Silliman_ch03.qxd 2/23/09 1:20 PM Page 41 3 Invasive Animals in Marshes biological agents of change James E. Byers Relative to other marine systems, salt marshes and estuaries are highly susceptible to invasion, and impacts by exotic species in these systems seem particularly pronounced. These impacts range from purely trophic and competitive effects that can lead to replacement of native species by exotics, to physical transformation by exotic species that engineer habitat and alter large-scale abiotic and hydrographic properties of the marsh environment. I discuss several examples of each of these, as well as three mechanisms that promote high establishment rates and strong competitive effects of nonnative species in marshes. Although the problem of exotic species in marsh systems is substantial, marshes’ tractable, discrete boundaries make intervention more successful than in other marine systems, such as the open coast. However, protocols and policies (ideally standardized at a national or international level) need to be in place for eradication or containment of incipient invasions, which often require fast action to be effective. Protocols and monitoring efforts should aim not only to detect newly introduced species but also to quantify the dynamics and impacts of established invaders to enable prioritization of intervention efforts. Estuaries and associated marshes are a heavily invaded habitat that must be well managed to mitigate the increasing ecological impacts of exotic species on native species and the valuable ecosystems services they provide. Salt marshes are one of the most anthropogeni- estate. Kennish (2001) calculates that more than cally impacted marine ecosystems (Cairnes 1993; 50 percent of original tidal salt marsh in the Kennish 2001; Nicholls 2004). Historically, phys- United States has been lost through such physical ical and chemical impacts to salt marshes have alterations. Chemical impacts on marshes largely been the most conspicuous and influential. result from the close proximity of this marine Physical changes include extensive habitat con- habitat to humans and the associated pollutants version (e.g., filling and dredging) and altered hy- they produce. That is, salt marshes are typically drography that stem, especially in recent times, the outlets for watersheds where large volumes of from marshes occupying valuable coastal real anthropogenic pollutants are deposited (Fox et al. 41 Silliman_ch03.qxd 2/23/09 1:20 PM Page 42 1999; Sanger, Holland, and Scott 1999; Holland recommendations for how to best monitor and et al. 2004). Furthermore, as the large number of manage salt marshes against these threats. marsh Environmental Protection Agency (EPA) Although the focus of this chapter is animal Superfund sites attests, marshes historically have invasions, most of the messages here are ger- directly received large quantities of contaminants mane to invasions of all types. because they were perceived as a convenient, val- ueless dumping ground. ESTUARIES, INCLUDING THEIR While these physical and chemical impacts ASSOCIATED MARSHES, ARE THE MOST have affected marsh biota for decades, only rela- INVADED MARINE HABITAT tively recently, with the surge in globalization and consequent transport of nonnative species, Compared to open coasts, a much higher num- have biological forces themselves been recog- ber and proportion of exotic species are found nized as major agents of change (e.g., Ruiz et al. in embayments, including associated marshes 1999). By skewing optimal environmental con- (see, e.g., Ruiz et al. 1997, 2000; Reise, Gollash, ditions away from conditions to which native and Wolff 2002; Nehring 2002). For example, in species are adapted, many physical and chemi- Elkhorn Slough, California, Wasson, Fenn, and cal impacts may have set the stage for more Pearse (2005) documented 526 invertebrates frequent and successful biological invasions. comprised of 443 natives, 58 exotics, and 25 Biological impacts of nonnative species range cryptogens (species whose geographic origin is from purely trophic and competitive effects that uncertain). The surrounding rocky intertidal can lead to replacement of native species by open coast contained 588 species, of which only exotics, to physical transformation by exotic 8 were exotic and 13 cryptogenic. The number species that engineer habitat and alter large- and proportion of exotics were significantly scale abiotic and hydrographic properties of higher in the estuary (11 percent) than on the ad- the marsh environment. Perhaps the best- jacent coast (1 percent). Furthermore, exotic documented example of the severe biological species in the estuary were not only more di- impacts possible from invasive species is San verse but also more abundant and