Questions on managing ecosystems sustainably

Question 1

The graph shows the mass a fish stock adds each year at each biomass. Give the carrying capacity of the stock and the maximum sustainable yield, and calculate the change in the biomass of the stock over a year that begins with 300 thousand tonnes in the water and in which 45 thousand tonnes are caught.

Accepted Answer

B1 the carrying capacity is 400 thousand tonnes, read from where the curve returns to zero, since a stock at its carrying capacity adds nothing B1 the maximum sustainable yield is 40 thousand tonnes a year, at the peak of the curve, and the peak sits at a biomass of 200 thousand tonnes, half the carrying capacity M1 reads the mass added at a biomass of 300 thousand tonnes off the curve as 30 thousand tonnes a year M1 change over the year = mass added − mass caught = 30 − 45 A1 the biomass falls by 15 thousand tonnes, to 285 thousand tonnes; a catch of 45 thousand tonnes is above the peak of the curve, so it cannot be replaced at any biomass and the stock goes on falling

Question 2

A campaigner argues that the way to get the largest catch from a fish stock, year after year, is to leave the population as large as possible by taking very little from it. Evaluate this claim, using the logistic growth model.

Accepted Answer

B1 for: a very large population is far from extinction, and taking very little from it clearly avoids the collapse that a much smaller population close to zero would risk B1 for: the half-capacity maximum is a model's figure, and the carrying capacity is estimated rather than measured and varies from year to year, so a stock kept large and fished lightly is protected against an overestimate or a poor breeding year, which is why managers set limits below the modelled maximum B1 against: in the simple logistic model, net population growth, and so the largest catch that can be taken and replaced, is greatest at about half the carrying capacity rather than at the carrying capacity itself B1 against: at the carrying capacity itself, births and deaths are equal and net growth is zero, so a population left at its largest possible size actually produces no surplus at all to harvest sustainably B1 judgement: the claim confuses population size with sustainable yield; the largest repeatable catch comes from holding the stock at about half its carrying capacity, not from leaving it as large as possible, though the stock must still be kept well clear of the very low levels where recruitment collapses

Question 3

A company plans to offset its emissions by planting a single fast-growing non-native tree species over 2000 hectares of upland that is currently grazed by sheep and includes areas of peat. Evaluate the plan as a way of managing the conflict between human needs and conservation.

Accepted Answer

B1 for: while the trees grow, photosynthesis exceeds respiration and decomposition, so carbon dioxide is removed from the air and stored in wood and soil, and a fast-growing species does this quickly B1 for: the plantation can supply timber sustainably if felling is matched by replanting, and the trees reduce soil erosion and run-off B1 against: a monoculture of a non-native species supports far fewer species than mixed native woodland, so it gains little for biodiversity B1 against: planting on peat dries it out, so the peat decomposes and releases carbon, possibly more than the trees take up; and the farmers lose grazing land and income, so the scheme may be resisted B1 judgement: the plan would store carbon, but it would work better with native mixed species, peat areas excluded and rewetted, and the local farmers paid or involved; the carbon is held only as long as the forest is protected

Question 4

Describe how a hazel wood is managed by rotational coppicing, and describe the effect this has on the number of habitats in the wood.

Accepted Answer

B1 the trees are cut down to a stump close to the ground, called a stool, and many new shoots grow from it, supplied by the established root system B1 the wood is divided into compartments and one compartment is cut each year, on a rotation of roughly seven to twenty years B1 after a full rotation the first compartment has regrown and is cut again, so the yield and the standing crop of the wood stay the same year after year B1 at any moment the wood contains compartments at every stage of regrowth, from open sunlit ground to shaded older growth, so habitat diversity and species diversity are both higher than in an unmanaged or clear-felled wood

Question 5

Evaluate the use of aquaculture, the farming of fish in cages, as a way of reducing pressure on wild fish stocks.

Accepted Answer

B1 in support: farmed fish are enclosed, so nothing is taken from the wild stock to supply them to consumers, no unwanted species are caught by accident and the sea bed is not damaged as it is by trawling B1 against: carnivorous farmed species such as salmon are fed on fishmeal made from wild-caught fish, so the pressure on wild stocks is moved rather than removed B1 against: dense stocking spreads parasites such as sea lice and diseases to wild fish passing the cages, uneaten food and faeces enrich the water beneath them, and escaped farmed fish interbreed with wild ones and dilute their local adaptations B1 a judgement: aquaculture reduces pressure on the stocks used directly for food, but it is only a net gain where the species farmed is not fed on wild fish and where waste, disease and escapes are controlled

Question 6

A government proposes to cut the quota for a declining fish stock by half. Discuss the arguments for and against the proposal.

