Biology 9700/41 — October/November 2010

Background Concept

Every species occupies a niche — the role it plays in its ecosystem, defined by what it eats, where it lives and how it interacts with other organisms. A species' niche is closely tied to its food web; if the organisms it eats decline, the predator declines too. Specialist feeders (those with a narrow diet) are far more vulnerable to such disruption than generalists, which can switch between food types.

Agricultural and industrial chemicals — pesticides, fertilisers and other pollutants — often enter waterways and accumulate in aquatic food chains. Insecticides in particular devastate insect populations, and bioaccumulation of substances such as organochlorines in fish then poisons the fish themselves. Anything that feeds on insects or fish therefore loses its food supply.

Understanding the Question

You are given Table 1.1, which lists five bird species around Lake Ontario, their feeding habits, and the percentage change in their numbers from 1995 to 2005. Four of the five species have declined (between –6.2 % and –18.7 %), while the mallard has increased by +10.0 %. The command word is suggest, so you should make reasonable ecological proposals, not just describe the figures. You have 4 marks, so you should make about four clear, distinct points.

Look at the feeding column: the four declining species all rely heavily on animals (insects, molluscs, fish, amphibia), whereas the mallard eats mainly plants and amphibia — it is the most plant-based feeder in the list. This contrast is the key to the answer.

Approach

1. Start with a one-sentence summary of the pattern in the data (this earns the first marking point).
2. For the four declining species, identify a common feature of their diet and link it to a plausible cause of decline (pesticides, pollution, water-quality change, loss of food).
3. For the mallard, explain why it has been less affected (its food is largely plants and amphibia, which are not eliminated by the same pollutants) and use the decline of other species to mention reduced competition.

The mark scheme lets you choose 2–3 reasons for the decline and 1–2 for the mallard increase, so you don't need to write about every possible cause — pick the strongest, most data-supported points.

Step-by-Step Reasoning

Marking point 1 — the pattern
Read across the right-hand column: –6.2, –12.3, –15.9, –18.7 versus +10.0. The mallard has gone up, the other four have gone down. State this clearly.

Marking points 2–6 (reasons for the decline — pick the strongest)

• Pesticides / pollution / fertilisers are the most likely culprits. Insecticides wipe out flying and aquatic insects; herbicides and fertilisers alter plant communities and trigger algal blooms that deoxygenate water; both routes collapse the food of the swallow, teal, grebe and tern. The fact that the two biggest decliners (grebe and tern) are the most fish-dependent supports this.
• Change in water temperature or pH — climate warming or acid rain can reduce invertebrate and fish populations directly. This is an alternative, not a contradictory, explanation.
• Lack of (named) food source — any of the categories above, named explicitly (e.g. fewer aquatic insects).
• Increased competition — e.g. introduced species such as zebra mussels outcompete native molluscs and invertebrates.
• Direct human interference — recreational activities, fishing, loss of reed-bed nesting habitat on the lake.

Marking points 7–8 (reasons for the mallard increase)

• The mallard does not depend on insects, molluscs or fish, so the same pollutants have not destroyed its food base. Plants and amphibia remain available.
• With the other four species all in decline, the mallard faces less interspecific competition for food, nest sites and other resources — a classic competitive-release situation. A generalist species benefits when its specialist competitors are removed.

Key Takeaways

• Specialist feeders (narrow diet) are more vulnerable to environmental change than generalists (broad diet).
• Aquatic food webs are easily disrupted by pesticides, fertilisers and pollution entering the water.
• When competing species decline, the survivors often increase because resources are freed up — competitive release.
• When asked to "suggest" reasons, propose plausible causes and link them to the data you have been given; don't just describe the trend.

Common Mistakes

• Just quoting the numbers ("the swallow went down by 6.2 %") without offering any biological reason — this is description, not suggestion, and scores zero.
• Giving only reasons for the decline and ignoring why the mallard increased. The mark scheme gives marks specifically for the mallard increase.
• Stating "pesticides" without linking them to the food of these particular birds — the examiners want you to connect the chemical to the lost prey (insects, molluscs, fish).
• Confusing cause and effect — saying "the birds died because there were fewer birds" rather than naming an external driver such as food loss or pollution.
• Vague answers like "climate change" or "human activity" without specifying which aspect (water temperature, pH, fishing, etc.) and why it would affect these particular species.

Things to Be Careful About

• Use the exact diet terms from the table when you explain which prey is lost — "insects", "molluscs", "fish" — not vague words like "animals".
• The mallard's diet in the table is amphibia and plants, so to score the "doesn't eat insects/molluscs/fish" point you should make this contrast explicit.
• "Suggest" allows you to propose, but you should still produce biological reasoning that would be true for many real ecosystems; the mark scheme does not credit pure speculation with no link to the data.
• Keep your answer concise: four well-chosen points beat eight rambling ones. This question is 4 marks, not 8.

Background Concept

Biodiversity is a measure of the variety of life in an area — the number of species (species richness) and the genetic variation within them. High biodiversity matters because ecosystems are networks of feeding and competitive relationships; remove a node and the network can unravel. A wide range of species also represents a wide range of genes, biochemicals and ecological roles that humans and other species can call on, now or in the future.

