Natural resources and their conservation – Population and Natural Resources (Senior 6 Biology) | Ganzaa

Population and Natural Resources

Lessons

Senior 6 · Biology

Population and Natural Resources

4 lessons

Lesson 4 of 4

Natural resources and their conservation

25 min

What is on the truck?

On many rural roads in Rwanda you will meet lorries piled high with sacks of charcoal, heading for the towns. In the 2022 census, 76 % of Rwanda's households cooked mainly with firewood; in urban areas, half used charcoal (NISR). Every sack began as a tree. Whether that is a problem depends on one question: are trees being grown as fast as they are being cut? That question, applied to every resource, is the subject of this lesson.

Definition

Natural resource: any material or energy source from nature that people use: land, soil, water, forests, wildlife, minerals, fossil fuels, sunlight and wind.

Renewable resource: a resource that is replaced by natural processes within a human lifetime, so it can be used indefinitely if it is not used faster than it is replaced. Examples: forests, fish, fresh water, soil (very slowly).

Non-renewable resource: a resource that exists in a fixed amount, or forms over millions of years, so using it uses it up. Examples: tin and tungsten ores, gold, peat, natural gas.

Sustainable use: using a resource in a way that meets today's needs without reducing its availability for future generations; for a renewable resource, taking no more than it can replace.

Table 1: Rwanda's natural resources

Resource	Renewable?	Main uses	Main threats
Forests and trees	Yes, if replanted	Firewood, charcoal, timber, soil and water protection, carbon storage	Cutting faster than regrowth; clearing for farms
Fresh water	Yes, through the water cycle	Drinking, irrigation, hydropower, industry	Rising demand, pollution, sediment, loss of wetlands and forest cover
Land and soil	Soil: only very slowly	Farming, housing, infrastructure	Erosion on steep slopes, rising population density
Wildlife and fisheries	Yes, if not over-harvested	Tourism, food (tilapia, isambaza sardines)	Poaching, over-fishing, habitat loss
Minerals: tin, tantalum (coltan), tungsten, gold	No	Exports for electronics and industry	Depletion; erosion and silt in rivers from mines
Energy: methane in Lake Kivu, peat, sunshine	Methane and peat: no; sunshine: yes	Electricity, cooking	Depletion of non-renewable stocks

Forests

In 2019 the Ministry of Environment reported that forests covered 30.4 % of Rwanda's land, 724 666 hectares, meeting the national target of 30 %. Figure 1 shows what that cover is made of.

Figure 1: The make-up of Rwanda's forest cover in 2019. Data: Ministry of Environment (2019). Hectares calculated by Ganzaa from the percentages.

Most of the forest cover is plantation, much of it eucalyptus and pine planted for wood, while natural forest, such as Nyungwe, Gishwati and Mukura, is under a fifth. The two are not equal: a plantation supplies wood and protects soil, but a natural forest holds far more species (Unit 1). Planting trees can therefore raise forest cover without restoring biodiversity, which is why protected natural forest must be kept separate from the woodlots that supply fuel.

Worked example 1: how big is the natural forest?

Using the Ministry of Environment's 2019 figures (forest cover 724 666 ha, 30.4 % of the land; 19 % of it natural forest), calculate (a) the area of natural forest, (b) the percentage of Rwanda's whole land area that is natural forest, and (c) the land area of Rwanda that these figures imply.

(a) $0.19 \times 724\,666 = 137\,687\,\mathrm{ha} \approx 138\,000\,\mathrm{ha}$ .
(b) $0.19 \times 30.4\,\% = 5.8\,\%$ of the land.
(c) $\frac{724\,666}{0.304} = 2\,383\,770\,\mathrm{ha} \approx 2.38 \times 10^6\,\mathrm{ha}$ , or about $23\,800\,\mathrm{km^2}$ (since $1\,\mathrm{km^2} = 100\,\mathrm{ha}$ ). This is less than Rwanda's total area of about $26\,300\,\mathrm{km^2}$ . Lakes and rivers (about $1\,700\,\mathrm{km^2}$ ) are not land, and they explain most of the gap. The rest is too large to be rounding: it shows that the 2019 assessment measured its percentage against a slightly different land total. Always check what a percentage is a percentage of.

A lorry piled high with sacks of charcoal on a dirt road in Gisovu, western Rwanda, between rows of tall eucalyptus trees. Charcoal is a renewable fuel only if the trees it comes from are replanted as fast as they are cut. Photo: JEsa, CC BY-SA 4.0, via Wikimedia Commons

Water

Rwanda receives plenty of rain, but it also has many people. The Rwanda Water Resources Board's National Water Resources Master Plan puts the long-term average renewable water available at about 670 m³ per person per year. Figure 2 compares that with the Falkenmark indicator: below 1 700 m³ per person per year a country faces water stress, and below 1 000 m³ water scarcity.

Figure 2: Water per person in Rwanda against the Falkenmark thresholds. Data: Rwanda Water Resources Board, National Water Resources Master Plan (long-term average).

Because the water supply is roughly fixed while the population grows, water per person falls every year. Protecting it means protecting the sources: forests and wetlands such as Rugezi that let rain soak in and release it slowly, as well as harvesting rainwater, irrigating efficiently and treating wastewater.

Worked example 2: water per person in 2052

Assume, for this calculation, that the 670 m³ per person applies to the 2022 population of 13 246 394, and that the total renewable water stays the same. Estimate the water per person for NISR's projected population of 23.6 million in 2052, and classify it.

Total renewable water $= 670 \times 13\,246\,394 = 8.88 \times 10^9\,\mathrm{m^3\,yr^{-1}}$ (3 s.f.).
Per person in 2052 $= \frac{8.875 \times 10^9}{23.6 \times 10^6} = 376\,\mathrm{m^3}$ per person per year (3 s.f.).
That is a fall of $\frac{670 - 376}{670} \times 100 = 44\,\%$ , taking Rwanda even further below the scarcity line of 1 000 m³.

Evaluate the assumptions: the 670 m³ figure is a long-term average and its base year may differ; rainfall may change with the climate; and water can be stored and reused. The calculation shows the direction of the problem, not an exact forecast.

Land, soil and minerals

Land is Rwanda's scarcest resource. Population density was 503 people per km² in 2022 and NISR's medium projection is about 894 per km² by 2052. More food must come from the same land, so the answers are higher yields rather than more fields: improved seed, fertiliser and manure, irrigation, terraces and agroforestry to hold the soil, and consolidating small plots so they can be farmed efficiently. Soil forms at a rate of millimetres per century, so soil washed off a hillside is, in practice, a non-renewable loss. The aerial photo below shows where much of it goes.

Minerals. Rwanda mines ores of tin (cassiterite), tantalum (coltan) and tungsten (wolframite), and gold. These are non-renewable: each tonne mined is gone. Conserving them means recycling metals, using them efficiently and making sure that mining does not destroy other resources. In October 2012, for example, the Ministry of Natural Resources suspended mining in part of the Western Province because the mines were endangering the Sebeya River.

Conserving natural resources

Harvest within the rate of renewal. For a renewable resource, keep the harvest below the growth rate. The logistic model of Lesson 2 shows that a population grows fastest at half its carrying capacity: this is the idea behind the maximum sustainable yield for fisheries and woodlots.
Reduce demand. Improved cookstoves use less wood; LPG, biogas and electricity replace wood and charcoal; efficient irrigation uses less water.
Reduce, reuse and recycle materials, especially metals and plastics.
Protect the sources: national parks, protected wetlands, buffer zones along rivers and lakes, and laws such as the ban on single-use plastics (Unit 1).
Restore what has been lost: tree planting (Rwanda holds a national tree planting season each year), terracing and the rehabilitation of wetlands and old mines.
Involve communities, who depend most on resources and whose support decides whether rules work.

Worked example 3: a sustainable woodlot

A cooperative owns a 3.0 ha woodlot. The trees add $18\,\mathrm{m^3}$ of wood per hectare per year (illustrative). Its members use $70\,\mathrm{m^3}$ of wood a year. (a) Is this sustainable? (b) After switching to improved cookstoves that use 40 % less wood, is it sustainable?

(a) Annual growth $= 3.0 \times 18 = 54\,\mathrm{m^3\,yr^{-1}}$ . Use, $70\,\mathrm{m^3}$ , is greater than growth, so the woodlot is shrinking by $16\,\mathrm{m^3}$ a year: not sustainable.
(b) New use $= 70 \times (1 - 0.40) = 42\,\mathrm{m^3\,yr^{-1}}$ , which is less than $54$ : sustainable, with $12\,\mathrm{m^3}$ a year to spare.

Reducing demand can turn an unsustainable harvest into a sustainable one without planting a single extra tree.

Warning

Common mistakes

"Renewable means unlimited." A renewable resource is used up if it is harvested faster than it renews: forests, fish and groundwater can all be exhausted.
"More forest cover means more biodiversity." A eucalyptus plantation raises forest cover but supports far fewer species than a natural forest.
Quoting a figure without its year. Resource statistics change; always give the year and the source, as in "30.4 % forest cover (Ministry of Environment, 2019)".

CrashCourse: "Human Population Growth" (Crash Course Ecology #3). Watch for how people have raised the Earth's carrying capacity for humans, and for the idea of an ecological footprint.

Tip

Exam technique for "suggest how this resource could be conserved". Give measures from different levels: the individual (use an improved stove), the community (plant a woodlot, protect a spring) and the nation (laws, protected areas, alternative energy). Link each to the resource: "Improved stoves reduce the wood burned, so trees are cut more slowly than they regrow."

✦

Practice: natural resources

Classify as renewable or non-renewable, and give a reason: (a) coltan, (b) tilapia in Lake Muhazi, (c) methane dissolved in Lake Kivu, (d) topsoil.
Explain why a renewable resource can still run out.
Using the 2019 figures, calculate the area of forest plantation in Rwanda, to 3 significant figures.
Explain why raising forest cover with plantations does not by itself conserve biodiversity.
Explain why water per person in Rwanda falls each year even if rainfall stays the same.
A fishery's fish population has a carrying capacity of 40 000 fish. At what population size will the growth rate be greatest, and why should managers aim to keep it there?
A school uses $25\,\mathrm{m^3}$ of firewood a year. Its woodlot grows $15\,\mathrm{m^3}$ a year. By what percentage must the school cut its wood use to make it sustainable?
Evaluate a proposal to drain a wetland to create farmland for a growing population.

Show answersHide answers

(a) Non-renewable: a mineral ore formed over geological time. (b) Renewable: the fish reproduce, if not over-fished. (c) Non-renewable on a human timescale: it accumulated over a very long time. (d) Effectively non-renewable: it forms at millimetres per century, far slower than it can erode.
If it is used faster than it is replaced (cutting faster than regrowth, fishing faster than breeding), the stock shrinks and can be exhausted.
$0.53 \times 724\,666 = 384\,073 \approx 384\,000\,\mathrm{ha}$ .
Plantations are usually one or two species (eucalyptus, pine) planted in rows; they support far fewer species than natural forest, which has many tree species, layers and niches. Biodiversity is conserved only by protecting and restoring natural forest.
The total renewable water is roughly fixed, but the population grows (by about 2.3 % a year from 2012 to 2022), so the same water is shared among more people.
At $\frac{K}{2} = 20\,000$ fish, where $\frac{dN}{dt}$ is greatest; harvesting the extra fish produced each year at this size gives the largest catch that can be taken year after year (maximum sustainable yield) without shrinking the population.
It must reduce use to $15\,\mathrm{m^3}$ : a cut of $\frac{25 - 15}{25} \times 100 = 40\,\%$ .
For: more land to grow food for a denser population. Against: wetlands store and filter water, reduce floods, feed rivers and lakes that supply hydropower, hold carbon (as peat) and shelter many species; Rugezi's drainage lowered the lakes below it. Draining peat also releases CO₂. Judgement: raising yields on existing farmland is usually better than draining a wetland whose services are worth more than the land gained.

Key points

Renewable resources renew within a human lifetime but can be exhausted if overused; non-renewable ones are used up.
Forest cover was 30.4 % of Rwanda's land in 2019, mostly plantations; natural forest is under a fifth of it.
Renewable water is about 670 m³ per person per year (Master Plan average), below the scarcity line, and falls as the population grows.
Conservation: harvest within renewal (maximum sustainable yield), reduce demand, recycle, protect sources and restore damaged land.

1What is on the truck?

2Forests

3Water

4Land, soil and minerals

5Conserving natural resources

What is on the truck?

Forests

Water

Land, soil and minerals

Conserving natural resources