Geographical range,heat tolerance and invasion success in aquatic species

Species with broader geographical ranges are expected to be ecological generalists, while species with higher heat tolerances may be relatively competitive at more extreme and increasing temperatures. Thus, both traits are expected to relate to increased survival during transport to new regions of the globe, and once there, establishment and spread. Here, we explore these expectations using datasets of latitudinal range breadth and heat tolerance in freshwater and marine invertebrates and fishes. After accounting for the latitude and hemisphere of each species’ native range, we find that species introduced to freshwater systems have broader geographical ranges in comparison to native species. Moreover, introduced species are more heat tolerant than related native species collected from the same habitats. We further test for differences in range breadth and heat tolerance in relation to invasion success by comparing species that have established geographically restricted versus extensive introduced distributions. We find that geographical range size is positively related to invasion success in freshwater species only. However, heat tolerance is implicated as a trait correlated to widespread occurrence of introduced populations in both freshwater and marine systems. Our results emphasize the importance of formal risk assessments before moving heat tolerant species to novel locations.     

Cohort effects in red squirrels: the influence of density, food abundance and temperature on future survival and reproductive success

1. Environmental conditions experienced early in life may have long-lasting effects on individual performance, thereby creating 'silver-spoon effects'. 2. We used 15 years of data from a North American red squirrel (Tamiasciurus hudsonicus Erxleben) population to investigate influences of food availability, density and spring temperature experienced early in life on reproduction and survival of female squirrels during adulthood. 3. We found that spring temperature and food availability did not affect female survival after 1 year of age, whereas higher squirrel densities led to lower survival, thereby affecting longevity and lifetime fitness. 4. In addition, both food availability experienced between birth and weaning, and spring temperature in the year of birth, had long-lasting positive effects on female reproductive success. These results emphasize the critical effect environmental conditions during the early life stages can have on the lifetime performance of small mammals. 5. These long-term effects of early food and temperature were apparent only once we controlled for conditions experienced during adulthood. This suggests that silver-spoon effects can be masked when conditions experienced early in life are correlated to some environmental conditions experienced later in life. 6. The general importance of silver-spoon effects for adult demographic performance might therefore be underestimated, and taking adult environment into account appears to be necessary when studying long-term cohort effects.     

Phototaxis in Daphnia magna: the influence of temperature and acidity on the phototactic behaviour of Daphnia genotypes

The influence of culture temperature, temperature shocks andpH shocks on the phototactic behaviour of four Daphnia magnaclones was studied. Test animals tended to become more positivelyphototactic when the pH of the medium was raised, and less positivelyphototactic when cultured at high (27°C) temperature. Responsesto temperature changes at the start of the experiment were clonedependent. Differences between genotypes remained highly significantdespite the broader environmental circumstances in which experimentswere carried out. Broad heritability estimates for the phototacticbehaviour of Daphnia under the different environmental conditionsstudied ranged from 0.78 to 0.88.     

Fitness optimization of Daphnia in a trade-off between food and temperature

In thermally stratified lakes with a deep chlorophyll maximum (DCM), Daphnia face a trade-off between food availability and optimum development temperatures. We hypothesize that Daphnia optimize their fitness by allocating the time spent in the different vertical habitats depending on the distribution of algal resources and the temperature gradient. We used the plankton towers (large indoor mesocosms) to study the vertical distribution of a population of Daphnia hyalina×galeata in three different temperature gradients with a DCM. Additionally, we determined the fitness of Daphnia in the epilimnion and hypolimnion by transferring water from these layers into flow-through systems where we raised Daphnia and assessed their juvenile growth rate as a measure of fitness. The fitness distribution was correlated with the vertical distribution. The vertical distribution most likely reflected the proportions of time Daphnia allocated to dwelling in the two vertical habitats.     

Plant invasion across space and time: factors affecting nonindigenous species success during four stages of invasion

Invasive nonindigenous plant species (NIPS) threaten native diversity, alter ecosystem processes, and may interact with other components of global environmental change. Here, a general framework is outlined that attempts to connect patterns of plant invasion to processes underlying these patterns at four well-established spatio-temporal stages of the invasion process: transport, colonization, establishment, and landscape spread. At each stage we organize findings and ideas about the filters that limit NIPS success and the interaction of these filters with historical aspects of introduction events, NIPS traits, and ecosystem properties. While it remains difficult to draw conclusions about the risk of invasion across ecosystems, to delineate universal 'invader traits', or to predict large-scale extinctions following invasions, this review highlights the growing body of research that suggests that the success of invasive NIPS is controlled by a series of key processes or filters. These filters are common to all invasion events, and will interact throughout the stages of plant invasion, although the relative importance of a filter may be stage, species or location specific. It is suggested that both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion. We further use the framework to briefly examine potential interactions between climate change and filters that limit NIPS invasion.     

