Edwin D. Grosholz

Tackling aquatic invasions: risks and opportunities for the aquarium fish industry

The aquarium trade is an important and rapidly growing vector for introduced species in the United States. We examined this vector by surveying pet stores in the San Francisco Bay–Delta region to compile a list of aquarium fish species commonly stocked. We identified which of these species might be able to survive in the Bay–Delta, and investigated store representatives’ knowledge and attitudes about biological invasions. A restrictive analysis using conservative estimates of fish temperature tolerances and environmental conditions found that the local aquarium trade includes 5 fish species that can survive in a temperate system such as the Bay–Delta. Under more inclusive parameters, up to 27 fish species met the criteria for survival in the Bay–Delta. We further explored these results by comparing potential invader incidence between different types of stores. In the more restrictive analysis, three national retail chains stocked significantly more potentially invasive species than independent aquarium stores, but there was no difference in the more inclusive analysis. A significantly higher percentage of fish taxa were easily identifiable and well-labeled in chain stores than in independent stores. Most aquarium store representatives indicated willingness to take action to reduce the threat of trade-related introductions, although chain store employees were more willing to assign responsibility for reducing this threat to the aquarium industry than were independent store employees. Management efforts for this vector should focus on (a) improving labeling and identification of fish species in stores, (b) expanding the often spotty data on fish physiological tolerances, especially for saltwater species, (c) educating customers and store employees about the risks posed by pet release, and (d) providing better options for responsible disposal of unwanted fish.

Influence of invasive Spartina growth stages on associated macrofaunal communities

In coastal wetlands, invasive plants often act as ecosystem engineers altering flow, light and sediments which, in turn, can affect benthic animal communities. However, the degree of influence of the engineer will vary significantly as it grows, matures and senesces, and surprisingly little is known about how the influence of an ecosystem engineer varies with ontogeny. We address this issue on the tidal flats of San Francisco Bay where hybrid Spartina (foliosa ×xa0 alterniflora) invaded 30xa0years ago. The invasion has altered the physico-chemical properties of the sediment habitat, which we predicted should cause changes in macrofaunal community structure and function. Through mensurative and manipulative approaches we investigated the influence of different growth stages of hybrid Spartina on macrobenthos and the underlying mechanisms. Cross-elevation sampling transects were established covering 5 zones (or stages) of the invasion, running from the tidal flat (pre-invasion) to an unvegetated dieback zone. Additionally, we experimentally removed aboveground plant structure in the mature (inner) marsh to mimic the ’unvegetated areas’. Our results revealed four distinct faunal assemblages, which reflected Spartina-induced changes in the corresponding habitat properties along an elevation gradient: a pre-invaded tidal flat, a leading edge of immature invasion, a center of mature invasion, and a senescing dieback area. These stages of hybrid Spartina invasion were accompanied by a substantial reduction in macrofaunal species richness and an increase in dominance, as well as a strong shift in feeding modes, from surface microalgal feeders to subsurface detritus/Spartina feeders (mainly tubificid oligochaetes and capitellid polychaetes). Knowledge of the varying influence of plant invaders on the sediment ecosystem during different phases of invasion is critical for management of coastal wetlands.

The influence of flood cycle and fish predation on invertebrate production on a restored California floodplain

Although floodplains are known to be tightly controlled by the flood cycle, we know comparatively little about how flooding influences predators and their consumption of secondary production, particularly in highly seasonal floodplains typical of Mediterranean climates. In this study, we investigate how the seasonal dynamics of a central California floodplain influence the timing and magnitude of fish predation and the abundance and composition of invertebrates. For 3xa0years (2000–2002), we compared changes in abundances and size distributions of invertebrates through the flood season (January–June) with seasonal changes in the abundance of larval and juvenile fishes. Using diet analysis of fishes and manipulative feeding experiments with fishes in field enclosures, we link specific changes in invertebrate populations directly to feeding preferences of seasonally abundant fish. Early in the flood season prior to March, we found little influence of fish predation, consistent with the near absence of larval and juvenile fishes during this period. Coinciding with the midseason increase in the abundance of larval and juvenile fishes in April, we found significant declines in zooplankton abundance as well as declines in the size of zooplankton consistent with fish feeding preferences. Our results were consistent with results from feeding enclosure experiments that showed that fish rapidly depressed populations of larger cladocerans with much less effect on smaller cladocerans and calanoid copepods. At the end of the flood season, zooplankton abundances rapidly increased, consistent with a switch in the feeding of juvenile fish to aquatic insects and subsequent fish mortality. We also found that zooplankton biomass on the floodplain reached a maximum 2–3xa0weeks after disconnection with the river. We suggest that floodplain restoration in this region should consider management strategies that would ensure repeated flooding every 2–3xa0weeks during periods that would best match the peaks in abundance of native fishes.

Does spatial heterogeneity and genetic variation in populations of the xanthid crab Rhithropanopeus harrisii (Gould) influence the prevalence of an introduced parasitic castrator

Abstract Potential differences between two populations of the xanthid crab, Rhithropanopeus harrisii (Gould), in susceptibility to infection by an introduced (exotic) parasite, the sacculinid barnacle, Loxothylacus panopaei (Gissler) were investigated. The frequency of L. panopaei infection, which causes castration of the crab host, has risen to 80% during the last 4 yr in one population, while the second population has never been parasitized. Using eggs collected from females of both infected and uninfected populations, broods of larval crabs were reared through metamorphosis, and then placed in field enclosures at the site of the infected population where they were exposed to ambient levels of infective stages of the parasite. We used differences in infection levels among sibling hosts from each population to estimate the heritability (h 2 ) of susceptibility to parasite infection. Comparisons of infection levels between populations showed no significant differences, nor were there significant differences in parasitism levels among families within a population. The estimate of heritability for susceptibility to parasitic castration was low (h 2 = 0.10), and not significantly different from zero, suggesting little additive genetic variance. In a second experiment, we examined how levels of parasite infection within a single population were affected by the spatial distribution of the crab population. We placed susceptible juvenile R. harrisii in enclosures either 0.1, 1.0, or 10.0 m from experimental aggregations of infected crabs, which acted as a potential source of infective parasite larvae. The spatial distribution significantly influenced levels of parasitic castration with significantly more infected juvenile crabs in enclosures nearest to experimental aggregations (0.1 m) than in enclosures 1.0 m and 10.0 m away. The lack of differences in susceptibility among populations, as well as families within populations, may be explained by several mechanisms including significant gene flow between populations and insufficient time for a measurable response to selection. The data are also consistent with the idea that equivalent generation times of host and parasite may allow the parasite to evolve infectivity as fast as the host can evolve resistance. The significant influence of the spatial distribution of the host population on the prevalence of the parasite suggests that both the rate of spatial spread and prevalence of this parasite in Chesapeake Bay may be influenced by the distance between aggregations of the host population.

Assessing population increase as a possible outcome to management of invasive species

Some efforts to reduce invasive populations have paradoxically led to population increases. This phenomenon, referred to as overcompensation, occurs when reduced intraspecific pressures increase juvenile survival or maturation rates, leading to increased population size. Overcompensation in response to eradication efforts could derail management efforts, so it would be beneficial to evaluate the likelihood of overcompensation prior to removal. We conducted a series of experiments to examine the potential for overcompensation of a non-native population of the European green crab, Carcinus maenas, which was being removed in Bodega Harbor, California. First, we examined the impact of adults on juvenile survival by measuring adult cannibalism on juveniles in the presence and absence of alternative prey, and the survival of tethered juveniles at varying adult densities. Second, we examined how adult presence affected juvenile short-term foraging and growth rates. Although adult presence reduced juvenile short-term foraging, we detected only minimal cannibalism and found no evidence that adults greatly reduce juvenile growth or survivorship. These results suggest that overcompensation is not likely to occur in this population in response to removal. We assessed this prediction using pre- and post-removal surveys of juvenile recruitment in Bodega Harbor compared to nearby populations, testing for evidence of overcompensation. Relative juvenile abundance did not statistically increase in removal compared to reference populations, consistent with our conclusion from the experiments. This experimental approach which focuses on an organism’s population biology provides a tool to assess capacity for assessing the capacity for overcompensation in management strategies for invasive species.