include predation (one organism hunts another), mutualism (both benefit), parasitism (one benefits, the other is harmed), and competition (organisms vying for the same limited resources).
- Competition can be intraspecific (between members of the same species) or interspecific (between members of different species).
- Photosynthetic organisms like plants and algae are the primary producers. They convert light energy into chemical energy, forming the base of the food web and creating the ecosystem's biomass.
Formulae
Population change = (births + immigration) - (deaths + emigration) To determine the net change in a population's size over a period. A positive result indicates growth, while a negative result indicates decline.
Definitions
- Ecosystem
- The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.
- Community
- All the populations of different species living and interacting in a particular habitat.
- Population
- A group of organisms of the same species living in the same area at the same time.
- Habitat
- The place where an organism lives.
- Abiotic Factor
- A non-living component of an ecosystem that affects the organisms within it, e.g., temperature or soil pH.
- Biotic Factor
- A living component of an ecosystem that affects other organisms, e.g., a predator or a pathogen.
Worked example
A population of squirrels in a woodland was estimated to be 800 at the start of a year. During the year, 240 squirrels were born and 60 immigrated from a neighbouring park. 180 squirrels died due to predation and disease, and 120 emigrated. What was the percentage change in the squirrel population by the end of the year?
- 1
Step 1:
Calculate the total additions to the population.
Add births and immigration:
240 + 60 = 300 - 2
Step 2:
Calculate the total removals from the population.
Add deaths and emigration:
180 + 120 = 300 - 3
Step 3:
Calculate the net change in population.
Subtract removals from additions:
300 - 300 = 0 - 4
Step 4:
The population size did not change.
Therefore, the percentage change is 0%.
Answer: 0%
Common mistakes
- ×When explaining population limits, students often mention only one factor (e.g., predation) but forget that multiple constraints like competition for food, disease, and abiotic factors act simultaneously.
- ×Misinterpreting predator-prey cycle graphs is common. Always identify which line represents the predator and which the prey (prey numbers peak first), and note the time lag before the predator population responds.
- ×Arithmetic errors in percentage change calculations are frequent. Always calculate the absolute change first, then divide by the *original* population size, and finally multiply by 100.
No-calculator tips
- ✓For percentage change, simplify the fraction before calculating. If a population goes from 50 to 70, the change is 20. The calculation is (20/50) × 100. Simplify 20/50 to 2/5. Since 1/5 is 20%, 2/5 is 40%.
- ✓When comparing populations on a graph, approximate values to the nearest ten or hundred to quickly identify trends and relative sizes before performing any precise calculations.