Molecular genetic analysis of Daphnia in the English Lake District: species identity, hybridisation and resting egg banks

• 1 Taxonomic issues in many Daphnia species complexes are often confused by a high degree of phenotypic plasticity and by interspecific hybridisation. Here, we employ molecular genetic tools to confirm the species composition and incidence of hybridisation in extant and resting egg populations of Daphnia from Windermere and Esthwaite Water in the English Lake District.
• 2 A combination of species‐diagnostic allozymes and mtDNA, confirms that contemporary populations are dominated by a single species, D. galeata.
• 3 We present the first account of genetic characterisation of dated ‘resting’ eggs using microsatellites and mtDNA, employing PCR‐based DNA recovery, thus providing a temporal dimension to taxonomic patterns. Thirty years ago, two species were present in Esthwaite, D. galeata and D. hyalina, but Windermere populations were dominated by D. galeata only.
• 4 The use of PCR‐based mtDNA RFLP analysis as a species‐diagnostic tool, and microsatellites to monitor clonal diversity, provide a valuable approach for long‐term studies, especially in populations free from the complicating effect of frequent hybridisation. The detailed limnological records available for many large lakes, and associated changes in land‐use, pollutants and climate, combined with long‐term ephippial molecular genetic data, provide opportunities for exploring natural and anthropogenic impacts on genetic and community structure.

     

Predator-released compounds, ambient temperature and competitive exclusion among differently sized Daphnia species

1. We studied the effects of fish water and temperature on mechanisms of competitive exclusion among two Daphnia species in flow‐through microcosms. The large‐bodied D. pulicaria outcompeted the medium sized D. galeata × hyalina in fish water, but not in the control treatment. Daphnia galeata × hyalina was competitively displaced 36 days earlier at 18 °C than at 12 °C. 2. It is likely that the high phosphorus content of fish water increased the nutritional value of detrital seston particles by stimulating bacterial growth. Daphnia pulicaria was presumably better able to use these as food and hence showed a more rapid somatic growth than its competitor. This led to very high density of D. pulicaria in fish water, but not in the controls. The elevated D. pulicaria density coincided with high mortality and reduced fecundity in D. galeata × hyalina, resulting in competitive displacement of the hybrid. 3. It is clear that the daphnids competed for a limiting resource, as grazing caused a strong decrease in their seston food concentration. However, interference may also have played a role, as earlier studies have shown larger Daphnia species to be dominant in this respect. The high density of large‐bodied D. pulicaria in fish water may have had an allelopathic effect on the hybrid. Our data are inconclusive with respect to whether the reached seston concentration was below the threshold resource level (R*) of the hybrid, where population growth rate and mortality exactly balance, as it would be set in the absence of interference, or whether interference actually raised the hybrid's R* to a value above this equilibrium particle concentration. 4. Our results do clearly show that fish‐released compounds mediated competitive exclusion among zooplankton species and that such displacement occurred at a greatly enhanced rate at an elevated temperature. Fish may thus not only structure zooplankton communities directly through size‐selective predation, but also indirectly through the compounds they release.     

Mycorrhizal species identity affects plant community structure and invasion: a microcosm study

Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can mediate plant interactions, thereby affecting plant community structure. Little is known, however, about whether the presence of different AMF species leads to differences in plant community structure or invasion success by introduced species. To investigate the effects of AMF species on community structure and invasion, we created replicate microcosms containing soil inoculated with one of three different AMF species (Glomus spurcum Pfeiffer, Walker & Bloss, Scutellospora erythropa (Koske & Walker) Walker & Sanders, or Scutellospora verrucosa (Koske & Walker) Walker & Sanders) or a mixture of all three AMF species. Seeds of seven naturally co‐occurring plant species (Ageratum conyzoides L., Cyperus compressus L., Chamaecrista nictitans (L.), Crotalaria incana L., Hyptis pectinata (L.) Poit., Sida rhombifolia L., Melinis repens (Willd.) Zizka) in Hawai‘i were sown equally into these microcosms, which were placed on outdoor benches. Plant community development was monitored over a season. Mid‐way through the experiment, an invader (Bidens pilosa L.) was added to the established communities to determine whether mycorrhizal species identity affected invasion success. Final aboveground and belowground phytomass were used to assess plant community differences among treatments. Although the identity of the dominant plant species (Melinis repens) remained the same in all treatments, community dominance, community productivity, plant species richness, Shannon index of diversity, and invasion success all varied with AMF species identity. Invasion success was not inversely related to species richness or diversity. Instead, increased richness, diversity, and invasion success all appeared to be related to decreased dominance by M. repens in the presence of certain AMF species. These results indicate that the composition of the AMF community belowground can influence the structure of the plant community aboveground, and may play a role in facilitating or repelling invasion.     

