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Feature

Genetic Roulette

Genetic diversity has been tagged as one of the primary goals of conservation, but its interpretation varies widely, causing problems for conservation efforts.

By Dr John Craig

Preservation of genetic diversity can be considered at its broadest interpretation as ensuring the continuation of all existing biodiversity. With limited resources, managers are left with the unenviable task of preserving as much and as representative a sample as possible.

Science can provide descriptions and categories to assist the conservationist, but it cannot say how much diversity should be conserved. Economic, social, cultural and political considerations must be considered as well.

What should we conserve -- species, subspecies or populations? What is the difference between these groups? There is no single answer to these questions. Scientists may use a "biological" species concept, where individuals interbreed but not between species. They may use a "phylogenetic" species concept, where animals and plants are just grouped together with similar organisms.

The concept of endangered species is well established in practice and in statute, and ensures that species and even subspecies have become the unit of conservation. Division of genetic diversity into separate taxa such as species is subjective and, for many scientists, is merely a hypothesis. As techniques change and information increases, taxonomy changes. It is important that managers consider revised taxonomic suggestions of scientists, but scientists have to be aware that managers may not reallocate resources in response to such changes.

Most taxonomies are based on morphological, or physical, characteristics, but there is an increasing use of molecular genetic techniques. It has been suggested that, given the high monetary and ecological stakes in endangered species management, a variety of criteria must be used to evaluate conservation strategies.

A problem of interpretation of data remains. Where morphology and molecular data do not agree, how can decisions be made? With molecular data, concepts of genetic distance -- the degree of relatedness -- are used, but no single value holds for all groups of animals or for all scientists.

Conservation in New Zealand is heavily biased both in practice and in legislation towards birds. Priorities are set for species and subspecies. Researchers have pointed out that the taxonomy of birds is based totally on morphology, and that it urgently needs reconsideration with the inclusion of molecular data. Other animal and plant groupings similarly require research.

Tuatara And Parakeets

Two good examples in New Zealand are the work on tuatara and parakeets. Genetic analysis shows that Forbes parakeets are very similar to red-crowned parakeets, but different from the yellow-crowned ones to which they were previously linked. Tuatara data shows that the Brothers Island animals are the most distinct of all tuatara populations, whereas the North Island groups are all very similar.

In both cases, researchers recommended the variability be recognised at species level. Because taxonomy is partly subjective, both these species may be challenged, but nevertheless it has important implications for management decisions.

Forbes parakeets are distinct in colour, but not measurements, from the red-crowned, although they do interbreed. The genetic distance separating them does not meet any recognised criteria. Yet, for some, the colour difference is seen as sufficient to warrant separate status and management. For others, the removal of hybrids is an unnatural interference.

The recent suggestion that tuatara be viewed as two species, one with three subspecies [Tuatara By Any Other Name, November 1990], is a good example of how conservation managers view molecular data. The researchers acknowledged the subjective nature of taxonomy, erected taxonomic division without any morphological support, and relied solely on genetic measures.

The genetic distance found is similar to that of the mainland and island populations of the Australian sleepy lizard, which are not considered distinct subspecies. However, the amount of variability of genetic distance among tuatara on 19 islands off the North Island and among the four remaining islands in Cook Strait is minute, relative to that separating either group from the Brothers Island tuatara. Hence, regardless of what different researchers call the distinction, the important management considerations are that the genetic stocks should not be mixed, and that if the greatest variability of tuatara is to be conserved, preservation of the Brothers Island tuatara should have precedence.

Tuatara researcher Dr Charles Daugherty has noted that where conservation managers fail to recognise separate taxa, forms may be lost through neglect. True, but the reverse suggests that, because conservation resources are limited, managers cannot ensure that all variants will be saved. Demands for greater subdivision of taxa may prolong neglect of less popular and less well known animals and plants.

Variability

Genetic techniques allow us to measure an insignificant fraction of total variability. Many researchers assume variability to be important for evolution, but the lack of demonstrable variability does not necessarily mean that there is insufficient for evolution and continued survival.

Conservation managers use guidelines based on models which assume that animals live in large groups that typically outbreed or have less than 1-3% chance of inbreeding; that there are no overlapping generations; and that all individuals contribute equally to future generations. A minimum of 50 individuals may be sufficient to avoid short-term inbreeding depression, and a single large population of 500 is thought to be required to avoid long-term erosion of genetic variation.

Species Recovery Plans for endangered animals in New Zealand use these guidelines, with population priorities of 500 for takahe, 400 for kokako, and over 500 for kakapo. A Department of Conservation report suggests that 50-100 founders are needed to maximise genetic diversity and that these should come from a number of donor populations.

This contrasts with the guidelines used by botanists. Inbreeding has long been accepted by botanists, as has local, presumably genetic, variation. They recommend using only local seed for replanting, often establishing a new population from a single plant. Some degree of inbreeding has ready acceptance among plant scientists, but is anathema to animal scientists.

Many Small Groups Needed

Ideas generated in the continental Northern Hemisphere with migrating animals may have little applicability in the conservation of small remnants of New Zealand populations that may have been inbreeding for well over 100 years. Including outbreeding by mixing previously isolated populations may be as problematic as continued inbreeding.

Establishing many small populations will ensure greater persistence and the preservation of greater amounts of genetic variability. This should be the focus of recovery efforts. It is more attainable and will ensure that the four large predator-cleared islands don't become little more than arks. Such arks care for endangered birds at the possible expense of lesser known animals or plants. While it is desireable to have some larger populations, a goal of many populations will allow the use of small islands as well.

A multi-population approach may require the acceptance that large genetic differences may occur as a result of genetic drift. We must be careful that the concerns of molecular genetics are balanced with others, or we may end up with replicate populations all being given separate taxonomic status. Each may then be required to have minimum populations of 500 -- clearly an impossible approach with the limited resources available.

The greater the number of populations, the greater the genetic variability and the greater the management costs. Managers have the difficult task of balancing the conservation of genetic variability within and between taxa. Given the many rare and endangered species we have, resources have to be spread a long way.

To ensure conservation of the greatest range of animals and plants, once 3-5 self-maintaining populations are established, resources can then be diverted to other areas. Daugherty and his colleagues have rightly pointed out the "avian imperialism" in island manipulation within New Zealand conservation. However, their concern with tuataras reads like a request to replace avian imperialism with tuatara imperialism. Surely a balance is preferable.

Debate about taxonomy and the importance of genetic variability is urgently needed. Such a debate must also take into account the political, social, cultural and financial realities that make conservation meet the needs of all New Zealanders.

Dr John Craig is an Associate Professor in Auckland University's Zoology Department.