Exotic Species Cover Introduction Sanctuary Program Accomplishments Beach Systems Rocky Intertidal and Subtidal Systems Open Ocean & Deep Water Systems The Physica Environment Wetlands and Watersheds Endangered and Threatened Species Marine Mammals Bird Populations Harvested Species Exotic Species Site Profile: Ano Nuevo Island Human Interactions Credits Exotic Invaders in Elkhorn Slough: Who Are They and Where Did They Come From? Large bays with international ports are known to harbor diverse populations of exotic species. For example, about 150 non-indigenous invertebrates have been documented in San Francisco Bay, 100 in Pearl Harbor, and fifty in Puget Sound. These invaders arrived by various means, but recently the most significant mechanism of introduction has probably been ballast water, which is pumped into vessels at one port and discharged at another. As a result, the fauna of many large bays has become homogenized, losing much of its regional character as native species are replaced or marginalized. While biological invasions of large bays have received much attention lately, very little is known about the extent of this problem in smaller embayments. Are estuaries without international shipping safe from exotic invaders? Figure 1: There was no decline in the cumulative number of exotic species collected at Elkhorn Slough over time (each sampling period represents only about ten hours of search effort), suggesting many more invaders remain to be found. In a nutshell, our study at Elkhorn Slough revealed that the answer is a resounding "no." We have recently documented fifty-five exotic invertebrate invaders in the Slough. Of these, twenty-four had not previously been known to occur there. We carried out a rather modest search for invasive invertebrates and have calculated that we collected one new exotic species for every 1.5 hours of search effort (Figure 1). Therefore, more hours spent searching will doubtless turn up more exotic invertebrate species. In addition, there are likely to be many other exotic species in other groups that we did not examine (dinoflagellates, fishes, bacteria, etc.). Overall, exotic species account for only about 10 percent of the Slough's rich invertebrate fauna. However, due to the incredible abundance of some of these species, we suspect that they account for a far more significant proportion of the total invertebrate biomass. For instance, the Japanese mud snail, Batillaria attramentaria, is found in virtually every high intertidal area of the Slough, often in great densities (see next article). Another conspicuous invader of the Slough is Hymeniacidon sinapium, a bright orange sponge that forms massive aggregations on Slough mudflats and channels (Figure 2). The filtering activities of this sponge may be influencing planktonic communities at the Slough. European green crabs (Carcinus maenas) also occur in the Slough. Voracious predators, these crabs have been shown to alter clam and worm communities at Bodega Bay dramatically. As a final example, the burrows of a small isopod resembling a pill bug, Spheroma quoyanum, riddle almost every bank around the Slough and likely increase the already high erosion rates in the Slough. Exotic invaders at the Slough therefore have the potential to influence native communities and physical habitat structure negatively. Some exotic species found in Elkhorn Slough were introduced with oysters, which were cultured there from the early 1900s to the 1970s. Japanese and Atlantic oysters were imported, and other creatures inadvertently hitched a ride with them. However, new introductions have continued unabated in the fifty years since oyster-culturing dwindled and ceased. Figure 2: Massive aggregations of an invasive sponge, Hymeniacidon sinapium, in the Elkhorn Sough Reserve. Due to the similarity in exotic species composition between the Slough and San Francisco Bay (about 90 percent of the invasive species found in the Slough are also found in the Bay), we suspect that most of the recent introductions into the Slough occurred indirectly via the Bay, rather than directly from distant waters. Many exotic species are first introduced to this coast at major harbors via international shipping. After becoming established and abundant, they may then spread up and down the coast, for instance on the hulls of regional boats or as larvae travelling by natural currents. The effect of international shipping, including ballast water dumping, is thus not limited to areas with major harbors but rather reverberates up and down the coast to smaller and more isolated estuaries. Appropriate management to limit future invasions involves not only minimizing direct transport vectors that bring exotic species from distant shores to major ports -- for instance by treating ballast water -- but also requires measures