Harm to Food Chains & Food Webs – Comprehensive Study Notes

Students should be able to:
- Recognise that some substances are harmful/toxic to living things.
- Explain how these substances can move through food chains and food webs, affecting multiple trophic levels.
- Describe ecological, health, ethical, and regulatory implications of contamination.

A food chain shows the linear order in which organisms obtain energy.
Energy originates from the Sun, is fixed by producers (via photosynthesis), and then moves to consumers.
At every transfer, a large proportion of energy is lost as heat (≈ $90\%$ ), leaving roughly $10\%$ for the next trophic level (the “ $10\%$ rule”).
Decomposers return nutrients to the abiotic environment, closing biogeochemical cycles.

Typical hierarchy (bottom ➜ top):
- Producers (plants, algae).
- Primary consumers (herbivores).
- Secondary consumers (small carnivores/omnivores).
- Tertiary consumers (top predators).
Energy pyramid visualises diminishing energy: each step upwards is dramatically narrower, emphasising energy loss.
Representative example from slide:
- Producer ➜ Primary consumer ➜ Secondary consumer ➜ Tertiary consumer ➜ Decomposer.

Excessive hunting, fishing, gathering can remove key species.
Depletion of target species (e.g.
commercial fish stocks) can cause population collapses.
Removing predators/prey destabilises ecological balance, triggering trophic cascades.
Destructive fishing (e.g. bottom trawling) damages physical habitat, compounding biological loss.

Conversion of forests, wetlands, grasslands into farms, cities, mines.
Consequences:
- Biodiversity loss, species extinctions.
- Fragmentation—isolates populations, increases vulnerability to invasive species, disease, environmental stress.
- Greenhouse-gas release (e.g. deforestation emits $\text{CO}_2$ ), feeding back to climate change.

Types:
- Plastics (macro & microplastics) – ingested at every trophic level; persist & biomagnify.
- Oil spills & industrial chemicals – acute toxicity; habitat smothering.
- Nutrient pollution (nitrates, phosphates) – algal blooms, hypoxia.
Pollutants can bioaccumulate in organisms and biomagnify up food chains (see “Biomagnification” section).

Temperature rise: shifts breeding, migration; creates timing mismatches (phenological asynchrony).
Altered precipitation: droughts/floods change resource availability.
Ocean acidification (dissolved $\text{CO}_2$ lowers pH) harms shell-forming plankton, coral — base of many marine chains.
Climate-enabled spread of invasive species which outcompete or prey on natives.

Natural and anthropogenic sources: volcanoes, coal combustion, mercury mines.
Atmospheric Hg deposits into oceans/soils.
In low-oxygen marine zones, bacteria convert inorganic Hg to monomethyl-mercury (MMHg) – a potent neurotoxin.
MMHg biomagnifies; highest concentrations found in tuna, swordfish, shark.
U.S. EPA consumption guidance (illustrative slide):
- “Unlimited” for low-Hg seafood (e.g.
  oysters, pollock).
- “Eat a few times per week” – salmon, halibut.
- “Only a few times per month” – shark, albacore.
Health impacts: neurological damage, developmental deficits in fetuses/children.

Sources: zinc/lead mining, batteries, industrial emissions, phosphate fertilisers.
Acute inhalation symptoms: flu-like fever, chest pain, pulmonary oedema.
Acute ingestion: nausea, vomiting, diarrhoea, abdominal cramps, tenesmus.
Chronic/long-term:
- Accumulates in kidneys & liver.
- Replaces calcium in bones ➜ fragility/osteoporosis; classic “Itai-Itai” disease in Japan.
- Rice grown in Cd-contaminated paddies concentrates Cd ⇒ dietary exposure pathway.

Plastic fragments < $5\,\text{mm}$ .
Enter plankton, benthic feeders, filter-feeders.
Adsorb persistent organic pollutants (POPs), acting as vectors of additional toxins.

Definition: chemicals designed to kill pests consuming crops.
Field scenario from slides:
- Corn (producer) sprayed with pesticide.
- Snails (herbivores) eat leaves, ingest pesticide.
- Birds eat snails; wildcats eat birds — pesticide passes upward.
- Example with units: bugs with $15$ units ➜ lizard with $5$ bugs = $75$ units ➜ eagle (2 lizards) $150$ units; results in weak eggshells (historical raptor declines).
DDT specifics:
- Used extensively post-WWII (malaria & agricultural pest control).
- Persistent organochlorine, fat-soluble, resists biodegradation.
- Bioaccumulates in adipose tissue; biomagnifies through trophic levels.
- Responsible for eggshell thinning in eagles, falcons, osprey (1960s population crash).
- Remnants still circulate globally; found in Arctic seals & polar bears despite regional ban.

Bioaccumulation: concentration of a substance in an organism exceeds that in environment.
Biomagnification: progressive increase in concentration from one trophic level to the next.
Quantitative river example (workbook):
- Pond-weed leaf $4$ units.
- Tadpole $20$ units (× $5$ ).
- Fish $240$ units (× $12$ ).
- Heron $500$ units.
General relationship:
$C<em>{n+1} = BAF \times C</em>n$
where $C$ = concentration, $BAF$ = bioaccumulation factor (> $1$ ).

Seafood advisories for Hg, POPs.
Indigenous Arctic diets reliant on marine mammals ⇒ risk of POP exposure.
Farming communities exposed to pesticide drift/run-off.
Ethical duty for sustainable harvesting, pollution control, environmental justice.
Economic costs: fishery collapses, health care, remediation.

International treaties (e.g. Minamata Convention on Hg, Stockholm Convention on POPs).
Marine protected areas (reduce overfishing, habitat damage).
Reforestation & sustainable forestry.
Green chemistry & biodegradable materials to replace persistent toxins & plastics.
Climate-change mitigation: emission cuts, renewable energy, carbon sequestration.
Integrated pest management (IPM) to minimise chemical pesticide use.

Law of Conservation of Mass: pollutants are redistributed, not destroyed.
Second Law of Thermodynamics: energy degradation explains small biomass of apex predators, hence greater pollutant concentration per unit biomass.
Systems thinking: food webs illustrate complex interdependence; disturbance propagates non-linearly (trophic cascades).

Vocabulary: toxic, accumulate, pesticides, environment, mercury.
True/False examples: Harmful substances move through food chains (T); can affect humans (T); some break down inside organisms (T/F context-dependent).
Calculations: Compare concentration differences between trophic levels.
DDT reflection: Explain why owls (top predators) succumb while frogs may survive.
Research extension: full chemical name of DDT = "dichloro-diphenyl-trichloroethane".

Food chains/webs are not only energy pathways but also contaminant conveyors.
Top predators (including humans) face highest toxin loads via biomagnification.
Prevention (source control) is more effective than remediation.
Understanding ecological networks is critical for informed policy, sustainable resource use, and public health protection.