4.16 - Conservation in zoos and seed banks

Species survival and genetic diversity

Conservation aims to maintain populations that can survive and reproduce, while retaining the inherited variation on which their future responses to environmental change may depend.

An endangered species faces a high risk of extinction. Its genetic diversity is the variation in its DNA, including the different alleles present within and between populations. Alleles are alternative versions of a gene. Losing a small population can therefore remove alleles even if the species survives elsewhere. Some variation may contribute to resistance to disease or tolerance of changed conditions; greater diversity preserves possibilities, rather than guaranteeing that every future threat will be overcome.

Protecting habitat can conserve many species, their interactions and the conditions in which natural reproduction and selection continue. It can include reducing exploitation, restoring breeding sites and maintaining connections between populations. However, a rapidly spreading threat may leave too little time for habitat measures alone. A managed insurance population or seed collection can preserve living material while threats in the wild are addressed.

Head count and genetic diversity answer different questions. A hundred plants produced as identical clones from one plant do not represent the same inherited variation as a hundred sexually produced plants from many different parents. Similarly, many zoo births can all descend from one family. Record numbers and reproduction, but also ancestry and genetic representation. A large collection is not, by itself, evidence that the species' genetic diversity has been secured.

Zoos and seed banks are therefore potential parts of an integrated conservation plan. A collection may buy time or supply organisms for restoration, but it does not automatically conserve a habitat, its pollinators, its food web or every wild allele. Its purpose and measurable outcomes must be stated.

Managing genes in captive breeding

A captive breeding programme must manage which individuals reproduce, because a growing population can still lose genetic diversity.

The founders are the individuals from which the managed population descends. A small founder sample may contain only a subset of the wild population's alleles. Relatedness describes shared ancestry: close relatives are more likely to carry copies of the same ancestral alleles. Mating close relatives increases homozygosity, having two identical alleles at a locus, which can expose harmful recessive alleles. The resulting reduction in survival or reproductive success is called inbreeding depression; it is a risk, not an inevitable identical outcome in every mating.

Small populations also experience genetic drift: random changes in allele frequencies as only some individuals and their alleles contribute to the next generation. An allele can disappear by chance. Inbreeding changes how alleles are paired, whereas drift can change which alleles remain. Neither process requires an allele to be useful or harmful. Producing more descendants of the same parents does not recover alleles that were absent from the founders or already lost.

Coordinated programmes use a studbook, a record of births, parentage, deaths and transfers, to select compatible breeding pairs and retain different founder lineages. DNA analysis can help resolve uncertain parentage or founder relatedness. Managers aim to limit close-relative matings and excessive contributions from a few successful breeders. They may arrange transfers between zoos, exchange gametes or use assisted reproduction where a species-specific method is feasible. Animals at different zoos must be considered as one managed population; separate zoo addresses do not prove unrelatedness.

For example, suppose most animals descend from founders A and B, while a healthy descendant of founder C has produced no offspring. Breeding another A–B pair may add animals while further overrepresenting those lineages. A compatible, less-related pairing that represents C may retain more of the remaining variation. This judgement needs reliable ancestry and health information, rather than appearance alone. Bringing in new founders can add variation but taking animals from a small wild population can damage it, and transfers can introduce disease.

Breeding decisions also depend on age, sex balance, social compatibility, space and lifelong care. More births are not automatically beneficial if suitable homes or release plans are absent. Captive conditions can favour traits useful there but unhelpful in the wild, such as tolerance of humans. Appropriate housing, natural foraging and careful management can reduce welfare and release problems, but require expertise and resources. EAZA's population management guidance describes coordinated breeding and transfer recommendations; these are tools whose outcomes still need evaluation.

Collecting and storing representative seeds

A seed bank conserves living embryos and their inherited variation, so collecting representative material matters as much as keeping a freezer cold.

