Restoration: Untangling the Food Web: Introduction

Introduction

Restoration has brought great changes to the Giacomini Wetlands landscape. Some are easy to see. There's more water, less pasture, more ducks and shorebirds, more frequent visits by pelicans, and even the occasional bald eagle. However, some of the changes that have occurred--or may occur in the future--are not so readily observable. Ironically, it's these changes that often have the strongest influence on whether these larger animals come to visit--and stay.

Animals need food. Some animals eat plants, but most on forage on smaller animals, some of which are too small to see with the naked eye. Most of the time, rather than these microscopic organisms being consumed directly by large animals, they are instead consumed by slightly larger organisms, which are, in turn, consumed by even larger organisms, thereby creating a food "chain" or "web" that links the smallest organism to the largest.

For example, larger birds such as northern harriers and red-tailed hawks will eat other birds and their young. Snowy egrets and other egrets and herons have a rather cosmopolitan diet born of opportunity, with snails, crabs, small fish, mice, and even amphibians on the menu. The diet of ducks tends to be slightly more restrictive. With the exact composition depending on the taxonomic tribe or subfamily (diving vs. dabbling ducks) or even species, ducks generally eat a mixture of aquatic insects and snails, aquatic invertebrates, and aquatic vegetation. For example, canvasback females often prefer immature aquatic insects and snails, while lesser scaups have developed specialized bills for straining small crustaceans (amphipods) from the water (Ehrlich et al. 1988).

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Shorebirds either feed in the water or either deeply or shallowly probe the mud for snails, worms, insects, small crabs, and even small fish. As with ducks, shorebirds have developed resource portioning or specialized diet interests that help them to get the food they need even if many hundreds or thousands of shorebirds are present. For example, as the tide recedes, least sandpipers will forage on insects in exposed or drier mudflat areas; dowitchers probe the mud in shallow water for mollusks; and greater yellowlegs feed in deeper water, snatching small fish from the water (NRCS 2000). In turn, the snails, worms, and crabs that are consumed by egrets, ducks, and shorebirds consume detritus, zooplankton, and phytoplankton--in fact, much the same diet as resident goby fish, which feed on these same items as well as worms. Zooplankton, small microscopic invertebrates swimming in the water, usually forage on detritus, phytoplankton, and even smaller species of zooplankton.

The cornerstone of the food web is phytoplankton, which receives all the energy it needs from sunlight. In this way, the largest organism (northern harrier and hawks) is linked to the smallest (phytoplankton) through a complicated web of food interactions. Interestingly, one of the more fervent debates in estuarine science revolves around whether phytoplankton or detritus--broken down plant material--really represents the cornerstone of the food web. Many studies now show that systems are not necessarily dominated by just one energy-transfer mechanism. Some systems show spatial heterogeneity in food chain preferences. In a Dutch estuary, the food web relied primarily on detritus in the upstream, brackish portions of the system, particularly around the estuarine turbidity maximum zone, while the downstream, seaward portions exhibited more of a dependence on seasonally fluctuating high primary production by phytoplankton (Hummel et al. 1988). In some cases, status can change with time: San Francisco Bay has been historically characterized as a low productivity estuary due to lower phytoplankton biomass estimates than other temperature estuaries, but decreases in turbidity and shellfish numbers have allowed phytoplankton biomass to explode in recent years (Cloern et al. 2006).

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While many species are generalists or capable of switching food sources over the seasons, others are more specialists or more reliant on particular diet items. The consequences of this diet item not being present or being present in low numbers can have devastating effects on these particular higher order organisms. The nearby Sacramento-San Francisco Bay Delta represents an incredible tale of how changes at the lowest level of the food web can reverberate up the chain and ultimately have a large impact on an ecosystem. Prior to the 1980's, the central or immediate areas of the Delta near Suisun were dominated by a phytoplankton-driven foodweb, a stable mesoplankton population dominated by a naturalized type of copepod called Eurytemora affinis, and large macrozooplankton typified by San Francisco bay shrimp (Crangon fransicanum) and mysids (Neomysis mercedis; Orsi and Mecum 1986, Modlin and Orsi 1997, Kimmerer 1998). These organisms fed native filter feeders such as the northern anchovy (Engraulis mordax), and planktivores such as delta smelt and juvenile salmon.

Since the 1980's, the San Francisco Bay food web has changed dramatically due to increases in turbidity and introduction of non-native aquatic species. The introduction of the Asian clam (Corbula amurensis) via the ballast water of foreign ships has since led to a 10-fold decline in plankton density and resulted into more transfer of food energy into the "benthos" or mud--where the clam lives--than into the water (Kimmerer 1996). Clam waste and other decaying organic material is then broken down by bacteria or grazed upon directly by flagellates, rotifers, and ciliates, which are in turn consumed by non-native microzooplankton such as Limnoithona tetraspina, a cyclopoid copepod that appeared in brackish zones of the San Franciso-Sacramento Estuary in the 1990s (Bouley 2006). Because of its small size, Limnoithona is generally not eaten by larger predators, particularly fish, making it a dead end of sorts in the "food chain."

