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Cryptic species are morphologically identical but genetically distinct, and are prominent across numerous phyla. The coexistence of such closely related species on local scales would seem to run counter to traditional coexistence and competition theory; it has been hypothesized as a consequence of differences in their resource use or tolerances to environmental conditions. We developed an individual-based model of a community of three cryptic Litoditis marina (nematode) species, to understand how individual-level interspecific and intraspecific interactions might explain the coexistence of these closely related species. The model incorporates individuals' reproduction, competition, dispersal and resource use. Data characterizing the cryptic species (growth rates, dispersal ability, competitive interactions and responses to changing environmental conditions) were obtained from laboratory experiments involving both mono- and multispecific nematode cultures, and are used to parameterize the model. Simulation studies are used to investigate which individual-level mechanisms of dispersal and interaction lead to the characteristic population-level patterns observed experimentally. Our results highlight the key role of intraspecific competition in mediating dispersal and therefore co-occurrence of the cryptic species. The differences in dispersal also influence the response of the cryptic species to competition, a combination of factors that provides an explanation for their co-occurrence. These results provide insights into how changes in individual-level processes can be amplified to affect population-level co-occurrence.

Scientists and policy makers have to establish criteria to distinguish ‘acceptable’ from ‘harmful’ levels of pollution. Earlier studies have shown that even amounts of heavy metal pollutants well below LC50 or EC50 concentrations, can affect the fitness of individual bacterivorous nematode species, as well as the balance of the horizontal interactions between them. Species interactions are critical in shaping community structure and promoting ecosystem functions like organic matter decomposition - a key process that drives the flow of energy and nutrients in ecosystems. In this paper, we exposed two bacterial feeding soil nematodes, Plectus acuminatus and Acrobeloides nanus , to different Cu concentrations in monospecific and two-species microcosms containing leaf litter of the common grass species Urochloa mutica for a period of 60 days. We demonstrate that toxicant concentrations well below LC50 not only impair the fitness of the nematodes, but may also affect the interspecific interactions between them as a result of their differential sensitivity to Cu. Both Plectus and Acrobeloides are bacterial feeders and may thus affect the decomposition of leaf litter by impacting on the abundance and composition of bacteria. We observed Cu effects on the decomposition of Urochloa , but in the absence of data on the microbial community, it is not possible to assign these to direct effects of Cu on the bacteria or indirect effects through the Cu impacts on nematodes and their interactions.

Climate change has an impact on all levels of biodiversity: genes, species, populations and ecosystems. This effect is expected to be dependent on the rate of change. When faced with unfavorable environmental conditions, individuals have three non-exclusive options by which they can mitigate these changes: temporal alterations in phenology, spatial tracking of optimal conditions or in situ adaptation. For the latter to occur requires the existence of variability among conspecifics with respect to traits that directly or indirectly influence individual performance and fitness. One such “functional” trait is thermal stress tolerance. We investigated the presence of intraspecific functional trait variability of isofemale lineages of the free-living marine nematode Litoditis marina in monoculture, double and quadruple mixtures under thermal stress, which was applied in a gradient and a sudden treatment over 20 days, corresponding to at least four generations. We found a high degree of intraspecific functional trait variability relevant to thermal tolerance. This type of variability has traditionally been considered negligible compared to interspecific variation, but our results demonstrate the importance of its inclusion within trait-based eco-evolutionary studies as this will confer increased accuracy when inputting physiological parameters to relevant models (e.g. species distribution models). Additionally, we found a significant effect of the temperature treatments. In accordance with the majority of experimental evidence, our populations showed a stronger response to a sudden versus a gradient increase in temperature. Contrary to our expectations, no significant differences were observed between monocultures, double and quadruple lineage mixtures suggesting that increased intraspecific trait variability did not lead to higher fitness in L. marina populations exposed to thermal stress.

Heavy metals are persistent pollutants, the concentrations of which have increased in many natural environments as a result of anthropogenic activities. Accumulation of heavy metals in coastal benthic habitats is common. Nematoda are the most abundant and species-rich phylum in these benthic habitats, and shifts in their assemblage composition are considered good indicators of environmental change. At the same time, single-species assays can be highly informative of the organismal and population-level effects of pollution. Litoditis marina is a marine nematode inhabiting decaying macroalgae in the littoral zone of coasts and estuaries; it represents a cryptic species complex, and local co-occurrence of several cryptic species is common. Impact studies often assume that phylogenetically related nematodes exhibit similar sensitivities to disturbance, and commonly used biomonitoring indices often utilize scores defined at the level of guilds or families. Nevertheless, studies on other phyla have shown that even closely related taxa such as the cryptic species analysed in this study, may differ in their tolerance to pollution. Here we test the effects of Cu and Pb on the growth, fecundity and behaviour of two cryptic species of L. marina, PmII and PmIV, in single-species assays. Pb is a neurotoxicant and we expected that it would strongly affect behaviour (measured here as taxis to food). Increased Cu concentrations cause oxidative stress and can interfere with osmoregulation and endocrine pathways; Cu was therefore expected to impact more on growth and fecundity. Both cryptic species of L. marina were sensitive to heavy-metal pollution, but PmIV was substantially more sensitive than PmII, especially to copper. In contrast to our expectation, copper yielded the strongest effects on all endpoints, including behaviour. Lead yielded more discrepant results, inhibiting reproduction but stimulating growth and taxis to food at low (PmII) to intermediate (PmIV) concentrations. This study demonstrates that also in nematodes, closely related species, such as the cryptic species PmII and PmIV analysed in this study, can substantially differ in their response to contaminants, necessitating species-level rather than family or guild-based analyses of pollution effects. This is further supported by a comparison with literature data on the sensitivity of other nematode species to the same heavy metals. This comparison highlights that sensitivity to pollution can vary equally strongly between as within nematode families and guilds, and thereby challenges many guild- or taxonomic-relatedness-based approaches in impact studies using nematodes.

