Conservation in situ and ex situ

In situ and ex situ conservation defined

Conservation is the protection and management of species and habitats so that they persist. It differs from preservation, which means leaving an area unaltered; many British habitats are maintained by continued cutting, grazing or burning.

Populations and species become extinct when deaths outnumber births for long enough, and Cambridge International names four causes. Habitat degradation and loss leave too little food, shelter or breeding space, and split what remains into fragments too small to persist. Hunting by humans removes animals faster than they are replaced, which hits large, slow-breeding species hardest. Competition, often from an introduced species, takes resources a native species depends on. Climate change shifts conditions faster than many species can move or adapt, and a species already confined to a mountain top has nowhere cooler to go.

In situ conservation

Conserving a species in its natural habitat, so that the community and the conditions it is adapted to are conserved with it.

Ex situ conservation

Conserving a species outside its natural habitat, in a zoo, a botanic garden, a seed bank or a captive breeding centre.

Endangered species

A species at very high risk of extinction in the wild, as assessed against criteria such as the IUCN Red List's.

Extinct in the wild

A species surviving only in cultivation, captivity or as a population well outside its past range.

Describe the drawing

Two large outlined cards side by side. The left card, in situ conservation, lists what conserving a species where it lives holds on to (the whole community, natural selection still operating, populations large enough to matter), and beneath it what that approach cannot touch: pollution from outside, climate change and a habitat that has already been destroyed. The right card does the same for ex situ conservation, from controlled conditions and planned matings down to the habitat itself, learned behaviour and genetic diversity.

In situ conservation

Conserving a species in its habitat conserves the community and conditions around it. National parks, nature reserves, Sites of Special Scientific Interest and marine conservation zones protect an area rather than a species, so unstudied species in the area are protected as well. Selection continues to act, so the population remains adapted to conditions that are themselves changing. Populations can be large, which slows the loss of genetic diversity. Per species conserved, the cost is much lower than captive breeding.

In situ conservation usually requires active management. A chalk grassland left unmanaged develops into scrub and then woodland within a few decades, and the orchids and butterflies it was designated for are lost. Reserves are therefore grazed, coppiced, mown or burned on rotation, holding succession at the stage that suits the species being conserved.

The limitations are the pressures a boundary does not exclude. Poaching continues inside reserves, and in some places a designation concentrates the animals and makes them easier to locate. Invasive species colonise reserves. Pollution and climatic change originate outside them. Reserves are often too small, or too isolated from one another, to hold a viable population of a wide-ranging animal, which is the reason for wildlife corridors linking fragments. And a reserve on land previously farmed or logged imposes a cost on the people who live there, so enforcement is difficult where local communities receive no benefit from it.

Invasive alien species are species moved by people outside their natural range that spread and harm the native community. They are controlled because they compete with native species for food and space, prey on species that evolved without them, carry disease and hybridise with close relatives: the grey squirrel outcompetes the red squirrel and carries squirrelpox, and rats introduced to seabird islands eat the eggs and chicks.

The IUCN, the International Union for Conservation of Nature, assesses species against standard criteria and publishes the Red List, which places each assessed species in a category from Least Concern through Near Threatened, Vulnerable, Endangered and Critically Endangered to Extinct in the Wild and Extinct. Governments and conservation bodies use those assessments to decide which species most need protection.

Two international agreements are named on the specifications. CITES, the Convention on International Trade in Endangered Species, has been in force since 1975 and regulates cross-border trade: species in Appendix I may not be traded commercially at all, those in Appendix II only under permit. It controls trade and nothing else, so it does not touch habitat loss, and it depends entirely on member states enforcing it at their own borders. The Convention on Biological Diversity, agreed at the Rio Earth Summit in 1992, is broader and less prescriptive: signatories commit to conserving biodiversity, using it sustainably, and sharing the benefits arising from genetic resources fairly with the countries of origin. It sets targets rather than prohibitions, and successive sets of targets have not been met in full.

Ex situ conservation

A zoo, a botanic garden or a captive breeding centre offers control that a reserve cannot. Temperature, diet and disease exposure are managed. Veterinary care is available. Breeding can be assisted with artificial insemination, embryo transfer or surrogacy, and matings can be selected. Captive breeding uses a studbook: a record of every individual's ancestry, used to pair the least related animals available and slow the loss of alleles. Animals can be moved between institutions for that purpose. For a species already extinct in the wild there is no alternative.

The main limitation is population size. A captive population is founded by a handful of individuals and held at a few hundred at most, so it passes through a bottleneck by construction, and alleles that were not in the founders are simply not there. Inbreeding raises the frequency of homozygous recessive genotypes, and with it the incidence of inherited disease and of reduced fertility. Captivity also selects: over generations, the animals that breed best in enclosures are not necessarily the ones that would survive outside them.

Behaviour is a second limitation. Hunting, predator avoidance, migration routes and, in many birds and mammals, the details of courtship are learned from other individuals. A hand-reared animal may not acquire them, so release programmes use puppet feeding, mentor birds, pre-release training pens and soft releases with continued support. These measures are expensive and release success rates remain low for many species.

Four assisted-reproduction methods are named on the specifications. Artificial insemination moves semen to a female without moving the male, which matters when the two animals are on different continents or when a pair will not mate. In vitro fertilisation, usually shortened to IVF, takes eggs from the female, fertilises them with collected semen in the laboratory, and grows the embryos far enough to be assessed before any of them is used. Embryo transfer then places an embryo into a female that did not produce the egg, so a single genetically valuable female can be the mother of many more offspring than she could carry herself. Where no female of the species is available to carry a pregnancy, a female of a closely related species can act as a surrogate, which has been done for wild cattle and for wild horses.

Those techniques are what make a frozen zoo, or cryobank, usable. Cryobanks store gametes, embryos or tissue at very low temperature. Samples taken from several individuals can preserve alleles without maintaining the same number of live animals. Material collected before further population decline may later reintroduce stored alleles, but it cannot recover alleles that were lost before sampling and requires suitable reproductive technology or surrogate animals.

In practice the material is held in liquid nitrogen at around minus one hundred and ninety six degrees Celsius, at which temperature metabolism is arrested and the cells remain viable for decades. Semen collected from a bull thirty years ago can be used on a living cow today, returning alleles to a population that has drifted since. Stored tissue also supports work unrelated to breeding, such as sequencing the genome of a species after it has become extinct.

The limitations belong in an evaluation. Freezing and thawing damage cells, and the protocols that work for cattle sperm do not simply transfer to a rhinoceros, so success rates for wild species are low and vary enormously between them. Eggs and embryos are much harder to freeze than sperm because they hold more water. A frozen sample can only be used where living animals are available to receive it, so a store with no surviving population and no suitable surrogate species functions as an archive rather than as a route to reintroduction. And cryobanking does not address the cause of endangerment, which is most often habitat loss.

Cost is a further consideration. Keeping a large mammal in a zoo costs more per animal per year than protecting a comparable area of habitat, which raises the allocation question directly: given a fixed budget, how much should be spent on captive populations and how much on land? Zoos hold that they also carry out research, breeding science and public education that returns funding and support to field work. Critics hold that a species whose habitat is not protected is being maintained as an exhibit.

Seed banks

For plants, ex situ conservation stores far more individuals per unit of space. Seeds are small and are already a dormant stage, and a single collection can hold tens of thousands of seeds from hundreds of separate parent plants, so a seed bank can store allelic variation from a large sample of a population within a building.

Describe the drawing

Five numbered cards stacked one above another with arrows running downward between them, setting out what happens to a seed in a seed bank: collection in the field with its locality and date, cleaning and X-ray screening to discard empty seeds, slow drying to about five per cent moisture, freezing at minus twenty degrees in sealed jars, and a germination test on a sample every ten years. A note underneath records that some seeds cannot be dried or frozen at all.

Seed is collected in the wild, from many individuals across a population, with the locality, date and habitat recorded, because an accession without that information cannot be put back anywhere in particular. It is cleaned and X-rayed so that empty or insect-damaged seeds are discarded. It is then dried slowly, typically at around 15 °C and 15 per cent relative humidity, down to roughly 5 per cent moisture content. Only then is it frozen, conventionally at −20 °C, in sealed containers. Drying first is not optional: water left in the seed forms ice crystals that rupture the cell membranes.

