Generalist Grazers Research Articles

Grazing strategies determine the size composition of phytoplankton in eutrophic lakes

AbstractAlthough the general impacts of zooplankton grazing on phytoplankton communities are clear, we know comparatively less about how specific grazing strategies interact with environmental conditions to shape the size structure of phytoplankton communities. Here, we present a new data‐driven, size‐based model that describes changes in the size composition of lake phytoplankton under various environmental constraints. The model includes an ecological trade‐off emerging from observed allometric relationships between (1) phytoplankton cell size and phytoplankton growth and (2) phytoplankton cell size and zooplankton grazing. In our model, phytoplankton growth is nutrient‐dependent and zooplankton grazing varies according to specific grazing strategies, namely, specialists (targeting a narrow range of the size‐feeding spectrum) vs. generalists (targeting a wide range of the size‐feeding spectrum). Our results indicate that grazing strategies shape the size composition of the phytoplankton community in nutrient‐rich conditions, whereas inorganic nutrient concentrations govern phytoplankton biomass. Under oligotrophic regimes, the phytoplankton community is dominated by small cell sizes and the grazers have little to no impact. Under eutrophic regimes, dominating specialist grazers push phytoplankton towards small cells, whereas dominating generalist grazers push phytoplankton towards large cells. Our work highlights that trait‐based modeling, based on realistic eco‐physiological trade‐offs, represents a valuable tool for disentangling the interactive roles played by nutrient regimes and grazing strategies in determining the size compositions of lake phytoplankton. Ultimately, our study offers a quantitative basis for understanding how communities of lake phytoplankton may reorganize in the future in response to changes in nutrient levels and zooplankton grazing strategies.

Epigenetic weapons in plant-herbivore interactions: Sulforaphane disrupts histone deacetylases, gene expression, and larval development in Spodoptera exigua while the specialist feeder Trichoplusia ni is largely resistant to these effects.

Cruciferous plants produce sulforaphane (SFN), an inhibitor of nuclear histone deacetylases (HDACs). In humans and other mammals, the consumption of SFN alters enzyme activities, DNA-histone binding, and gene expression within minutes. However, the ability of SFN to act as an HDAC inhibitor in nature, disrupting the epigenetic machinery of insects feeding on these plants, has not been explored. Here, we demonstrate that SFN consumed in the diet inhibits the activity of HDAC enzymes and slows the development of the generalist grazer Spodoptera exigua, in a dose-dependent fashion. After consuming SFN for seven days, the activities of HDAC enzymes in S. exigua were reduced by 50%. Similarly, larval mass was reduced by 50% and pupation was delayed by 2-5 days, with no additional mortality. Similar results were obtained when SFN was applied topically to eggs. RNA-seq analyses confirm that SFN altered the expression of thousands of genes in S. exigua. Genes associated with energy conversion pathways were significantly downregulated while those encoding for ribosomal proteins were dramatically upregulated in response to the consumption of SFN. In contrast, the co-evolved specialist feeder Trichoplusia ni was not negatively impacted by SFN, whether it was consumed in their diet at natural concentrations or applied topically to eggs. The activities of HDAC enzymes were not inhibited and development was not disrupted. In fact, SFN exposure sometimes accelerated T. ni development. RNA-seq analyses revealed that the consumption of SFN alters gene expression in T. ni in similar ways, but to a lesser degree, compared to S. exigua. This apparent resistance of T. ni can be overwhelmed by unnaturally high levels of SFN or by exposure to more powerful pharmaceutical HDAC inhibitors. These results demonstrate that dietary SFN interferes with the epigenetic machinery of insects, supporting the hypothesis that plant-derived HDAC inhibitors serve as "epigenetic weapons" against herbivores.

Antelope space-use and behavior indicate multilevel responses to varying anthropogenic influences in a highly human-dominated landscape.

