Why Garden with Natives

Genetic Diversity


Is That Plant Really Native?
Genetic Considerations for Conservation, Restoration and Horticulture

Dr. Paul G. da Silva

As we humans have learned more about the world around us, we have become increasingly aware of complexities that were previously unknown. In the process, we have gained new insights that can help us protect our environment more effectively. Our relationship with native plants in our landscape provides some wonderful examples.

Many years ago, people began to discover the devastating effects of deforestation and erosion. One solution was reforestation. This often used exotic species, such as blue gum eucalyptus. (In fact, there may be some Marin CNPS members who remember doing this in their youth!) However, although there may have been some benefits associated with such projects, in time people began to realize that the large scale planting of exotic species also presented many risks. Some of these are potential for spread of new weeds, loss of habitat resources for native animals and microorganisms, increased fire danger, and altered soil chemistry.

The founding of CNPS coincided with a growing understanding of the important roles of native plants in our environment. Conservation of our native plants in their habitats and encouragement of planting of native plants in gardens and landscaping are two important goals of our organization. Progress toward both of these goals reduces risks associated with spread of exotic species.

However, it has become clear in work with many kinds of organisms all over the world that just focusing on the species category is not sufficient to ensure good stewardship of our planet. As we step back and look at the bigger picture, we see that species belong to communities and ecosystems and that these categories have characteristics that we cannot ignore. Likewise, there are also complexities below the level of the species that demand our attention.

In particular, at these lower levels we must pay attention to the question of genetic diversity. Just as all humans are not identical, even though we belong to the same species, not all individuals of a plant species are the same. If a species has great genetic diversity, the genes of its individual members are very different. It often follows that these different individuals behave very differently, even when planted in the same location.

Two major threats to genetic diversity exist. One is reduction of diversity by elimination of individuals and their genes; this produced greater and greater genetic uniformity. The other is introduction of genes from closely-related species or from very distant individuals from the same wide-ranging species. This is often called genetic contamination. Both kinds of threats are important in the important areas of conservation, restoration and horticulture.

In conservation, people now agree that a species is not necessarily “saved” if a few members of the species are still alive. This has become most clear with large mammals in zoos and in small fragments of natural habitats. In many of these cases, the genetic diversity has become so reduced that the health of the remaining individuals suffers from adverse effects of inbreeding. Furthermore, this genetic uniformity means that a species has less flexibility in coping with environmental changes, increasing the chances of its extinction. Although this may be most obvious in cases of large animals such as the Florida panther, it is also important for more obscure plants such as our local Baker’s larkspur. We must conserve multiple, genetically diverse populations of native plants in their native habitats!

Genetic contamination is also important in conservation. With the big mammals, the best-known case may be of the interbreeding of the American bison and domestic cattle. The most famous plant parallel may now involve pollen of genetically engineered varieties of corn reaching non-engineered traditional varieties. In Marin county, the best known case may be the hybridization of our native cordgrass with the Atlantic cordgrass. In order to avoid genetic contamination, we must be very careful not to allow potential sources of exotic pollen or seeds to become established close enough to populations of native plants of concern to spread their genes to them.

In restoration, the goal is to re-establish native species in order to recreate a community that functions as much as possible as the original community that was damaged or eliminated. If we accept that different individuals of the same species behave differently because of their different genetic constitutions, then maintenance of genetic diversity becomes very important in restoration projects.

This understanding was behind the controversial decision to remove all of the California condors from the wild to begin a captive breeding project. This action provided the maximum genetic diversity for the breeding colony that was established. In this case, because there was no risk of another species interbreeding with the condors, or from possibly maladapted genes coming in from distant California condor populations, genetic contamination was not an issue.

However, in many native plant restoration projects, both genetic uniformity and genetic contamination are potential issues. Since it has not been feasible to do exhaustive genetic analysis of all species used, a simple rule of thumb has been developed. This is to make multiple collections of seeds or other propagules within the same watershed (or at least a neighboring watershed) as the basis for establishing the plants that will be used in the project. This maximizes genetic diversity and minimizes contamination from plants that may not be the best suited to the habitat. The best local example of this procedure is the network of native plant nurseries in the Golden Gate National Recreation Area. Although economies of scale might dictate one large central nursery, genetic diversity concerns prompted the establishment of a decentralized system.

Awareness of the importance of genetic diversity has also invigorated and improved native plant horticulture. At the time of the founding of CNPS, many people who wanted to use native plants in landscaping ended up using exotic plants because native plants were not commercially available. Furthermore those “native” plants available in nurseries and garden supply stores often represented genetic stocks native to areas hundreds or thousands of miles away. For example, the majority of California poppy seeds sold have always been from collections in the deserts or interior regions of the state, where the flowers are the deep orange most consumers prefer.

The native plant sales sponsored by so many CNPS chapters were of enormous importance in promoting the planting of native plants appropriate to local habitats. First, they convinced people that manzanita, ceanothus, and buckwheat could be just as useful in horticulture as boxwood, cotoneaster and oleander. Second, as more chapters began to propagate their own material and label it with location of origin, they made people aware of the diversity that existed within these genera and also within the individual species. In addition, experience accumulated by horticulture groups within the various chapters has been useful to people working on conservation and restoration projects statewide.

Current recommendations for maintaining genetic diversity for native plants in gardens are not as strict as those for conservation or restoration projects. If the garden is isolated from natural stands of related native plants, the genetic contamination threat to native neighbors is low, and if the gardener is willing to give extra care to the garden plants, even native plants not well adapted to that particular site may perform well. Still, it is always good to try to find out the origin of all planting stock and to plant those plants most similar to the original native plants present on the site. This recommendation becomes more important the closer a native plant garden is to undisturbed natural areas.

In summary, in all activities involving native plants, genetic diversity is a pertinent topic. Yes, it is complex, but once we begin to understand it, it further enriches our experience of the world around us!

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