Accepted Answer

B1 for: the stock is declining, so the present catch is above the sustainable yield, and every year that continues the population falls further and the same quota takes a larger share of it B1 for: allowing the biomass to climb back towards half the carrying capacity raises the sustainable yield itself, so the long-term catch is larger than it would otherwise be B1 against: the loss of income and of jobs falls on the fishing communities immediately, while the benefit arrives years later and may be taken by somebody else, and stock estimates carry real uncertainty B1 a judgement: the cut is justified because a collapse costs every job for decades, as at the Grand Banks, but it should come with support during the recovery and with the fleet involved in setting and policing the rules, or it will not hold

Question 7

Compare selective felling with clear felling as ways of taking timber from a forest.

Accepted Answer

B1 selective felling identifies individual mature trees and removes them, leaving the canopy and the structure of the forest around them standing, whereas clear felling takes every tree in an area at one time B1 under selective felling the soil stays covered and held by the roots of the trees left behind, so it is not eroded or leached, whereas a clear-felled area is left bare and loses soil and mineral ions B1 the forest goes on fixing carbon and remains a habitat under selective felling, and the gaps are filled by seed from the trees around them, whereas clear felling removes the habitat and the carbon fixation together and the area has to be replanted B1 clear felling yields far more timber per hectare per visit and costs less per tree extracted, whereas selective felling yields less and costs more, which is why it is not used everywhere

Question 8

A no-take zone is set up over the spawning ground of a heavily fished stock, and fishing goes on everywhere outside it. Suggest why the catch taken from the water around the zone may rise after several years, and suggest one reason the zone might fail to help.

Accepted Answer

B1 nothing is removed inside the zone, so individuals survive to spawn more than once and the biomass inside climbs back towards the carrying capacity B1 a large old female produces far more eggs than a small one, so a recovered stock releases eggs and larvae out of all proportion to the area of the zone, and those larvae drift out into the water that is fished B1 as the density inside rises, adults and juveniles move out across the boundary, so the fished stock outside is supplied from inside and catches near the boundary rise B1 it may fail if the zone is smaller than the area a fish moves over, if it is not policed, or if the same fleet simply fishes the water that is left open, so the same total is taken from a smaller area

Question 9

A government proposes to ban logging inside a tropical forest reserve that borders several villages whose only cash income comes from timber cut there. Discuss how the reserve might be managed so that the rules are actually followed.

Accepted Answer

B1 for a strict ban: the forest is protected in full only if no logging happens at all, since any legal cutting sets a precedent that is hard to police or to limit once it is allowed B1 against a strict ban with no alternative: rules that remove a community's only cash income without support are poorly complied with, since the people enforcing the ban do not depend on the resource while the people who do have every reason to keep logging B1 an alternative: successful schemes usually involve local people in setting the rules, give them a share of the benefit, for example from licensed harvesting, tourism or payment for managing the land, and keep the rules simple enough to be checked B1 judgement: a ban imposed without local involvement or an alternative income is likely to be broken or resented, so a managed, locally-involved scheme is more likely to protect the reserve in the long run than an unsupported ban, even though it permits some harvesting

Question 10

Bioethanol made from sugar cane is sometimes described as carbon neutral. Explain why it is described in this way, and explain why the description is not fully accurate.

Accepted Answer

B1 the carbon dioxide released when the ethanol burns was taken from the air by photosynthesis in the cane a short time before B1 the next crop takes up the same amount again, so in principle there is no net addition to the atmosphere, unlike a fossil fuel, which releases carbon removed from the cycle millions of years ago B1 growing, fertilising, harvesting, transporting and processing the crop use energy, mostly from fossil fuels, and fertilised soil releases nitrous oxide B1 if forest or other natural vegetation is cleared to grow the crop, the carbon stored in it is released, which can outweigh the saving for many years

Question 11

Discuss the use of food crops such as maize and wheat to make biofuels, in terms of the conflict between human needs and conservation.

Accepted Answer

B1 biofuels are renewable and replace fossil fuels, so they can reduce net emissions of carbon dioxide and reduce a country's dependence on imported oil B1 land used to grow fuel cannot grow food, so food supply falls or prices rise, which harms the poorest people most B1 more land may be cleared to make up the food, destroying habitats and releasing stored carbon, and large monocultures reduce biodiversity B1 the conflict is reduced by making biofuels from wastes and residues such as straw, used cooking oil and manure, or from crops on land too poor for food

Question 12

Explain why the maximum sustainable yield from a fish stock is obtained when the population is held at about half its carrying capacity.