Experimental effect of consumer identity on the invasion success of a non-native cladoceran

An important goal of ecologists is to identify the characteristics of native communities that help to either promote or resist the invasion of non-native taxa. In this study, experimental mesocosms were used to determine how the richness and species identity of freshwater consumers influenced the invasion success of Daphnia lumholtzi (DL), a cladoceran that has successfully invaded aquatic habitats throughout the United States. Three species of native zooplankton were used to create a gradient of richness in replicated mesocosms that included all species in monoculture, all two species combinations, and all three species together. DL was then added to each mesocosm and its biomass was monitored over time. While additional research is needed to better determine how richness affects the invasion success of DL, the presence of native zooplankton resulted in reduced levels of DL biomass in all treatments relative to the zooplankton free controls. One species (Daphnia magna, DM) was able to reduce the biomass of DL to lower levels than the other two species. DL also appeared to be unable to establish or persist over time in mesocosms with DM, unlike in mesocosms with the two other species. These species–specific effects on invasion success appeared to be related to DM’s ability to reduce algal biomass to lower levels than the other two species. Therefore, while the presence of native zooplankton helps to reduce the biomass of DL once it is introduced into novel systems, the degree to which DL is able to establish and maintain populations over time within individual systems will likely depend on the presence or absence of particular species that can have particularly strong impacts on invasibility. Using such data, we may be able to predict which habitats are most likely to be invaded based on the presence or absence of individual native species.     

Habitat partitioning of native and exotic Daphnia in gradients of temperature and food: mesocosm experiments

1. Invasion of tropical zooplankton into temperate lakes provides an interesting opportunity to explore habitat segregation in a thermal gradient. 2. We explored differing vertical positioning of native and exotic Daphnia (Daphnia mendotae and Daphnia lumholtzi) in a large indoor mesocosm system (Plön plankton towers) during 2 month‐long experiments. The two towers were manipulated to provide gradients of both temperature (15–29 °C) and algal food (0.05–0.58 mg C L^?1) and a day–night cycle. 3. Both juvenile and adult D. lumholtzi showed a ‘typical’ vertical migration pattern, with higher densities in the epilimnion at night than during the day. They avoided the food‐poor middle layer. In contrast, D. mendotae adults showed little tendency to migrate into the epilimnion at night, remaining in the cooler hypolimnion while juveniles migrated. The vertical distribution of D. mendotae appeared unaffected by the presence of D. lumholtzi. 4. The strong migration behaviour of D. lumholtzi in the absence of fish cues suggests that this behaviour may be a constitutive trait. Habitat partitioning of the two species is probably the result of different thermal tolerances, with D. mendotae constrained to remaining in deeper water by high temperatures in the epilimnion and the tropical D. lumholtzi able to use the warm epilimnion at night.     

Prey DNA detection success following digestion by intraguild predators: influence of prey and predator species

Intraguild predation (IGP) has been increasingly recognized as an important interaction in ecological systems over the past two decades, and remarkable insights have been gained into its nature and prevalence. We have developed a technique using molecular gut-content analysis to compare the rate of IGP between closely related species of coccinellid beetles (lady beetles or ladybirds), which had been previously known to prey upon one another. We first developed PCR primers for each of four lady beetle species: Harmonia axyridis, Coccinella septempunctata, Coleomegilla maculata and Propylea quatuordecimpunctata. We next determined the prey DNA detection success over time (DS(50) ) for each combination of interacting species following a meal. We found that DS(50) values varied greatly between predator-prey combinations, ranging from 5.2 to 19.3 h. As a result, general patterns of detection times based upon predator or prey species alone are not discernable. We used the DS(50) values to correct field data to demonstrate the importance of compensation for detection times that are specific to particular predator-prey combinations.     

Synergistic effects of different food species on life-history traits of Daphnia galeata

In this paper we describe the life history consequences of feeding Daphnia galeata with different food types in different concentrations. We fed the animals with four concentrations of two green algae Scenedesmus obliquus and Chlamydomonas globosa, given separately as well as in a 1:1 mixture. Growth, reproduction and condition of the animals and consequently the intrinsic rate of population increase, r, increased with the concentration of the food for all three experimental series. The life history parameters measured were lowest in the animals raised on C. globosa. Animals fed on S. obliquus showed higher values for r. We observed a distinct synergistic effect of the two algal species: growth, reproduction, condition and r were significantly higher for the animals raised on the mixed media as compared to the animals raised on the mono-algal media. We conclude that the two algal species represented complementary food resources.     

