10.1 The nature of ecosystems

Define ecosystem

• All the organisms living in a particular area, and all the non-living conditions found there

• Can vary from very large to very small (e.g. deep sea to garden pond)

Define & name the trophic levels

• Describes an organism’s feeding relationships with other organisms (i.e. its position in a food chain)

• Producer → primary consumer → secondary consumer → tertiary consumer → decomposer

Give three ways we can represent ecosystem structure

• Pyramids of number (counting)

• Pyramids of biomass (weighing)

• Pyramids of energy

What are the advantages & disadvantages of the three pyramids?

• Number = easiest to measure, but can be distorted by large organisms

• Biomass = more accurate, but dry mass has to be used

• Energy = most accurate, but hardest to measure

Suggest methods of assessing abundance & distribution of organisms

• Quadrats = square frames placed at random in area to be investigated

• Transects = line or belted that runs across the area to be investigated

Suggest different ways abundance can be quantified

• Percentage cover

• Frequency (individual counting)

• ACFOR scale (abundant, common, frequent, occasional, rare)

Why might we calculate a Spearman’s rank correlation coefficient?

To measure correlation between two variables (i.e. the extent to which changing one variable affects the other variable)

Explain how Spearman’s rank results are interpreted

• Closer to 1 = more positive correlation

• Closer to -1 = more negative correlation

• Around 0 = no correlation

Why might we calculate a t-test?

To determine if the means (averages) of two sets of data are significantly different from each other

Explain how t-test results are interpreted

• The t vale obtained is compared to a critical value (found in a table) for a particular p value chosen by the researcher

• If the t value is greater than the critical value, the difference is said to be statistically significant