8 minute read
Biodiversity in the Living Collection
By: Christina Varnava, Living Collections Curator
California is a biodiversity hot spot, with thousands of species, subspecies, and varieties of plants that can be found nowhere else on Earth.
You have probably read that fact in an Ironwood article before or maybe you’ve seen it on our website or perhaps you have heard it in a talk, and it’s certainly true. You may have also been left pondering a simple question: Why is that the case? What exactly makes California (and especially the California Floristic Province, or CA-FP) so special? It’s not a simple answer, and botanists have been discussing this puzzle for a long time. Papers and books have been written; tears have been shed ... We could create an epic tome on this subject, but for now, we’ll just start with the broad strokes. I’ll include some sources at the end of the article if you want to do a deeper dive. Read on, and we will explore some of the reasons that California has such incredible plant diversity, while highlighting some of the incredible specimens you can find in Santa Barbara Botanic Garden’s Living Collection.
Why Is California So Special?
To begin to answer the question of why California is so rich with plant diversity, we’ll need to think a bit about what California is like. Visualize traveling from the south to the north and think about all the changes in the landscape that you would experience. California has a lot of diversity in habitat, topography, geology, and climate. In a relatively short space, you can traverse from a sandy sea-level habitat with cool breezes to blazing-hot alkaline soil in the deserts to high-elevation coniferous forests with granitic soils.
For plants, these multifaceted habitats mean lots and lots of niches. In ecological terms, an organism’s niche is a specific set of environmental conditions where it thrives. Organisms in the wild are all competing for a shared pool of resources. For example, plants need specific amounts of light, water, and nutrients in the soil. More niches mean that many different types of plants can coexist together since each species can get the amount of water, nutrients, and light it needs without facing as much competition. You can think of these niches as little islands of habitat where plants can thrive. These islands have two different effects that boost plant diversity: They can help shelter species that might have otherwise gone extinct, and they can catalyze or accelerate the formation of new species. These factors aren’t the whole story, but they are a huge part of why California’s flora is so special.
The Singularity of California’s Flora
Although many new species form here, California’s amazing biodiversity is boosted thanks to low rates of extinction. New species form, and older ones may become extinct, and these events generally happen over a very long time at a rate that can be analyzed by scientists. In comparison with other places, the rate of extinction in California over the past several million years has been relatively low. In other words, California’s diverse landscape has served as a refuge for many species that may have otherwise died out.
An especially interesting part of plant biodiversity in California is due to so called “relictual” plants. These are types of plants that have no remaining close relatives. Generally, these groups once had more living relatives which went extinct sometime in the distant past for various reasons. Some of the most fascinating and iconic members of the California flora are these types of plants that lack any surviving close relatives. These no longer occur anywhere else on Earth, but they have survived here in the CA-FP. The towering coast redwood (Sequoia sempervirens), Matilija poppies (Romneya coulteri and R. trichocalyx), Channel Islands ironwood (Lyonothamnus floribundus), tree anemone (Carpenteria californica), and the California bay laurel (Umbellularia californica) are all examples of these relict groups that you can find growing in the Garden. There are others you might find as you explore California, including the fascinating snow plant (Sarcodes sanguinea) and the massive giant sequoia (Sequoiadendron giganteum).
The diverse landscape of California can also help create new species in addition to sheltering older lineages. As plant populations spread and encounter various soil types, climates, and elevations, those plants can adapt and change to be more suited to these environments, eventually evolving into new species. California is the center of diversity for a few groups of plants. This means that more species of these groups are found in California than anywhere else.
Manzanitas (Arctostaphylos spp.) are a great example of this phenomenon. Manzanitas are an iconic part of the California flora: an astonishing 106 of our 107 known types can be found within the CA-FP. Our Living Collection has representatives of 60 species and subspecies, as well as 34 hybrids, selections, and cultivars. Plants growing in our Manzanita Section encompass habitat from the lush redwood forests in Del Norte County to Otay Mountain near California’s border with Mexico.
Finally, perhaps the most special example of California’s plant biodiversity: the Channel Islands. The Channel Islands are a major contributor to the diversity of the California flora. Islands in general are well known to have high levels of biodiversity. New species can form more easily on islands, since any organisms that get to them are automatically isolated from other populations. The Channel Islands have many unique plants. Some of these are new species that have formed, whereas others are very old lineages of plants that have managed to survive there while they went extinct elsewhere. For example, fossil evidence indicates that in the distant past the island oak (Quercus tomentella) was once a far more widespread species. Over time it became isolated to the Channel Islands, where it thrives today. The Channel Islands ironwood shares a similar story, and its unique leaves can be found in fossils as far east as the Rocky Mountains. The same goes for other iconic species like the Santa Rosa Island Torrey pine (Pinus torreyana ssp. insularis).
Our Living Collection’s Role in Conservation
Our Living Collection is a thriving example of biodiversity conservation in action, with multiple specimens of each of these unique plants mentioned that come from across their native ranges in the wild. Some of these plants are highly desirable for the home gardener, but others are more challenging to cultivate or not suited for home garden conditions.
The Garden is home to the largest collection of Channel Islands plants on the mainland, which includes each of those species mentioned and many others. All told our collection houses just over half of all species endemic to the Channel Islands. The collection is also home to many specimens of widespread plants that happen to occur on the islands. These plants do not represent new species, but they still harbor genetic diversity due to the fact that the populations are isolated on the islands. For example, a few of the bigleaf maple trees (Acer macrophyllum) that live in the Backcountry Section were collected from Santa Cruz Island. These trees have adapted to different conditions than some of their mainland relatives and display very different leaves with much narrower lobes and a far more tough and leathery texture, a sign of adaptation to drought conditions. This collection is an incredible testament to decades spent by researchers at the Garden studying and protecting these plants in the wild.
California Plants and People Need Biodiversity
These pieces are all part of the story of California’s rich plant diversity. From new species emerging to ancient lineages finding shelter, California’s plants seem to appreciate the diverse landscapes as much as humans do. As you explore the state, I hope this perspective helps you to appreciate the plant diversity that California has to offer.
If you’d like to read more about the Garden’s (and other botanic gardens’) role in plant conservation, visit: BGCI.org/about/botanic-gardens-and-plant-conservation.
Kier, G., Kreft, H., Lee, T. M., Jetz, W., Ibisch, P. L., Nowicki, C., Mutke, J., & Barthlott, W. (2009, June 9). A global assessment of endemism and species richness across island and mainland regions. PNAS, 106(23). https://doi.org/10.1073/pnas.0810306106
Lancaster, L. T., & Kay, K. M. (2013, April 1). Origin and diversification of the California flora: Re-examining classic hypotheses with molecular phylogenies. Evolution, 67(4), 1041–1054. https://doi.org/10.1111/evo.12016
Raven, P. H., & Axelrod, D. I. (1995). Origin and Relationships of the California Flora. California Native Plant Society.
