Community structure of caprellids (Crustacea: Amphipoda: Caprellidae) on seagrasses from southern Spain

The community structure of caprellids inhabiting two species of seagrass (Cymodocea nodosa and Zostera marina) was investigated on the Andalusian coast, southern Spain, using uni and multivariate analyses. Three meadows were selected (Almería, AL; Málaga, MA; Cádiz, CA), and changes in seagrass cover and biomass were measured from 2004 to 2005. Four caprellid species were found; the density of Caprella acanthifera, Phtisica marina and Pseudoprotella phasma was correlated to seagrass biomass. No such correlation was found for Pariambus typicus, probably because this species inhabits sediments and does not cling to the seagrass leaves. We recorded a significant decrease in seagrass cover and biomass in MA due to illegal bottom trawling fisheries. Phtisica marina and P. typicus were favoured by this perturbation and increased their densities after the trawling activities. A survey of reports on caprellids in seagrass meadows around the world showed no clear latitudinal patterns in caprellid densities (ranging from 6 to 1,000 ind/m² per meadow) and species diversity. While caprellid abundances in seagrass meadows are often very high, the number of species per meadow is low (range 1–5).     

Density-dependent resistance of the gypsy moth <Emphasis Type="Italic">Lymantria dispar</Emphasis> to its nucleopolyhedrovirus,and the consequences for population dynamics

The processes controlling disease resistance can strongly influence the population dynamics of insect outbreaks. Evidence that disease resistance is density-dependent is accumulating, but the exact form of this relationship is highly variable from species to species. It has been hypothesized that insects experiencing high population densities might allocate more energy to disease resistance than those at lower densities, because they are more likely to encounter density-dependent pathogens. In contrast, the increased stress of high-density conditions might leave insects more vulnerable to disease. Both scenarios have been reported for various outbreak Lepidoptera in the literature. We tested the relationship between larval density and disease resistance with the gypsy moth (Lymantria dispar) and one of its most important density-dependent mortality factors, the nucleopolyhedrovirus (NPV) LdMNPV, in a series of bioassays. Larvae were reared in groups at different densities, fed the virus individually, and then reared individually to evaluate response to infection. In this system, resistance to the virus decreased with increasing larval density. Similarly, time to death was faster at high densities than at lower densities. Implications of density–resistance relationships for insect–pathogen population dynamics were explored in a mathematical model. In general, an inverse relationship between rearing density and disease resistance has a stabilizing effect on population dynamics.     

Associations Between Coinfection Prevalence of <Emphasis Type="Italic">Borrelia lusitaniae</Emphasis>, <Emphasis Type="Italic">Anaplasma</Emphasis> sp., and <Emphasis Type="Italic">Rickettsia</Emphasis> sp. in Hard Ticks Feeding on Reptile Hosts

An increasing number of studies reveal that ticks and their hosts are infected with multiple pathogens, suggesting that coinfection might be frequent for both vectors and wild reservoir hosts. Whereas the examination of associations between coinfecting pathogen agents in natural host–vector–pathogen systems is a prerequisite for a better understanding of disease maintenance and transmission, the associations between pathogens within vectors or hosts are seldom explicitly examined. We examined the prevalence of pathogen agents and the patterns of associations between them under natural conditions, using a previously unexamined host–vector–pathogen system—green lizards Lacerta viridis, hard ticks Ixodes ricinus, and Borrelia, Anaplasma, and Rickettsia pathogens. We found that immature ticks infesting a temperate lizard species in Central Europe were infected with multiple pathogens. Considering I. ricinus nymphs and larvae, the prevalence of Anaplasma, Borrelia, and Rickettsia was 13.1% and 8.7%, 12.8% and 1.3%, and 4.5% and 2.7%, respectively. The patterns of pathogen prevalence and observed coinfection rates suggest that the risk of tick infection with one pathogen is not independent of other pathogens. Our results indicate that Anaplasma can play a role in suppressing the transmission of Borrelia to tick vectors. Overall, however, positive effects of Borrelia on Anaplasma seem to prevail as judged by higher-than-expected Borrelia–Anaplasma coinfection rates.     

