Managing ecosystems sustainably

Sustainable yield and the logistic model

A sustainable yield is a harvest that does not cause a long-term decline in the population under the conditions considered. In the simple logistic model, replacement growth is greatest at about half the carrying capacity, but real carrying capacity and recruitment vary. In practice, managers monitor the stock, set cautious limits and combine several controls, rather than treating the modelled maximum as a safe quota.

Anything harvested from a wild population, whether timber, fish or game, is taken from a population that replaces itself. The sustainable yield is the amount that can be removed in a given period without reducing the size of the population, so that the same amount can be removed again in the next period. A smaller harvest allows the population to grow; a larger one reduces it, and repeated harvests above the sustainable yield reduce it further each period.

The growth curve gives the size at which the sustainable yield is largest. In the simple logistic model, net population growth is zero at carrying capacity, because births and deaths are equal. So a population left at that size produces no surplus to take. A population reduced almost to nothing is not growing much either, because there are too few individuals to reproduce. In between is the steep part of the sigmoid curve, where there are plenty of individuals and resources are not yet limiting. That is where the population adds the most each year: net growth is greatest at about half the carrying capacity.

Harvesting moves a population below its carrying capacity, into that range. A sustainable harvest must not exceed the population's replacement growth at its current size. The half-K maximum applies only under the assumptions of that model.

Describe the drawing

Two graphs side by side. On the left, a cyan curve of the growth a population adds each year rises from nothing at an empty stock, peaks at half the carrying capacity where an amber dot and dashed line mark the largest catch that can be replaced, and falls back to nothing at a full stock. On the right, stock size is plotted against sixty years of fishing: the cyan line, taken at a quota below the peak, dips and then holds steady, while the coral line, taken at a quota only slightly above it, falls all the way to zero.

That is why a fixed quota above the maximum sustainable yield has consequences out of proportion to the excess. In the model, a catch above the peak exceeds the replacement growth available at any population size, so the population falls each year and the same quota takes a larger share of what remains. Managers therefore set limits below the modelled maximum, both for that reason and because the carrying capacity is estimated rather than measured and varies with conditions.

Sustainable yield

The amount that can be harvested from a population in a given period without reducing the size of that population, so the same harvest can be repeated indefinitely.

Maximum sustainable yield

The largest sustainable yield available from a population. In the simple logistic model it occurs at about half the carrying capacity, where net growth is greatest; that position depends on the model's assumptions.

Sustainable management

Using an ecosystem so that its resources are still available to future generations, and the ecosystem itself is not degraded.

Timber management

Four named techniques manage woodland for a repeatable timber yield.

Coppicing is cutting a broadleaved tree down to a stump, called a stool, close to the ground. Hazel, ash, willow, sweet chestnut and lime all respond by sending up many new shoots from the stool, which grow far faster than a seedling would because they are supplied by a root system that is already established. After a rotation of somewhere between seven and twenty years, depending on the species and on what the poles are wanted for, the shoots are cut again. The tree is not killed; coppice stools are among the oldest living things in Britain, and some have been cut on a cycle for many centuries.

Rotational coppicing turns that into a steady supply. The wood is divided into compartments and one compartment is cut each year, so that after a full rotation the first compartment is ready again. The wood as a whole therefore holds the same standing crop every year, the yield is the same every year, and at any moment the wood contains patches at every stage of regrowth.

Describe the drawing

A grid of seven rows and seven columns, one row for each plot of a coppiced wood and one column for each year of the rotation. Every cell holds a small bar whose height is the age of the regrowth in that plot in that year, coral where the plot has just been cut, cyan while it grows and amber in the year it is ready. The coral cells step diagonally across the grid, so exactly one plot is cut each year and the wood as a whole carries the same standing crop every year.

The resulting patchwork has a conservation value alongside the timber. A recently cut compartment is open and sunlit, and the woodland flowers that respond to it, primroses, bluebells and violets, bring in the butterflies that feed on them. An older compartment is shady and suits different species. A wood managed on a rotation therefore holds more habitat diversity, and in surveys of British coppice more species, than a comparable unmanaged or clear-felled wood; the comparison depends on the woodland type and on what is measured. This is deflected succession applied to management, and for that reason conservation bodies fund coppicing in woods where the poles have no market.