Accepted Answer

B1 at the carrying capacity births and deaths are equal, so the population adds nothing each year and there is no surplus to harvest B1 at a very low population size there are too few individuals reproducing, so the number added each year is also small B1 at about half the carrying capacity the population is on the steepest part of the sigmoid curve, with many reproducing individuals and resources not yet limiting, so it adds the greatest number each year and that surplus is the largest catch that can be replaced

Question 13

Explain how increasing the mesh size of a fishing net and closing a fishery during the spawning season each help a fish stock to recover.

Accepted Answer

B1 a larger mesh allows smaller, immature fish to escape from the net B1 those fish survive to reach reproductive age and breed at least once before they can be caught, so recruitment to the stock continues B1 a closed season prevents adults from being taken while they are spawning, so the eggs are laid before any of those fish are removed from the population

Question 14

Describe pollarding, and describe why it is used instead of coppicing where grazing animals such as deer or cattle have access to the wood.

Accepted Answer

B1 pollarding is the same cutting operation as coppicing, encouraging new shoots to grow from the cut, but carried out about two metres above ground rather than at ground level B1 the new shoots then grow out at that height, above the reach of grazing animals such as deer, cattle or sheep B1 a stool cut near the ground would have its new shoots eaten as they grew, so pollarding is used instead wherever the wood cannot be fenced against grazing

Question 15

Explain two reasons a quota, on its own, may fail to keep a fish stock at a sustainable level.

Accepted Answer

B1 the total allowable catch is set from a survey estimate of the stock, and if that estimate is wrong the quota set from it may be higher than the stock can actually sustain B1 fish caught in excess of a boat's quota are often thrown back dead rather than landed, unless a landing obligation applies, so fish are still removed from the stock without being limited by the quota B1 a quota limits the total mass taken but does nothing about which fish are taken, so young, immature fish can still be caught and removed before they have had a chance to breed even once

Question 16

Explain how limiting fishing effort, for example by capping the number of days a boat may spend at sea, is intended to protect a fish stock, and explain why this measure can still fail to reduce the catch.

Accepted Answer

B1 capping days at sea, engine power or the number of licensed boats is intended to limit how much fishing pressure is applied to the stock, on the assumption that less time or fewer boats fishing means a smaller catch B1 boats become more efficient over time, for example through better sonar or larger nets, so the same nominal effort, the same number of days or boats, is able to catch more fish than before B1 if efficiency rises fast enough, a capped effort can still take a catch above the sustainable yield, so limiting effort alone does not guarantee the limit on catch that was intended

Question 17

A plantation of young trees is left unthinned and grows far more densely than a natural woodland would. Suggest what effect this has on the trees produced, and suggest why the owner should thin the plantation.

Accepted Answer

B1 at high density each tree competes intensely with its close neighbours for light, water and nutrients, so individual trees grow more slowly and the plantation produces many thin poles rather than fewer thick trunks B1 timber production usually requires fewer, thicker trunks rather than many thin poles, so an unthinned plantation yields a lower-value crop even though the total number of trees standing is high B1 thinning removes some trees, usually the smaller or poorer ones, reducing competition and letting the remaining trees grow larger and thicker, giving a more valuable crop by the time of felling

Question 18

Drained peatland releases carbon dioxide. Explain, using your knowledge of the carbon cycle, why blocking the drains to rewet the peat can reduce atmospheric carbon dioxide.

Accepted Answer

B1 draining lets air into the peat, so aerobic decomposers respire the dead organic matter and release carbon dioxide B1 rewetting makes the peat waterlogged and anaerobic again, which slows decomposition, so the transfer of carbon from the peat to the air falls B1 bog plants such as mosses keep photosynthesising, and their dead remains build up instead of decomposing, so carbon is moved into a long-term store

Question 19

Explain why a newly planted forest removes carbon dioxide from the atmosphere faster than an old forest that has reached its climax, and why the old forest should still be protected.

Accepted Answer

B1 in a young forest photosynthesis exceeds respiration and decomposition, because biomass and soil organic matter are increasing, so there is a net transfer of carbon out of the air B1 in a climax forest biomass is roughly constant, so photosynthesis is largely matched by respiration and decomposition and there is little net uptake B1 the old forest holds a large store of carbon in wood and soil, which would be released to the air by felling, burning or decay if it were cleared

Question 20

State what is meant by a sustainable yield, and state what is meant by the maximum sustainable yield.

Accepted Answer

B1 a sustainable yield is 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 taken again B1 the maximum sustainable yield is the largest such harvest available from the population, which in the simple logistic model occurs at about half the carrying capacity