Waste and Biodiversity
A pesticide that is barely detectable in the water of a bay can build up, level after level, until it reaches dangerous concentrations in the birds that feed on the fish of that same bay. Substances that flow through food webs can become a problem, and the same is true of the waste that reaches the sea and of the species we carry to places where they do not belong. On this page you will see how that imbalance happens.
Xenobiotics, bioaccumulation, and biomagnification
In an estuary, the concentration of DDT (an insecticide that was used in Mexico in campaigns against malaria) was measured at several levels: water ($0.00005$ ppm), zooplankton ($0.04$ ppm), small fish ($0.5$ ppm), and fish-eating birds ($25$ ppm). Why does a contaminant that is almost undetectable in the water reach harmful levels in the top predators?
The reason is that each predator eats many individuals from the level below, and the substance is neither broken down nor eliminated: it stays in fatty tissues. Since at each level only a small fraction of the matter is converted into biomass of the next, the same amount of contaminant is spread over less biomass, and its concentration rises.
Define what a xenobiotic is and give two examples, explaining why decomposing organisms find it difficult to degrade these substances.
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.
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…
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.
Waste in the ocean: plastics and microplastics
A PET bottle abandoned in a river in Oaxaca can end up in the Gulf of Mexico or the Pacific. What happens to plastic when it reaches the sea?
Since removing microplastics from the sea is almost impossible, the effective approach is to keep them from getting there. Two highly effective measures:
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At the source: use filters on washing machines to catch synthetic fibers, prefer clothing made of natural fibers, and avoid cosmetics with plastic microbeads.
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On land: improve waste sorting and collection (organic/inorganic), reduce single-use plastics, clean up rivers and ravines, and avoid open-air dumps, because rivers carry the trash to the sea.
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.
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 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…
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.
Biodiversity, invasive species, and keystone species
On the coasts of the Mexican Caribbean, the lionfish (native to the Indo-Pacific) eats juvenile fish on the reefs and has almost no predators. By contrast, in the fields of Jalisco, the lesser long-nosed bat pollinates the agave: if it disappears, many plants stop reproducing. Why can some species transform an entire ecosystem?
What makes a species invasive
For an alien species to become invasive, it usually has several characteristics: it reproduces quickly and in large numbers, tolerates a wide range of conditions, eats almost anything, and arrives in a place without predators or parasites to keep it in check. In that case it competes with native species for food and space, and displaces them.
Two examples that are mentioned in Mexico:
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Zebra mussel (freshwater mollusk): it reproduces very quickly, attaches in masses to rocks, pipes, and boat hulls, and filters plankton from the water. In this way it takes food away from native mollusks and fish and clogs water intakes at dams and treatment plants.
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Tiger mosquito (Aedes albopictus): it lays hardy eggs in any puddle or container with water, tolerates different climates, and competes with other mosquitoes. It can also transmit dengue and other viruses, so it affects people's health as well.
How they are controlled
The cheapest and most effective control is prevention. Next come early detection and, if the species has already become established, direct control:
| Measure | What it does | Example |
|---|---|---|
| Prevention | Keep it from arriving | Clean and dry boats and equipment before moving from one dam to another; inspect vessels and cargo |
| Early detection | Find it when there are only a few individuals | Monitoring at dams, ovitraps for mosquito eggs, citizen reports |
| Physical control | Remove it by hand or with barriers | Scrape mussels off pipes; get rid of containers and tires holding standing water |
| Chemical control | Kill with substances | Larvicides and insecticides; they must be used carefully because they can harm native species |
| Biological control | Use a natural enemy | Fish that eat mosquito larvae; it must be studied beforehand, because the introduced enemy could become another invasive species |
Ecosystem services are the benefits that nature provides to people. There are provisioning services (food, water, wood), regulating services (pollination, flood control, climate), cultural services (recreation, identity), and supporting services (nutrient cycles, soil formation). When biodiversity is lost, those services degrade: without pollinators, the regulating service (pollination) fails and fruit and seed production falls; without mangroves, protection against hurricanes is lost and the regulating service degrades.
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…
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 zebra mussel and tiger mosquito are invasive alien species in Mexico. Explain what characteristics make a species invasive, how these two damage biodiversity and…
The zebra mussel and tiger mosquito are invasive alien species in Mexico. Explain what characteristics make a species invasive, how these two damage biodiversity and ecosystems, and what control measures are applied against them.
The Earth's spheres and the biosphere
The rain that falls on Popocatépetl, the smoke from a truck, and the soil of a milpa seem like separate things. How are they connected?
The Earth can be studied as four systems that exchange matter and energy:
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Geosphere: rocks, soil, and the solid interior of the planet.
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Hydrosphere: all the water (oceans, rivers, lakes, ice, aquifers, and vapor).
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Atmosphere: the layer of gases (N₂, O₂, CO₂, water vapor) that regulates temperature and climate.
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Biosphere: all living things and the places where they live.
The atmosphere and the hydrosphere are fluid layers: they move and carry energy and substances very far. They surround the geosphere and make life possible.
Following a diesel spill in a peri-urban sandy soil, three in situ bioremediation plots are tested over 90 days, monitoring the concentration of total petroleum…
Following a diesel spill in a peri-urban 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:
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Plot A (natural attenuation, no intervention): final TPH = 13 800 mg/kg; degrading bacterial population = 10⁴ CFU/g.
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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.
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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.
- a) Interpret the results by comparing B and C: justify which factor was limiting biodegradation in the original soil and identify why bioaugmentation failed.
- 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.
- 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.
One Health and water quality
Spills, diseases that pass from animals to people, and antibiotic resistance show that human health depends on the health of ecosystems. To decide between two waste-management proposals, it helps to compare the volume avoided, the risk of contamination, and the cost, distinguishing measured data from estimates.
When a landfill or a dump contaminates water, it is detected by measuring indicators:
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Electrical conductivity (µS/cm): pure water conducts little; dissolved salts (chlorides, sulfates, ammonium, metals) make it conduct more. It is compared with the baseline value of the site, that is, the one the water had before it was contaminated. If the baseline is 450 µS/cm and it now reads 2,400, the water has about 5 times more dissolved salts: a sign of contamination.
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Ammonium (mg/L): it comes from the decomposition of organic matter and from leachate. It is compared with the limit set by the regulation. If the limit is 0.5 mg/L and the water has 12 mg/L, the limit is exceeded 24 times.
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Dissolved oxygen (mg/L): the bacteria that break down ammonium and organic matter consume oxygen. If it drops to 1.8 mg/L, many fish and invertebrates cannot survive (in general they need several mg/L).
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…
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.
- 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.
- b) A town councillor proposes "pumping clean water into the landfill to dilute the groundwater contamination". Refute his proposal technically, demonstrating what hydrogeological risks it would trigger.
- 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.
Go back to the beginning: the pesticide that is almost undetectable in the water becomes dangerous for the birds because it does not break down and becomes concentrated in each link of the food chain.