A primary means of conserving a species or a habitat in a human‐dominated landscape is through promoting coexistence with humans while minimizing conflict. For this, we should understand how wildlife is impacted by direct and indirect human activities. Such information is rare in areas with high human densities. To investigate how animals respond to altered ecological conditions in human‐dominated landscapes, we focused on a wild herbivore of conservation concern in the Krishnasaar Conservation Area (KrCA) in Nepal. Here, blackbuck Antilope cervicapra, a generalist grazer, lives in refugia located with a growing human population. We studied the impacts of humans on habitat use and behavior of blackbuck. We laid 250 × 250 m grid cells in the entire KrCA and carried out indirect sign surveys with three replications for habitat use assessment. We observed herds of blackbuck for 89 h in different habitat types using scan sampling methods. Our habitat‐use survey showed that habitats under intensive human use were hardly used by blackbuck, even when high‐quality forage was available. Habitat openness was the major predictor of habitat use inside the core area, where levels of human activities were low. We also found a positive correlation between habitat use by blackbuck and livestock. Blackbuck were substantially more vigilant when they were in forest than in grassland, again indicating an influence of risk. Overall, blackbuck appear to be sensitive to the risk associated with both natural and anthropogenic factors. Our findings have direct implications for managing human–wildlife interactions in this landscape, specifically regarding strategies for livestock grazing in habitats highly used by blackbuck and concerning predictions of how changing land use will impact the long‐term persistence of blackbuck. Our work suggests that wild herbivores may be able to persist in landscapes with high human densities so long as there are refuges where human activities are relatively low.

Intraspecific variation in resource use is not explained by population persistence or seasonality.

Populations of generalist grazers often contain genotypes with "powerful" and "efficient" strategies. Powerful genotypes grow rapidly on rich-quality resources, but slowly on poorer-quality ones, while efficient genotypes grow relatively better on poorer resources but cannot exploit richer resources as well. Via a "power-efficiency" trade-off, variation in resource quality could maintain genetic diversity. To evaluate this mechanism, we sampled six populations of the freshwater cladoceran Daphnia pulicaria. In persisting (year-round) populations, Daphnia consume resources that vary in quality, whereas in non-persisting (spring-only) populations, Daphnia primarily encounter rich-quality resources. We hypothesized that non-persisting populations harbor no efficient clones (hence should show lower growth on poor-quality resources). Although individuals from non-persisting populations remained smaller than individuals from persisting populations, no evidence arose for a trade-off between powerful and efficient strategies. In fact, growth rates on the two diets were positively correlated (instead of negatively, as predicted). Furthermore, in the persisting populations, we predicted that clonal selection from spring to summer should shift the distribution of genotypes from powerful (specialists on richer spring resources) to efficient (poorer, summer resources). Genetic composition of populations shifted from spring to summer, but not toward more efficient genotypes. Therefore, in these lakes, maintenance of variation among genotypes must stem from more complicated factors than population persistence patterns or seasonal shifts in resource quality alone.

The Role of Mixotrophy in Southern Ocean Ecosystems

We investigate the influence of mixotrophy on the dynamical properties of a six-population model of a three–trophic level Southern Ocean ecosystem. We find that including mixotrophic interactions between the lowest trophic level populations can significantly influence the dynamics of the highest trophic level populations, and in extreme cases lead to extinctions. Significantly, not only is the strength of the mixotrophic interaction important, it matters how it is included in the model, as a specialist or generalist grazer. We note in particular that the generalist formulation is inappropriate for “green” mixotrophs that fuel the majority of their growth by photosynthesis. The model can have complicated dynamics when subject to large amplitude, regular forcing, suggesting the sea ice—salps link may be obfuscated by endogenous population oscillations. Further, we observe that constructing the model within the Conservative Normal framework allows insights into the bifurcation behaviour of the model.

