Illustration of the steps within the analysis of density dependence.

<p>A: regression through observed data of <i>Macoma balthica</i> for the time interval May to June 1997. B: preset slope forced through observed data (in this example slope = 1 for density independence). C: deterministic data calculated from observed data to lie on hypothetical regression line from previous step. D: regression after measurement error, estimated by bootstrapping, was added to deterministic data, one example of 10000 simulations. E: additionally, before adding measurement error, process error was added to log<i>D_t</i>₊₁, to arrive at observed residual deviance from first step, one example of 10000 simulations. F: modelled slope values with prediction intervals for various preset slopes, simulated with observed measurement error alone (white line for slope values and grey bootstrap interval) and observed process error additionally (black dots and whiskers). The dashed line gives the observed slope from step A. In this case the observed slope falls within the modelled prediction interval, whatever the preset slope (i.e. assumed true slope) is.</p

Mortality in relation to initial density.

<p>Range of log densities per m² at the beginning of a time interval, and total mortality (mean log<i>D_t</i> - mean log<i>D_t</i>₊₁) during the time interval. Solid line: <i>Macoma balthica</i> (bold: density dependence detected). Broken line: <i>Cerastoderma edule</i>.</p

Scheme of an asymptotic adult-recruit relationship.

<p>Reproductive output of the adult stock is proportional to the stock size, but through density-dependent mortality during the pre-recruit phase, a stock-recruitment relationship is lacking (after <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102491#pone.0102491-Bos1" target="_blank">[12]</a>).</p

Sampling time scheme with corer areas (cm²).

<p>Sampling time scheme with corer areas (cm²).</p

Simulation results for <i>Cerastoderma edule</i> in the time intervals of density decline.

<p>Dashed line: slope value of the original regression through the observed data. White line: average slope values resulting when measurement error is added to deterministic data on preset regression lines. grey area: corresponding 95% confidence intervals. Black dots: average slope values resulting when process error and measurement error are added to deterministic data on preset regression lines. Whiskers: corresponding 95% confidence intervals. If the lower end of the confidence interval for the preset slope 1 would end above the observed slope value, the observed slope would differ significantly from 1 and density dependence would be concluded.</p

Appendix B. Scatterplot of median grain size (MGS) and inundation time (IT).

Scatterplot of median grain size (MGS) and inundation time (IT)

Impact of the invasive parasitic copepod <i>Mytilicola orientalis</i> on native blue mussels <i>Mytilus edulis</i> in the western European Wadden Sea

<p>Invasive species can indirectly affect native species by modifying parasite–host dynamics and disease occurrence. This scenario applies to European coastal waters where the invasive Pacific oyster (<i>Magallana gigas</i>) co-introduced the parasitic copepod <i>Mytilicola orientalis</i> that spills over to native blue mussels (<i>Mytilus edulis</i>) and other native bivalves. In this study, we investigated the impact of <i>M. orientalis</i> infections on blue mussels by conducting laboratory experiments using controlled infections with larval stages of the parasitic copepod. As the impact of infections is likely to depend on the mussels’ food availability, we also tested whether potential adverse effects of infection on mussels intensify under low food conditions. Blue mussels that were experimentally infected with juvenile <i>M. orientalis</i> had a significantly lower body condition (11–13%) compared with uninfected mussels after nine weeks of infection. However, naturally infected mussels from a mixed mussel and oyster bed did not significantly differ in body condition compared with uninfected mussels. Contrary to effects on mussel condition, we did not find an effect of experimental infections on clearance rates, shell growth or survival of blue mussels and no clear sign of exacerbating effects of food limitation. Our study illustrates that invasive species can indirectly affect native species via parasite co-introductions and parasite spillover. The results of this study call for the integration of such parasite-mediated indirect effects of invasions in impact assessments of invasive species.</p

Parasite body length (corrected for host size using linear regression) for both introduced <i>Mytilicola</i> species.

<p><b>Females (left) and males (right) of</b><i>M</i>. <i>intestinalis</i> (grey) and <i>M</i>. <i>orientalis</i> (white) in each surveyed region. The boxes represent the interquartile range, the whiskers denote the lowest and highest values within the 1.5 interquartile range, the black line in each box denotes the median, the large black dots represent the mean and the smaller dots outside the boxes are outliers.</p

Sampling locations of blue mussel (<i>Mytilus edulis</i>) and Pacific oyster (<i>Magallana</i> (previously <i>Crassostrea</i>) <i>gigas</i>) hosts.

<p>Left: The sampled regions in the Dutch Delta and the Wadden Sea (shaded area), with the islands Sylt (north) and Texel (south). Above right: Sampling locations around the islands of Sylt and Texel in the Wadden Sea. Below right: Sampling locations in the Dutch Delta. For exact coordinates see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0193354#pone.0193354.s003" target="_blank">S1 Table</a>.</p

Boxplots of <i>Mytilicola orientalis</i> body length (μm) in both host species.

<p>Female (left) and male (right) copepods originating from oysters (<i>Magallana gigas</i>) in grey and from mussel (<i>Mytilus edulis</i>) hosts in white, from the Dutch Delta and Wadden Sea. The boxes represent the interquartile range, the whiskers denote the lowest and highest values within the 1.5 interquartile range, the black line in each box denotes the median, the large black dots represent the mean and the smaller dots outside the boxes are outliers.</p