Trophic interactions in rockpool food webs: regulation of zooplankton and phytoplankton by Notonecta and Daphnia

We studied trophic interactions in experimental rockpools with three different food web structures: phytoplankton and small-bodied zooplankton; phytoplankton, small-bodied zooplankton and Daphnia ; and phytoplankton, small-bodied zooplankton, Daphnia and Notonecta . Nutrients, primary productivity, chlorophyll a and zooplankton species composition and biomass were measured over eight weeks.
2. Daphnia had a negative impact on other zooplankton and reduced the phytoplankton biomass and primary productivity. In the absence of Daphnia , small-bodied zooplankton species were abundant, in particular cyclopoid copepods. Concentrations of dissolved nutrients were lower and the standing crop of primary producers was higher when Daphnia was absent.
3. The presence of the invertebrate predator Notonecta produced a top-down effect which was similar to that reported for planktivorous fish, i.e. a selective reduction of daphnids followed by an increase of small-bodied zooplankton species and phytoplankton biomass.
4. The study showed that consumer regulation of Daphnia by Notonecta and of algae by Daphnia are important, but also demonstrated that trophic level biomasses were controlled by a combination of predation and resource limitation.     

The parasitic chytrid, Zygorhizidium, facilitates the growth of the cladoceran zooplankter, Daphnia, in cultures of the inedible alga, Asterionella

In food-web studies, parasites are often ignored owing to their insignificant biomass. We provide evidence that parasites may affect trophic transfer in aquatic food webs. Many phytoplankton species are susceptible to parasitic fungi (chytrids). Chytrid infections of diatoms in lakes may reach epidemic proportions during diatom spring blooms, so that numerous free-swimming fungal zoospores (2-3 microm in diameter) are produced. Analysis shows that these zoospores are rich in polyunsaturated fatty acids and sterols (particularly cholesterol), which indicates that they provide excellent food for zooplankters such as Daphnia. In life-table experiments using the large diatom Asterionella formosa as food, Daphnia growth increased significantly in treatments where a parasite was present. By grazing on the zoospores, Daphnia acquired important supplementary nutrients and were able to grow. When large inedible algae are infected by parasites, nutrients within the algal cells are consumed by these chytrids, some of which, in turn, are grazed by Daphnia. Thus, chytrids transfer energy and nutrients from their hosts to zooplankton. This study suggests that parasitic fungi alter trophic relationships in freshwater ecosystems and may be the important components in shaping the community and the food-web dynamics of lakes.     

Notonecta exhibit threat-sensitive,predator-induced dispersal

Dispersal is a central process determining community structure in heterogeneous landscapes, and species interactions within habitats may be a major determinant of dispersal. Although the effects of species interactions on dispersal within habitats have been well studied, how species interactions affect the movement of individuals between habitats in a landscape has received less attention. We conducted two experiments to assess the extent to which predation risk affects dispersal from an aquatic habitat by a flight-capable semi-aquatic insect (Notonecta undulata). Exposure to non-lethal (caged) fish fed conspecifics increased dispersal rates in N. undulata. Moreover, dispersal rate was positively correlated with the level of risk imposed by the fish; the greater the number of notonectids consumed by the caged fish, the greater the dispersal rate from the habitat. These results suggest that risk within a habitat can affect dispersal among habitats in a landscape and thus affect community structure on a much greater scale than the direct effect of predation itself.     

Ecological determinants of the occurrence and dynamics of Vibrio parahaemolyticus in offshore areas

The life cycle of Vibrio parahaemolyticus has been conventionally associated with estuarine areas characterized by moderate salinity and warm seawater temperatures. Recent evidence suggests that the distribution and population dynamics of V. parahaemolyticus may be shaped by the existence of an oceanic transport of communities of this organism mediated by zooplankton. To evaluate this possibility, the presence of V. parahaemolyticus in the water column of offshore areas of Galicia was investigated by PCR monthly over an 18-month period. Analysis of zooplankton and seawater showed that the occurrence of V. parahaemolyticus in offshore areas was almost exclusively associated with zooplankton and was present in 80% of the samples. The influence of environmental factors assessed by generalized additive models revealed that the abundance and seasonality of V. parahaemolyticus in zooplankton was favoured by the concurrence of downwelling periods that promoted the zooplankton patchiness. These results confirm that offshore waters may be common habitats for V. parahaemolyticus, including strains with virulent traits. Additionally, genetically related populations were found in offshore zooplankton and in estuaries dispersed along 1500 km. This finding suggests that zooplankton may operate as a vehicle for oceanic dispersal of V. parahaemolyticus populations, connecting distant regions and habitats, and thereby producing impacts on the local community demography and the spread of Vibrio-related diseases.     

