Concept of an Ecosystem

Lessons

Senior 6 · Biology

Concept of an Ecosystem

5 lessons

Lesson 5 of 5

Human impact, pollution and conservation

25 min

A marsh that was drained, and saved

The Rugezi wetland, about 6 700 hectares of peat marsh at around 2 050 m in the north of Rwanda, filters and regulates the water that flows into Lakes Burera and Ruhondo, and so supports hydroelectric power generation downstream. In past decades, parts of it were drained for farming and for water management. The marsh's water table dropped and the levels of the lakes below it fell. Recognising its value, Rwanda listed Rugezi–Burera–Ruhondo as its first Wetland of International Importance under the Ramsar Convention on 1 December 2005 and set out to restore it.

Rugezi shows the three themes of this lesson: people need land and food from ecosystems; using them carelessly damages the services they provide; and damaged ecosystems can be protected and restored.

Definition

Pollution: the release by human activity of substances or energy into the environment in amounts that harm living things or the services ecosystems provide. The harmful substance is a pollutant.

Ecosystem services: the benefits people get from ecosystems, such as food, clean water, flood control, pollination and carbon storage.

In-situ conservation: protecting species in their natural habitat, such as in a national park. Ex-situ conservation: protecting species outside their natural habitat, such as in a zoo, botanic garden or seed bank.

Biochemical oxygen demand (BOD): the mass of oxygen used by microorganisms to break down the organic matter in one litre of water, usually over 5 days at $20\,^\circ\mathrm{C}$ . Unit: $\mathrm{mg\,dm^{-3}}$ . A high BOD means much organic pollution.

Agriculture and food supply

Rwanda's population depends heavily on farming, on a hilly land with little room to expand. Every method that increases food supply also changes ecosystems. Table 1 sets out the main effects and how they can be reduced.

Table 1: Farming practices, their ecological effects and mitigation

Practice	Ecological effect	Mitigation
Clearing forest, draining wetlands for new fields	Habitat loss, the main cause of falling biodiversity; less water storage and carbon storage	Raise yields on existing farmland instead; protect forests and wetlands by law
Monoculture (one crop over large areas)	Low diversity; pests and diseases spread easily	Crop rotation, intercropping, agroforestry
Inorganic fertiliser	Runoff of nitrate and phosphate causes eutrophication	Apply the right amount at the right time; leave buffer strips of vegetation beside rivers
Pesticides	Kill non-target species such as bees; persistent ones biomagnify (Lesson 3)	Integrated pest management, biological control, less persistent chemicals
Cultivating steep slopes	Soil erosion: fertile topsoil and its phosphate are washed into rivers and lakes	Terraces, contour hedges of grasses such as napier, mulching, trees on the slopes

Pollution and its effects

Organic pollution and oxygen. Untreated sewage and waste from food factories such as coffee washing stations add organic matter to rivers. Aerobic bacteria break it down and multiply, using dissolved oxygen faster than it can dissolve back from the air. Figure 1 shows the result below a sewage outfall: the oxygen sag curve.

Figure 1: The oxygen sag curve below a sewage outfall (illustrative, modelled). Blue: dissolved oxygen. Orange, dashed: BOD. Green band: clean water upstream.

Eutrophication. When nitrate and phosphate from fertiliser, eroded soil or sewage enter a lake, a chain of events follows:

The extra nutrients remove a limiting factor, so algae and cyanobacteria grow rapidly: an algal bloom covers the surface.
The bloom shades the water below, so submerged plants cannot photosynthesise and die.
Algae are short-lived; dead algae and plants sink and are decomposed by aerobic bacteria, whose populations grow.
The bacteria use up the dissolved oxygen: the BOD rises.
Fish and other aerobic animals die from lack of oxygen, adding still more dead matter. Some cyanobacteria also release toxins.

Other pollutants. Heavy metals such as mercury and lead from mining and waste are toxic and can biomagnify. Carbon dioxide and methane from fuel burning, cattle and rice fields enhance the greenhouse effect (Lesson 4). Plastics persist for centuries, block drains and break into microplastics that animals swallow.