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In addition to transferring more of the food energy into the mud, introduction of non-native species and increases in turbidity of waters have changed the composition of invertebrates within San Francisco Bay waters. So-called secondary production is now dominated by copepods rather than meso- and macrozooplankton. For example, the naturalized native calanoid copepod, Eurytemora, that was prevalent in the 1980s has largely been replaced by an introduced calanoid copepod, Pseudodiaptomus forbesi (Orsi and Walter 1991, Kimmerer 1996). This organism persists by maintaining a source population in upstream, more freshwater areas where the Asian clam does not occur due to salinity tolerances (Durand 2006). Because Eurytemora is a more brackish species and does not occur upstream, it is more vulnerable to predation by the clam and to apparent competition with Pseudodiaptomus. Effects of invasion are not just restricted to smaller zooplankton: larger species such as Neomysis have also been affected, with this native species largely being replaced by the introduced Acanthomysis bowmani, which persists at lower densities (Modlin and Orsi 1997, Kimmerer 1998).

All of these changes have seemingly had huge ramifications for the San Francisco Bay-Sacramento Delta food web, with the impacts of shifts in micro and macro invertebrates reverberating right up the web to larger organisms. These changes are not surprisingly one of the causes attributed to declining fish stocks. For example, the northern anchovy, Engraulis mordax, was until the 1980's quite abundant in the Low Salinity Zone, until its range in the Estuary became restricted to the Central and South Bays (Kimmerer 2006). This shift in occurrence is believed to have occurred because of the decline in plankton availability in areas where the Asian clam occurs. Mysid decline has been linked to the subsequent decline in a number of fish species in the Estuary in the 1980's and 90's, because they represented an energetic conduit between plankton and planktivorous fishes, including juvenile fishes, sturgeon, Chinook salmon, and American shad (Orsi and Knutson 1979, Modlin and Orsi 1997). Several key species, including delta smelt, longfin smelt, striped bass, and threadfin shad, have been declared "species of interest" because of a stepwise decline in abundance beginning in 2001 that is coincident with changes in the estuarine food web.

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Even Tomales Bay has not been immune to the impacts of invasion. With the opening of the estuary lying right off one of the major shipping lanes for San Francisco Bay, organisms contained in ballast water are likely to reach its waters. While there are no definitive numbers, the recent all-taxa biological inventory in Tomales Bay found a considerable number of non-native species. However, unlike San Francisco Bay, the historical ecology of the Bay is not well known, so it is difficult to determine how much impact to the historical food web that these recent invaders have had. Some of the more well-known invasive species within the southern portion of the watershed near Giacomini Wetlands include the green crab (Carcinus maenas), signal crayfish (Pacifastacus leniusculus), Korean shrimp (Palaemon macrodactylus), New Zealand boring isopod (Sphaeroma quoyanum), and a number of non-native fish species, including mosquitofish (Gambusia affinis), yellowfin goby (Acanthogobius flavimanus), and game fish such as largemouth (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu).

Ultimately, understanding the structure of the prey base or food selection available both prior to and after restoration will help us to better understand changes in use of the restored habitat by larger animals--ones that are more easily seen. For a number of years prior to restoration, monitoring of zooplankton, benthic invertebrates, and fish was conducted to determine what species were using the Giacomini Ranch during its dairy period and what their relative numbers were (Pre-Restoration). This monitoring was continued during the period when dairying was discontinued, but there was either greatly reduced grazing or no grazing at all, but levees had not been removed (Passive Restoration). Here, we present some of the results from the first two years after Full Restoration to evaluate how--if at all--assemblages and numbers of invertebrates and fish have changed in response to the rapid conversion of pasture to salt, brackish, and, in some cases, freshwater marsh.

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Changes in Zooplankton, Benthic Invertebrate, and Fish Communities

As described earlier, zooplankton are typically pelagic or water-associated micro-invertebrates, although many of the same species can also occur within the sediment or benthic. Certain non-zooplankton species such as nematodes and insects can also occur in the water. Benthic invertebrates live either on the surface or within the mud itself, with the latter being either near the surface or deeper within the mud. Invertebrates are separated into several orders such as Calanoids, Copepods, and others and are often difficult to identify conclusively beyond this level. Zooplankton and benthic invertebrates differ in the salinities or substrate type (e.g., mud or sand) that they can tolerate--or prefer. In this aspect, zooplankton and benthic invertebrates often occur in distinct assemblages that change with substrate type or salinity or the location of the system along the freshwater-saltwater gradient. These assemblages are influenced by other aspects, as well, including how polluted or contaminated waters are, with certain species being better able to tolerate more polluted conditions. In reality, in complicated systems such as Giacomini, where salinities dramatically vary both spatially and temporally, assemblages tend to be more indistinct than they would be in systems with a more uniform or consistent salinity.

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Last updated: February 28, 2015

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