Differential wind dispersal of cladoceran ephippia in a rock pool metacommunity

Dispersal connects patches within metapopulations and is crucial to the persistence of many species, particularly those living in discontinuous habitat. Rock pools are excellent habitats in which to study dispersal in time as well as space, because many of the organisms that live within them make resistant long-lived dormant stages, they are often abundant, and they are easy to sample. The rock pools on Appledore Island, Gulf of Maine, USA, are home to several cladocerans, including Moina macrocopa and Daphnia pulex × pulicaria hybrids. Both taxa exist in extremely high abundances in some pools and make diapausing eggs enclosed in ephippia that are dispersed in time by hatching long after they are produced, and are also known to spatially disperse via pool overflows and by adhering to gulls. I hypothesized that ephippia of both taxa would also be spatially dispersed by wind. I found that while Moina are present in more pools, more abundant in those pools, and produce more ephippia, many more Daphnia ephippia dispersed into traps placed around the island. This may be explained, in part, by differences in the buoyancy of ephippia between the two species. A higher propensity to disperse may result in Daphnia relying more heavily on the spatial context of rock pools than Moina.     

Variation in ephippial buoyancy in Daphnia pulicaria

1. Freshwater zooplankton often produce diapausing eggs to survive environmental stress. The diapausing eggs of Daphnia (Crustacea, Cladocera) are encased in an ephippium that either floats at the surface or sinks to the sediment. These two types of ephippia may represent different strategies between spatial (floaters) and temporal (sinkers) dispersal of offspring. 2. We observed floating and sinking characteristics of ephippia obtained from eight lakes. We then conducted an experiment with 26 Daphnia pulicaria clones obtained from six of these lakes and observed the production of buoyant and non‐buoyant ephippia under high and low food conditions. 3. Ephippia were more often non‐buoyant than buoyant both from females caught in nature and those reared in the laboratory. The experiment revealed that each clone was able to produce both types of ephippia, but that there was considerable among‐clone variation in the percentage of non‐buoyant ephippia produced. 4. We conclude that production of non‐buoyant versus buoyant ephippia may be driven by both genetic and environmental factors.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing environmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk increased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.     

The interactive effects of competition and predation risk on dispersal in an insect

Dispersal dynamics have significant consequences for ecological and evolutionary processes. Previous work has demonstrated that dispersal can be context-dependent. However, factors affecting dispersal are typically considered in isolation, despite the probability that individuals make dispersal decisions in response to multiple, possibly interacting factors. We examined whether two ecological factors, predation risk and intraspecific competition, have interactive effects on dispersal dynamics. We performed a factorial experiment in mesocosms using backswimmers (Notonecta undulata), flight-capable, semi-aquatic insects. Emigration rates increased with density, and increased with predation risk at intermediate densities; however, predation had minimal effects on emigration at high and low densities. Our results indicate that factorial experiments may be required to understand dispersal dynamics under realistic ecological conditions.     

Interactions between Nematodes and Earthworms: Enhanced Dispersal of Steinernema carpocapsae

Dispersal of the nematode Steinernema carpocapsae (All strain), applied on the top or the bottom of soil columns, was tested in the presence or absence of two earthworm species, Lumbricus terrestris or Aporrectodea trapezoides. Nematode dispersal was estimated after a 2-week period with a bioassay against the greater wax moth, Galleria mellonella. Vertical dispersal of nematodes was increased in the presence of earthworms. When nematodes were placed on the surface of soil columns, significantly more nematodes dispersed to the lower half of the columns when either earthworm species was present than when earthworms were not present. When nematodes were placed on the bottom of soil columns, significantly more nematodes dispersed to the upper half of the columns when L. terrestris was present than when A. trapezoides was present or in the absence of earthworms. Because nematodes were found on the exterior and in the interior of earthworms, nematode dispersal may be enhanced by direct contact with the earthworms.     

