A comparative study of the population biology of the American immigrant triclad Dugesia tigrina (Girard) in two British lakes

The population biology of the American immigrant triclad Dugesia tigrina was investigated in two British eutrophic lakes: Colemere, England, harbours an asexually reproducing population and Llyn Coron, North Wales, a population that reproduces both sexually and asexually. Monthly samples of triclads were taken from the undersides of stones in both lakes, and from ceramic tiles and plastic trays in Colemere and Llyn Coron, respectively, over two years. In both populations, body size-structure decreased slightly over winter, with an absence of very large and very small animals. Individual growth occurred in spring, and over summer the proportion of large and small individuals declined and increased, respectively. During September to November, size-structure was fairly stable. Increases in the size structure of the populations were associated with higher temperatures and an increased food supply; stability in size structure during the autumnal post-reproductive period, when temperatures were favourable for growth, to the occurrence of intraspecific competition for food; and decreases in size structure to lower temperatures during the winter. In both populations the density of triclads increased in summer to a peak in autumn, thereafter declining to a nadir in spring. The numerical peak in autumn was due to recruitment from reproduction, and the subsequent decline to deaths following the occurrence of intraspecific competition for food in late autumn, some invertebrate predation and winter mortality. Asexual reproduction (fission) started in June and finished during August in both populations. Laboratory experiments indicated that the occurrence and rate of fission is influenced by temperature and the amount of available food. The critical temperature for the initiation and cessation of fission lay between 14 and 16 °C in laboratory studies, and in the field fissioning started as temperatures approached 15 °C. In Llyn Coron, cocoon deposition commenced in June, peaked in July with only a very few produced in August. Triclads collected from the field outside these months could not be induced to lay cocoons in the laboratory. Laboratory experiments showed that cocoons were deposited at temperatures of 17.5, 20, and 25 °C, but not at 12.5 °C. Thus, in the field, cocoon production started when temperatures approached 15 °C in June, but had ceased whilst temperatures were still in excess of 15 °C in late August. Termination of cocoon dsposition is associated with the disappearance of large triclads in excess of 20 mm², below which animals are not sexually mature; disappearance of large animals was due to fissioning, death after sexual reproduction, and probably shrinkage due to severe intraspecific competition for food. Laboratory experiments indicated that the Colemere population had, potentially, greater fission production than the Llyn Coron population. Despite this, the numerical size of the Colemere and Llyn Coron populations increased seven-fold and eleven-fold, respectively, over the reproductive period. Whether or not this difference in reproductive potential between the populations will continue is speculative; the introduction of D. tigrina to Llyn Coron is relatively recent and the population may still be expanding to reach the carrying capacity of the habitat. Its high reproductive output must have helped its successful invasion of the lake.     

Implications of sensory ecology for species coexistence: biased perception links predator diversity to prey size distribution

Inherent to sensory systems is a discrepancy between the perceived and the actual environment. We modelled prey perception in different species of echolocating bats and show that differences in sensory systems can be important for shaping the niches of animals and for structuring animal communities. We argue that sensory specialization can lower interspecific competition by making the same world appear different. We specifically raise the claim that it is important to consider the interaction of sensory bias and the distribution of (prey) resource size. Using a modeling approach we assessed the potential contribution of sensory bias for species coexistence for the example of bat echolocation. We show that even relatively small sensory differences among coexisting species can translate into significant differences in access to food resources, if prey size distribution is skewed towards small prey. Specifically, for the prey size distribution occurring most frequently in nature, differences in sensory access to resources seem large enough to relax competition and facilitate species coexistence. Interaction between sensory bias and prey size distribution in a way that enhances species coexistence may be a general phenomenon not limited to bat echolocation.     

Spatiotemporal population distributions and their implications for species coexistence in a variable environment

A population experiences environmental variation both directly, through effects on life history parameters such as fecundity, and indirectly, through effects on the population distributions of competitors and thus on the distribution of competition. Which spatial and temporal scales of environmental variation most influence the coexistence of two species thus depends in part on the degree to which the resident population responds to different scales of variation. In this paper, I calculate an approximation for a spatiotemporal population distribution as the result of a filter function convolved with the environmental variation. I find that there is no straightforward connection between spatial or temporal scales inherent to an organism's life history, such as mean lifetime or dispersal distance, and the population's sensitivity to variation at different scales. Rather, life history traits interact sensitively with the way environmental variation affects the organism. I comment on the implications for variation-mediated coexistence.     

