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Ecological succession

By Shannon Lee

Although you might not be familiar with the term “ecological succession,” you’ve seen it play out if you live here in the Valley of the Moon. The biodiverse mosaic of grassland and oak woodland habitats in our county are the product of ecological succession. It all starts with disturbance. From a biological perspective, disturbance is anything that disrupts the nature of interactions between living organisms, their abundances, or distributions. These disturbances can be grand (hurricanes, tsunami) or small (tornado, single tree fall) on a spatial scale. They can be frequent (seasonal storms) or very infrequent (glacier scour) on a temporal scale. They can be profound (lava flow) or minimal (frost event) on an impact scale. Disturbances can be natural (dust storm) or human-caused (dredging of a wetland). No matter the scale or driver, a disturbance changes the biological community. The pattern of community recovery or redevelopment after the disturbance is known as ecological succession.

Even before the more recent firestorms, our natural areas have felt the impacts of a variety of disturbances: flood, drought, human development, earthquakes, tree falls, landslides, etc. We can see the impact around us; there are clues in the ecosystems. The biological communities in our oak woodlands show us the patterns of ecological succession if we look closely. Some areas up on the Mayacamas Range and Bennett Valley Ridge are still very barren, having been stripped down to a primary successional state by the intensity of the 2017 fires. Other areas where the fire wasn’t as intense are in varying stages of recovery. On the eastern slope of Sonoma Mountain, our forest is mostly in a stable late successional state with no record of fire disturbance in over one hundred years. Extremely intense disturbances can strip all life from a habitat. At its most profound, only the bare rock remains. Recovery of what was there before (if even possible) is a long process, requiring the work of organisms like lichen that can break down the rock into soil, and microbes which make that soil hospitable to the germination of seeds. Most disturbances are not on that extreme intensity scale and, at the very least, leave behind soil, bacteria, fungi, spores, and seeds. The first organisms to take hold in this cleared landscape are called pioneer species. They may be plants that have seeds easily dispersed into disturbed areas or they may have sprouted from seeds that remained safe deep in the soil. These are typically fast-growing opportunists. Some animals, such as burrowing mammals, insects, or those on the wing, can also be pioneers. Next in succession come other organisms, and the competition for resources (space, light, water, prey) can really take off. These intermediate stages of succession are biologically fascinating due to the complexity of interactions seen between and within species. Over time, only the best locally habitat- and community-adapted, plus the most competitively dominant organisms, remain. Stability ensues and the community is said to be at climax.

When asked about disturbance and succession, Sonoma Ecology Center’s (SEC) senior project manager Caitlin Cornwall says, “In the natural landscapes of California, fire acts as a reset. If fires burn too frequently for a particular plant community — this is happening in the chaparral of southern California — biodiversity can plummet, but generally, biodiversity and wildlife are helped by fire. SEC is using controlled fire to manage invasive grasses and rejuvenate wildflowers at the Van Hoosear Wildflower Preserve, fostering community science projects on fire recovery in Sugarloaf Ridge State Park (most of which burned twice within three years), and teaching landowners how to create beautiful, biodiverse defensible space through our Resilient Landscapes program.” A biological concept that she is hinting at here ties together the process of ecological succession with measures of biodiversity. In areas with frequent or profound disturbance and in areas with very infrequent or minor disturbance, biodiversity is low in the biological community. In areas with intermediate intensity and frequency of disturbance, biodiversity is higher. This concept, developed by California ecologist Dr. Joseph Connell, is known as the Intermediate Disturbance Theory.

If you take a hike up at Sugarloaf State Park, you see a landscape shaped by these biological processes. Over the course of just a few hours you can experience habitats that are in varying stages of ecological succession: a primary state landscape with obvious charring and barren of most plant life; an area bursting with fast-growing pioneer plants and wildflowers that have waited for fire to trigger their return; biodiverse intermediate-stage sections of grassland and forest teeming with the bustle of predator-prey relationships and competitive interactions; and finally, areas that haven’t been touched by disturbance in a very, very long time. These are the climax communities.

Sonoma State University professor Dr. Lisa Patrick Bentley, together with her graduate students, explores ecophysiology, plant-environment interactions, fire ecology, and forest dynamics in the context of climate change. Inherent to her lab’s research are disturbance and ecological succession. When several of her newly established research sites in the county burned in 2017, a natural side-experiment was dropped in her lap. The mosaic of recovery and change can now be tracked through time and compared to nearby undisturbed sites. As Dr. Bentley says, “Natural wildfire wasn’t an element of our original experimental design, but field research often throws you curves and opportunities, and so flexibility is key. Now we are writing grants and expanding our terrestrial laser scanning measurements to quantify forest structure and evaluate change following disturbance in Sonoma County.”

Ecological succession is not specific to a single type of habitat, nor is it restricted to terrestrial systems. In the ocean, we study successional transitions in the redevelopment of a kelp forest or within the tidepools along the shore. In tropical areas, a major area of study is ecological recovery of coral reefs and sea grass beds after hurricane disturbances. An important takeaway message is that biological communities are in constant flux and disturbances are natural “adapted” aspects within these environments.

Shannon Lee, PhD, is a Glen Ellen resident and an instructor in the Biology department at Sonoma State University. She has been a science educator for 20 years, having taught previously at UCLA and California State University Northridge.