Biology • Year 11 • Module 4 • Lesson 16
Biodiversity — Measurement, Importance and Ecosystem Stability
Apply the concepts of species diversity, redundancy, and biodiversity loss to real data sets and ecological scenarios.
1. Compare species diversity between two sites
Two regenerating bushland sites were surveyed for reptile species. The table below shows the results. 9 marks
| Site | Species | Individuals counted |
|---|---|---|
| P (Intact woodland) | Blue-tongue lizard | 14 |
| Eastern water skink | 11 | |
| Lace monitor | 9 | |
| Eastern bearded dragon | 12 | |
| Shingleback lizard | 14 | |
| Q (Cleared paddock) | Blue-tongue lizard | 51 |
| Eastern water skink | 7 | |
| Common garden skink | 2 |
1.1 Calculate the species richness for Site P and Site Q. 2 marks
1.2 Without calculating, identify which site has higher species evenness. Justify your answer by referring to the data. 2 marks
1.3 Which site would receive a higher Simpson’s Diversity Index score? Explain your reasoning. 2 marks
1.4 A new fungal disease kills blue-tongue lizards across both sites. Predict which site would be more affected and explain why, using the concept of functional redundancy. 3 marks
2. Interpret a graph — biodiversity and ecosystem recovery
The graph below models the rate of ecosystem recovery after a fire event, comparing sites with high biodiversity and low biodiversity. 7 marks
Figure 2.1. Modelled vegetation recovery after a fire event for sites with contrasting biodiversity. Illustrative data only.
2.1 Describe the difference in recovery rate between the two sites during the first three years after the fire. Include specific values from the graph. 2 marks
2.2 By year 10, the high-biodiversity site has reached approximately 85% vegetation cover while the low-biodiversity site has only reached approximately 35%. Using the concept of functional redundancy, explain why the high-biodiversity site recovers more completely. 3 marks
2.3 Identify one limitation of this model as evidence for the relationship between biodiversity and ecosystem resilience. 2 marks
3. Apply to a new scenario — the collapse of a pollinator community
A market gardener reports that crop yields have halved over three years. In the same period, pesticide use on neighbouring farms has eliminated six of the seven native bee species that previously visited the crops. One introduced European honeybee species remains. 8 marks
3.1 Using lesson vocabulary, identify which component of species diversity has changed most dramatically and explain what has happened to it. 2 marks
3.2 Explain why the elimination of six native bee species would reduce functional redundancy in the pollination function. 3 marks
3.3 If the remaining European honeybee colony collapses next season, predict the ecological and economic consequences for the market garden. Connect your answer to the concept of ecosystem services. 3 marks
Q1.1 — Species richness
Site P: 5 species. Site Q: 3 species. 1 mark per site (max 2).
Q1.2 — Species evenness
Site P has higher evenness. At Site P, individuals are distributed across 5 species with counts of 14, 11, 9, 12, and 14 — no single species dominates. At Site Q, the blue-tongue lizard accounts for 51 of 60 individuals (~85%), so one species overwhelmingly dominates and evenness is very low. 1 mark for identifying Site P; 1 mark for a data-supported justification referencing the dominance of the blue-tongue lizard at Site Q.
Q1.3 — Simpson’s Diversity Index
Site P would receive a higher Simpson’s D score. The index reflects both richness and evenness — Site P has both more species and a more even distribution, both of which increase the index. Site Q has fewer species and very uneven distribution (one species dominant), giving a lower index. 1 mark for Site P; 1 mark for reasoning that both richness and evenness contribute to a higher D.
Q1.4 — Impact of disease and functional redundancy
Site Q would be more affected. At Site Q, blue-tongue lizards make up ~85% of all individuals, so the function they perform (e.g. insect predation, seed dispersal) is concentrated in one species with very few others to compensate [1]. At Site P, multiple species perform overlapping functions; when the blue-tongue is lost, other lizard species maintain some level of those ecological functions — this is functional redundancy [1]. Site Q has almost no functional redundancy in the roles the blue-tongue fills, so losing it destabilises the community significantly [1]. Award marks for: identifying Site Q as more affected (1); linking to the lack of redundant species at Site Q (1); explaining how Site P’s diversity buffers the loss (1).
Q2.1 — Recovery rate description
In the first three years, the high-biodiversity site recovers from 0% to approximately 50% vegetation cover, whereas the low-biodiversity site recovers to only approximately 15%. The high-biodiversity site’s recovery rate is noticeably steeper (faster) during this early phase. 1 mark for comparing the two curves over years 0–3; 1 mark for citing at least one approximate figure from each curve.
Q2.2 — Why high biodiversity site recovers more completely
The high-biodiversity site contains many species performing overlapping functions such as nutrient cycling, seed dispersal, and pollination [1]. When a fire kills some species, others performing the same functions remain and maintain ecosystem processes, allowing vegetation to rebuild [1]. In the low-biodiversity site, fewer species mean less redundancy: losing a key species to fire can disrupt a critical function with no backup species to compensate, slowing or halting vegetation recovery [1]. Award marks for: identifying overlapping/redundant functions in the high-diversity site (1); explaining how these functions persist after disturbance (1); contrasting with the low-diversity site where function loss is not buffered (1).
Q2.3 — Limitation of the model
The model is illustrative/stylised and does not represent a specific real ecosystem, so it cannot be used to make precise quantitative predictions about actual recovery rates [1]. Other variables (soil type, rainfall, fire severity, nearby seed sources) that also affect recovery are not controlled or represented in the model, so we cannot attribute the difference solely to biodiversity [1]. Accept any one reasonable limitation with appropriate justification. Award 1 mark for identifying a limitation; 1 mark for explaining why it limits the evidence. Accept: sample size, lack of replication, no time-series replication, model ≠ real data.
Q3.1 — Component of species diversity most changed
Species richness has changed most dramatically — falling from 7 native bee species to effectively 1 (the introduced honeybee). This represents a ~86% reduction in the number of pollinator species present at the site. 1 mark for identifying species richness; 1 mark for explaining the direction and magnitude of the change using data from the scenario.
Q3.2 — Functional redundancy in pollination
Previously, seven bee species all performed the pollination function [1]. When six are eliminated, only one species remains to carry out that function; the redundancy (multiple species doing the same job) has been almost entirely removed [1]. If the remaining honeybee is stressed, diseased, or absent on any given day, there are no backup species to continue pollination, so any disruption to the honeybee immediately disrupts the crop [1]. Award marks for: identifying multiple species previously sharing the function (1); explaining that six of seven species performing that function have been removed (1); linking the loss of redundancy to increased vulnerability if the final species is affected (1).
Q3.3 — Consequences if honeybee colony collapses
Ecologically: pollination ceases entirely, preventing sexual reproduction in the crop plants, eliminating fruit and seed set, and disrupting the provisioning ecosystem service the garden depends on [1]. Economically: crop yields drop to near zero because no fruit is produced, directly threatening the viability of the farm [1]. This illustrates the link between biodiversity loss, reduced ecosystem services (in this case a regulating/provisioning service), and human welfare — the loss of the final pollinator removes the last remaining regulating service (pollination) that underpins the provisioning service (food) [1]. Award marks for: identifying ecological consequence of no pollination (1); linking to economic/provisioning service loss (1); using the term “ecosystem services” or “regulating/provisioning” correctly in context (1).