Salinity-temperature tolerance of two closely related triclad species,Dugesia lugubris and D. polychroa (Turbellaria), in relation to their distribution in The Netherlands

The tolerance of adult specimens of Dugesia lugubris and D. polychroa for 13 different chlorinities ranging from 15.0–3.8 and for two temperatures, viz. 4 and 23 °C, was tested.At chlorinities of 7.5 and lower, the survival time of both species was considerably longer than at higher chlorinities (a few hours at 7.5, one to several days at 6.6 and lower concentrations). It is assumed that this is determined by the osmoregulatory capacity of the planarians.It was found that at low chlorinities combined with a high temperature D. polychroa survived longer than D. lugubris, while at the same chlorinities the opposite was true for a low temperature. The effect of temperature on survival at low chlorinities was more drastic for D. lugubris than for D. polychroa.The results correlate with data on the distribution of both species in The Netherlands. Outside areas with an average chlorinity below 2 the two species were rarely found.     

Metabarcoding for the parallel identification of several hundred predators and their prey: Application to bat species diet analysis

Assessing diet variability is of main importance to better understand the biology of bats and design conservation strategies. Although the advent of metabarcoding has facilitated such analyses, this approach does not come without challenges. Biases may occur throughout the whole experiment, from fieldwork to biostatistics, resulting in the detection of false negatives, false positives or low taxonomic resolution. We detail a rigorous metabarcoding approach based on a short COI minibarcode and two‐step PCR protocol enabling the “all at once” taxonomic identification of bats and their arthropod prey for several hundreds of samples. Our study includes faecal pellets collected in France from 357 bats representing 16 species, as well as insect mock communities that mimic bat meals of known composition, negative and positive controls. All samples were analysed using three replicates. We compare the efficiency of DNA extraction methods, and we evaluate the effectiveness of our protocol using identification success, taxonomic resolution, sensitivity and amplification biases. Our parallel identification strategy of predators and prey reduces the risk of mis‐assigning prey to wrong predators and decreases the number of molecular steps. Controls and replicates enable to filter the data and limit the risk of false positives, hence guaranteeing high confidence results for both prey occurrence and bat species identification. We validate 551 COI variants from arthropod including 18 orders, 117 family, 282 genus and 290 species. Our method therefore provides a rapid, resolutive and cost‐effective screening tool for addressing evolutionary ecological issues or developing “chirosurveillance” and conservation strategies.     

The role of seasonal timing and phenological shifts for species coexistence

Shifts in the phenologies of coexistence species are altering the temporal structure of natural communities worldwide. However, predicting how these changes affect the structure and long‐term dynamics of natural communities is challenging because phenology and coexistence theory have largely proceeded independently. Here, I propose a conceptual framework that incorporates seasonal timing of species interactions into a well‐studied competition model to examine how changes in phenologies influence long‐term dynamics of natural communities. Using this framework I demonstrate that persistence and coexistence conditions strongly depend on the difference in species’ mean phenologies and how this difference varies across years. Consequently, shifts in mean and interannual variation in relative phenologies of species can fundamentally alter the outcome of interactions and the potential for persistence and coexistence of competing species. These effects can be predicted by how per‐capita effects scale with differences in species’ phenologies. I outline how this approach can be parameterized with empirical systems and discuss how it fits within the context of current coexistence theory. Overall, this synthesis reveals that phenology of species interactions can play a crucial yet currently understudied role in driving coexistence and biodiversity patterns in natural systems and determine how species will respond to future climate change.     

Fecundity of the newts Triturus cristatus, T. marmoratus and their natural hybrids in relation to species coexistence

The two larger newt species in Europe, Triturus cristatus and T. marmoratus , have largely exclusive distributions but live in sympatry in a small area in western France, where they also hybridize. Triturus cristatus is increasing in abundance in the area at the expense of T. marmoratus , although this study shows that T. marmoratus has higher relative lifetime reproductive success. Triturus marmoratus lays more eggs than its relative under controlled conditions, and more T. marmoratus eggs in relation to its relative abundance are found in natural breeding sites. The hybrids are even more fecund, but their eggs and embryos have a lower survival rate.
The data are compared for age at maturity and species longevity, and estimates of lifetime reproductive success show that T. marmoratus females are more fecund than T. cristatus females. There are no differences between the species in survival of the embryos. It is therefore not clear why T. cristatus is replacing T. marmoratus , but changes in terrestrial habitats due to human activity might be the explanation.     

The effect of prey mobility, prey contrast, turbidity and spectral composition on the reaction distance of Gobiusculus flavescens to its planktonic prey

The reaction distance of Gobiusculus flavescens to mobile and immobile copepod prey of different transparency was studied in water conditions with different turbidity levels or spectral composition. Both prey contrast (red or transparent Calanus spp.) and prey mobility caused reaction distance of G. flavescens to increase. The effect of prey mobility or prey transparency on reaction distance was independent of turbidity level. Longest reaction distances were measured for mobile and red prey at intermediate turbidity levels (10–20 JTU). Contrast threshold decreased with increasing turbidity. Although contrast of red and transparent copepod prey increased with increasing wavelength, longest reaction distances were observed at shorter wavelengths (blue–green). Lowest contrast threshold was found at shorter wavelengths. Visual pigments of G. flavescens are adapted to its natural environment. Both turbidity and spectral composition influenced G. flavescens contrast threshold.     

