Carles Alcaraz

Incongruence between test statistics and P values in medical papers

BackgroundGiven an observed test statistic and its degrees of freedom, one may compute the observed P value with most statistical packages. It is unknown to what extent test statistics and P values are congruent in published medical papers.MethodsWe checked the congruence of statistical results reported in all the papers of volumes 409–412 of Nature (2001) and a random sample of 63 results from volumes 322–323 of BMJ (2001). We also tested whether the frequencies of the last digit of a sample of 610 test statistics deviated from a uniform distribution (i.e., equally probable digits).Results11.6% (21 of 181) and 11.1% (7 of 63) of the statistical results published in Nature and BMJ respectively during 2001 were incongruent, probably mostly due to rounding, transcription, or type-setting errors. At least one such error appeared in 38% and 25% of the papers of Nature and BMJ, respectively. In 12% of the cases, the significance level might change one or more orders of magnitude. The frequencies of the last digit of statistics deviated from the uniform distribution and suggested digit preference in rounding and reporting.ConclusionsThis incongruence of test statistics and P values is another example that statistical practice is generally poor, even in the most renowned scientific journals, and that quality of papers should be more controlled and valued.

Life-history traits of invasive fish in small Mediterranean streams

We compared the life-history traits of native and invasive fish species from Catalan streams in order to identify the characters of successful invasive fish species. Most of the exotic fish species were characterized by large size, long longevity, late maturity, high fecundity, few spawnings per year, and short reproductive span, whereas Iberian native species exhibited predominantly the opposite suite of traits. Species native to the southeastern Pyrenees watershed were also significantly different from species native to the rest of the Iberian Peninsula but not native to this watershed. Iberian exotic species come predominantly from large river basins, whereas Catalan streams (and other small, coastal river basins) correspond to basins and streams of a smaller size and different hydrology, with differences in species composition and life-history traits of fish. The occurrence and spread of invasive species was not significantly related to life-history traits but to introduction date. The successful prediction of future invasive species is limited due to small differences in life-history and ecological traits between native and exotic species. Fecundity, age at maturity, water quality flexibility, tolerance to pollution and habitat seem the most discriminating life-history variables.

Salinity mediates the competitive interactions between invasive mosquitofish and an endangered fish

The interplay of abiotic factors and competition has a long history in ecology, although there are very few studies on the interaction of salinity and competition in fish. Mosquitofish (Gambusia holbrooki) are among the most invasive fish worldwide, with well documented ecological impacts on several taxa such as amphibians and small native fish. It has been previously hypothesized, based on field observations, that salinity limits the invasive success of mosquitofish and provides a competitive refuge for Mediterranean cyprinodonts. We experimentally tested this hypothesis by examining the agonistic behaviour and food competition between mosquitofish and an endangered native cyprinodont (Aphanius fasciatus) at three salinities (0, 15, 25‰). Intraspecific aggressive behaviour for both species was not significantly affected by salinity. As salinity increased, mosquitofish decreased their aggressive behaviour towards cyprinodonts and captured less prey. In contrast, the cyprinodonts did not change their behaviour with different salinity treatments, with the possible exception of increased defensive acts in higher salinities, but captured more prey with increasing salinity because of the reduced efficiency of mosquitofish. Our study confirms previous field observations that salinity limits the invasive success of mosquitofish and provides one of the few experimental demonstrations that it may mediate behavioural and competitive interactions between fish species. Condition-specific competition of mosquitofish might be expected with other species and ecosystems worldwide and illustrates the importance of integrating biotic and abiotic factors in the study of interspecific interactions.

Procambarus clarkii as a bioindicator of heavy metal pollution sources in the lower Ebro River and Delta

In the Ebro River basin, point and diffuse pollution of heavy metals stems mainly from industry and agriculture. Bioaccumulation patterns were examined under different pollution sources (point and diffuse) using levels of heavy metals (As, Cd, Cr, Cu, Hg, Pb and Zn) in abdominal muscle tissue of Procambarus clarkii. P. clarkii captured under point source effects presented the highest concentrations of Hg, Pb and As; and were related with distance to the source of industrial waste sediments. Mean Hg levels in crayfish exposed to point sources of metals significantly exceeded legal allowed values established by the European Union legislation. In the Ebro Delta, high levels of As, Cr, Cu and Zn were associated with traditional agriculture activity (diffuse pollution) as well. These results demonstrate the potential of P. clarkii to bioaccumulate heavy metals from both point and diffuse sources and hence potentially transfer these metals to higher trophic levels.