Synergistic reduction of a native key herbivore performance by two non-indigenous invasive algae

Native generalist grazers can control the populations of non-indigenous invasive algae (NIIA). Here, it was found that the simultaneous consumption of two co-occurring NIIA, Caulerpa cylindracea and C. taxifolia var. distichophylla, hinders the grazing ability of the main Mediterranean herbivorous, the native sea urchin Paracentrotus lividus. The ingestion of any of the two NIIA alone did not produce any difference in sea urchin righting time with respect to usual algal diet. In contrast, the simultaneous consumption of both NIIA, which grow intermingled in nature and are consumed by P. lividus, retarded its righting behavior. Such result reveals substantial physiological stress in the sea urchin, which resulted in reduced motility and coordination. The reported findings reveal the potential of NIIA co-occurrence to escape the supposed control exerted by the main native generalist grazer in Mediterranean sublittoral communities, which in turn can be locked in an “invaded” state.

Mixotrophs: Dynamic disrupters of plankton systems?

We consider a simple plankton model with one phytoplankton autotroph population, one mixotrophic phytoplankton population that both photosynthesises and consumes the other phytoplankton, and one zooplankton predator population. The zooplankton predator may be either a specialist or a generalist grazer on the phytoplankton. We examine the influence on the system's equilibrium points and dynamical properties of varying degrees of mixotrophy by the phytoplankton and grazing strategy by the zooplankton, both with and without the presence of seasonal forcing.We find that the strength of the mixotrophy interaction does not substantially change the equilibrium properties of the model across all grazing strategies, but it can have a substantial effect on the dynamical properties. Further, external forcings on typical phytoplankton or zooplankton time scales can in some circumstances substantially modify these dynamics. Our analyses suggest that plankton models developed to represent bio-geochemical processes in the ocean for applications in climate change modelling should be subjected to thorough equilibrium and dynamical analyses before being used for climate prediction.

Grazing and growth rate of a cyclopoid copepod fed with a phytoplankton diet constituted by a filamentous cyanobacterium

Many tropical freshwaters experience persistent cyanobacterial blooms throughout the year. However, to date, most of the cyanobacteria–zooplankton interaction studies focused on temperate regions using large generalist grazers, which rarely coexist with cyanobacteria. Here a short-term (3 h) laboratory experiment was conducted to examine the grazing rate of Thermocyclops decipiens exposed to different combinations of phytoplankton (T1 = 100% Nitzschia-0% Cylindrospermopsis, T2 = 75% Nitzschia-25% Cylindrospermopsis, T3 = 50% Nitzschia-50% Cylindrospermopsis, T4 = 25% Nitzschia-75% Cylindrospermopsis and T5 = 0% Nitzschia-100% Cylindrospermopsis). In addition, an 8-day experiment was performed to determine the effect of the diet treatments on Thermocyclops survivorship and growth. The total biomass of phytoplankton in treatments with higher proportion of Nitzschia (i.e., T1 and T2) showed abrupt decline (i.e., down to 61% and 71%, respectively) over time when compared to other treatments. While Thermocyclops largely excluded Cylindrospermopsis when Nitzschia is abundant, they were capable of shortening the longer filaments of Cylindrospermopsis into significantly smaller fragments and ingesting them when provided as a sole food. This, however, negatively influenced their survivorship and growth. Our results generally suggest that dominantly occurring selective grazers in tropical and warmer-temperate lakes may shift the phytoplankton community composition to favor cyanobacteria by grazing on the alternative food source, at least before the cyanobacteria becomes dominant.

High phenolic content fails to deter mesograzer consumption of Myriophyllum spicatum (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the USA. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of herbivores in determining milfoil invasion success via the potential mechanisms of enemy release and biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured macrophyte phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was more than two times higher than in the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory. Furthermore, amphipod consumption rates in our study were an order of magnitude higher than amphipod consumption rates reported in milfoil’s native range, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis in choice experiments. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed 2.5 times more milfoil than E. canadensis in a choice feeding trial. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance.

Selective Grazing by a Tropical Copepod (Notodiaptomus iheringi) Facilitates Microcystis Dominance.

