Priority effects and habitat complexity affect the strength of competition

Both habitat complexity and priority effects can influence the strength of competitive interactions; however, the independent and synergistic effects of these processes are not well understood. In Moorea, French Polynesia, we conducted a factorial field experiment to quantify the independent and combined effects of priority effects and habitat complexity on the strength of intraspecific competitive interactions among recently settled individuals of a coral reef fish (Thalassoma quinquevittatum: Labridae). Simultaneous arrival of focal individuals with competitors resulted in a 2.89-fold increase in survival relative to reefs where focal individuals arrived 5 days later than competitors (i.e., a priority effect). Increasing habitat complexity resulted in a 1.55-fold increase in survivorship when focal individuals arrived simultaneously with or before competitors. However, increasing habitat complexity did not affect the survivorship of focal individuals arriving 5 days later than competitors. Behavior observations showed that survivorship was negatively correlated with aggression. Aggression by prior residents towards focal individuals was significantly greater when focal individuals arrived 5 days later than competitors than when they arrived simultaneously. Increasing habitat complexity did not reduce aggression. Our results suggest that, when competitors arrive simultaneously, competitive interactions are weak and subordinates are not displaced from complex habitat; increasing habitat complexity increases survival by disrupting predation. Conversely, when competitors arrive at different times, aggression intensifies and increasing habitat complexity does not disrupt predation because competitive subordinates are excluded from habitat resources. This study demonstrates that the strength of competition can be context-dependent and may vary with the timing of competitive interactions and habitat complexity.     

Priority effects: How the order of arrival of an invasive grass,Bromus tectorum,alters productivity and plant community structure when grown with native grass species

Theories and models attempt to explain how and why particular plant species grow together at particular sites or why invasive exotic species dominate plant communities. As local climates change and human‐use degrades and disturbs ecosystems, a better understanding of how plant communities assemble is pertinent, particularly when restoring grassland ecosystems that are frequently disturbed. One such community assembly theory is priority effects, which suggests that arrival order of species into a community alters plant–plant interactions and community assembly. Theoretically, priority effects can have lasting effects on ecosystems and will likely be altered as the risk of invasion by exotic species increases. It is difficult to predict how and when priority effects occur, as experimental reconstruction of arrival order is often difficult in adequate detail. As a result, limited experimental studies have explored priority effects on plant community assembly and plant invasions. To determine if and how priority effects affect the success of invasive species, we conducted a greenhouse study exploring how the arrival order of an invasive grass, Bromus tectorum, affects productivity and community composition when grown with native grasses. We found evidence for priority effects, as productivity was positively related to dominance of B. tectorum and was greater the earlier B. tectorum arrived. This suggests that priority effects could be important for plant communities as the early arrival of an invasive species drastically impacted the productivity and biodiversity of our system at the early establishment stages of plant community development.     

Priority effects in a planktonic bloom-forming marine diatom

Priority effects occur when a species or genotype with earlier arrival has an advantage such that its relative abundance in the community or population is increased compared with later-arriving species. Few studies have dealt with this concept in the context of within-species competition. Skeletonema marinoi is a marine diatom that shows a high degree of genetic differentiation between populations over small geographical distances. To test whether historical events such as priority effects may have been important in inducing these patterns of population differentiation, we performed microcosm experiments with successive inoculation of different S. marinoi strains. Our results show that even in the absence of a numerical advantage, significant priority effects were evident. We propose that priority effects may be an important mechanism in initiating population genetic differentiation.     

Synergistic effects of multiple mechanisms drive priority effects within a tadpole assemblage

Mounting evidence suggests that the history of species arrival to a locality can have important effects on species performance but the mechanism(s) through which priority effects are produced is not always clear. Differences in the developmental time of frog tadpoles provide an opportunity to examine mechanisms through which priority effects may influence fitness components of a late arriving taxon. Specifically, tadpoles of the southern leopard frog (Rana) can often require more than a year to complete metamorphosis so they overwinter in a pond and compete with newly deposited tadpoles in the spring. We conducted an experiment in artificial ponds to evaluate mechanisms through which overwintering Rana tadpoles influence fitness components of southern toad (Bufo) tadpoles deposited into ponds during the spring. We found that overwintered Rana reduced Bufo performance while newly hatched Rana that enter a pond simultaneously with Bufo did not. The production of this priority effect was primarily the result of Rana depleting algal resources in a pond during the winter prior to Bufo arrival. The performance of Bufo did not correspond with variation in the abundance of algae present in a pond during the spring and we found evidence to indicate that, in the absence of resource exploitation during the winter, overwintered Rana do not compete strongly with Bufo during the spring when both species co‐occur. When Rana deplete algal resources during the winter, however, interactions between Rana and Bufo during the spring became much more important as both the development rate and survival of Bufo was reduced to a greater extent than what would have been predicted by resource exploitation alone. Our results demonstrate that priority effects can result from early colonists depleting resource availability prior to the arrival of other species which can intensify behavioral/physical interactions between species when they co‐occur.     