conspicuous Francisco Bay, where more than 240 nonindige- than on the open coast (e.g., the mudsnail nous species now reside, and 90 to 95 percent of Batillaria attramentaria, the orange sponge the biomass is exotic in many areas of the bay Hymeniacidon, the reef-building tube worm (Cohen and Carlton 1998; Lee, Thimpson, and Ficopomatus enigmaticus). Similarly, while more Lowe 2003). Clearly, to fully understand modern than 240 nonnative species are known from San impacts on both the function and the taxonomic Franscico Bay, fewer than 10 are found on the composition of estuaries and salt marshes, im- adjacent outer coast (Ruiz et al. 1997). pacts of exotic species must be considered. Perhaps even more enigmatic is the observa- Relative to other marine systems, salt tion that in many cases, even species that are marshes and estuaries are highly susceptible to typically open coast residents in their native invasion, and impacts by exotic species in these habitats often remain in embayments and systems seem particularly pronounced. I begin marshes in their introduced range (Griffiths this chapter by discussing underlying proper- 2000; Robinson et al. 2005; Wasson et al. ties of estuaries that may be important in ex- 2005). For example, the snail Littorina saxatilis, plaining these patterns. I then highlight some which is almost exclusively coastal in eastern prominent impacts driven by invasive animals North America and Europe where it is native, in salt marshes, using examples of nonindige- has not left the confines of San Francisco Bay nous species that exert large engineering effects where it has been established for more than a and ones whose impacts are limited to trophic decade (Carlton and Cohen 1998; W. Miller, or competitive effects. I briefly conclude with personal communication). 42 invasions in north american salt marshes Silliman_ch03.qxd 2/23/09 1:20 PM Page 43 Clearly this pattern of a higher number of with particularly high per capita impact. That invaders in marshes and estuaries may be due is, having more exotic species essentially cre- in part to a sampling bias. Embayments and ates a sampling effect, whereby more species marshes are located nearshore in close proxim- simply means species with greater impacts have ity to humans, who as a result have examined a higher probability of being among those these habitats more than most other marine established. habitats (e.g., Ruiz et al. 1997, 2000; Cohen and However, even after we standardize for the Carlton 1998; Hewitt et al. 1999). In addition, number of introduced species, we may still ex- compared to open marine systems, salt pect marshes to experience larger impacts from marshes have discrete boundaries and a high exotic species for at least two additional reasons. proportion of benthic, tractable species. First, as emphasized previously (and through- However, three general, spatially variable fac- out this book in general), salt marshes are one tors likely contribute to a true pattern of higher of the most anthropogenically disturbed habi- numbers of introduced species in estuaries and tats. Pollutants, eutrophication, habitat filling, marshes. First, bays and their associated drainage, dredge spoils dumping, and channel- marshes receive vast quantities of exotic propag- ization are a few of the many, often severe abi- ules from ballast water. In U.S. ports alone, tens otic alterations humans have imposed on this of millions of metric tons of ballast water from habitat type (e.g., Kennish 1992, 2001; Valiela, ports of origin all over the world are discharged Rutecki, and Fox 2004; see also chaps. 8 and 9, yearly, with each liter containing up to ten this volume). The novel and sustained environ- zooplankton organisms (Verling et al. 2005). mental changes that anthropogenic distur- Second, bays are retention zones where larvae bances impose may often be enough to move a often are not advected away. Byers and Pringle species out of the parameter space that defined (2006) demonstrated that advection typical of its evolutionary history and to which it was open coastlines makes retention, and thus estab- adapted—a process dubbed selection regime lishment, difficult and may be largely responsi- modification (SRM) (Byers 2002a). A native ble for the dearth of invasive species in those marsh species may therefore suddenly find it- environs. Third, marsh and estuarine habitats self in an environment that in key ways is just as best match the habitat from which most non- novel as it is to a nonindigenous species (Byers indigenous species propagules are exported. Two 2002a). SRM can thus accentuate competitive of the biggest vectors for nonindigenous marine impacts of exotics on natives by eliminating a species are ballast water and aquaculture, partic-