Density of conspecifics is considered as one of the main conditions affecting dispersal behavior and leading to a stabilization of population dynamics. Density-dependent dispersal can be induced by local competition at different phases during development and/or by density-related sources of social information. Here, we assessed the importance of population density on emigration rates and the degree to which the presence of silk threads at dispersal takeoff locations affects immediate dispersal decision making in the spider Erigone atra. By quantifying behaviors in wind tunnels under standardized laboratory conditions, silk-assisted long- and short-distance dispersal is quantified before the actual onset of the dispersal event. Increased densities during juvenile development only affected short-distance dispersal behavior. In females, short-distance dispersal increased with the female density experienced during development, whereas responses in males increased under combined high male/low female-experienced densities. Elevated densities at the onset of dispersal led to a general increase of predispersal behaviors. The presence of silk threads at takeoff platforms similarly induced an increase of dispersal displays, with specifically an increase in long-distance dispersal in both sexes. Our results demonstrate that the spider E. atra uses information related to density during development, most probably to avoid competition by performing short-distance dispersal. In contrast, density-related cues at the time of dispersal (i.e., increased densities and the presence of silk threads) increase general dispersal activities and long-distance ballooning events. Short- and long-distance dispersal strategies are consequently guided by differential density-related information use. Key words: araneae, density, dispersal, information, silk. [Behav Ecol]

Competition is one of the main drivers of dispersal, which can be an important mechanism to achieve permanent or temporal coexistence of multiple species. This coexistence can be achieved by a dispersal-competition tradeoff, spatial store effects or neutral dynamics. Here we test the effect of inter- and intraspecific competition on dispersal of four species of the marine nematode species complex Litoditis marina. A previous study in closed microcosms without a possibility for dispersal had demonstrated pronounced interspecific competition, leading to the exclusion of one species. We now investigated whether 1) the dispersal is affected by interspecific interactions, by intraspecific competition (density) or by food availability, 2) the dispersal dynamics influence assemblage composition and can lead to co-occurrence of the species, and 3) the abiotic environment (here salinity) can affect these dynamics. We show that density is the main driver for dispersal in two of the four species. Dispersal of a third species always started at the same time irrespective of density, whereas in the fourth species interspecific interactions accelerated dispersal. Remarkably, this fourth species was not a strong competitor, suggesting that a dispersal–competition tradeoff does not explain the observed coexistence. Salinity did not alter the timing of dispersal when interspecific interactions were present but did affect assemblage composition. Consequently, spatial store effects may influence coexistence. All four species co-occurred in fairly stable abundances throughout the present experiment indicating the importance of species specific dispersal strategies for coexistence. Co-occurrence can be facilitated because competition is postponed or avoided by dispersal. Neutral dynamics also played a role as intra- and interspecific competition were of similar importance in three of the four species. We conclude that dispersal is a driver of the coexistence of closely related nematode species, and that population density and interspecific interactions shape these dynamics.

Co-occurrence of closely related species may be achieved in environments with fluctuating dynamics, where competitively inferior species can avoid competition through dispersal. Here we present an experiment in which we compared active dispersal abilities (time until first dispersal, number and gender of dispersive adults, and nematode densities at time of dispersal) in Litoditis marina, a common bacterivorous nematode species complex comprising four often co-occurring cryptic species, Pm I, II, III, and IV, as a function of salinity and food distribution. The experiment was conducted in microcosms consisting of an inoculation plate, connection tube, and dispersal plate. Results show species-specific dispersal abilities with Pm I dispersing almost one week later than Pm III. The number of dispersive adults at time of first dispersal was species-specific, with one dispersive female in Pm I and Pm III and a higher, gender-balanced, number in Pm II and Pm IV. Food distribution affected dispersal: in absence of food in the inoculation plate, all species dispersed after ca four days. When food was available Pm I dispersed later, and at the same time and densities irrespective of food conditions in the dispersal plate (food vs no food), suggesting density-dependent dispersal. Pm III dispersed faster and at a lower population density. Salinity affected dispersal, with slower dispersal at higher salinity. These results suggest that active dispersal in Litoditis marina is common, density-dependent, and with species, gender- and environment-specific dispersal abilities. These differences can lead to differential responses under suboptimal conditions and may help to explain temporary coexistence at local scales.

Dispersal, and especially movement between the natal and the new environment, is a costly process. Costs are incurred not only during the preparation or transfer phase but also on arrival in a new environment. Such costs comprise, for instance, time costs because of the need to search for a suitable microhabitat or integration costs when immigration occurs in already densely populated habitat. The failure to settle or to integrate may consequently inflate the costs already incurred during emigration and transfer. Therefore, we expect emigrating individuals to have evolved enhanced integration capacities. In our laboratory experiments, dispersive phenotypes of the agrobiont spider Erigone atra were characterized by (1) faster settlement in empty habitat and (2) increased willingness to compete for ownership of already occupied sites. These correlated behaviours point to dispersive phenotypes (1) being likely to experience a selective advantage at settlement and (2) not being a random subsample from the population. These correlated behaviours are predicted to have a strong impact on spatial population dynamics and gene flow.