Flying shells: historical dispersal of marine snails across Central America

The geological rise of the Central American Isthmus separated the Pacific and the Atlantic oceans about 3 Ma, creating a formidable barrier to dispersal for marine species. However, similar to Simpson's proposal that terrestrial species can 'win sweepstakes routes'-whereby highly improbable dispersal events result in colonization across geographical barriers-marine species may also breach land barriers given enough time. To test this hypothesis, we asked whether intertidal marine snails have crossed Central America to successfully establish in new ocean basins. We used a mitochondrial DNA genetic comparison of sister snails (Cerithideopsis spp.) separated by the rise of the Isthmus. Genetic variation in these snails revealed evidence of at least two successful dispersal events between the Pacific and the Atlantic after the final closure of the Isthmus. A combination of ancestral area analyses and molecular dating techniques indicated that dispersal from the Pacific to the Atlantic occurred about 750 000 years ago and that dispersal in the opposite direction occurred about 72 000 years ago. The geographical distribution of haplotypes and published field evidence further suggest that migratory shorebirds transported the snails across Central America at the Isthmus of Tehuantepec in southern Mexico. Migratory birds could disperse other intertidal invertebrates this way, suggesting the Central American Isthmus may not be as impassable for marine species as previously assumed.     

Dispersal of Symbiodinium by the stoplight parrotfish Sparisoma viride

Environmental reservoirs of zooxanthellae are essential for coral larvae settlement; understanding where they occur and how they are maintained is important for coral reef ecology. This study investigated the dispersal of Symbiodinium spp. by the stoplight parrotfish Sparisoma viride, which had high mean densities of viable and cultivable Symbiodinium (3207-8900 cells ml(-1)) in faeces. Clades A, B and G were detected using amplified chloroplast ribosomal sequences (cp23S-HVR), and corresponded with diet preferences of fish and the environmental Symbiodinium diversity of the region. Cells are constantly dispersed in the water column and deposited in the substrate at a local level (86 ± 17.8 m(2)), demonstrating that parrotfishes are vectors for short-distance dispersal of zooxanthellae. Such dispersal could constitute a key role in the maintenance of environmental Symbiodinium reservoirs.     

Effect of vertebrate and invertebrate kairomones on the life history of Daphnia magna Straus (Crustacea: Branchiopoda)

The history of selection of Daphnia magna populations living in North African temporary ponds may differ from populations inhabiting permanent ponds. Laboratory experiments were conducted to examine the effect of fish Gambusia holbrooki and invertebrate Notonecta glauca kairomones on the life history traits of the freshwater Cladocera Daphnia magna Straus. With fish kairomones, Daphnia reproduced early and had a significantly smaller size at first reproduction (SFR) and a smaller size of neonates compared to control. In contrast, daphnids reared in water treated with Notonecta glauca had no effect on the age at first reproduction but females were also smaller and produced smaller neonates.     

Induced tolerance expressed as relaxed behavioural threat response in millimetre-sized aquatic organisms

Natural selection shapes behaviour in all organisms, but this is difficult to study in small, millimetre-sized, organisms. With novel labelling and tracking techniques, based on nanotechnology, we here show how behaviour in zooplankton (Daphnia magna) is affected by size, morphology and previous exposure to detrimental ultraviolet radiation (UVR). All individuals responded with immediate downward swimming to UVR exposure, but when released from the threat they rapidly returned to the surface. Large individuals swam faster and generally travelled longer distances than small individuals. Interestingly, individuals previously exposed to UVR (during several generations) showed a more relaxed response to UVR and travelled shorter total distances than those that were naive to UVR, suggesting induced tolerance to the threat. In addition, animals previously exposed to UVR also had smaller eyes than the naive ones, whereas UVR-protective melanin pigmentation of the animals was similar between populations. Finally, we show that smaller individuals have lower capacity to avoid UVR which could explain patterns in natural systems of lower migration amplitudes in small individuals. The ability to change behavioural patterns in response to a threat, in this case UVR, adds to our understanding of how organisms navigate in the ‘landscape of fear’, and this has important implications for individual fitness and for interaction strengths in biotic interactions.     

