ALTERATION OF CALIFORNIA WETLAND HABITAT BY INVASIVE INVERTEBRATES
KEYWORDS: Exotic species, Habitat modification, Sediments, Wetlands ecology
In recent years the pace of exotic species invasion has accelerated. Coastal systems, including tidal flats and salt marshes, have been particularly susceptible, possibly because they are normally high-stress, species-poor environments. Invasive species may affect coastal ecosystems through alteration of the physical environment, trophic interactions, competitive displacement, parasitism, and modification of genetic structure.
Our research has focused on intertidal habitat modification by two invasive, sediment-dwelling invertebrates, and on the consequences of this alteration for associated fauna. The Australasian isopod, Sphaeroma quoyanum, is a bioeroder capable of burrowing into a large variety of substrates, including soft rock, wood and mud. We have studied this species in the banks of salt marsh channels and in marsh edge habitat of San Francisco and San Diego Bays. The Asian mussel, Musculista senhousia, colonizes sand flats, mud flats, marsh channels and seagrass beds in the intertidal and shallow subtidal. It forms dense mats bound with byssal threads, feces, detritus and sediment. Experimental studies of mussel habitat invasion, biodeposition and synecology have been conducted intertidally in Mission Bay, California.
Densities and Burrows. A variety of measurements were made in San Diego and San Francisco Bays to examine (a) S. quoyanum habitat preferences, (b) S. quoyanum effects on sediment properties and erosion rates, and (c) S. quoyanum influence on other infauna. Isopod densities in San Diego Bay during 1998 averaged 7,931 ± 3,938 ind. m-2 (mean + 1 SE). Isopod densities in San Francisco Bay averaged 13,375 ± 3035 ind. m-2. The isopod was found in both restored and natural sites of both bays. Density estimates were found to be correlated with burrow number and complexity, as revealed by resin casting (Fig. 1) and visual observations. Isopod burrows were also visualized by x-radiography (Fig. 2).
Habitat Alteration. Channel and marsh-edge banks were found to exhibit one of three morphologies: sloped, vertical, or undercut. Sloped banks typically had low densities of S. quoyanum, vertical banks had intermediate densities and undercut banks invariably exhibited the highest S. quoyanum densities. Increased densities corresponded to increased extent of bank undercutting. The presence of isopod burrows was caused a 15% loss of sediments from creek bank walls, although the porosity (water-holding capacity) was not affected. Isopods were found to reduce sediment shear strength, destabilizing banks and increasing susceptibility to erosion. Although there was considerable variation in sediment shear strength (measured by torque meter) among the marshes studied, patches of high S. quoyanum density in each marsh were almost always associated with reduced shear strength. Isopods tended to have the largest effects on sediment stability in firmer sediments (as indicated by high shear strengths). Resin casts (Fig. 1) and x-radiographs (Fig. 2) clearly reveal the source of the sediment instability.
Reduction in sediment shear strength and increased undercutting leads to eventual collapse of banks (Fig. 3). Measures of marsh edge loss made over 6 month periods in San Francisco and San Diego Bays revealed significant erosion in both embayments, but considerable variability at both sites (San Diego 8±2cm; San Francisco 18±10cm). Overall, rates of bank habitat loss may be somewhat greater in San Francisco Bay (up to 90 cm in 6 months) than in San Diego Bay (up to 30 cm in 6 months). This is possibly due to the more exposed nature (stronger waves), longer isopod residence time, and higher densities of isopods in the San Francisco Bay habitats.
Associated Fauna. Isopod density was weakly correlated with densities
of other peracarid crustaceans (3 1 mm) in channel bank and marsh edge
banks of both bays. These taxa living in and around isopod burrows may
benefit from the oxygenation and increased flushing associated with the
S. quoyanum burrows, as well as from the increased structural complexity
of the habitat.
Previous research has demonstrated that the presence of M. senhousia and their mats can transform sand flats into muddy organic-rich sediments. In the process, the associated infaunal community is modified, with enhancement of some taxa but inhibition of large bivalves. Our recent research has focused on the ability of the mussel to alter habitat properties directly, through biodeposition, and indirectly, through physical effects of the shell or mat structure.
Biodeposition. Biodeposition studies, using tubes containing either living mussels or plaster-filled mussel shells (mimics) near the tube tops, were conducted within sediments on the Mission Bay tidal flat for a 6 wk period. After 3 weeks, mussel presence enhanced the relative amount of C and N in deposited sediments by 50%. After 6 weeks (and following a heavy rain event), there were no significant differences in carbon content or nitrogen content between the living and mimic mussel deposits. However, tubes with living mussels had more sediment than tubes with mussel mimics, indicating a mussel-induced increase in the flux of material from the water column to the benthos.
Structural vs Direct Influence. A manipulative experiment was conducted to compare the relative effects of living mussels to the effects of the physical modification of habitats by mussel shells and mats. Four experimental treatments and a control were introduced to tidal flat sediments: (1) artificial mats and living mussels, (2) artificial mats and mussel mimics (plaster filled shells), (3) living mussels, (4) mussel mimics, and (5) undisturbed sediments. After 6 wk the greatest alteration of sediment organic matter content (which increased) and grain size (which decreased) was observed in the treatment with both living mussels and mats. Mats / mussel mimics and living mussels alone also led to habitat alteration relative to controls. Increased organic matter content and increased percentage of fine particles (silt and clay) were associated with greater macrofaunal density and species richness in the experiment.
Ecosystem consequences. Our experiments indicate that alteration of sediment properties by M. senhousia results from both structural effects, as a result of mat formation, and from active deposition by the living mussels. Studies of S. quoyanum reveal that isopod burrowing activities are largely responsible for habitat alteration. By increasing the complexity of benthic habitats, structures created by both species (burrows and mats) can lead to small-scale enhancement of infaunal polychaete, mollusc and crustacean densities. However, on larger scales, the habitat alteration has negative effects on native plant and animal species. These include competitive displacement of large, filter feeding bivalves by Musculista and reduction in cordgrass and pickleweed habitat by Sphaeroma.
Habitat alteration by invading species is likely to increase in the coastal
zone as these ecosystems become degraded and exotic species continue to
invade by means such as ballast water discharge. Population explosions
of exotic species may be more likely in settings stressed by pollution,
altered circulation, or fragmentation, as well as in created or restored
systems. Future concerns include (1) assessing the modification of wetland
habitat by invasive plants (e.g., Spartina alterniflora, Zostera japonica,
Phragmites australis) and animals (e.g., Eriocheir sinensis, Ficopomatus
enigmaticus) and (2) determining ways to minimize impacts of these
species in natural and restored systems.
Lisa A. Levin1, Theresa Talley1, Matthias Saladin1 , Jeff Crooks2
1Scripps Institution of Oceanography
2Smithsonian Environmental Research Center
September 7, 2006
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