Pollarding is the same operation carried out about two metres up, so that the new shoots grow out of reach of deer, cattle and sheep. It is the shape of trees on old parkland and along roadsides, and it is used wherever the wood cannot be fenced against grazing animals.

Selective felling is used for larger timber. Rather than clearing an area, individual mature trees are identified and removed, leaving the canopy and the structure of the forest essentially intact. Compared with clear felling it keeps the habitat, leaves the soil covered so it is not eroded or leached, keeps the forest fixing carbon, and lets the gaps fill from the surrounding trees. It yields less per hectare per visit and costs more per tree extracted, so it is not universal.

Two supporting practices come up in the same questions. Trees are replanted, at least one for each removed and usually more, and planting native species suited to the site matters more than the number. And trees are given room, since a plantation grown too dense produces many thin poles rather than the fewer thick trunks that timber requires, so plantations are thinned as they grow.

Fishery management

A fishery presents four difficulties a wood does not. The stock cannot be observed directly, it moves across international boundaries, and it is a common resource that many parties take from. And fishing effort has increased faster than the stocks are replaced. Several North Atlantic stocks have been fished to collapse, among them the Grand Banks cod fishery off Newfoundland. It was closed in 1992 after the stock fell to about one per cent of its former biomass. The stock has not recovered to its former level, although a small commercial fishery was reopened in 2024.

The management measures are separate controls, each with its own effect and its own limitation.

The measures are complementary rather than alternatives. A mesh size protects the young fish but not the total taken; a quota limits the total taken but says nothing about which fish; a closed season protects spawning but not the rest of the year. Fisheries that have recovered, such as North Sea herring after its 1977 closure or Icelandic cod under strict quota management, have generally used several at once and enforced them.

Aquaculture is offered as the way out, and it now supplies about half the fish people eat. It has real advantages. The stock is enclosed so nothing is caught by accident, and food conversion can be managed. Harvesting does not damage the sea bed as trawling does, and pressure on the wild stock is reduced.

It also has four documented problems, which belong in an evaluation. Carnivorous farmed species such as salmon are fed on fishmeal made from wild-caught fish, so the pressure is moved, not removed. Dense stocking spreads parasites such as sea lice and diseases into wild populations swimming past. Uneaten food and faeces enrich the water beneath the cages, which is eutrophication in miniature. And escaped farmed fish interbreed with wild ones, diluting the local adaptations of a wild stock with alleles selected for a cage.

Distributional effects and governance

Each measure above imposes a present cost in order to protect a future yield, and the distribution of those costs is contested. A quota that saves a stock over twenty years may put a fishing community out of work this year, and a family whose income has gone does not get it back when the stock recovers. The Newfoundland closure put tens of thousands of people out of work in one announcement.

The same conflict occurs in land management, between protected areas and the people whose livelihoods depend on the land. A national park in which grazing is restricted protects a habitat and reduces a herder's income. A logging ban protects a forest and removes the only cash a village has. Rules written by people who do not depend on the resource, and enforced on people who do, are less well complied with. Successful schemes therefore tend to share three features. Local people are involved in setting the rules. They get a share of the benefit, whether from tourism, licensed harvesting or payment for managing the land. And the rules are simple enough to be checked.

Sustainable management therefore combines a biological quantity with an economic decision. The biology gives an estimated sustainable yield, and a stock harvested above it declines regardless of what has been agreed. The decisions are about who takes that yield, over what period the costs are borne, and who compensates those asked to take less in the short term.

Using the carbon cycle to lower carbon dioxide

Atmospheric carbon dioxide rises because the transfers into the atmosphere are larger than the transfers out of it. Every method of reducing it does one of two things to the cycle: it makes a transfer into the air smaller, or it makes a transfer out of the air larger and puts the carbon into a store that keeps it.

Two principles from the cycle decide how much a method achieves. First, a store is only as good as its permanence. Carbon in a fossil fuel left underground is out of the cycle for good. Carbon in a tree is out of the air for as long as the tree and the wood made from it last. Second, a growing store is a sink and a full one is not. A young forest takes up more carbon than it releases, because its biomass and soil carbon are increasing. A mature forest near its climax is close to balance, its photosynthesis largely matched by respiration and decomposition. Its value is as a large store to be kept, not as a continuing sink. Reforestation and biofuels are taken further in the next section.

