Jeb Byers, Associate Professor of Zoology, travels to Australia

Professor James (Jeb) Byers spent the 2007-08 academic year in Australia conducting research on a highly invasive alga species

Chapter 8 Comparative Biogeography of Marine Invaders Across Their Native and Introduced Ranges

Biological invasions continue to exert extensive environmental and economic impacts. Understanding why some introduced species become invasive is critical to their management. Determining the mechanisms underpinning invasion success has focussed on aspects of the ecology and physiology of the species in the introduced range. Through the application of biogeographic approaches, however, a growing body of research highlights insights that stem from studying invasion success as a biogeographic issue. In particular, a comparison of both biogeographic regions (i.e., the native and invasive ranges) allows exclusive insight into seven different major biogeographic hypotheses that we identified to explain invader success. These include the enemy release hypothesis, niche shifts, trait differences, the evolution of invasiveness, native allies, environmental matching, and genetic diversity. All imply a difference or gradient between the ranges that may mechanistically explain an invader’s differential performance. This review summarises the support for these seven different theories underpinning the biogeography of marine invasions, and also provides case studies for different theories addressing the comparative biogeography of marine invasions. Additionally, we catalogue the geographic regions of the invasive species used in biogeographic comparisons and the diversity of species, habitats and climate zones examined. Finally, we highlight critical knowledge gaps and suggest future research directions for improving our understanding the processes driving invasion success

Variable direct and indirect effects of a habitat-modifying invasive species on mortality of native fauna

Habitat-modifying invasive species can influence rates of predation on native prey either directly by providing protective structure or indirectly by modifying traits of prey species responding to the habitat. The alga Caulerpa taxifolia is one of the most successful invasive species of shallow-water marine systems globally, often provisioning habitat in areas previously lacking in vegetated structure. We experimentally evaluated the direct effect of Caulerpa to provide refuge for the native clam Anadara trapezia and how this balances with its influence on two trait-mediated indirect interactions that may increase Anadara\u27s susceptibility to predators. Specifically, Caulerpa\u27s alteration of physical and chemical properties of the surrounding water and sediment deteriorate Anadara\u27s condition and predator resistance properties and also cause Anadara, though normally buried, to project from beneath the sediment, exposing it to predators. Our results show that Anadara are somewhat (but not consistently) protected from predators by living among Caulerpa. Shallow burial depth did not counteract this protective effect. However at times of year when predator activity diminishes and conducive environmental conditions develop, negative effects of Caulerpa habitat such as hypoxia and lowered flow may dominate. Under such situations, poor clam condition accentuates Anadara\u27s susceptibility to mortality. Ultimately, a slight and inconsistent positive effect of Caulerpa to protect Anadara from predators is exceeded by the strong negative effect of Caulerpa on clam mortality, which is heightened by clams\u27 weakened condition produced by chronic exposure to Caulerpa. Our results show that invasive habitat-modifying species can affect mortality of native species not simply through obvious positive direct effects of their protective structure, but indirectly through contrasting negative modification of the traits of prey species responding to the habitat

Invasive ecosystem engineer selects for different phenotypes of an associated native species

Invasive habitat-forming ecosystem engineers modify the abiotic environment and thus represent a major perturbation to many ecosystems. Because native species often persist in these invaded habitats but have no shared history with the ecosystem engineer, the engineer may impose novel selective pressure on native species. In this study, we used a phenotypic selection framework to determine whether an invasive habitat-forming ecosystem engineer (the seaweed Caulerpa taxifolia) selects for different phenotypes of a common cooccurring native species (the bivalve Anadara trapezia). Compared to unvegetated habitat, Caulerpa habitat has lower water flow, lower dissolved oxygen, and sediments are more silty and anoxic. We determined the performance consequences of variation in key functional traits that may be affected by these abiotic changes (shell morphology, gill mass, and palp mass) for Anadara transplanted into Caulerpa and unvegetated habitat. Both linear and nonlinear performance gradients in Anadara differed between habitats, and these gradients were stronger in Caulerpa compared to unvegetated sediment. Moreover, in Caulerpa alternate phenotypes performed well, and these phenotypes were different from the dominant phenotype in unvegetated sediment. By demonstrating that phenotype-performance gradients differ between habitats, we have highlighted a role for Caulerpa as an agent of selection on native species

Chapter 8 Comparative Biogeography of Marine Invaders Across Their Native and Introduced Ranges

Biological invasions continue to exert extensive environmental and economic impacts. Understanding why some introduced species become invasive is critical to their management. Determining the mechanisms underpinning invasion success has focussed on aspects of the ecology and physiology of the species in the introduced range. Through the application of biogeographic approaches, however, a growing body of research highlights insights that stem from studying invasion success as a biogeographic issue. In particular, a comparison of both biogeographic regions (i.e., the native and invasive ranges) allows exclusive insight into seven different major biogeographic hypotheses that we identified to explain invader success. These include the enemy release hypothesis, niche shifts, trait differences, the evolution of invasiveness, native allies, environmental matching, and genetic diversity. All imply a difference or gradient between the ranges that may mechanistically explain an invader’s differential performance. This review summarises the support for these seven different theories underpinning the biogeography of marine invasions, and also provides case studies for different theories addressing the comparative biogeography of marine invasions. Additionally, we catalogue the geographic regions of the invasive species used in biogeographic comparisons and the diversity of species, habitats and climate zones examined. Finally, we highlight critical knowledge gaps and suggest future research directions for improving our understanding the processes driving invasion success

Parasites Alter Community Structure

Parasites often play an important role in modifying the physiology and behavior of their hosts and may, consequently, mediate the influence hosts have on other components of an ecological community. Along the northern Atlantic coast of North America, the dominant herbivorous snail Littorina littorea structures rocky intertidal communities through strong grazing pressure and is frequently parasitized by the digenean trematode Cryptocotyle lingua. We hypothesized that the effects of parasitism on host physiology would induce behavioral changes in L. littorea, which in turn would modulate L. littorea\u27s influence on intertidal community composition. Specifically, we hypothesized that C. lingua infection would alter the grazing rate of L. littorea and, consequently, macroalgal communities would develop differently in the presence of infected versus uninfected snails. Our results show that uninfected snails consumed 40% more ephemeral macroalgal biomass than infected snails in the laboratory, probably because the digestive system of infected snails is compromised by C. lingua infection. In the field, this weaker grazing by infected snails resulted in significantly greater expansion of ephemeral macroalgal cover relative to grazing by uninfected snails. By decreasing the per-capita grazing rate of the dominant herbivore, C. lingua indirectly affects the composition of the macroalgal community and may in turn affect other species that depend on macroalgae for resources or habitat structure. In light of the abundance of parasites across systems, we suggest that, through trait-mediated indirect effects, parasites may be a common determinant of structure in ecological communities

Responses of an oyster host (Crassostrea virginica) and its protozoan parasite (Perkinsus marinus) to increasing air temperature

Background Changes in climate are predicted to influence parasite and pathogen infection patterns in terrestrial and marine environments. Increases in temperature in particular may greatly alter biological processes, such as host-parasite interactions. For example, parasites could differentially benefit from increased reproduction and transmission or hosts could benefit from elevated immune responses that may mediate or even eliminate infections. In the southeastern United States, the Eastern oyster, Crassostrea virginica, is infected by the lethal protozoan parasite, Perkinsus marinus. Under field conditions, intertidal (air-exposed) oysters have been found to have significantly higher P. marinus infection intensity and marginally higher infection prevalence than subtidal (submerged) oysters. During summer, air temperatures are much warmer than water and this exposure of intertidal oysters to higher temperatures is a suggested mechanism for increased infection intensity. Methods We simulated intertidal exposure using controlled laboratory experiments to determine how host traits (survival and immune response) and parasite infection intensity will respond to elevated air temperature ranging from 27 °C to 53 °C during emersion at low tide. In Georgia, where our work was conducted, the average summer water temperature is 29 °C and the average maximum high air temperature in July is 33 °C (though oysters have been shown to survive at much higher air temperatures). Results Host survival declined as temperature increased, with a definitive drop-off between 39–43 °C. Negative effects of air temperature on host immune response (phagocytic activity) were detectable only at extremely high temperatures (47–50 °C) when hosts were suffering acute mortality. Parasite infection intensity peaked at 35 °C. Discussion Our results suggest that an increase in average summer air temperature to 35 °C or higher could affect oyster survival directly through temperature-related impacts in the short-term and indirectly through increased P. marinus infection intensity over the long-term

Development and characterization of microsatellite loci for the haploid–diploid red seaweed Gracilaria vermiculophylla

Microsatellite loci are popular molecular markers due to their resolution in distinguishing individual genotypes. However, they have rarely been used to explore the population dynamics in species with biphasic life cycles in which both haploid and diploid stages develop into independent, functional organisms. We developed microsatellite loci for the haploid–diploid red seaweed Gracilaria vermiculophylla, a widespread non-native species in coastal estuaries of the Northern hemisphere. Forty-two loci were screened for amplification and polymorphism. Nine of these loci were polymorphic across four populations of the extant range with two to eleven alleles observed. Mean observed and expected heterozygosities ranged from 0.265 to 0.527 and 0.317 to 0.387, respectively. Overall, these markers will aid in the study of the invasive history of this seaweed and further studies on the population dynamics of this important haploid–diploid primary producer

Genetic by environmental variation but no local adaptation in oysters ( Crassostrea virginica )

Functional trait variation within and across populations can strongly influence population, community, and ecosystem processes, but the relative contributions of genetic vs. environmental factors to this variation are often not clear, potentially complicating conservation and restoration efforts. For example, local adaptation, a particular type of genetic by environmental (G*E) interaction in which the fitness of a population in its own habitat is greater than in other habitats, is often invoked in management practices, even in the absence of supporting evidence. Despite increasing attention to the potential for G*E interactions, few studies have tested multiple populations and environments simultaneously, limiting our understanding of the spatial consistency in patterns of adaptive genetic variation. In addition, few studies explicitly differentiate adaptation in response to predation from other biological and environmental factors. We conducted a reciprocal transplant experiment of first-generation eastern oyster (Crassostrea virginica) juveniles from six populations across three field sites spanning 1000 km in the southeastern Atlantic Bight in both the presence and absence of predation to test for G*E variation in this economically valuable and ecologically important species. We documented significant G*E variation in survival and growth, yet there was no evidence for local adaptation. Condition varied across oyster cohorts: Offspring of northern populations had better condition than offspring from the center of our region. Oyster populations in the southeastern Atlantic Bight differ in juvenile survival, growth, and condition, yet offspring from local broodstock do not have higher survival or growth than those from farther away. In the absence of population-specific performance information, oyster restoration and aquaculture may benefit from incorporating multiple populations into their practices