Top-down grazer control of cyanobacteria is a controversial topic due to conflicting reports of success and failure as well as a bias toward studies in temperate climates with large generalist grazers like Daphnia. In the tropical lowland lakes of Brazil, calanoid copepods of the Notodiaptomus complex dominate zooplankton and co-exist in high abundance with permanent blooms of toxic cyanobacteria, raising questions for grazer effects on bloom dynamics (i.e., top-down control vs. facilitation of cyanobacterial dominance). Accordingly, the effect of copepod grazing on the relative abundance of Microcystis co-cultured with a eukaryotic phytoplankton (Cryptomonas) was evaluated in a series of 6-day laboratory experiments. Grazer effects were tested in incubations where the growth of each phytoplankton in the presence or absence of the copepod Notodiaptomus iheringi was monitored in 1 L co-cultures, starting with a 6-fold initial dominance of Cryptomonas by biomass. Compared to the no grazer controls, N. iheringi reduced the growth of both phytoplankton, but Cryptomonas growth was reduced to negative values while Microcystis growth continued positively despite grazers. Hence, in a matter of 6 days selective grazing by N. iheringi increased the biomass of Microcystis relative to Cryptomonas by an order of magnitude compared to controls, and thus, facilitated the dominance of this cyanobacterium. To account for the potential effect of allelopathy, we performed a secondary experiment comparing the abundance and growth rate of Microcystis and Cryptomonas in single and mixed co-cultures in the absence of grazers. The growth rate of Microcystis was unaffected by the presence or relative abundance of Cryptomonas, and vice versa, indicating no allelopathic effects. Our results suggest that selectively grazing zooplankton may facilitate cyanobacteria blooms by grazing on their eukaryotic phytoplankton competitors in nature. Given that selective grazers predominate zooplankton biomass in warmer waters, grazer facilitation of blooms may be a common but poorly understood regulator of plankton dynamics in a warmer and more eutrophic world.

The role of grazer predation strategies in the dynamics of consumer-resource based ecological models

We analyse a simple plankton system to provide a heuristic for more complex models such as Dynamic Green Ocean Models (DGOMs). Zooplankton foraging is either by generalist grazers that consume whatever they bump into or specialist grazers that actively seek particular prey. The zooplankton may further be classified as either facultative grazers that can survive on any of their prey or obligate grazers that depend on the presence of specific prey.A key result is that different prey dependencies can result in dramatically different impacts of grazing strategies on system outcomes. The grazing strategy can determine whether a system with obligate grazers will be stable, have regular, predictable cycles or be chaotic. Conversely, whether facultative zooplankton functioned as specialist or generalist grazers makes no qualitative difference to the dynamics of the system. These results demonstrate that the effect of different grazing strategies can be critically dependent on the grazer's dependency on specific prey. Great care must be taken when choosing functional forms for population interactions in DGOMs, particularly in scenarios such as climate change where parameters such as mortality and growth coefficients may change. A robust theoretical framework supporting model development and analysis is key to understanding how such choices can affect model properties and hence predictions.

Habitat selectivity influences the reactive responses of African ungulates to encounters with lions

Prey mortality depends on the likelihood of encountering a predator and of being killed following such an encounter. While the proactive responses of large herbivores to the risk of predation have been well documented, the reactive responses of prey following encounters have been less well studied and restricted mostly to northern temperate ecosystems. We postulated that the reactive response should depend on the habitat selection of the prey species as well as the hunting tactic (cursorial versus stalking) of the predator. We investigated the reactive responses of a selective grazer, the blue wildebeest, Connochaetes taurinus, and a generalist grazer, the plains zebra, Equus quagga, to encounters with lions, Panthera leo, their main predator, in Kruger National Park. We used simultaneous global positioning system (GPS) locations of prey and predators to identify encounters and assess their frequency and characteristics (e.g. time of day). We compared both the immediate response (stay or flee) and subsequent movement (remain in the habitat patch or relocate) of the two ungulate species following encounters with lions. Despite being more predictably located in open short grass areas, wildebeest encountered lions no more frequently than zebra. However, being more selective for these particular habitat types, wildebeest more often stayed near the encounter site than zebra. Specifically, wildebeest were less likely to leave when located in more open areas with short grass providing both high-quality food and little concealment for stalking lions. We suggest that this is because wildebeest incur higher risks and greater feeding costs than zebra should they relocate, owing to their narrower preference for areas of short grass. Our findings show how the adaptive responses of wildebeest and zebra following encounters with their major predator depend on their habitat requirements.

