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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.