Predation and priority effects in experimental zooplankton communities

During colonization of new habitats, the sequence of arrival among species is in many cases determined by chance. Priority effects imply that differences in arrival time may lead to long-lasting differences in species dominance. To evaluate the importance of priority effects, we performed a community assembly experiment, manipulating the inoculation order of three large cladoceran zooplankton species. The inoculation treatments were crossed with a predation treatment to test whether the presence of a predator (larvae of the phantom midge Chaoborus ) results in changes in the strength of species sorting and priority effects. Our results clearly demonstrate priority effects, but also that the presence of a predator impacts both community assembly and the strength of priority effects. In the predation-free treatments, communities were dominated by either Daphnia magna or Daphnia obtusa , depending on the species arrival sequence. Whenever D. obtusa was inoculated after D. magna , the species displayed negative growth. In the presence of Chaoborus predation, the communities were generally dominated by the third species, Simocephalus vetulus . Here, the growth of D. magna was negative when the species was inoculated as second or third. Overall, our results underscore the importance of both priority effects and species sorting during community assembly.     

Environmental variability counteracts priority effects to facilitate species coexistence: evidence from nectar microbes

The order of species arrival during community assembly can greatly affect species coexistence, but the strength of these effects, known as priority effects, appears highly variable across species and ecosystems. Furthermore, the causes of this variation remain unclear despite their fundamental importance in understanding species coexistence. Here, we show that one potential cause is environmental variability. In laboratory experiments using nectar-inhabiting microorganisms as a model system, we manipulated spatial and temporal variability of temperature, and examined consequences for priority effects. If species arrived sequentially, multiple species coexisted under variable temperature, but not under constant temperature. Temperature variability prevented extinction of late-arriving species that would have been excluded owing to priority effects if temperature had been constant. By contrast, if species arrived simultaneously, species coexisted under both variable and constant temperatures. We propose possible mechanisms underlying these results using a mathematical model that incorporates contrasting effects of microbial species on nectar pH and amino acids. Overall, our findings suggest that understanding consequences of priority effects for species coexistence requires explicit consideration of environmental variability.     

Fat stores of American redstarts Setophaga ruticilla arriving at northerly breeding grounds

Many long-distance passerine migrants arrive with more fat stores than necessary to have reached their northerly breeding grounds. Researchers have argued for adaptive advantages associated with arriving with 'surplus' migratory fat, including increased breeding performance and insurance against adverse weather, and reduced food resources during the days following arrival. The objectives of this work were to document fat stores in American redstarts Setophaga ruticilla arriving to breed in northern Michigan and to test predictions associated with the hypothesis that arrival fat serves an insurance role. Results suggest that redstarts arrived in northern Michigan with fat stores sufficient to have continued migrating an additional distance greater than 1000 km. Significant yearly variation in arrival fat corresponded in part with environmental conditions measured at the breeding grounds. Birds arrived with the most fat in the year with the coolest temperatures and the lowest food abundance at the time of arrival to the site. Further, an inverse relationship between arrival fat and arrival day in males indicated that early arrivals carried more fat than later arrivals. Birds that arrived early faced cooler temperatures and lower densities of terrestrial invertebrates, and arrival fat may have provided a mechanism to overcome poor early season foraging conditions. However, our results are not entirely consistent with the hypothesis that arrival fat serves only as insurance. Arrival fat appeared important even during the most benign year of this study. Further, evidence suggests that females may have derived more benefit from arrival fat than males. These results highlight the connection between phases of a bird's annual cycle. Migrants that do well en route , arrive at breeding grounds in better condition, which may contribute to survival and reproductive success.     

Timing is everything: priority effects alter community invasibility after disturbance

Theory suggests that communities should be more open to the establishment of regional species following disturbance because disturbance may make more resources available to dispersers. However, after an initial period of high invasibility, growth of the resident community may lead to the monopolization of local resources and decreased probability of successful colonist establishment. During press disturbances (i.e., directional environmental change), it remains unclear what effect regional dispersal will have on local community structure if the establishment of later arriving species is affected by early arriving species (i.e., if priority effects are important). To determine the relationship between time‐since‐disturbance and invasibility, we conducted a fully factorial field mesocosm experiment that exposed tundra zooplankton communities to two emerging stressors – nutrient and salt addition, and manipulated the arrival timing of regional dispersers. Our results demonstrate that invasibility decreases with increasing time‐since‐disturbance as abundance (nutrient treatments) or species richness (salt treatments) increases in the resident community. Results suggest that the relative timing of dispersal and environmental change will modify the importance of priority effects in determining species composition after a press disturbance.     