Palatability and chemical defences of benthic cyanobacteria to a suite of herbivores

Nuisance blooms of toxic cyanobacteria are a common occurrence in many tropical and subtropical locations. Benthic marine cyanobacteria of the genera Lyngbya, Okeania, and Moorea are frequently observed in both Florida and throughout the Caribbean, sometimes forming large mats, and are prolific producers of bioactive secondary metabolites that often act as feeding deterrents to generalist herbivores. Little is known regarding the ecological roles of the secondary metabolite chemistry and the palatability of benthic cyanobacteria to grazers. This study examines the palatability of benthic cyanobacterial species from Florida (IRL1, IRL2, IRL3 and Okeania erythroflocculosa) and Belize (BEL1, BEL2) to a range of macro- and mesograzers in Florida and Belize. Pair-wise feeding assays using artificial diets of Gracilaria tikvahiae or fish food coated with cyanobacterial extracts and a control were used to determine palatability of extracts to Floridian and Belizean generalist grazers. The extracts of IRL1, IRL2, IRL3 and O. erythroflocculosa from Florida did not deter feeding by invertebrate grazers. Reef fish, however, were deterred by the non-polar extracts of IRL1, IRL3 and O. erythroflocculosa. Stylocheilus striatus was stimulated to feed on IRL2 extracts and non-polar extracts from IRL3. Non-polar extracts of BEL1 stimulated feeding in S. striatus; however, no significant difference was observed between BEL2 extracts and the control. Most generalist invertebrate grazers, sympatric and non-sympatric, appear indifferent to cyanobacteria extracts whilst reef fish are more likely to be deterred by cyanobacterial extracts, which may affect species interaction within communities with fluctuating or dominating benthic cyanobacterial blooms.

Spatially nested niche partitioning between syntopic grazers at foraging arena scale within overlapping home ranges

Niche separation among species with similar resource requirements can be expressed at various spatiotemporal scales, from the resource components selected at feeding sites to habitat and home range occupation and ultimately geographic distribution ranges. African large herbivores present a challenge to niche theory because multiple species commonly overlap both spatially and in vegetation components consumed. Aided by GPS telemetry, we investigated the space use patterns of two large grazers that are frequently associated in mixed‐species aggregations. Specifically, we compared a generalist grazer with hindgut fermentation (plains zebra) with a similar‐sized grazing ruminant (blue wildebeest) in west‐central Kruger National Park, South Africa. We found that herds of the two species overlapped substantially in the home ranges that they occupied, but exploited spatially distinct foraging arenas for periods lasting several days or weeks within these ranges. Moreover, wildebeest and zebra differed in duration of settlement, extent of areas occupied during settlement, consequent exploitation intensity per unit area, proportion of time spent within foraging arenas relative to roaming interludes, and movement rates while within these arenas. In particular, wildebeest herds concentrated within small areas for prolonged periods, while zebra herds used more foraging arenas but exploited them for briefer periods. Both species overlapped substantially in habitat use, although wildebeest more strongly favored gabbro uplands and sodic sites presenting short grass lawns while zebra made greater use of areas with a taller grass cover. Hence resource partitioning was expressed mainly through behavioral distinctions in patch exploitation at foraging arena scale rather than in home range or habitat separation. Although zebra may have been partially excluded from the grasslands kept short by wildebeest, these sites formed only a small part of the wider ranges utilized by zebra, thereby restricting the competitive consequences. Hence spatially nested resource partitioning of this form contributes to the coexistence of these two grazers, and may be a mechanism enabling niche separation among other species.

