Rare plant introductions can take years of planning, and it is very difficult to know how plants will respond. Unlike animals, plants can't move to more desirable locations to avoid predators or avoid threats like human trampling or weeds. When considering the introduction or “release” of plants into the wild, this sedentary reality dictates many of our actions.
Of course, the exact placement, time of year, and site quality play a big part in these choices. We must also consider the species’ ability to reproduce. Because plants themselves don’t move, the flow of genetic information among populations is limited by the movement of pollinators. Some plants are pollinated by wind, some by bees and butterflies, others by tiny flies or beetles. Genetic exchange can be hindered or stopped by physical distance or barriers between populations, or by a decline in the pollinator population. Because of this, it’s relatively easy for plant populations to become genetically isolated from one another, especially in developed areas.
When introducing rare plants to the wild, we want there to be a mechanism for pollination among individuals in a population, but we don’t want to unintentionally influence the genetics of isolated populations. Ideally, we look to genetic testing to inform us of the relatedness of in certain populations, and their relative isolation from one another. We wouldn’t mix seeds from different populations, for example, unless we knew that the populations were currently sharing genetic information, or that they were recently isolated and showed little to no genetic divergence.
Taking this all into consideration, planning for a rare plant introduction is no easy task. Working with our partners California State Parks and The Chaparral Lands Conservancy, it took a few years before we were prepared to reintroduce a tiny local species in early January of 2019. It had never been done before.
Throughout the past 4 years, Dudleya brevifolia has been a focal species of the San Diego Zoo Global Plant Conservation team. This tiny succulent species is known from just five existing populations. All five are located in an area that is roughly eight square miles, centered around Torrey Pines State Park. It grows on the edges of sandstone bluffs, in soil known as Torrey sandstone.
This tiny plant lives in one of the most beautiful locales on Earth, with expansive views of Pacific waves, wind swept pines, and the setting sun, but humans love this view too. While once more widespread, suitable habitat for this species has largely been destroyed, bulldozed to build homes, roads, sidewalks, and restaurants.
With information about the species’ population genetics forthcoming, our current conservation plan for Dudleya brevifolia maintains the genetics of each population independently from the others. We do not currently mix seed or individuals from separate populations.
These plants are tiny, and though the five populations are geographically close, they are separated by large physical barriers like river valleys or roads and housing developments. It is unlikely that the tiny pollinators travel among populations, and unclear how long they have been separated and growing without this genetic flow. Distinct populations may have developed unique traits that could be lost if the genetics of populations were comingled.
The downside of this conservative treatment is that the larger, more stable populations are currently not being used as a source of seed for the smaller, threatened populations.
This is where we can help.
Relegated to a small cluster of intact bluff edges, the smallest and most imperiled population of this species contains an alarmingly small number of individuals. Current estimates suggest there are around 50 individuals or fewer in the population, and after the low rainfall season in 2017-2018, fewer than five individuals were counted.
When we began working with this species we soon discovered it was a good candidate for seed bulking and population augmentations. Seeds germinated very quickly, grew at a rapid rate, and were reproductive within 5-6 months. One seed collected from the wild could be grown into a plant that produced hundreds of seeds relatively easily.
Wild seed from the small population was grown into a population of 132 individuals at our facilities in 2017, and San Diego Zoo Global was housing more than double the number of individuals than were found in the wild population.
After much discussion and planning, we eventually decided to use 46 of these plants as part of a reintroduction experiment. We would augment the small, imperiled population using our wild, captive grown plants.
On a cool, crisp winter morning, after a few days of light rain, we were ready to “release” our Dudleya back into the wild!
TO BE CONTINUED...