Dispersal as a means of inbreeding avoidance in a wild bird population

The long-term study of animal populations facilitates detailed analysis of processes otherwise difficult to measure, and whose significance may appear only when a large sample size from many years is available for analysis. For example, inbreeding is a rare event in most natural populations, and therefore many years of data are needed to estimate its effect on fitness. A key behaviour hypothesized to play an important role in avoiding inbreeding is natal dispersal. However, the functional significance of natal dispersal with respect to inbreeding has been much debated but subject to very few empirical tests. We analysed 44 years of data from a wild great tit Parus major population involving over 5000 natal dispersal events within Wytham Woods, UK. Individuals breeding with a relative dispersed over several-fold shorter distances than those outbreeding; within the class of inbreeding birds, increased inbreeding was associated with reduced dispersal distance, for both males and females. This led to a 3.4-fold increase (2.3-5, 95% CI) in the likelihood of close (f=0.25) inbreeding relative to the population average when individuals dispersed less than 200m. In the light of our results, and published evidence showing little support for active inbreeding avoidance in vertebrates, we suggest that dispersal should be considered as a mechanism of prime importance for inbreeding avoidance in wild populations.     

P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

Dispersal of introduced house sparrows Passer domesticus: an experiment

An important issue concerning the introduction of non-indigenous organisms into local populations is the potential of the introduced individuals to spread and interfere both demographically and genetically with the local population. Accordingly, the potential of spatial dispersal among introduced individuals compared with local individuals is a key parameter to understand the spatial and temporal dynamics of populations after an introduction event. In addition, if the variance in dispersal rate and distance is linked to individual characteristics, this may further affect the population dynamics. We conducted a large-scale experiment where we introduced 123 house sparrows from a distant population into 18 local populations without changing population density or sex ratio. Introduced individuals dispersed more frequently and over longer distances than residents. Furthermore, females had higher probability of dispersal than males. In females, there was also a positive relationship between the wing length and the probability of dispersal and dispersal distance. These results suggest that the distribution and frequency of introduced individuals may be predicted by their sex ratio as well as their phenotypic characteristics.     

Seasonal dynamic of phototrophic epibionts on crustacean zooplankton in a eutrophic reservoir with cyanobacterial bloom

Seasonality of burden and prevalence of phototrophic (microalgal) epibionts Characidiopsis ellipsoidea, Colacium vesiculosum and Colacium sp. on dominating crustacean zooplankton (Daphnia longispina, Cyclops vicinus and Mesocyclops leuckarti) were studied in a small reservoir Bugach with cyanobacterial bloom. The correlations between the seasonal dynamics of prevalence and the dynamics of others biotic and abiotic factors were calculated. The conclusions were as follows. The substrate species, that determined the development of the epibionts on the three studied crustacean zooplankton, was Daphnia longispina (Cladocera). Despite intensive epibiotic infestation of crustacean zooplankton, epibionts did not appear to have caused non-consumptive mortality of the crustacean zooplankton. But they could have contributed to the Daphnia summer decline by increasing mortality due to its consumption by planktivorous fishes. The phototropic epibionts may successfully coexist with cyanobacterial bloom. The possible role of the epibionts in changing nutrient fluxes in pelagic food web is discussed.     

Historical and contemporary gene dispersal in wild carrot (Daucus carota ssp. carota) populations

Background and Aims

Wild carrot is the ancestor of cultivated carrot and is the most important gene pool for carrot breeding. Transgenic carrot may be released into the environment in the future. The aim of the present study was to determine how far a gene can disperse in wild carrot populations, facilitating risk assessment and management of transgene introgression from cultivated to wild carrots and helping to design sampling strategies for germplasm collections.

