At the Convergence of Ecoregions
By Conservancy Science Director Mike White, PhD
One of the things that we at the Conservancy like to talk about when discussing the biodiversity of Tejon Ranch is its location at the “convergence of ecoregions.” What does that mean and why does it matter?
An ecoregion, short for ecological region, is a relatively large area that supports characteristic and unique groups of organisms. For example, if you have been there, you know that the Mojave Desert ecoregion supports plants and animals that are very different from those that occur in the Sierra Nevada ecoregion. This is because the climate, topography, geology and soils, and evolutionary history of the land within the Mojave region is different than that of the Sierra Nevada region, and different assemblages of plants and animals have evolved in those regions as a result.
Descriptions of ecoregions are developed by multidisciplinary teams of scientists to provide information to support the work of natural resource planners and managers. In places where ecoregions meet each other or converge, plants and animals from the adjacent ecoregions can co-occur, resulting in higher numbers of species (i.e., a higher biodiversity). Often unique groupings of species drawing from both ecoregions occur in the convergence zone as well.
Jepson Floristic regions of California. From "A Flora of California."
The 270,000-acre Tejon Ranch sits at a unique location in California where four of California’s 10 major floristically defined ecological regions converge. These ecoregions are: Mojave Desert, Sierra Nevada, Southwestern California, and Great Central Valley. The convergence of these four ecoregions at Tejon Ranch is in large part responsible for the very high biological diversity that we see on the Ranch. Since species from each of these ecoregions occur on Tejon, the overall number of species is higher than if the Ranch was located within any one of the ecoregions alone.
As an illustration of the high biodiversity at Tejon, Nick Jensen—a doctoral student at Rancho Santa Ana Botanic Garden—has been developing a “Flora of Tejon”, or a list of every plant present on the Ranch. (Or at least those that Nick has been able to find in four years!) Nick has identified 1,030 taxa (species, subspecies, and varieties) of plants on Tejon, of which 885 (or 86%) are native. This represents over 13% of the native plants known throughout California, on a piece of property 0.25% of the area of the State. This is what conservationists refer to as a “biodiversity hotspot,” or an area that has a disproportionately high number of species relative to its area. Biodiversity hotspots are often targeted by conservationists because of their high conservation “bang for the buck” and Tejon is no exception.
Jepson floristic ecoregions converging on Tejon Ranch (red outline).
Grasslands and oak woodlands of the Great Central Valley.
Blue oak woodlands are characteristic of the foothills around the Central Valley.
White fir forests are characteristic of high elevations of the Sierra ecoregion.
Joshua trees and yellow-blooming rabbitbrush, characteristic of the Mojave Desert ecoregion, co-occur with grey pines and Valley oaks from the Sierra Nevada and Great Central Valley ecoregions in Sacatara Canyon.
The convergence of ecoregions also produces unique groups or communities of species. By definition, we are at the edges of ecoregions in areas where they converge. So, species that are characteristic of an ecoregion, think Joshua trees in the Mojave, are often at the limits of their range in convergence zones, and can co-occur with their neighbors from the adjacent ecoregion. Therefore, in these ecoregional convergence zones, we have unique mixtures of species that we often don’t see anywhere else. A classic case at Tejon is the co-occurrence of Joshua trees, rabbitbrush, Valley oaks, grey pines, and chaparral shrubs representing the convergence of the Mojave, Sierra Nevada, and Southwestern California ecoregions.
Joshua trees and yellow-blooming rabbitbrush, characteristic of the Mojave Desert ecoregion, co-occur with grey pines and Valley oaks from the Sierra Nevada and Great Central Valley ecoregions in Tejon’s Sacatara Canyon.
Joshua trees are characteristic of the Mojave ecoregion.
As hard as it is to imagine, it is also important to remember that the boundaries of these ecoregions and the characteristic suites of species they support have changed over very long time frames from millennia of plate tectonics, mountain building and erosion, changes in global temperatures and ocean circulation patterns, and ongoing evolutionary processes. (Remember Tejon Ranch was once ocean-front property!) The biodiversity that we see today at places like Tejon is just a snapshot in time of a long, complex, and ongoing process unfolding around us, shaping the biodiversity of the landscape. We at the Conservancy are stewarding this biodiversity for future generations and hope that you will join our efforts by becoming a member today.