A comparative study on the ability of tropical micro-crustaceans to feed and grow on cyanobacterial diets

Controlling noxious phytoplankton blooms by biomanipulation largely depends on the presence of large-sized (.3000 mm) generalist grazers, not common in the tropics. Therefore, we compared the ability of small ( 2000 mm) microcrustaceans Ceriodaphnia dubia, Moina macrocopa, Daphnia pulex, Simocephalus vetulus and Heterocypris incongruens to feed and grow on cyanobacterial diets. We studied their feeding preferences on phytoplankton from natural lake water with a dominance of Microcystis sp., Fragilaria sp. or Planktothrix sp. to which we added Scenedesmus acutus. Also tested were the ability of D. pulex, S. vetulus and H. incongruens to reduce cyanobacterial densities by grazing, and the rate of grazing and demographic responses of S. vetulus and H. incongruens on diets of S. acutus, Microcystis sp. and Planktothrix sp. All species fed on small colonies of Microcystis sp., but not on filamentous Planktothrix sp. or colonial Fragilaria sp. Simocephalus vetulus and H. incongruens were generalists and most capable of reducing cyanobacterial densities. Demographic variables of S. vetulus, but not H. incongruens, were significantly lower on the cyanobacterial diet; for instance, the population growth rates were between 0.07– 0.2 day on cyanobacterial diets, but 0.31 on S. acutus while for H. incongruens they ranged between 0.07 and 0.08 day, regardless of whether the diet was a cyanobacteria or S. acutus. Our study warrants further tests on the ostracod H. incongruens in order to test its efficacy in reducing cyanobacterial densities in shallow tropical ponds.

Metabolomic assessment of induced and activated chemical defence in the invasive red alga Gracilaria vermiculophylla.

In comparison with terrestrial plants the mechanistic knowledge of chemical defences is poor for marine macroalgae. This restricts our understanding in the chemically mediated interactions that take place between algae and other organisms. Technical advances such as metabolomics, however, enable new approaches towards the characterisation of the chemically mediated interactions of organisms with their environment. We address defence responses in the red alga Gracilaria vermiculophylla using mass spectrometry based metabolomics in combination with bioassays. Being invasive in the north Atlantic this alga is likely to possess chemical defences according to the prediction that well-defended exotics are most likely to become successful invaders in systems dominated by generalist grazers, such as marine macroalgal communities. We investigated the effect of intense herbivore feeding and simulated herbivory by mechanical wounding of the algae. Both processes led to similar changes in the metabolic profile. Feeding experiments with the generalist isopod grazer Idotea baltica showed that mechanical wounding caused a significant increase in grazer resistance. Structure elucidation of the metabolites of which some were up-regulated more than 100 times in the wounded tissue, revealed known and novel eicosanoids as major components. Among these were prostaglandins, hydroxylated fatty acids and arachidonic acid derived conjugated lactones. Bioassays with pure metabolites showed that these eicosanoids are part of the innate defence system of macroalgae, similarly to animal systems. In accordance with an induced defence mechanism application of extracts from wounded tissue caused a significant increase in grazer resistance and the up-regulation of other pathways than in the activated defence. Thus, this study suggests that G. vermiculophylla chemically deters herbivory by two lines of defence, a rapid wound-activated process followed by a slower inducible defence. By unravelling involved pathways using metabolomics this work contributes significantly to the understanding of activated and inducible defences for marine macroalgae.

Community composition and consumer identity determine the effect of resource species diversity on rates of consumption

The effect of species diversity on ecosystem function is commonly studied within a single trophic level, but less is known about how resource diversity affects species interactions between trophic levels. We conducted a grazing experiment to determine how resource species diversity affects rates of consumption by three species of freshwater zooplankton consumers. We measured the effect of resource diversity on rates of consumption for several resource community compositions. These compositions varied in terms of palatability for the consumers. The effect of resource diversity on consumption rates depended on the diet breadth of the consumer species (from specialist to generalist) and the community composition of resources. Overall, high resource diversity commonly caused a decline in consumption rates of consumers. The most selective grazer showed reduced consumption for nearly all community compositions, whereas the most generalist grazer showed accelerated consumption when all resource species were palatable. Our results demonstrate that resource species diversity can modulate rates of consumption through the action of multiple different mechanisms.