The benefits of biodiversity can be grouped into a few familiar categories:

• Ecosystem services — processes that ecosystems perform "for free", such as nutrient cycling, pollination, climate buffering and erosion control.
• Direct resources — food, timber, fibres, medicines.
• Future potential — species whose value we do not yet know; losing them forecloses options.
• Genetic reservoir — a diverse gene pool gives populations the variation needed to evolve in response to new pressures (pathogens, climate change).
• Cultural, aesthetic and ethical value — biodiversity has recreational, spiritual and moral worth to humans.

Understanding the Question

The stem of the question has already defined biodiversity as a wide range of species. Part (b) asks you to explain the benefits of maintaining biodiversity, for 4 marks. "Explain" means you must do more than just name a benefit — you should say why it is a benefit, or how it follows from biodiversity. The mark scheme accepts up to ten different creditable points and lets you pick any four, so the easiest strategy is to choose one point from each of the broad categories so your answer is balanced and shows breadth.

Approach

Pick four distinct benefits, one from each of these categories so the examiner can see you understand biodiversity has many kinds of value:

1. Ecosystem function (e.g. food webs, nutrient cycling, climate stability).
2. Resource for humans (e.g. food, wood, fibres, medicines).
3. Future potential (e.g. unknown medicines, gene pool for future breeding).
4. Cultural/ethical/economic (e.g. ecotourism, moral reasons, leisure).

For each, write one sentence that names the benefit and adds the why (the explanation). Four sentences, four clear points, full marks.

Step-by-Step Reasoning

Point 1 — Ecosystem stability and food webs.
Biodiversity means many species occupy many different niches, so food chains and food webs are complex and have redundancy — if one species is lost, others can perform a similar role. This makes the whole ecosystem more resistant to disturbance (e.g. a single crop pest is less likely to wipe out a whole community). Nutrient cycling and protection against erosion also depend on diverse communities of decomposers and plants.

Point 2 — Resources for humans.
Humans take food (crops, fish, livestock, wild foods), materials (wood for building, cotton and other plant fibres for clothes, rubber, oils) and medicines from biodiversity. Many of the drugs in modern use (aspirin, morphine, quinine, antibiotics) originated from wild species. Maintaining biodiversity keeps these supplies available.

Point 3 — Future use / potential resources.
We have studied only a tiny fraction of the world's species. A species that today looks unremarkable may, when its chemistry is examined, yield a cure for a disease or a useful industrial compound. Destroying a species forecloses options that future generations might value — the precautionary "option value" argument.

Point 4 — Genetic variation / large gene pool.
A diverse population contains a wider range of alleles, including some that may confer resistance to a new disease, tolerance of a new temperature, or any other future selective pressure. This genetic variation is the raw material for natural selection and selective breeding. Without it, populations are more vulnerable to extinction when conditions change. Wild relatives of crop plants are an important source of such genes for plant breeding.

Alternative points the mark scheme credits:

• Cultural / aesthetic / leisure — nature has recreational, inspirational and artistic value; people enjoy watching birds, walking in forests, etc.
• Moral / ethical — many people believe other species have a right to exist and that humans have a duty to prevent extinction.
• Ecotourism — biodiversity attracts tourists, providing income to local communities and an incentive to conserve habitats.
• Economic benefits — directly, through resource use; indirectly, through ecosystem services that save money (e.g. pollination of crops, flood prevention by mangroves).
• Climate stability — forests and oceans buffer atmospheric CO₂ and local climates; biodiversity supports these systems.

Key Takeaways

• Biodiversity has multiple kinds of value: ecological, economic, ethical, cultural and "option" value for the future.
• Ecosystem services (food webs, nutrient cycling, climate regulation) depend on biodiversity.
• A large gene pool is biodiversity's insurance against future change.
• Many modern medicines originated from wild species — biodiversity is a pharmaceutical resource.
• When asked to "explain the benefits", do not just list single words; add the reason the benefit matters.

Common Mistakes

• Listing one-word benefits ("food, medicine, wood") without explaining how biodiversity provides them. "Explain" demands a reason, not just a label.
• Repeating the same idea in different words — "food for humans" and "resources for humans" score as one point, not two. Make sure your four points are genuinely different.
• Missing the ecosystem-services category — many candidates give only economic and ethical reasons and forget that biodiversity functions in the ecosystem (food webs, cycling, stability).
• Confusing biodiversity with abundance of one species — biodiversity is the number and variety of species, not the population size of any single one.
• Forgetting the gene-pool / genetic-variation point — this is a CIE favourite and easy marks if remembered.

Things to Be Careful About

• The mark scheme lists 10 possible points; you only need 4, so be selective and distinct.
• A single sentence per point is enough — do not pad; the question is 4 marks.
• "Maintain a large gene pool" is a stronger formulation than "more variation" — use the technical term.
• Where possible, name a concrete example (e.g. aspirin from willow, rubber from Hevea trees) — the mark scheme accepts these and they show real understanding.
• Do not credit points that simply describe what biodiversity is (e.g. "many different species") — these earn nothing for an "explain the benefits" question.