such as frequent cleaning of small boats to reduce intraregional transport from port areas to other smaller embayments. Kerstin Wasson1, Chela Zabin2, Laura Bedinger3, Cristina Diaz3, and John Pearse3 1Elkhorn Slough National Estuarine Research Reserve 2Department of Zoology, University of Hawaii 3Institute of Marine Sciences, University of California Santa Cruz A Battle between Snails Elucidates Details of Biological Invasion A non-native Japanese mudsnail, Batillaria attramentaria, was introduced to Elkhorn Slough and other sites along the West Coast of North America fifty to seventy years ago with imports of Japanese oysters. Batillaria has been displacing a highly similar native mudsnail, Cerithidea californica (sometimes called the "California horn snail"), in several salt marshes along our coast, such as Tomales Bay and Bolinas Lagoon. Cerithidea however no longer exists in Elkhorn Slough; its absence there is suspected to be due to competitive exclusion by Batillaria. Batillaria achieves superiority over Cerithidea through a combination of three major advantages. First, Batillaria is a better competitor for the diatoms that each species grazes off the surface of the marsh mud. Notably, Batillaria processes its food more efficiently and can thus convert the shared, limited food resource into snail tissue at a faster rate than Cerithidea. Second, both snails are susceptible to infection by trematode parasites, but Batillaria is typically infected at a lower rate. Because these parasites disable a snail's reproductive system, this lower infection in Batillaria equates to a major advantage in the number of eggs that a Batillaria individual can produce in its lifetime compared to a Cerithidea. (Interestingly, one of the parasites infecting Batillaria is itself a non-indigenous species, thus adding to the growing list of non-indigenous species in Elkhorn Slough.) Finally, Batillaria has a lower background mortality rate; that is, Batillaria simply has a lower probability of death than Cerithidea of the same age. sIn certain pockets of Elkhorn Slough Batillaria achieves its densest populations (up to 10,000 snails/m2 a) in its entire non-native distribution along the North American Pacific coast. In order to test the relative importance of Batillaria's three advantages over Cerithidea, we defined the biology and interactions of the snail species as connected mathematical expressions. Output of this mathematical model of the snails' interactions identified which advantage was most responsible for the exclusion of Cerithidea by Batillaria, and thus suggests means of human intervention that may more successfully control or delay the impact of the exotic species. The model indicated that displacement and ultimate local exclusion of Cerithidea by Batillaria takes between fifty-five to seventy years. Historical and museum records and analyses of long-term population patterns corroborate this result, indicating that the model accurately captures the biology of the species. Importantly, the long time to Cerithidea exclusion highlights that, over short time scales, harmful invaders may appear quite innocuous. Additionally, the model indicated that competition and susceptibility to parasites are relatively weak mechanisms in driving the overall success of Batillaria. Rather, we learned that Batillaria's lower background mortality rate plays the key role governing its displacement of Cerithidea. Therefore, creative management techniques can focus attention on equalizing species-specific differences in mortality to neutralize Batillaria's invasion most effectively. Despite the impending local extinction of Cerithidea and an immediate increase in the invader's population, all aspects of Cerithidea's biology in the model were disconcertingly slow to exhibit signs of impact from the invasion. Most of Cerithidea's biological parameters (such as density, population biomass, egg production, mean size, proportion parasitized, and individual growth rate) took at least twenty to twenty-five years from Batillaria's first introduction to show significant declines. With such a pronounced delay between an invader's introduction and the manifestation of its harmful effects, biological monitoring programs currently used to prioritize intervention for imperiled native species appear inadequate. Rather, the studies of Batillaria-Cerithidea interactions indicate that mechanistic field experiments and mathematical modeling of native-invader interactions may provide a better, earlier warning of impending invader impact on native biota. Jeb Byers University of California Santa Barbara University of Washington Home | Introduction | Visitors | Education | Research | Protection | Calendar Foundation | Search Credits For comments or question please refer to the Webmaster Last modified on: Jan 15, 2000