The significance of the Loosdrecht lakes research project for eutrophication policy in The Netherlands

The Project on the Water Quality Research in the Loosdrecht Lakes (WQL) has come up to the expectations of the Netherlands Environment Ministry. The results reaffirm the main lines of national eutrophication policy drawn up in 1979 (Policy Document on Phosphates) and further developed in the eighties. Interesting new insights have been gained, for example into the role of sediment and seston as well as into the relative importance of trophic levels. It is not possible, however, to definitely establish the effect of WQL on eutrophication policy.The Loosdrecht project is an example of genuine ecological research, incorporating several disciplines, placing the object of research into its surroundings, emphasising the relation nature-culture and committing itself to certain value judgements and policy choices. As a consequence, there is a striking resemblance between the evolution of the project itself and that of ecosystems.All in all the Loosdrecht project should be regarded as a paradigm for future ecosystem studies.     

英国的侧盘菌属:属的概况及大型孢子种的研究(英文)

对侧盘菌属在英国的研究概况进行了评述,研究侧重于Otidea apophysata和O.platyspora两个具有大型子囊孢子的种。同时,对4个错误地用于大型孢子种的名称进行了订正:O. abietina(Pseudotis属的模式种)是含糊名称(nomen ambiguum);O.cochleata也为含糊名称;O. felina是O.alutacea的同物异名,并为后者指定了选模式;将O.umbrina处理为O.bufonia的同物异名。此外,确定了Otidea violacea的分类地位。     

The significance of the asymmetric effect of crowding for coexistence in a mixed temperate forest

Abstract. The coexistence of coniferous (mostly Abies homolepis) and broad-leaved tree species (mostly Fagus crenata) in a mixed temperate old-growth forest in Japan was simulated by a size-structure dynamics model incorporating the asymmetrical (one-sided) effect of shading between these two life-form guilds. The model assumes that the crowding effect due to one-sided competition for light on a tree of a given size regulates the rate of size growth and recruitment. The cumulative basal area of trees larger than a given tree in the forest is employed to express the intensity of one-sided competition on that tree. Cumulative basal areas of both guilds negatively affected the growth rate of any tree. The shading effect by conifers on the growth rate of either guild was stronger than that by broad-leaved species. Two types of model were tested for recruitment; an additive and a reciprocal model. A reciprocal model, where basal area density of conifers and broad-leaved species has a negative effect on the recruitment of its own guild but has a positive effect on that of the other guild, fit the observed data better than an additive model where total basal area of the two guilds suppresses recruitment rates. Simulations using these models showed that, within a particular range of the set of recruitment rates, the two guilds could coexist. The tendency for reciprocal replacement, incorporated in the reciprocal model, substantially widened the range of coexistence and shortened the time required for convergence.     

Differences in refuge function for prey and tolerance to crayfish among macrophyte species

The invasive crayfish Procambarus clarkii is an omnivore and an ecosystem engineer whose feeding mechanism has reduced the abundance of many native invertebrates and macrophytes. Since macrophytes provide refuges for aquatic insects, macrophyte depletion by crayfish might have indirect negative effects on animal prey in aquatic habitats. We postulated that the prey refuges provided by macrophytes and macrophyte tolerance to crayfish cutting and feeding vary among macrophyte species. We conducted two experiments to (1) investigate differences in macrophyte refuge function for dragonfly larvae against crayfish, and (2) test the tolerance to crayfish cutting and feeding among macrophyte species. Elodea nuttallii (submerged plant), Potamogeton crispus (submerged plant), and Carex idzuroei (emergent plant) had greater refuge effects than Trapa japonica (floating-leaved plant), an effect that might result from the larger total cover of E. nuttallii, P. crispus, and C. idzuroei, and the hardness of C. idzuroei leaves. Tolerance to crayfish cutting and feeding was greater in C. idzuroei than in the other species. As the macro-invertebrate assemblages in submerged vegetation are more abundant and species-rich than those in emergent and floating-leaf vegetation, conservation of E. nuttallii and P. crispus should be prioritized for restoring native aquatic animals in ecosystems invaded by the introduced crayfish.     

The role of predators in the stable coexistence of several alga species

Experiments were conducted on growing mixed cultures of green Ankistrodesmus arcuatus Korsch and diatom Diatoma elongatum (Lyngb.) algae with and without ciliates Cyclidium glaucoma Ehrenberg and Paramecium caudatum Ehrenberg in accumulative and perfusion cultures. Both alga species successfully developed in the presence of ciliates despite their grazing on both alga species; however, in their absence, Ankistrodesmus arcuatus were suppressed and did not develop. It is supposed that ciliates contribute to the stabilization of the community not only as predators but also by excreting dissolved reduced nitrogen into the environment as a result of their vital functions.