In 1992 Canada closed the Atlantic cod fishery off Newfoundland, putting about 30,000 fishing and plant workers out of a living. For years the catch had been "within quota". The quotas were set from stock estimates that were too high, and the fish had been dwindling underneath them. Nobody was breaking a rule. The rule itself was the problem.
That is the pattern conservation science exists to catch. A protected area that protects the wrong ground, a "decline of 73%" that does not mean what it sounds like, a harvest plan that works until a bad year, a tree-planting pledge that counts seedlings rather than forests. Applied ecology takes the equations of Ecology (35/48) and asks what a government, a community or a fishing fleet should actually do with them.
Clear most of a forest and you do not lose most of its species the next morning. The species-area relation of Ecology (35/48) says that, once things settle, the share of species that remain is the share of area that remains, raised to a small power z, usually about 0.15 to 0.35:
"Eventually" is the important word. A remnant forest starts with the species it had, then loses them as small populations blink out through bad luck, inbreeding and isolation, over decades or centuries. David Tilman and colleagues named the gap the extinction debt in 1994: species that are still present but already doomed, unless the habitat is restored or reconnected. Debt cuts two ways. It means counts made today understate what past clearance will cost, and it means there is still time to act on the species that are still hanging on.
The method has critics. In 2011 Fangliang He and Stephen Hubbell argued in Nature that running the species-area curve backwards overestimates extinctions from habitat loss, because the curve counts species present in an area, while extinction needs species found only in the cleared part. Others answer that observed extinctions and the debt are real, and that the question is the size. Treat the 56% above as the textbook calculation, not a forecast for any real forest.
Textbook model, illustrative: share of species = eq + (1 − eq)·e−t/τ, where eq = (1 − lost)z. Real relaxation times differ by species, patch size and isolation.
With 90% cleared, z = 0.25 and a 40-year time constant, 83% of the species are still there after 20 years and 60% after 100, on the way down to 56%. The first photograph after clearing looks fine. That is the trap.
The logistic model of Ecology (35/48) says growth is fastest at half the carrying capacity, so the largest steady catch, the maximum sustainable yield (MSY), is rK/4 and is taken by keeping the stock near K/2. That idea (Michael Schaefer's surplus-production work in the 1950s) became the backbone of fisheries law: the 1982 UN Convention on the Law of the Sea refers to maximum sustainable yield, and many national laws still do.
It has been criticised from the start. William Larkin's 1977 paper was titled "An Epitaph for the Concept of Maximum Sustained Yield". The reasons are practical. Stock size is estimated, not counted. Fish species depend on one another. Recruitment swings with ocean conditions. And MSY sits right at the edge: the same arithmetic that gives the biggest catch gives the smallest safety margin. The Peruvian anchoveta, the biggest fishery in the world by weight in the 1960s, crashed in 1972–73 during an El Niño year after years of record catches.
What matters is how you set the catch. A fixed quota (take 120 tonnes whatever you find) is easy to administer. A fixed fraction (take a quarter of the stock) falls automatically when the stock does. Try both with a bad year in year 10, where a fraction of the stock simply dies or leaves.
Schaefer-style model, K = 1000, simulated in 0.05-year steps for 40 years from the equilibrium the rule would give. Illustrative; real fisheries add age structure, noise and uncertain estimates. Fixed fraction means the catch is that fraction of the current stock.
At r = 0.5, MSY is 125 a year. A quota of 120 holds the stock at 600, with an unstable tipping point at 400. A 30% bad year drops it to 420, just above the line, and it recovers. A 50% bad year pushes it under, and the stock collapses by about year 17, while the quota keeps being taken. A quota of 125, the exact MSY, has no safety margin at all. A fixed fraction of 0.25 settles at 500, the same 125 a year, and survives a 50% bad year with a low point of 252, because the catch falls when the stock does.
Where do the world's fisheries stand? FAO's 2024 report on the state of world fisheries put 37.7% of assessed marine stocks as overfished in 2021, up from 35.4% in 2019, with the remainder within biologically sustainable levels. Later FAO work using a finer list of stocks gives somewhat different shares, and by weight of landings the picture is better, around three-quarters of landings coming from sustainable stocks. Scientists disagree about how bleak that is. Daniel Pauly and others argue that official catch data hide declines and that many unassessed stocks are worse. Ray Hilborn and others reply that well-assessed fisheries in North America, Europe and Oceania have been recovering under strict limits, and the biggest problems are in poorly managed, unassessed fisheries, many of them in the tropics. Both camps agree that management works where there is data, enforcement and the will to cut catches.
The headline everyone knows: the WWF and Zoological Society of London's Living Planet Report 2024 says monitored vertebrate populations fell on average by 73% between 1970 and 2020. The Living Planet Index (LPI) is built from tens of thousands of population time series, for several thousand species of mammals, birds, reptiles, amphibians and fish. For each population it takes the rate of change, then combines the rates with a (weighted) geometric mean, which is the average of the logarithms of the ratios. So it averages percentage changes, with every monitored population counting as one vote.
That design has consequences, and reading the index well means knowing them. It is not "73% of animals" or "73% of species" gone. It is an average change in a sample, which is heavy with birds and with Europe and North America, where monitoring is easiest. Regions differ widely: the report puts Latin America and the Caribbean at declines above 90%, Europe and Central Asia far lower. Freshwater populations show the steepest fall of the three realms. And in 2020 a McGill University team (Leung and colleagues) showed that a small share of populations, under 3%, with extreme collapses, drove much of the earlier headline decline; other researchers criticised that re-analysis in turn, and more recent work has asked whether the index's mathematics exaggerates or hides trends. The direction of travel is not in dispute among the groups that studied it. The exact size is.
24 invented populations, sorted from worst to best. Grey bars are excluded. Remove the steepest decliners or risers and watch the index.
Invented numbers, not data. Index = exp(mean of ln(end ÷ start)) over the populations kept; the real LPI adds smoothing and weighting by species and realm.
In the toy set the index is 0.62, an average fall of 38%, even though the total number of animals has risen by about 3%: a few small populations have collapsed while the big ones are steady. Exclude the six steepest decliners and the index turns slightly positive. Neither number is "the truth". They answer different questions, and a good conservationist asks which one the decision needs: how many populations are in trouble, how many animals are left, or how many species are at risk. The IUCN Red List answers the last, by classifying species one at a time.
The Kunming–Montreal Global Biodiversity Framework, adopted in December 2022, set a target (Target 3) to conserve at least 30% of land, inland waters, coast and ocean by 2030, "30×30". The Protected Planet Report 2024 (UNEP-WCMC and IUCN) put coverage at 17.6% of land and inland waters and 8.4% of the ocean, with gains of under half a point since 2020 in either. Fewer than 5% of the world's land lies in protected areas whose management effectiveness has been assessed, and 51 countries and territories already exceed 30% on land.
The target counts area, and area is easy to hit in the wrong way. Protected areas worldwide tend to sit on land that is high, steep, cold, dry and far from cities, because that is where it is cheap and politically easy to protect: "residual protection" (Joppa and Pfaff, 2009, is the classic paper). Species that live in fertile lowlands are the ones most likely to be left out. The better tool is systematic conservation planning: choose areas to cover as many species as possible at least cost, using complementarity (pick the next site that adds the most species not already covered), not richness alone.
Invented 16×16 landscape with 60 invented species whose ranges are small clusters, mostly in the lowlands on the left (darker cells, more species). Remote and cheap = the right-hand cells first. Illustrative: the point is the method, not the numbers.
In this toy landscape, protecting 30% of the cells at random keeps some part of 54 of the 60 species and at least 30% of the range of 42 of them. Choosing the remote, cheap cells keeps part of 12 species and 30% of the range of 10, the same area for a fifth of the benefit. Choosing by complementarity covers all 60 and 30% of the range of 52. That gap is why the 30×30 text also asks for areas that are ecologically representative, well connected and equitably governed, and why whether the target works depends on which 30% is chosen.
Early national parks were often made by removing the people who lived there. Yellowstone (1872) was created on land used by Indigenous nations; the Maasai were moved from the Serengeti National Park when its boundaries were set in 1959. Critics of "fortress conservation" say the model is unjust and often fails, because it ignores local knowledge and turns neighbours into opponents. Defenders of strict protected areas answer that some places, and some species, do need people kept out, and that weak local governance can fail too. Most conservation scientists now take a middle position: both models work somewhere, and the question is which and where.
The scale is large. A 2018 study in Nature Sustainability by Stephen Garnett and colleagues estimated that Indigenous Peoples manage or hold tenure rights over at least 38 million km² in 87 countries, more than a quarter of the world's land, and that these lands overlap about 40% of terrestrial protected areas and ecologically intact landscapes. Studies from the Amazon and elsewhere generally find lower forest loss inside recognised Indigenous territories, although how much of that comes from the title and how much from remoteness is argued.
Management here is active, not absence. Aboriginal communities in northern Australia have used fire for tens of thousands of years, and since 2005 the West Arnhem Land Fire Abatement project has paired cool, early dry-season burning with carbon credits, reducing the large, hot late-season fires. In Namibia, communal conservancies (the first registered in 1998) give rural communities rights over wildlife, and wildlife numbers have risen in many of them. In Canada, Indigenous Protected and Conserved Areas, for example the Tribal Parks that the Tla-o-qui-aht declared in 1984 on Meares Island, are being used to count towards national targets. The 30×30 text itself recognises Indigenous and traditional territories, a change from the older, park-only view.
The United Nations Decade on Ecosystem Restoration runs from 2021 to 2030, and the Bonn Challenge, launched in 2011, has a target of 350 million hectares of degraded land under restoration by 2030. The evidence says two things at once. Recovery can be fast and impressive: studies of tropical forests regrowing on abandoned land (Poorter and colleagues, 2016) found that soil fertility and biomass can approach old-growth levels within decades, though the species mix takes a century or more. Costa Rica's forest cover fell to roughly a quarter in the 1980s and has since recovered to more than half, helped by payments for ecosystem services, cattle-sector changes and protection.
But the way planting is counted matters. Tree-planting pledges count seedlings; what ecosystems need is surviving, diverse, locally suited trees. Monoculture plantations store carbon but are poor habitat, planting trees on natural grassland or savanna can destroy a biome that was fine, and the African Union's Great Green Wall, launched in 2007 with a goal of 100 million hectares by 2030, has fallen well behind schedule. Often the cheapest and best method is to protect land and let it regrow by itself (natural regeneration), which is also what farmer-managed regrowth in Niger did. Ask three questions of any restoration claim: what was the reference ecosystem, how many survive, and who owns and keeps it.
Conservation is measurement under pressure. The same few ideas answer most of it: species vanish with a lag, harvests need margins, indices need denominators, targets need locations, and land needs its keepers. This is information, not a verdict on any policy; choosing between protection, use and restoration also involves values that the numbers cannot settle.
Try this: find a local protected area or nature reserve and ask what it was chosen for and what is left out of it. Then check who lives or works around it. The answers say more than the percentage on a map.
Sources. The species-area relation and extinction debt (Tilman and colleagues, Nature, 1994) and its critique (He and Hubbell, Nature, 2011); Schaefer's surplus-production model; Larkin, "An Epitaph for the Concept of Maximum Sustained Yield" (1977); FAO State of World Fisheries and Aquaculture 2024 (37.7% of assessed stocks overfished in 2021) and later FAO assessments; Living Planet Report 2024 (WWF and ZSL) and Leung and colleagues (Nature, 2020); Protected Planet Report 2024 (UNEP-WCMC and IUCN) and the Kunming–Montreal framework (December 2022); Garnett and colleagues, Nature Sustainability 2018; Joppa and Pfaff, 2009; Poorter and colleagues, Nature 2016. Statements about Costa Rica, the Great Green Wall, Namibia, Arnhem Land and Canada are given in outline and should be checked against current reports before being quoted. All interactive numbers were computed in node; the models and data in them are illustrative.