Collectors identify the species and obtain the relevant permission and local cooperation. They sample ripe seeds from many suitably spaced parent plants and, where appropriate, several populations across the species' range. This is more likely to capture variation than collecting the same number of seeds from one heavily fruiting plant. Collection must leave enough seed for the wild population to regenerate. A very small population may require a more cautious, repeated collection strategy instead of a large one-off harvest.

Each collection needs an identity and provenance: records of its species, location, date, source population and sampling method. Keeping population identities traceable allows later users to select material suited to a restoration site. Different populations may have different adaptations; mixing everything without records prevents an informed decision. Sampling many parents improves representation but cannot guarantee collection of every rare allele.

Orthodox seeds tolerate drying and storage at low temperature. After cleaning out debris and damaged or empty material, an appropriate drying regime lowers seed moisture. Drying and cooling slow deterioration and keep growth suspended. Suitable seeds are sealed in airtight containers and stored cold; conventional long-term seed banks commonly use about −20 °C. A geographically separate duplicate reduces the risk that one fire, flood or equipment failure destroys the entire collection. These procedures, including provenance, duplication and viability monitoring, are set out in Kew's Seed Conservation Standards.

Thousands of dormant seeds can often be kept in much less space, with less daily care, than an equivalent living plant collection. They can supply material for research, multiplication and eventual restoration. However, the stored seeds are a sample taken at a particular time. They do not continue to reproduce and adapt to a changing wild habitat while stored, and their lifespan is finite. Counting full containers is therefore an incomplete measure of conservation.

Viability, regeneration and storage limits

Stored seeds must remain capable of developing into plants, and the storage method must suit the species' biology.

Viability means being alive and capable of development under suitable conditions. A bank tests a representative subsample at the start and at intervals, commonly by germination under a validated combination of moisture, temperature and light. Some viable seeds are dormant: they do not germinate until a particular cue or treatment removes dormancy. Failure to germinate in one unsuitable test is therefore not proof that every ungerminated seed is dead. Research may establish the required cue or use another validated viability assessment.

To compare two storage treatments, randomly allocate comparable seeds from the same well-mixed collection to each treatment, with independent replicate containers. Keep other storage conditions the same. At the same elapsed time, test representative, equal-sized subsamples under the same suitable germination conditions and record the proportion developing into normal seedlings over the same justified period. Use an initial viability test as a baseline. Independent containers help distinguish treatment effects from an accidental problem in one container; repeatedly checking one container is not the same replication. With scarce endangered material, use enough seeds for useful evidence while minimising depletion of the collection. A pilot on more abundant material can establish methods first. No online planning exercise establishes that practical work has been performed.

If viability or the remaining seed number becomes too low, staff may recollect from the wild where safe, or regenerate the collection by growing plants and collecting newly produced seed. Regeneration is reproduction under cultivation, so it brings genetic risks resembling captive animal breeding. Growing only a few surviving seeds creates another bottleneck; unequal seed contributions, unintended crosses or selection for success in cultivation may change the collection. Maintain sufficient representative parent plants, manage pollination appropriately for the species, retain records and avoid repeated unnecessary regeneration. A jar replenished with descendants of one plant has regained seed number without regaining its original diversity.

Recalcitrant seeds are sensitive to drying and cannot be conserved by the usual dry, cold seed-bank procedure. Storage behaviour is not a simple guarantee even for all orthodox seeds: longevity varies, and some species store poorly. Alternatives include living plant collections, tissue culture under controlled conditions, and species-specific cryopreservation of suitable embryos, tissues or other material at very low temperature. These methods require research to show that material survives storage and can regenerate whole plants. Cloning one genotype does not conserve the variation of many genotypes, and living collections need space, disease protection and genetic management. Kew describes these exceptional-species limitations and alternative research.

From a collection to a wild population

Reintroduction succeeds only if released organisms can establish a population in a suitable habitat, rather than merely survive the day of release.

Reintroduction aims to re-establish a species within an area of its former natural range from which it has disappeared. Adding individuals to a population that remains there is reinforcement. Zoo-bred animals and plants grown from banked seeds can supply individuals for these programmes, but availability alone does not establish suitability.