Low temperature and low water content slow metabolism almost to a stop, so respiration barely proceeds, reserves are not used up and enzymes do little damage. Seeds of many species stay viable for decades or centuries this way. But viability does fall, so a sample from each accession is germinated at intervals, of which every ten years is a common cycle, and if the germination percentage has dropped too far, seed is grown on to produce a fresh batch.

The Millennium Seed Bank at Wakehurst in Sussex, run by Kew, holds seed from tens of thousands of species with a particular emphasis on wild plants rather than crops; the Svalbard Global Seed Vault, cut into permafrost inside an Arctic mountain, exists as a backup copy of the world's crop collections. The advantages over conserving the plants themselves are less space, less labour, lower risk from fire or a disease outbreak in a living collection, and the storage of many more individuals and therefore many more alleles. Storage risk is reduced rather than eliminated: freezer failure, flooding and funding loss have all affected collections, which is why duplicate accessions are held at separate sites.

One limitation restricts which species can be stored. Seeds that tolerate drying and freezing are called orthodox; a substantial minority, the recalcitrant seeds, do not. Acorns, horse chestnuts, avocados, mangoes, cocoa and a large proportion of tropical rainforest trees have seeds that are shed with a high water content and die if dried. They must be kept as living collections, or by freezing extracted embryos in liquid nitrogen, or in tissue culture, all of which are more expensive and less secure. The habitats richest in species are the ones whose seeds a seed bank finds hardest to take.

Four arguments for maintaining biodiversity

Reasons for maintaining biodiversity are conventionally grouped under four headings. Each heading is a position that is argued for, and each has a standard objection; an evaluation should give both.

Describe the drawing

Four cards in a two by two arrangement, one for each kind of case made for conserving biodiversity: economic, ecological, ethical, and aesthetic and cultural. Each card carries the claim in pale text and, beneath it in coral, the standard objection that case runs into: that a species with no known use becomes expendable, that the important species are identified too late, that intrinsic value cannot rank two species against each other, and that beauty favours large and photogenic animals.

The economic case. Wild species are a stock of useful material. Vincristine and vinblastine, still used against childhood leukaemia and Hodgkin lymphoma, came from the Madagascar periwinkle; paclitaxel came from the bark of the Pacific yew. Wild relatives of crops carry disease resistance that breeders need. A wild rice from India supplied the resistance to grassy stunt virus that was bred into cultivated varieties. Habitats also do work for nothing, known as ecosystem services: wetlands hold flood water, forests hold soil, insects pollinate crops. Variety in the crops themselves pays too: growing the same crop on the same land year after year, continuous monoculture, depletes the soil of the nutrients that crop draws on, which a rotation of different crops avoids. The objection is that the argument applies in both directions. If a species is worth conserving because it may prove useful, a species shown to have no use has no claim under this argument, and most species will never be assessed for usefulness at all.

The ecological case. Species are connected, and removing one can lead to the loss of others. Some species have effects out of proportion to their abundance, of which sea otters limiting sea urchin numbers in kelp forests is the standard example, and a community that loses species loses the functional redundancy that allows it to absorb a disturbance. The objection is practical: which species have these effects is generally not known in advance, and often becomes apparent only after the species has been lost. The argument therefore supports precaution generally rather than a method of prioritising funding.

The ethical case. This case holds that a species is the outcome of a very long evolutionary history and should not be ended by human action irrespective of its usefulness, and that present generations have obligations to future ones. The argument does not depend on the species being useful or conspicuous. The objection is that it provides no basis for choosing. Budgets are finite, and an argument that every species has an equal and absolute claim cannot decide between two species when only one can be funded.

The aesthetic and cultural case. People value landscapes and animals, travel to see them, write about them and build identities around them; ecotourism turns that into income that can pay for protection. The objection is that this valuation is measurably uneven. It favours large mammals and birds over invertebrates, fungi and plants, which is close to the reverse of where most species diversity lies, and it makes funding dependent on public interest.

The four arguments do not converge on a single ranking and they conflict in practice. An evaluation should state which argument it relies on, and why, rather than listing all four.