Genotype‐dependent interactions among sympatric Microcystis strains mediated by Daphnia grazing

Natural populations of the bloom forming cyanobacterium Microcystis are typically composed of several distinct genotypes. Using Microcystis strains that differ in growth rate, microcystin production and colony formation, we conducted a laboratory experiment in the presence and absence of a grazer, the water flea Daphnia, to investigate whether interactions among strains can be predicted from functional traits, and whether the outcome of competition between strains is influenced by a grazer. Two toxic and two non‐toxic Microcystis strains, isolated from a single lake, were grown during four weeks as single strains, in all possible combinations of two strains and all together, in the presence and absence of Daphnia magna. The relative abundance of strains in the populations was assessed using denaturing gradient gel electrophoresis, and the growth rate of each strain in mixed populations was compared to its growth rate in monoculture to determine interactions between strains. The observed interactions were strain‐specific, and the relative abundances of strains in mixed populations could be partially explained by taking toxicity and colony formation into account. Importantly, some of the interactions were strongly altered by the presence of Daphnia. Daphnia induced colony formation in one strain, which then became a better competitor. Daphnia grazing also caused a higher evenness in the populations, both through a weakening of competitive interactions as well as by facilitation effects. Strong facilitation effects were due to non‐toxic strains benefiting from the protection offered by toxic strains in the presence of predation.Overall, our results emphasize the presence of strong competitive interactions between Microcystis strains in the absence of grazing, whereas indirect positive interactions are prevalent in the presence of a generalist grazer. Our results suggest that differences in functional traits and grazer‐mediated facilitation effects may enhance coexistence of Microcystis strains, including toxic and non‐toxic strains.

SPECIES-SPECIFIC DAPHNIA PHENOTYPES: A HISTORY OF INDUSTRIAL POLLUTION AND PELAGIC ECOSYSTEM RESPONSE

Phenotypic differences among species, even closely related species, may translate into distinct effects on ecosystem dynamics. In lakes, the generalist grazer genus Daphnia often has marked effects on the abundance of primary producers, the rate of primary production, and rates of nutrient cycling. The effects are particularly distinct during the clear-water phase (CWP) when algal biomass is driven to extremely low values as Daphnia densities undergo an annual population increase. Here we show that the timing of the CWP in Onondaga Lake, New York, USA, has depended upon which Daphnia species were present in the water column. An analysis of the ephippia and diapausing eggs from the sediments reveals that long-term changes in the zooplankton species assemblage tracks a history of chemical (especially salt waste) pollution. Prior to 1930 the assemblage was dominated by native D. pulicaria and D. ambigua. From 1930 to 1980, these species were replaced by D. exilis and D. curvirostris, two salinity-tolerant ex...

Effect of substrate stability and canopy cover on stream invertebrate communities

Disturbance can be an important determinant of stream benthic invertebrate community structure. Whether this is a result of the direct loss of invertebrates, an alteration to the food base of the community by the disturbance, or a combination of both, is never clear. We examined this question by conducting a disturbance experiment under artificial cover where low light limited periphyton growth and minimised the effect of disturbance on periphyton abundance. The experiment was conducted using baskets of natural substrate in a spring‐fed Taranaki Ring Plain stream, Taranaki, New Zealand during February and March 2000. Baskets were placed in the open or under cover and half were disturbed every week for 4 weeks. The artificial cover dramatically reduced periphyton biomass. Presence or absence of cover was the primary determinant of what invertebrate taxa dominated in the baskets, most likely as a result of differences in food resources. The filter feeding Coloburiscus humeralis dominated numerically in the covered baskets, whereas the generalist grazers Deleatidium spp., Beraeoptera roria, Orthocladiinae, and Diamesinae dominated numerically in the open baskets. However, cover had no effect on the overall abundance and species richness of the experimental baskets but disturbance reduced abundance and to a lesser extent species richness.