Rwanda's plastics law. Rwanda banned polythene bags under Law N° 57 of 2008. Law N° 17 of 2019, in force from 23 September 2019, extends the ban to single-use plastic items, with exceptional permission handled by the Rwanda Environment Management Authority (REMA).

Worked example 1: reading the oxygen sag curve

Use Figure 1. (a) State the minimum dissolved oxygen concentration and where it occurs. (b) Explain why it occurs downstream of the outfall rather than at it. (c) Explain why the animals change along the river.

(a) About $1.5\,\mathrm{mg\,dm^{-3}}$ , about 28 km downstream.
(b) At the outfall the bacteria have only just met the sewage. As the water flows on, the bacteria multiply and respire, using oxygen faster than it dissolves from the air. The oxygen reaches its lowest point where the rate of use falls to equal the rate at which oxygen dissolves back in. Beyond that, as the organic matter (BOD) runs out, re-aeration wins and the oxygen recovers.
(c) Stonefly and mayfly nymphs need water rich in oxygen, so they live only upstream: they are indicator species of clean water. Tubifex worms and red chironomid larvae tolerate low oxygen (chironomid larvae have haemoglobin with a high affinity for oxygen), so they dominate the sag, where they have little competition and much food.

Warning

Common mistakes about eutrophication

"The algae use up the oxygen." Living algae release oxygen by day. It is the aerobic bacteria decomposing the dead algae that use it up.
"Fertiliser poisons the fish." Nitrate and phosphate at these levels are not the direct cause; fish die from lack of oxygen.

Conservation: in situ and ex situ

Table 2: Comparing the two approaches

In-situ conservation	Ex-situ conservation
Examples: national parks (Volcanoes, Nyungwe, Akagera, Gishwati–Mukura); protected wetlands such as Rugezi; forest reserves	Examples: zoos with captive breeding; botanic gardens and arboreta; seed banks; frozen sperm, eggs and embryos
Advantages: protects the whole ecosystem and its interactions; species keep adapting to their natural conditions; many species are protected at once	Advantages: safe from poaching, habitat loss and disease outbreaks; allows breeding programmes; seeds keep for decades in a small space
Disadvantages: needs large areas and constant protection against poaching and encroachment; may conflict with local land needs	Disadvantages: only a few individuals, so little genetic diversity; animals may lose natural behaviour; expensive; does not protect the habitat

Case study: bringing species back to Akagera. Akagera National Park is managed jointly by the Rwanda Development Board and the conservation organisation African Parks. Lions had been lost from the park; they were reintroduced from South Africa from July 2015. Eastern black rhinoceroses, absent for about ten years, returned in May 2017, when 18 were moved from South Africa, and in June 2019 five more arrived from zoos in Europe. White rhinoceroses followed in 2021 and 2025. The 2019 rhinos show the two approaches working together: animals bred ex situ in zoos were released in situ, adding new genes to the wild population. Such reintroductions succeed only because the park is fenced, patrolled and supported by the communities around it, who share in the park's revenue.

Worked example 2: why small populations lose genetic diversity

In a small population, some alleles are lost by chance each generation (genetic drift). A standard population-genetics result gives the expected heterozygosity (a measure of genetic diversity) after $t$ generations:

H_t = H_0 \left( 1 - \frac{1}{2N_e} \right)^t

where $N_{e}$ is the effective population size (roughly, the number of breeding adults). Compare a captive group with $N_{e} = 10$ and a wild population with $N_{e} = 50$ , each over 10 generations.

$N_{e} = 10$ : $\frac{H_{10}}{H_0} = \left( 1 - \frac{1}{20} \right)^{10} = 0.95^{10} = 0.599$ . About 40 % of the genetic diversity is lost.
$N_{e} = 50$ : $\frac{H_{10}}{H_0} = \left( 1 - \frac{1}{100} \right)^{10} = 0.99^{10} = 0.904$ . About 10 % is lost.
Evaluation: small captive or reintroduced groups lose diversity fast, which raises inbreeding and lowers the ability to adapt to disease or climate change. That is why conservationists bring animals from several source populations, as with Akagera's rhinos from South Africa and from European zoos, and swap animals between zoos using stud books.

