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Lorraine Parsons One of the most striking similarities between results of recent monitoring of some relatively disparate indices of ecosystem health is the sense that the restored wetland may be approaching some type of equilibrium. The initial post-restoration period was characterized by a heady, almost uniquely American sense of unlimited potential and growth. The initial cautious estimates of the wetland taking 10–20 years to evolve into a system even remotely resembling natural marshes was quickly replaced by a sense of wonderment, as thousands of waterbirds showed up in the first month after levees were breached, and salt marsh vegetation rapidly encroached into dying pasture grasses. In the first two to three years after restoration, numbers associated with all sorts of parameters kept quickly climbing—waterfowl, wading birds, invertebrates, salt marsh vegetation, tidewater goby, California red-legged frogs, native fish, special status breeding birds. The sky was, seemingly, the limit. However, much like the dot com phenomenon, these expectations quickly came down to earth. What has become evident at this intermediate restoration time period is that unrestrained growth is not sustainable, whether it be economic or ecological. Birds, fish, plant, invertebrates, all responded to the ecological promise of the restored wetland, but, eventually, newly created niches were occupied; overly abundant resources were depleted; and the wetland reached a sort of ecological status quo. One of the primary drivers of this initial ecological euphoria was really disturbance. Creation of new channels, removal of levees, scraping of pastures to create new intertidal marsh, and re-flooding of the historic wetland represent disturbance or a perturbation to the existing ecosystem. Many species, both native and non-native, take advantage of the new opportunities associated with disturbance to colonize or use these habitats. However, as species move in, conditions begin to stabilize, leading to fewer opportunities for species in the future. A perfect example of this can be found with some of the salt marsh rare plant species. Within the first few years after restoration, the extent of habitat colonized by rare marsh annuals such as Point Reyes bird's-beak and Humboldt Bay owl's-clover jumped significantly, spreading from existing seed wherever tidal waters reached. At this point, vegetation communities were in transition, with saltwater killing off former pasture grasses, which led to either bare ground or sparse cover. Rare plants moved into these disturbance niches and quickly spread, with the largest acreage re-distribution occurring within 1–2 years after restoration. However, by Year 10, the spatial extent of rare plants had contracted considerably. This contraction appears to coincide with continued evolution of salt marsh plants into a more mature, densely vegetated community that leaves less room for disturbance-adapted rare annual plants. A second example would be California red-legged frog use of the newly created Tomasini Triangle freshwater marsh. As this marsh was literally excavated into former pasturelands, vegetation at the outset was limited to some perimeter plantings of emergent marsh species. Open water colonized by small, floating emergent marsh species dominated most of the marsh. However, over time, already existing stands of cattails have encroached into these open waters, making habitat less attractive to breeding red-legged frogs, as well as greatly complicating efforts to track this species. This may be why some of the best remaining habitat in Point Reyes for frog ponds tends to be agricultural stock ponds that are maintained to keep emergent vegetation at a minimum. What this means is that, somewhere between Year 5 and Year 10, the Giacomini Wetlands reached some kind of dynamic equilibrium condition. As was discussed in a previous website posting from almost 10 years ago, several different models exist about ecosystem evolution. The classic Gradual Continuum Model assumes a linear model of change from disturbed condition (i.e., diked pasture) to a more natural state (e.g., tidal marsh). One of the most famous theoretical constructs of ecological change is the so-called Clementsian succession model, in which forests convert to meadow during fires and then evolve over time back to forest, the "climax" community, through successional processes. While not necessarily envisioned as a classic successional process, the evolution of the Giacomini Wetlands was anticipated to occur more or less in a linear fashion or along some type of continuum of change (i.e., A→AB→B). However, early on, it became apparent that the transition to tidal marsh would not necessarily follow this Gradual Continuum Model of habitat evolution, but more of what is being called a Threshold or Dynamic Regime Model (Hobbs and Suding 2008). While certain threshold-type models also assume progression from a disturbed condition to a more natural state with removal of disturbance factors, the progression is not necessarily assumed to be linear or to occur along a continuum of conditions. Rather, under these models, change occurs in a more discrete, step-like fashion, often with abrupt transitions between different states or conditions that require certain "thresholds" to be passed for movement from one state or condition to the next. In addition, certain perturbations can actually cause movement between states or conditions to be negative or to revert habitat back to an earlier condition. In 2009, the perturbation factor predicted to transition the wetland towards a more stable salt marsh system was high stormflow. Flooding would carve out new channels, erode old levees, and drop new sediment for rare plants and salt marsh plants to colonize. Ten years later, the Giacomini Wetlands still have yet to experience an ecosystem changer in terms of flooding. While 2017 and 2019 were very wet years, the previously dry years meant that upstream reservoirs were empty, so peak stormflows were captured behind dams and not allowed to flow down Lagunitas Creek to its wetland destination. By the time the reservoirs were full, the winter storms had eased off for the season. While disturbance is often perceived as a negative influence on ecosystem health, particularly as it can open the door to unwanted non-native plants or animals, many native ecosystems are adapted to some level of episodic perturbation or disturbance. Chaparral and some native forests require periodic fires to regenerate. Grassland health appears to be tied some level of grazing by ungulates. Based on research done in other systems, the health of tidal wetlands appears strongly associated with periodic flooding of an intensity that can widen existing channels, carve out new ones, open up vegetation canopy, and deposit new sediment for establishment of rare plants and other plants that are less able to compete in dense salt marsh communities. The importance of flooding for ecosystem development, called Flood Pulsing Theory, has been recognized both for riverine and tidal wetland systems (Middleton 2002). The lack of high-intensity flood events in Giacomini since breaching of the levees 10 years ago is driving the wetland ecosystem towards a more stable and perhaps even stagnant condition that lessens opportunities for disturbance-adapted species. This same phenomenon is observed in San Francisco Bay marshes that are highly managed or hydrologically constrained and often dominated by monocultures of either single or a few highly uber-competitive plant species such as pickleweed or saltgrass that preclude establishment of more uncommon or rare marsh plant species. Granted, disturbance can be double-edged sword, with flood flows sometimes bringing in unwanted invasives such as perennial pepperweed, as it did at Giacomini. Until more natural hydrologic, disturbance-promoting regimes can be achieved at Giacomini, the wetlands are likely to remain in somewhat of a Dynamic Equilibrium, with more tangible indices of wetland health such as birds, fish, and invertebrates unlikely to change unless there are some major climatic or hydrologic changes at a watershed or regional level. While numbers of birds, native fish, and benthic invertebrates may have reached a sustainable status quo of sorts, it should be acknowledged that less tangible indices of wetland health such as biogeochemical processes and nutrient cycles may be still evolving and not have reached any type of equilibrium as yet. Future monitoring may enable us to test these theories of ecological evolution and determine how the successional trajectory for the Giacomini Wetlands is altered by external factors such as flood pulsing, sea level rise, and increases in salinity levels within Tomales Bay waters. |
Last updated: January 8, 2024