Reforestation and biofuels

The Edexcel A specification names two examples of managing the conflict between human needs and conservation: reforestation, and the use of sustainable resources including biofuels. A sustainable resource is one used no faster than it is replaced, so that it is still available to later generations. Both examples are attempts to meet a human need, for land, timber or fuel, while reducing the damage that meeting it does.

Reforestation is replanting trees, or allowing them to regrow, on land where forest has been cleared; afforestation is planting forest where there was none in recent times. While the trees grow, photosynthesis exceeds respiration and decomposition across the site, so carbon moves from the air into wood, roots and soil. A forest also holds soil against erosion, slows the run-off of rain and so reduces flooding downstream, and provides habitat. Felling a forest and replanting it in a planned cycle gives a sustainable supply of timber, as the coppice rotation above does on a small scale.

How much a scheme achieves depends on how it is done. A plantation of a single fast-growing species, often a non-native conifer or eucalyptus, fixes carbon quickly and yields timber. But it supports far fewer species than a mixed native forest. Planting it on an existing habitat of value, such as peat bog or species-rich grassland, can cost more habitat than it provides. Planting trees on drained peat can release more carbon from the drying peat than the trees take up.

Native species suited to the site give more biodiversity per hectare, especially when planted to join up existing woodland so that animals can move between fragments. And the carbon is out of the air only for as long as the forest lasts, so a scheme needs protection from fire, felling and grazing for decades.

Reforestation competes with farming for land. Where it is imposed on land that local people farm or graze, it takes away their food or income, and schemes have failed when the trees were cut or grazed by people who gained nothing from them. Successful schemes tend to pay landowners or communities to plant and protect the trees, allow sustainable harvesting of timber or other forest products, or mix trees with crops, an arrangement called agroforestry.

Biofuels are fuels made from recently grown biomass rather than from fossil remains. Bioethanol is made by yeast fermenting the sugars in crops such as sugar cane, maize and wheat, and is blended with petrol; Brazil has used ethanol from sugar cane as a vehicle fuel on a large scale since the 1970s. Biodiesel is made from plant oils such as rapeseed oil and palm oil, or from waste cooking oil. Biogas, mostly methane, is produced by anaerobic digestion of manure, sewage and food waste.

The case for biofuels comes from the carbon cycle. The carbon dioxide released when a biofuel burns was taken from the air by the crop a few months or years earlier. The next crop takes up the same amount again, so in principle the fuel adds no carbon to the atmosphere. A fuel with that property is called carbon neutral. Burning a fossil fuel instead returns carbon that has been out of the cycle for millions of years. Biofuels are also renewable, and can be grown at home by a country that would otherwise import oil.

In practice no biofuel is fully carbon neutral, for three reasons. Growing, harvesting, transporting and processing the crop use energy, most of it from fossil fuels, and making nitrogen fertiliser uses a great deal. Fertilised soils release nitrous oxide, a powerful greenhouse gas. And if forest or peat is cleared to grow the crop, the carbon released from the cleared store can exceed the savings the biofuel makes for decades. Oil palm planted on drained tropical peat forest is the worst case.

The conflict with human needs is sharpest over food. Land used to grow fuel cannot grow food. Diverting maize or wheat into ethanol raises the demand for grain and so can make food dearer, which falls hardest on the poorest. Large monocultures of a fuel crop also replace habitat and reduce biodiversity. These problems are smaller for biofuels made from wastes and residues, such as straw, forestry waste, used cooking oil and manure, which need no extra land. They are also smaller for crops grown on land too poor for food. Fuels made from the cellulose of whole plants, instead of only their sugar or oil, aim to use more of each crop for the same reason, but are harder to make economically.

Reforestation

Replanting or allowing the regrowth of trees on land where forest has recently been cleared.

Biofuel

A fuel made from recently grown biomass, such as bioethanol from fermented crop sugars or biodiesel from plant oils.

Carbon neutral

Describes a process that adds no net carbon dioxide to the atmosphere, because the carbon it releases is balanced by carbon taken up.

Sustainable resource

A resource used no faster than it is replaced, so that it remains available to future generations.