Exercises: Waste and biodiversity

Define what a xenobiotic is and give two examples, explaining why decomposing organisms find it difficult to degrade these substances.

Differentiate between bioaccumulation and biomagnification using a three-level marine food chain of your choice.

List the waste management hierarchy (reduce, reuse, recycle, recover, and dispose) ordered from most to least recommended, and give an everyday example of each level.

Explain the difference between a sanitary landfill and an open dump, and point out two environmental risks of the latter (leachate, gases, fires, proliferation of disease vectors).

Explain what the circular economy is and how it differs from the linear "extract, manufacture, use and throw away" model, giving an example of a product redesigned according to circular criteria.

What is composting? Indicate which waste it accepts and which it does not, explain what biological process occurs in the pile (aerobic decomposition), and what product is obtained and what it is used for.

Define biodiversity and explain its three levels — genetic, species, and ecosystem diversity — giving an example of each.

Explain what an exotic species is and under what conditions it comes to be considered invasive, citing an example of a non-invasive exotic species in Spain and an example of an invasive one.

The diagram shows the concentration of a persistent pesticide in a bay food chain. Calculate the biomagnification factor between water and fish-eating birds (marked with '?'), and explain why apex predators are always the most affected.

Explain what microplastics are, what their two main sources are, and why it is so difficult to remove them from the ocean. Propose two effective measures to reduce their entry into the sea.

Relate loss of biodiversity to loss of ecosystem services using two specific examples (for example pollinators, wetlands, or reefs), and indicate which type of service is degraded in each case.

Federal health agencies (the FDA and EPA) recommend limiting the consumption of tuna, swordfish and orange roughy, especially for pregnant women and children, because of their mercury content. Explain, using the concepts of persistence, bioaccumulation and biomagnification, why mercury concentrates precisely in large predatory fish and not in sardines.

Explain what energy recovery from waste is (incineration with energy recovery) and analyze its advantages (less landfilling, electricity) and its drawbacks (emissions, ashes, disincentive to recycle) compared with prevention and recycling.

The so-called "plastic islands" form at the center of the great ocean gyres. Explain how the currents concentrate floating waste there, why plastic does not disappear but fragments into microplastics, and what difficulties its large-scale cleanup faces.

The current rate of species extinction is estimated to be tens or hundreds of times higher than the background rate of the fossil record. Explain what the background extinction rate is, name the major causes of the current crisis (habitat destruction, overexploitation, invasive species, pollution, climate change) and discuss why there is talk of a possible sixth mass extinction.

Explain what ecodesign is and describe two examples of products redesigned to generate less waste (repairable appliances with spare parts, single-material packaging, refillable products), indicating which principle of the waste hierarchy they apply.

A landfill closed 15 years ago on fractured karst limestone lacks a basal geomembrane. Located 800 m downslope is a spring that supplies a town and feeds a protected wetland. Analysis of the spring water shows: electrical conductivity of 2400 µS/cm (natural baseline value: 450 µS/cm), ammonium at 12 mg/L (legal limit: 0.5 mg/L), traces of benzene, and a drop in dissolved oxygen to 1.8 mg/L in the wetland.

1. a) Justify hydrogeologically why the lithology of the substrate accelerates the impact, and predict the consequences of this chemical cocktail on the wetland's food web.
2. b) A city councilmember proposes "pumping clean water into the landfill to dilute the groundwater contamination". Refute his proposal technically, demonstrating what hydrogeological risks it would trigger.
3. c) Design an urgent three-phase action plan (immediate hydraulic containment, leachate sealing/collection, and bioremediation), justifying the chronological order of each measure to prevent the collapse of the ecosystem.

In a hydroelectric reservoir, a zebra mussel (Dreissena polymorpha) larval density of 45 000 individuals/m² is detected. In parallel, phytoplankton concentration has dropped by 80%, Secchi disk transparency has increased from 1.2 m to 5.8 m, and a massive proliferation of submerged macrophytes and toxic benthic cyanobacteria is observed.

1. a) Interpret the ecological cascade: why is a drastic increase in water clarity not a sign of environmental improvement, but rather of severe imbalance? Relate it to the reproductive (r/K) and trophic strategies of the bivalve compared to native naiads.
2. b) A company proposes introducing Asian black carp (Mylopharyngodon piceus), a natural predator of the mussel in its native basin, as a biocontrol method. Critically evaluate this proposal, predicting its consequences according to the principles of invasion ecology.
3. c) Design a combined control strategy for a water intake infrastructure that minimizes chemical impact (avoiding continuous massive chlorination) on the downstream river stretch.

Following a diesel spill in a suburban sandy soil, three in situ bioremediation plots are tested over 90 days, monitoring the concentration of total petroleum hydrocarbons (TPH) starting from an initial value of 15 000 mg/kg:

• Plot A (natural attenuation, no intervention): final TPH = 13 800 mg/kg; degrading bacterial population = 10⁴ CFU/g.