Soil moisture mediated interaction between <Emphasis Type="Italic">Polygonatum biflorum</Emphasis> and leaf spot disease

Fungal pathogens can regulate the abundance and distribution of natural plant populations by inhibiting the growth, survival, and reproduction of their hosts. The abiotic environment is a crucial component in host–pathogen interactions in natural plant populations as favorable conditions drive pathogen development, reproduction, and persistence. Foliar plant pathogens, such as fungal lesions referred to generically as “leaf spot disease,” are particularly responsive to increased moisture levels, but the manner in which the abiotic environment drives disease dynamics, and how these diseases regulate natural plant populations, is not fully understood. We investigate (1) the impact of ambient soil moisture and diffuse light on the prevalence of a leaf spot pathogen (Phyllosticta sp.) in a natural population of Polygonatum biflorum, an understory herb native to deciduous forest understories in the eastern US, and (2) the effects of the fungal pathogen on the survival, growth, and abundance of the plants. We tracked six P. biflorum populations and disease incidence, as well as soil moisture and diffuse light, between 2003 and 2005 in the understory deciduous forest of the southern Appalachian Mountains, North Carolina, USA. Results show that both the occurrence of P. biflorum and the prevalence of P. biflorum leaf spot disease are highest where soil moisture is intermediate and diffuse light is lowest. Disease occurrence depends upon plant presence, but it also adversely impacts plant survival, abundance, and growth. These results suggest that leaf spot disease is likely to impact population dynamics, which in turn vary as a function of environmental drivers.     

Resilience of Zostera marina habitats and response of the macroinvertebrate community to physical disturbance caused by clam harvesting

The consequences of physical disturbances to seagrasses depend on disturbance frequency relative to the capacity for recolonization and recovery following fragmentation. In a subtidal seagrass meadow of Zostera marina L., following a season of clam harvesting, we compared the temporal change of shoot density and biomass of this seagrass together with the community structure of the associated macroinvertebrates, at two sites differing in the intensity of the physical disturbance. The impacted site showed significantly lower shoot density and total biomass than the non-impacted site initially. The increase in above-ground biomass over four months (May to September) of this species was significantly higher (46%) at the impacted site than in the area not affected by the disturbance. Four months after cessation of the extraction activity, the biomass and density values of Z. marina reached similar values to those measured in the non-impacted site. The sexual reproductive effort of the seagrass population affected by the disturbance (4%) was significantly lower than at the non-impacted site (10%), which could influence genetic diversity and the seed bank. The community structure of molluscs showed 54% similarity between sites at the beginning of the study. Four months later, mollusc communities increased to a similarity of 74%. The current closure season (four months annually) established for the recovery of the exploited stocks of bivalves allowed the recovery of Z. marina density and biomass. Nevertheless, other population properties, such as those related to reproductive patterns, remained altered by the disturbance.     

Host-selective toxins produced by <Emphasis Type="Italic">Stagonospora nodorum</Emphasis> confer disease susceptibility in adult wheat plants under field conditions

Stagonospora nodorum, causal agent of Stagonospora nodorum blotch (SNB), is a destructive pathogen of wheat worldwide. As is true for many necrotrophic host–pathogen systems, the wheat-S. nodorum system is complex and resistance to SNB is usually quantitatively inherited. We recently showed that S. nodorum produces at least four proteinaceous host-selective toxins that interact with dominant host sensitivity/susceptibility gene products to induce SNB in seedlings. Here, we evaluated a population of wheat recombinant inbred lines that segregates for Tsn1, Snn2, and Snn3, which confer sensitivity to the toxins SnToxA, SnTox2, and SnTox3, respectively, to determine if compatible host–toxin interactions are associated with adult plant susceptibility to SNB foliar disease under field conditions. Artificial inoculation of the population in 2 years and two locations with a fungal isolate known to produce SnToxA and SnTox2 indicated that compatible SnToxA–Tsn1 and SnTox2–Snn2 interactions accounted for as much as 18 and 15% of the variation in disease severity on the flag leaf, respectively. As previously reported for seedlings, the effects of these two interactions in conferring adult plant susceptibility were largely additive. Additional adult plant resistance QTLs were identified on chromosomes 1B, 4B, and 5A, of which, the 1B and 5A QTLs were previously reported to be associated with seedling resistance to SNB. Therefore, in this population, some of the same QTLs are responsible for seedling and adult plant resistance/susceptibility. This is the first report showing that host-selective toxins confer susceptibility of adult plants to SNB, further substantiating the importance of compatible toxin–host interactions in the wheat-S. nodorum pathosystem. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture.     