Life history and parasites of the invasive mosquitofish (Gambusia holbrooki) along a latitudinal gradient

The eastern mosquitofish (Gambusia holbrooki) is among the most invasive fish worldwide and yet, while very abundant in most Mediterranean countries, it is unable to tolerate the colder winters of northern and central Europe. Understanding the effects of latitude on its life-history traits is essential to predict the potential for its invasion of central Europe in current scenarios of climate change. We studied the variation of life-history traits and parasite load in the eastern mosquitofish along a latitudinal gradient from southern France to southern Spain, sampling mosquitofish populations in eight Mediterranean river mouths ranging 5° in latitude. Southern mosquitofish populations displayed higher catch rates, allocated more energy to reproduction (gonadosomatic index and gonadal weight after accounting for fish size) and had a lower condition (total weight and eviscerated weight after accounting for fish size) than in northern populations. Despite variability among populations, size-at-maturity (L50) significantly varied with latitude and northern individuals matured at smaller size (lower L50). Parasite prevalence ranged from 0.0 to 26.7% but parasite richness was very low; all the parasites identified were larvae of pleurocercoid cestodes belonging to the order Pseudophyllidea. The abundance of mosquitofish parasites decreased with latitude and the presence and number of parasites infecting the mosquitofish had a significant negative effect on fish condition. The significant effects of latitude on the catch rates, life history and parasites of mosquitofish highlight the importance of latitudinal studies of invasive species to understand the interactive mechanisms of climate change and biological invasions.

Food of an endangered cyprinodont (Aphanius iberus): ontogenetic diet shift and prey electivity

SynopsisWe studied the ontogenetic diet shift and prey electivity of an endangered cyprinodontid fish endemic to the Iberian Peninsula, the Spanish toothcarp (Aphanius iberus). The toothcarp’s diet was omnivorous, dominated by harpacticoid copepods (Mesochra lilljeborgi and Tisbe longicornis), copepod nauplii and detritus. Diet composition varied greatly among habitats, depending on prey availability. In a rarely inundated habitat (glasswort), there was more consumption of the isopod Protracheoniscus occidentalis and the harpacticoid copepod Mesochra lilljeborgi, while in algal mats another harpacticoid (Tisbe longicornis), chironomid dipterans and invertebrate eggs were more important in diet. Although a benthic feeding habitat has previously been suggested, in our study the diet was based rather on water column organisms for both glasswort and algal mat habitats. There was also an ontogenetic diet shift, with an increase of mean prey length with fish length, clearly linked to a microhabitat change. Smaller fish showed positive electivity and greater reliance on planktonic prey (e.g. copepod nauplii, the harpacticoid copepods Mesochra lilljeborgi and Tisbe longicornis, the rotifer Brachionus plicatilis, and ostracods), while larger fish elected and preyed on more benthic organisms (e.g. Canuella perplexa, Mesochra rapiens, and ephydrid dipterans).

Regime shift from phytoplankton to macrophyte dominance in a large river: Top-down versus bottom-up effects.

The lower Ebro River (Catalonia, Spain) has recently undergone a regime shift from a phytoplankton-dominated to a macrophyte-dominated system. This shift is well known in shallow lakes but apparently it has never been documented in rivers. Two initial hypotheses to explain the collapse of the phytoplankton were considered: a) the diminution of nutrients (bottom-up); b) the filtering effect due to the colonization of the zebra mussel (top-down). Data on water quality, hydrology and biological communities (phytoplankton, macrophytes and zebra mussel) was obtained both from existing data sets and new surveys. Results clearly indicate that the decrease in phosphorus is the main cause of a dramatic decrease in chlorophyll and large increase in water transparency, triggering the subsequent colonization of macrophytes in the river bed. A Generalized Linear Model analysis showed that the decrease in dissolved phosphorus had a relative importance 14 times higher than the increase in zebra mussel density to explain the variation of total chlorophyll. We suggest that the described changes in the lower Ebro River can be considered a novel ecosystem shift. This shift is triggering remarkable changes in the biological communities beyond the decrease of phytoplankton and the proliferation of macrophytes, such as massive colonization of Simulidae (black fly) and other changes in the benthic invertebrate communities that are currently investigated.

Bioaccumulation of pollutants in the zebra mussel from hazardous industrial waste and evaluation of spatial distribution using GAMs.

In the Flix Reservoir (Ebro River, Spain), ca. 300,000tons of industrial waste were dumped because of the activity of a factory plant in Flix. Within the recovery program implemented, this exceptional situation provides a unique opportunity to test the value of zebra mussel as sentinel organism. Ten metal concentrations were measured in mussels from different sites to assess spatial redistribution of metals and bioavailability to the food web. Our results showed an important metal uptake by mussels; metal concentrations (except As) measured in impacted sites were up to 10 times higher than in control sites, and Mn and Hg exceeded several times the levels previously reported for polluted waters. Concentrations increased downstream showing the metal mobilization from polluted sediments in Flix Reservoir. The higher metal concentrations measured in zebra mussel individuals clearly indicated their bioavailability to the food web, allowing the toxics transfer to predators and occasionally to humans. Thus, zebra mussel is a valuable sentinel organism to identify highly polluted waters, transport routes and trophic transfer.

Use of a flooded salt marsh habitat by an endangered cyprinodontid fish (Aphanius iberus)

We report the first data on the use of occasionally inundated habitats in a Mediterranean coastal lagoon by the Spanish toothcarp (Aphaniusiberus), a cyprinodontid fish in danger of extinction. During a flooding period, we sampled the fish population and the macroinvertebrate community in three contrasting habitats: mats of green algae, open water, and a habitat dominated by glasswort (Salicornia patula) that is occasionally inundated. The three habitats displayed strong variation in density and species composition of invertebrates in the water column and the benthos. In general, algal mats had higher invertebrate biomasses, but glasswort had higher diversity of organisms, in part of terrestrial origin. The density of toothcarp was very low in the open water. The habitat that is occasionally inundated (glasswort) significantly had the highest density of mature toothcarp, while immature fish were similarly abundant in the glasswort and algal mat habitats. Condition (weight–length relationship) and total food biomass in the gut contents of immature toothcarp was significantly higher in algal mats than in glasswort, whereas there were no such differences for mature fish. Therefore, the occasionally flooded habitat (glasswort) was positively selected by large mature toothcarp but seemed a disadvantageous habitat for immature individuals.

Herbivory and seasonal changes in diet of a highly endemic cyprinodontid fish (Aphanius farsicus)

Trophic ecology is essential to understand ecosystem functioning and structure and assist biological conservation. Here we investigate, for the first time, the feeding ecology of Aphanius farsicus, a cyprinodontid fish endemic of a single landlocked river basin in central Iran. We sampled monthly a population of this fish species during a year and examined differences in food across seasons, sexes and sizes. Similarly to other cyprinodonts, A. farsicus showed sexual dimorphism and more abundance of females. Size structure and individual condition varied across seasons, with larger fish in spring and better condition in summer and less in winter. We found no empty guts, suggesting that these fish feed all year round due to the warm climate of its native distribution. Farsi toothcarp diet was based on detritus, algae (particularly diatoms, green algae, and cyanobacteria), and small invertebrates. Seasonal variation in diet was more important than variation due to fish size and the Farsi toothcarp consumed more green algae in spring and early summer and more diatoms and insects the rest of the year. Herbivory was considerable, similarly to a few other cyprinodonts, and increased with fish size, particularly because of higher consumption of green algae. As with species composition in diet, season was more important than size in the variation of number, biovolume, mean size, and diversity of prey captured, with higher number, richness and size of prey captured in summer. The ontogenetic diet shift was less marked in this cyprinodont than in many other Aphanius species, probably due to its reduced size and the resource availability of its habitat, but was also shown by size-dependent feeding selectivity for a few invertebrates.