• Plot B (biostimulation with the addition of N and P at a C:N:P ratio of 100:10:1 and periodic tilling for aeration): final TPH = 1200 mg/kg; degrading population = 10⁸ CFU/g.

• Plot C (bioaugmentation with a commercial inoculum of allochthonous degrading bacteria, without nutrient correction or tilling): final TPH = 11 500 mg/kg; introduced bacteria drop to undetectable levels after 20 days.

1. a) Interpret the results by comparing B and C: justify which factor was limiting biodegradation in the original soil and identify why bioaugmentation failed.
2. b) Reasonably predict what would have happened in Plot B if the soil had become waterlogged due to continuous torrential rains, indicating which microbial metabolic pathways would be affected.
3. c) If the hydrocarbons had migrated to the saturated zone (a deep anoxic aquifer), justify why physicochemical methods (such as multi-phase extraction or permeable reactive barriers) would compete advantageously against standard aerobic biostimulation.

A ravine receives discharges from tanneries containing hexavalent chromium [Cr(VI)]. Ecotoxicological analyses reveal:

• Surface water: total Cr = 0.08 mg/L (virtually all in the form of Cr(VI)).

• Surface sediment (top 10 cm): total Cr = 1450 mg/kg.

• Benthic macroinvertebrates (mayfly larvae): Cr = 320 mg/kg dry weight.

• Benthic fish (barbels, muscle): Cr = 45 mg/kg; nektonic carnivorous fish (trout, muscle): Cr = 18 mg/kg.

1. a) Interpret the distribution pattern: justify why the concentration in the sediment exceeds that of the water by four orders of magnitude. Analyze the data for barbels and trout: are we observing a bioaccumulation or a trophic biomagnification process? Contrast this behavior with that of organochlorine pollutants such as DDT.
2. b) A technical report suggests "applying phytoremediation by planting willows directly in the ravine bed". Discuss the feasibility and the risks of transferring the pollutant to terrestrial food webs via leaf litter.
3. c) Propose a technical remediation sequence that combines an initial reducing chemical treatment (Cr(VI) to Cr(III)) with subsequent phytostabilization, justifying the change in solubility and bioavailability between both ionic forms.

An ecotoxicological trial evaluates the impact of the neonicotinoid insecticide imidacloprid on foraging honey bee colonies (Apis mellifera):

• Median lethal concentration (48-h LC50 in the laboratory): 40 ng/bee.

• Mean concentration detected in pollen and nectar of treated crops: 4 ng/g.

• In field trials at doses of 1.5 ng/g: the rate of return to the hive after 1-km flights decreases by 62% and the frequency of the "waggle dance" drops by 75%.

• After 6 months, hives in the treated agricultural environment suffer a 40% loss of their brood mass and a drastic increase in the opportunistic parasite Nosema ceranae.

1. a) Interpret these data: demonstrate why using the LC50 as the sole environmental safety indicator masks population collapse caused by sublethal effects, and how this relates to immunosuppression in the colony.
2. b) Analyze the conflict of interests: a farmer argues that replacing this preventive seed treatment with conventional broad-spectrum aerial spraying increases production costs and kills insects through direct contact. What are the medium-term consequences of each model on pest resistance and the loss of wild pollinators?
3. c) Design an Integrated Pest Management (IPM) program for that agricultural region that dispenses with preventive chemical treatment, incorporating economic injury levels and ecological infrastructure measures (hedgerows and reservoir floral strips).

Four years after the natural recolonization of the Iberian wolf (Canis lupus) in a Cantabrian mountain district, a scientific team gathers the following quantitative data:

• The density of red deer and roe deer has decreased by 22%, but the time spent by ungulates in riverbanks and open clearings has fallen by 70% ("landscape of fear" effect).

• The regeneration of willows, birches, and yews along riverbanks has increased by 55%, reducing summer river turbidity by 40% and doubling the diversity of aquatic macroinvertebrates.

• Wild boar damage to peri-urban corn crops has decreased by 35% due to changes in their nocturnal foraging routes.

• Concurrently, recorded attacks on extensive sheep farms without guarding have increased from 2 to 31 per year.

1. a) Explain the concept of a top-down trophic cascade using these data, distinguishing the direct predatory effect from the indirect behavioral effect on river morphology and biodiversity.
2. b) Refute both the stance of those who advocate for the "total eradication of the predator to save the rural environment" and those who promote "strict protection without management or compensation", using arguments from ecosystem ecology and rural socioeconomics.
3. c) Design a coexistence proposal based on predator behavior (use of livestock guardian dogs trained with the flock of sheep, electrified night enclosures, and birthing synchronization) and biologically justify why non-selective hunting or poison usually disrupts wolf family groups and paradoxically increases attacks on domestic livestock.