Population structure of <Emphasis Type="Italic">Apis cerana</Emphasis> in Thailand reflects biogeography and current gene flow rather than <Emphasis Type="Italic">Varroa</Emphasis> mite association

Concordance between the mitochondrial haplotypes of the Eastern honey bee, Apis cerana, and its ectoparasitic Varroa mites across the Isthmus of Kra in Thailand has suggested that local host–pathogen co-evolution may be responsible for the geographic distribution of particular genotypes. To investigate nuclear microsatellites population structure in A. cerana, single workers of A. cerana colonies from Thailand were genotyped at 18 microsatellite loci. The loci showed intermediate to high levels of heterozygosity and a range of allele numbers. The analyses confirmed a fundamental subdivision of the Thai A. cerana population into the “Asia Mainland” and “Sundaland” regions at the Isthmus of Kra. However, the nuclear microsatellite differentiation was less distinct than mtDNA haplotype differences, suggesting male-biased dispersal and population admixture. Overall, samples showed a weak isolation-by-distance effect. The isolated population on Samui island was most differentiated from the other samples. The results do not support our initial hypothesis of local host–pathogen co-evolution, which predicts a strict correspondence between the nuclear genome and the lineage of parasitic Varroa mite of the A. cerana samples, because the gene flow indicated by our nuclear microsatellite markers should also mix potential Varroa resistance alleles among subpopulations. Instead, our study suggests that the coincidental distribution of Varroa lineages and A. cerana population structure in Thailand are the result of biogeographic history and current migration patterns.     

Identification and characterization of a novel host–toxin interaction in the wheat–<Emphasis Type="Italic">Stagonospora nodorum</Emphasis> pathosystem

Stagonospora nodorum, casual agent of Stagonospora nodorum blotch (SNB) of wheat, produces a number of host-selective toxins (HSTs) known to be important in disease. To date, four HSTs and corresponding host sensitivity genes have been reported, and all four host–toxin interactions are significant factors in the development of disease. Here, we describe the identification and partial characterization of a fifth S. nodorum produced HST designated SnTox4. The toxin, estimated to be 10–30 kDa in size, was found to be proteinaceous in nature. Sensitivity to SnTox4 is governed by a single dominant gene, designated Snn4, which mapped to the short arm of wheat chromosome 1A in a recombinant inbred (RI) population. The compatible Snn4–SnTox4 interaction is light dependent and results in a mottled necrotic reaction, which is different from the severe necrosis that results from other host–toxin interactions in the wheat–S. nodorum pathosystem. QTL analysis in a population of 200 RI lines derived from the Swiss winter wheat varieties Arina and Forno revealed a major QTL for SNB susceptibility that coincided with the Snn4 locus. This QTL, designated QSnb.fcu-1A, explained 41.0% of the variation in disease on leaves of seedlings indicating that a compatible Snn4–SnTox4 interaction plays a major role in the development of SNB in this population. Additional minor QTL detected on the short arms of chromosomes 2A and 3A accounted for 5.4 and 6.0% of the variation, respectively. The effects of the three QTL were largely additive, and together they explained 50% of the total phenotypic variation. These results provide further evidence that host–toxin interactions in the wheat–S. nodorum pathosystem follow an inverse gene-for-gene model.     

The combined influence of trematode parasites and predatory salamanders on wood frog (<Emphasis Type="Italic">Rana sylvatica</Emphasis>) tadpoles