The influence of food quality on the phototactic behaviour of Daphnia magnaStraus

We examined the influence of food quality on the phototactic behaviour of Daphnia magna. Cohorts of a positively phototactic D. magna clone were fed nine diets differing in quality. Diets were obtained by substitution of a fraction of unicellular algae ( Scenedesmus acutus) by a biomass-equivalent of fresh yeast ( Saccharomyces cerevisiae). Some of the diets were enriched with an inoculum of ciliates and organic compounds added as a hay infusion filtrate. Animals fed with a diet containing at least 25% algae showed a similar phototactic behaviour as animals fed with a diet that contained 100% algae. Addition of ciliates to the yeast diets resulted in a lower mortality and a higher reproductive rate compared to diets without a ciliate supplement. The presence of ciliates did not influence phototactic behaviour. Experiments testing for a maternal effect showed that the phototactic behaviour of the animals was strongly influenced by diet during the early developmental stages, but was not influenced by the diet of the mother. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modes and mechanisms of a Daphnia invasion

Whether exotic species invade new habitats successfully depends on (i) a change in the invaded habitat that makes it suitable for the invader and (ii) a genetic change in the invading taxon that enhances its fitness in the new habitat, or both. We dissect the causes of invasions of Swiss lakes, north of the Alps, by Daphnia galeata (a zooplankter typical of eutrophic lakes, e.g. those south of the Alps, which are also warmer) by comparing the fitness performance of eight geographically distributed clones that were fed algal-food typical of oligotrophic versus eutrophic conditions at two temperatures. Daphnia longispina, native to oligotrophic Swiss lakes, served as a reference. Daphnia galeata requires eutrophic food to persist, whereas D. longispina survives and grows on oligotrophic food but does even better on eutrophic food. Invasion by D. galeata is further explained because invading clones from the north perform better on eutrophic food and at cooler temperatures than native clones from the south, suggesting a local response to countergradient selection. Our data support the hypothesis that populations of the invader in northern lakes are dominated by well-adapted genotypes. Our results illustrate how environmental change (i.e. eutrophication) and local adaptation can act together to drive a successful invasion.     

Decay of multiple species of seagrass detritus is dominated by species identity, with an important influence of mixing litters

No studies of seagrass decay have examined effects of the number of species contained within the detrital pool. Given the importance of decay for nutrient cycling and long-term productivity, we tested how three seagrass species affected decay in litterbags. Experimental results clearly showed that species identity was the predominant driver of mass loss, with total loss not differing between bags containing one, two and three species. Furthermore, we show there were also non-additive effects of mixing litter that were not predicted from single-species decay rates. The nature of the non-predictable mass loss varied both through time and with the number of species in the mixtures. This indicates that species richness indeed plays a role in this important ecosystem function but one that is of less importance than species identity. The concordance of these results with terrestrial studies suggests that mechanisms responsible for mixed-species decay rates may be broadly applicable across ecosystems.     

The response of three chydorid species to temperature, pH and food

The responses of three chydorid species, Chydorus sphaericus(O.F. Müller), Alona affinis (Leydig) and Alonopsis elongata (Sars) to temperature, pH and food type were examined. Egg development time of all species decreased with increasing temperature, although the degree of change was different for each species. C. sphaericus had the fastest development time at all temperatures, and A. elongata the slowest. pH also affected the egg development time of each species differently. A. elongata failed to reproduce at low and medium pH, the egg development time of C. sphaericus was fastest at high pH while that of A. affinis was fastest at low pH. Food type was found to have significant effects on the population growth of individual species. C. sphaericus populations grew equally well in all three food types provided, and grew more than the other two species when fed on an algae culture, and filtered pond water. A. affinisand A. elongata populations grew best when fed on a detritus food source. The responses of each species to the different variables tested are discussed in relation to field observations of their distribution and abundance.     

Distribution of Daphnia in a trade-off between food and temperature: individual habitat choice and time allocation

1. In a thermally stratified water column with a deep‐water algal maximum, Daphnia face a trade‐off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known. 2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10 °C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep‐water algal maximum. 3. The migration behaviour of individual neonate and egg‐bearing Daphnia hyalina × galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion. 4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg‐bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg‐bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions.