Discuss the impacts of persistent or non-biodegradable pollutants, such as DDT or mercury, on ecosystems [9 marks]

Indicative Content	Guidance
Bioaccumulation: Definition of bioaccumulation / explanation of how it occurs Provide examples of persistent pollutants → e.g. mercury/DDT/PCBs Some pollutants tend to accumulate in fatty tissues Certain factors influence rate of bioaccumulation → e.g. body size / metabolism of organism Biomagnification: Definition of biomagnification / explanation of how it occurs → due to decrease of biomass/energy along food chains Use of examples → e.g. mercury in aquatic food chains Explain how pollutants become more concentrated as they move up the food chain → affects top predators Effects on trophic levels / feeding relationships: Discuss how persistent pollutants disrupt higher trophic levels in ecosystems → top predators are particularly vulnerable to pollutant accumulation Use of specific case studies to show how certain species are affected by these pollutants → e.g. eagles/ospreys/peregrine falcons Explanation of how these disruptions can lead to imbalances in ecosystem dynamics → prey population increases (or declines) Long-term ecological risks: Discuss long-lasting ecological damage associated with persistent pollutants Use of real-world examples → e.g. impact of DDT on bald eagle populations Explanation of how shifts in species composition / biodiversity loss / ecosystem instability can result from long-term exposure to these substances Potential human health risks: Explanation of how pollutants can enter the human food chain → e.g. mercury in fish Discuss human health risks → e.g. neurological damage / cancer Mention of regulatory measures → e.g. Stockholm Convention on Persistent Organic Pollutants → aimed at reducing human exposure to these pollutants	You will always have to answer two 9 mark questions from Section B in your IB ESS SL Paper 2 exam This is a 'level of response' answer. The examiner will award you a combined mark out of 9 for knowledge, application, analysis and evaluation ‘Discuss’ (for 9 marks) requires: Knowledge of concepts, principles and models Application to the context using relevant and focused examples Analysis showing logical and coherent chains of reasoning of a range of issues Evaluation showing critical thinking linked to the context Depth is more important than breadth It is sensible to discuss three or four main points but to apply, analyse and evaluate them fully For top-level evaluation, informed judgement is required This can be demonstrated in a conclusion that synthesises and weighs up the arguments to form a supported answer to the question
Model Answer
Understanding the impacts of persistent or non-biodegradable pollutants, such as DDT or mercury, on ecosystems is crucial for limiting the environmental and ecological consequences of these substances [An]. This discussion will explore the key aspects of bioaccumulation, biomagnification, effects on higher trophic levels and the long-term ecological and human health risks associated with these pollutants [K]. Many persistent pollutants can result in bioaccumulation, accumulating in organisms over time, often in fatty tissues [K]. For example, in Minamata, Japan, methylmercury contamination in shellfish led to severe mercury poisoning, known as the Minamata disease. This demonstrates how pollutants can enter the human food chain through bioaccumulation [Ap]. Biomagnification, the increase in pollutant concentration up the food chain, is also a major problem [K]. For example, industrial pollutants like PCBs (polychlorinated biphenyls) in the Great Lakes of North America led to exceptionally high levels in top predators like lake trout, impacting their health and populations [Ap]. The effects of biomagnification of persistent pollutants on trophic levels can be severe. The pesticide DDT, for instance, caused thinning and weakening of eggshells in birds of prey such as bald eagles. This led to high offspring mortality and population declines, causing shifts in ecosystem dynamics. The DDT-induced decline of the bald eagle population had ripple effects, affecting prey populations and causing ecological imbalances [An]. Long-term risks include ecosystem instability and changes in species composition, as pollutants persist long after their introduction [K]. Regulatory and health policy measures, like the Stockholm Convention on Persistent Organic Pollutants, are essential to mitigate these risks and reduce human exposure [Ap]. In conclusion, persistent pollutants have profound impacts on ecosystems, as seen in the Minamata disease, PCB contamination in lake trout, and DDT-induced eggshell thinning in birds of prey. Understanding these examples is vital for managing and mitigating the ecological and human health consequences of these pollutants [E].
Commentary
This answer is an example - yours may be different The answer discusses multiple impacts of non-biodegradable pollutants on ecosystems and finishes with a supported final judgement The response is annotated to show where the skills of knowledge (K), application (Ap), analysis (An) and evaluation (E) have been demonstrated

Communities & Ecosystems (SL IB Environmental Systems & Societies (ESS))

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