Methods

Wild carrots were sampled from Meijendel and Alkmaar in The Netherlands and genotyped with 12 microsatellite markers. Spatial autocorrelation analyses were used to detect spatial genetic structures (SGSs). Historical gene dispersal estimates were based on an isolation by distance model. Mating system and contemporary pollen dispersal were estimated using 437 offspring of 20 mothers with different spatial distances and a correlated paternity analysis in the Meijendel population.

Key Results

Significant SGSs are found in both populations and they are not significantly different from each other. Combined SGS analysis indicated significant positive genetic correlations up to 27 m. Historical gene dispersal σ_g and neighbourhood size N_b were estimated to be 4–12 m [95 % confidence interval (CI): 3–25] and 42–73 plants (95 % CI: 28–322) in Meijendel and 10–31 m (95 % CI: 7–∞) and 57–198 plants (95 % CI: 28–∞) in Alkmaar with longer gene dispersal in lower density populations. Contemporary pollen dispersal follows a fat-tailed exponential-power distribution, implying pollen of wild carrots could be dispersed by insects over long distance. The estimated outcrossing rate was 96 %.

Conclusions

SGSs in wild carrots may be the result of high outcrossing, restricted seed dispersal and long-distance pollen dispersal. High outcrossing and long-distance pollen dispersal suggest high frequency of transgene flow might occur from cultivated to wild carrots and that they could easily spread within and between populations.     

Environmental correlates of the intrinsic rate of natural increase in primates

Morphological and life history traits of two clones of the cladoceran Daphnia pulex were measured in the presence and absence of size-selective insect predators, the midge larva Chaoborus flavicans, which preys on small Daphnia, and the water bug Notonecta glauca, which preys on large Daphnia. The aim was to detect predator-induced phenotypic changes, particularly the effect of simultaneous exposure to both types of predators. Other work has shown that in the presence of Chaoborus americanus, Daphnia pulex produce a socalled neck spine which may carry several teeth. The morphological modifications are supposed to serve as an anti-predator device. Furthermore, females exposed to Chaoborus often delay their maturation; this has been interpreted as a cost that balances the benefits of the neck teeth. In this investigation, females of both clones produced fewer but larger offspring than control animals when reared in the presence of Chaoborus flavicans. The offspring showed the typical neck spine and delayed first reproduction. In the presence of Notonecta glauca, one of the clones produced more and smaller offspring, and maturation occurred at earlier instars. The other clone also produced more offspring than the control but there was no size difference. When both predators were present, in most cases the reactions of the daphnids were similar to those in the Notonecta experiment. The response to Chaoborus appeared to be suppressed. The observed modifications are interpreted as evolved strategies that reduce the impact of size-selective predation. They are consistent with predictions of life-history theory.     

Dynamic energy budget theory and population ecology: lessons from Daphnia

Dynamic energy budget (DEB) theory offers a perspective on population ecology whose starting point is energy utilization by, and homeostasis within, individual organisms. It is natural to ask what it adds to the existing large body of individual-based ecological theory. We approach this question pragmatically--through detailed study of the individual physiology and population dynamics of the zooplankter Daphnia and its algal food. Standard DEB theory uses several state variables to characterize the state of an individual organism, thereby making the transition to population dynamics technically challenging, while ecologists demand maximally simple models that can be used in multi-scale modelling. We demonstrate that simpler representations of individual bioenergetics with a single state variable (size), and two life stages (juveniles and adults), contain sufficient detail on mass and energy budgets to yield good fits to data on growth, maturation and reproduction of individual Daphnia in response to food availability. The same simple representations of bioenergetics describe some features of Daphnia mortality, including enhanced mortality at low food that is not explicitly incorporated in the standard DEB model. Size-structured, population models incorporating this additional mortality component resolve some long-standing questions on stability and population cycles in Daphnia. We conclude that a bioenergetic model serving solely as a 'regression' connecting organismal performance to the history of its environment can rest on simpler representations than those of standard DEB. But there are associated costs with such pragmatism, notably loss of connection to theory describing interspecific variation in physiological rates. The latter is an important issue, as the type of detailed study reported here can only be performed for a handful of species.