Predators can have important impacts on host–parasite dynamics. For many directly transmitted parasites, predators can reduce transmission by removing the most heavily infected individuals from the population. Less is known about how predators might influence parasite dynamics in systems where the parasite relies on vectors or multiple host species to complete their life cycles. Digenetic trematodes are parasitic flatworms with complex life cycles typically involving three host species. They are common parasites in freshwater systems containing aquatic snails, which serve as obligate first intermediate hosts, and multiple trematode species use amphibians as second intermediate hosts. We experimentally examined the impact of predatory salamanders (Ambystoma jeffersonianum) and trematode parasites (Echinostoma trivolvis and Ribeiroia ondatrae) on short-term survival of wood frog tadpoles (Rana sylvatica) in 150-L outdoor pools. Two trematode species were used in experiments because field surveys indicated the presence of both species at our primary study site. Parasites and predators both significantly reduced tadpole survival in outdoor pools; after 6 days, tadpole survival was reduced from 100% in control pools to a mean of 46% in pools containing just parasites and a mean of 49% in pools containing just predators. In pools containing both infected snails and predators, tadpole survival was further reduced to a mean of 5%, a clear risk-enhancement or synergism. These dramatic results suggest that predators may alter transmission dynamics of trematodes in natural systems, and that a complete understanding of host–parasite interactions requires studying these interactions within the ecological framework of community interactions.     

From the inverse density–area relationship to the minimum patch size of a host–parasitoid system

The minimum amount of suitable habitat (MASH) is an important concept in conservation biological control. Two methods for estimating the MASH have been proposed by McCoy and Mushinsky based on an inverse density–area relationship. Using data of the population densities of aphid host–parasitoid–hyperparasitoid collected from wheat fields of different habitat sizes, we argued that the inverse density–area relationship may be an artifact. Significant correlations between population densities and patch sizes from all three trophic levels were found once the population density had been log-transformed. We could not obtain the same results if the population density had not been log-transformed. We estimated that the MASH for the aphid M. avena, S. graminum, A. avenae, A. gifuensis, P. aphidis, and Alloxysta sp. were 246, 246, 479, 495, 949, and 835 m² according to the methods of McCoy and Mushinsky. The scale-dependence and the systematic spatial variations of the host–parasitoid interaction suggests that we can achieve an optimal effect of biological control by manipulating the habitat patch sizes, although not based on the inverse density–area relationship.     

Enhancement of juvenile Caribbean spiny lobsters: an evaluation of changes in multiple response variables with the addition of large artificial shelters

Shortage of natural crevice shelters may produce population bottlenecks in juvenile Caribbean spiny lobsters (Panulirus argus), a socially gregarious species. We conducted a field experiment to test enhancement of a local population of juvenile P. argus with the addition of artificial shelters (“casitas”) that mimic large crevices (1.1 m² in surface area and 3.8 cm in height). Changes in density and biomass of juvenile lobsters 15–50 mm carapace length (CL) were assessed with a multiple before-after control-impact (MBACI) analysis. Separate analyses were also conducted on small (15–35 mm CL) and large (35.1–50 mm CL) juveniles to assess size-related effects. First, we carried out 13 lobster surveys on nine fixed 1-ha sites over a shallow reef lagoon (“before” period). Then, we deployed ten casitas in each of five sites and left four sites as controls, and conducted 22 further surveys (“after” period). Deployment of casitas resulted in a sixfold increase in juvenile density (76% contributed by small and 24% by large juveniles) and a sevenfold increase in biomass (40 and 60%, respectively). Capture–recapture results revealed that enhancement was achieved not by promoting individual growth but by increasing survival, persistence, and foraging ranges of small and large juveniles. Casitas both mitigated shortage of natural shelter and increased sociality, allowing for cohabitation of smaller, more vulnerable juveniles with larger conspecifics that have greater defensive abilities. Casitas may help enhance local populations of juvenile P. argus in Caribbean seagrass habitats, typically poor in natural crevice shelters. The use of MBACI and the simultaneous assessment of multiple interrelated response variables may be a powerful analytical approach to test shelter limitation in other species and to examine the function of structural habitat in other systems.     