Condition: the formula assumes random mating, no migration, no mutation and no selection, so it is a model, not a prediction.

Two rhinoceroses crossing a track between grass and bush in Rwanda. All of Rwanda's wild rhinos live in Akagera National Park, where they were reintroduced from 2017 onwards. Photo: Nkunda102, CC0, via Wikimedia Commons

CrashCourse: "Conservation and Restoration Ecology" (Crash Course Ecology #12). Watch for the difference between protecting an ecosystem and restoring one, and compare the examples with Rugezi and Akagera.

Tip

Exam technique for "evaluate" questions on conservation. Give points on both sides, then a judgement with a reason: "Ex-situ breeding saves the species from immediate extinction, but it cannot replace habitat; the best strategy combines both, as at Akagera." A one-sided answer, however detailed, cannot reach the top marks.

✦

Practice: human impact and conservation

Define biochemical oxygen demand, and state whether clean river water has a high or a low BOD.
Put these stages of eutrophication in order: fish die; algal bloom; nitrate and phosphate enter the lake; bacteria decompose dead algae; submerged plants die from lack of light; dissolved oxygen falls.
Give two farming practices that reduce soil erosion on Rwanda's hills, and explain how one of them works.
Using Figure 1, estimate the BOD 40 km downstream, and explain why BOD falls with distance.
A captive group has $N_{e} = 8$ . Calculate the fraction of its heterozygosity expected to remain after 5 generations.
Give two advantages and two disadvantages of a seed bank as a way of conserving plants.
Explain how the 2019 transfer of rhinos from European zoos to Akagera combined ex-situ and in-situ conservation.
Evaluate the view that "banning plastic bags is enough to solve Rwanda's waste pollution."

Show answersHide answers

The mass of oxygen used by microorganisms to break down the organic matter in one litre of water over 5 days at $20\,^\circ\mathrm{C}$ . Clean water has a low BOD.
Nitrate and phosphate enter the lake → algal bloom → submerged plants die from lack of light → bacteria decompose dead algae → dissolved oxygen falls → fish die.
Any two of: terracing, contour hedges, mulching, agroforestry, cover crops. Terraces turn a slope into level steps, so rain soaks in instead of running downhill and carrying soil with it.
About $10\,\mathrm{mg\,dm^{-3}}$ (from the curve: 9.8). BOD falls because bacteria break down the organic matter as the water flows, so less remains to be decomposed.
$\left( 1 - \frac{1}{16} \right)^5 = 0.9375^5 = 0.724$ : about 72 % remains.
Advantages: seeds of many species and varieties stored in a small space; they stay viable for decades when dried and cooled; safe from habitat loss and disease. Disadvantages: some seeds (for example of many tropical trees) cannot survive drying; stored seeds do not evolve with changing conditions; seeds must be regrown and tested regularly; does not protect habitats or pollinators.
The rhinos were bred ex situ, in European zoos, protected from poaching; they were then released in situ, into the protected wild habitat of Akagera, where they add new genetic diversity to the reintroduced population.
The ban removes a major, persistent pollutant that blocked drains and harmed livestock, so it is valuable. But it is not enough: other wastes, such as sewage, food waste, other plastics, electronic waste and farm runoff, need collection, treatment, recycling and good farming practice. A judgement: the ban is an important part of the solution, not the whole of it.

Key points

Farming feeds people but causes habitat loss, eutrophication, pesticide harm and erosion; each has practical mitigation.
Organic pollution raises BOD and creates an oxygen sag; nutrient pollution causes eutrophication, where bacteria decomposing dead algae use up the oxygen.
In-situ conservation protects whole ecosystems; ex-situ conservation protects species from immediate threats; the best programmes combine both.
Small populations lose genetic diversity: $H_t = H_0 \left( 1 - \frac{1}{2N_e} \right)^t$ .

1A marsh that was drained, and saved

2Agriculture and food supply

3Pollution and its effects

4Conservation: in situ and ex situ

A marsh that was drained, and saved

Agriculture and food supply

Pollution and its effects

Conservation: in situ and ex situ