Perspective of practical biological control and population theories

We have not yet had sufficient theoretical explanation for successful biological control in which a key pest is controlled after an introduction of natural enemies. I compare here real features of successful biological control and theoretical host–parasitoid population models to reduce the gap between theory and practice. I first review the historical interaction between classical biological control projects and theoretical population models. Second, I consider the importance of host refuges in host–parasitoid population dynamics as concerns the mechanisms of low and stable host density. The importance of density–dependent parasitism through parasitoid reproduction in multivoltine host–parasitoid systems and supplemental generalist natural enemies are also discussed. Finally, I consider the difference in tactics for classical biological control and for augmentation of natural enemies in annual crop systems. Received: December 20, 1998 / Accepted: January 15, 1999     

Coevolution at multiple spatial scales: Linum marginale–Melampsora lini – from the individual to the species

Coevolutionary processes are intrinsically spatial as well as temporal, and occur at many different scales. These range from single populations dominated by demographic and genetic stochasticity, to metapopulations in which colonisation/extinction dynamics have a large influence, and larger geographic regions where phylogenetic patterns and historical events become important. We present data for the genetically and demographically well-characterised plant host–pathogen interaction, the Linum marginale–Melampsora lini system, and use this to demonstrate the varying nature of resistance and virulence structure across these spatial scales. At the within population level, our results indicate considerable variability in resistance and virulence, but little evidence of coordinated changes in host and pathogen. Studies involving comparisons among multiple demes within a single metapopulation show that adjacent populations often have asynchronous disease dynamics and large differences in diversity and frequency of resistance and virulence phenotypes. Nevertheless, at this scale, there is also evidence of spatial structure in that more closely adjacent host populations are significantly more likely to have similar resistance phenotypes and mean levels of resistance. At larger scales, comparisons among adjacent metapopulations indicate that quantitative differences in host mating system and other life history features can have further major consequences for how host and pathogen variation is packaged. Finally, comparisons at continental and among host-species levels show variation consistent with specialisation and speciation in the pathogen. This revised version was published online in July 2006 with corrections to the Cover Date.     

Phaeocystis and its interaction with viruses

Over the years, viruses have been shown to be mortality agents for a wide range of phytoplankton species, including species within the genus Phaeocystis (Prymnesiophyceae). With its polymorphic life cycle, its worldwide distribution, and the capacity of several of the Phaeocystis species to form dense blooms, this genus is a key player for our understanding of biogeochemical cycling of elements. This paper provides an overview of what is know to date about the ecological role of viruses in regulating Phaeocystis population dynamics. It explores which variables affect the algal host–virus interactions, and examines the impact of virally induced cell lysis of Phaeocystis on the function and structure of the pelagic food web as well as on the flow of organic carbon and nutrients.     

Morphological and physiological variation among seagrass (Zostera marina) genotypes

Intraspecific variation in habitat-forming species can have important ecological consequences at the population, community, and ecosystem level. However, the contribution of genetic variation among individuals to these effects is seldom documented. We quantified morphological and physiological variation among genotypes of a marine foundation species, the seagrass Zostera marina. We grew replicate shoots of eight genetically distinct Zostera individuals collected from Bodega Bay, California, in a common garden environment and then quantified shoot production and morphology, nutrient uptake, and key photosynthetic parameters. We found that genotypes differed in shoot production, biomass, and both root and shoot nutrient uptake rates, even when corrected for genotype-specific biomass differences. In addition, the rank order of uptake ability differed for ammonium and nitrate, indicating that genotypes may exhibit resource partitioning of different forms of nutrients. Our results suggest that both niche complementarity among genotypes and the sampling/selection effect could contribute to previously observed positive effects of seagrass clonal diversity on resource utilization and biomass production. Further, they highlight that genotypic variation in key traits of habitat-forming species could have measurable effects on community structure and function.     

Testing Mechanisms of the Dilution Effect: Deer Mice Encounter Rates, Sin Nombre Virus Prevalence and Species Diversity

Species diversity has been shown to decrease prevalence of disease in a variety of host–pathogen systems, in a phenomenon termed the Dilution Effect. Several mechanisms have been proposed by which diversity may decrease prevalence, though few have been tested in natural host-pathogen systems. We investigated the mechanisms by which diversity influenced the prevalence of Sin Nombre virus (SNV), a directly transmitted virus in deer mice (Peromyscus maniculatus). We monitored both intra and interspecific encounters of deer mice using foraging arenas at five sites in the Great Basin Desert with disparate levels of species diversity to examine two potential mechanisms which may contribute to the dilution of SNV prevalence: (1) reduced frequency of encounters between deer mice, or (2) reduced duration of contacts between deer mice. We also investigated the relationship between deer mouse density and these mechanisms, as density is often predicted to influence both inter and intraspecific encounters. Results of our study indicate that frequency of intraspecific interactions between deer mice was reduced with increased diversity. Species diversity did not impact average duration of encounters. Density was correlated with absolute, but not relative rates of encounters between deer mice, suggesting that encounters may be influenced by factors other than density. Our study indicates that species diversity influences the dynamics of SNV by reducing encounters between deer mice in a trade-off between intra and interspecific interactions.     

Genetic analysis of disease susceptibility contributed by the compatible Tsn1–SnToxA and Snn1–SnTox1 interactions in the wheat-Stagonospora nodorum pathosystem

Stagonospora nodorum is a foliar pathogen of wheat that produces several host-selective toxins (HSTs) and causes the disease Stagonospora nodorum blotch (SNB). The wheat genes Snn1 and Tsn1 confer sensitivity to the HSTs SnTox1 and SnToxA, respectively. The objectives of this study were to dissect, quantify, and compare the effects of compatible Snn1–SnTox1 and Tsn1–SnToxA interactions on susceptibility in the wheat-S. nodorum pathosystem. Inoculation of a wheat doubled haploid population that segregates for both Snn1 and Tsn1 with an S. nodorum isolate that produces both SnTox1 and SnToxA indicated that both interactions were strongly associated with SNB susceptibility. The Snn1–SnTox1 and Tsn1–SnToxA interactions explained 22 and 28% of the variation in disease, respectively, and together they explained 48% indicating that their effects are largely additive. The Snn1–SnTox1 interaction accounted for 50% of the variation when the population was inoculated with an S. nodorum strain where the SnToxA gene had been mutated, eliminating the Tsn1–SnToxA interaction. These results support the theory that the wheat-S. nodorum pathosystem is largely based on multiple host–toxin interactions that follow an inverse gene-for-gene scenario at the host–toxin interface, but disease exhibits quantitative variation due to the mainly additive nature of compatible interactions. The elimination of either Snn1 or Tsn1 toxin sensitivity alleles resulted in decreased susceptibility, but the elimination of both interactions was required to obtain high levels of resistance. We propose the use of molecular markers to select against Snn1, Tsn1, and other toxin sensitivity alleles to develop wheat varieties with high levels of SNB resistance.     

The effect of landscape heterogeneity on host–parasite dynamics

Environmental heterogeneity has been shown to have a profound effect on population dynamics and biological invasions, yet the effect of its spatial structure on the dynamics of disease invasion in a spatial host–parasite system has received little attention. Here we explore the effect of environment heterogeneity using the pair approximation and the stochastic spatially explicit simulation in which the lost patches are clustered in a fragmented landscape. The intensity of fragmentation is defined by the amount and spatial autocorrelation of the lost habitat. More fragmented landscape (high amount of habitat loss, low clustering of lost patches) was shown to be detrimental to the parasitic disease invasion and transmission, which implies that the potential of using artificial disturbances as a disease-control agency in biological conservation and management. Two components of the spatial heterogeneity (the amount and spatial autocorrelation of the lost habitat) formed a trade-off in determining the host–parasite dynamics. An extremely high degree of habitat loss was, counter-intuitively, harmful to the host. These results enrich our understanding of eco-epidemiological, host–parasite systems, and suggest the possibility of using the spatial arrangement of habitat patches as a conservation tool for guarding focal species against parasitic infection and transmission.     

Desiccation,Moisture Content and Germination of Zostera marina L. Seed

Zostera marina is the only seagrass species whose seeds have been successfully used in large‐scale restoration. Although progress has been made in refining Z. marina restoration protocols, additional information on Z. marina seed physiology is necessary as the science of seagrass restoration evolves. We tested the germination rates of Z. marina seeds under different relative humidities and temperatures for different periods of time. Z. marina seed moisture content (MC) and germination rates were also tested when seeds were exposed to a temperature of 25°C and relative humidity of 50%. Z. marina seeds suffered higher mortality when exposed to lower relative humidity and higher temperature for longer period of exposure time. A significant negative correlation was detected between seed germination rate and MC. Z. marina seeds are sensitive to desiccation exposure and long periods of exposure to air should be prevented to minimize seed mortality when seeds are used in restoration projects.