Population size dependence, competitive coexistence and habitat destruction

1. Spatial dynamics can lead to coexistence of competing species even with strong asymmetric competition under the assumption that the inferior competitor is a better colonizer given equal rates of extinction. Patterns of habitat fragmentation may alter competitive coexistence under this assumption.
2. Numerical models were developed to test for the previously ignored effect of population size on competitive exclusion and on extinction rates for coexistence of competing species. These models neglect spatial arrangement.
3. Cellular automata were developed to test the effect of population size on competitive coexistence of two species, given that the inferior competitor is a better colonizer. The cellular automata in the present study were stochastic in that they were based upon colonization and extinction probabilities rather than deterministic rules.
4. The effect of population size on competitive exclusion at the local scale was found to have little consequence for the coexistence of competitors at the metapopulation (or landscape) scale. In contrast, population size effects on extinction at the local scale led to much reduced landscape scale coexistence compared to simulations not including localized population size effects on extinction, especially in the cellular automata models. Spatially explicit dynamics of the cellular automata vs. deterministic rates of the numerical model resulted in decreased survival of both species. One important finding is that superior competitors that are widespread can become extinct before less common inferior competitors because of limited colonization.
5. These results suggest that population size–extinction relationships may play a large role in competitive coexistence. These results and differences are used in a model structure to help reconcile previous spatially explicit studies which provided apparently different results concerning coexistence of competing species.     

Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival

Exotic species are sometimes phenologically distinct from native species in the invaded community, allowing them to be active when there may be reduced competition for resources. In southern California, annual species are particularly problematic invaders, and prior work has shown that these species germinate earlier in the growing season, giving them a competitive advantage over later‐germinating native species. This result begs the question, if being active earlier is advantageous, why have not native species adapted earlier cues for germination? We hypothesized native species would benefit less from earlier germination than exotic species (potentially due to slower growth following germination), thus negating potential selection for early germination. Here we manipulated planting time for common native and exotic species, growing them in all possible species pairs, to evaluate how competitive outcomes were altered by the time of arrival and the origin of competing species. In contrast to our hypotheses, the exotic species often had lower biomass when planted first, potentially due to disturbance when the second species was planted. In contrast, three out of our four native species benefited from earlier planting (a priority effect). Unlike the potential benefit of arriving early, we found no evidence that being planted one week later resulted in a competitive disadvantage, when compared to being planted simultaneously with a competitor. Further, we found that the magnitude and even direction of priority effects varied depending on the identity of the interacting species. Together these results suggest that a lack of directional selection may prevent adaptation towards earlier germination times of native species. Although this experiment was conducted with a limited suite of species, the results show that the role of seasonal priority effects varies among species, and that native species could benefit from seasonal priority effects in restoration efforts even when in competition with fast‐growing exotic annual species.     

Impact of the invasive cane toad (Bufo marinus) on an Australian frog (Opisthodon ornatus) depends on minor variation in reproductive timing

Invasive species are widely viewed as unmitigated ecological catastrophes, but the reality is more complex. Theoretically, invasive species could have negligible or even positive effects if they sufficiently reduce the intensity of processes regulating native populations. Understanding such mechanisms is crucial to predicting ultimate ecological impacts. We used a mesocosm experiment to quantify the impact of eggs and larvae of the introduced cane toad (Bufo marinus) on fitness-related traits (number, size and time of emergence of metamorphs) of a native Australian frog species (Opisthodon ornatus). The results depended upon the timing of oviposition of the two taxa, and hence the life-history stages that came into contact. Growth and survival of O. ornatus tadpoles were enhanced when they preceded B. marinus tadpoles into ponds, and reduced when they followed B. marinus tadpoles into ponds, relative to when tadpoles of both species were added to ponds simultaneously. The dominant tadpole-tadpole interaction is competition, and the results are consistent with competitive priority effects. However, these priority effects were reduced or reversed when O. ornatus tadpoles encountered B. marinus eggs. Predation on toxic toad eggs reduced the survival of O. ornatus and B. marinus. The consequent reduction in tadpole densities allowed the remaining O. ornatus tadpoles to grow more rapidly and to metamorphose at larger body sizes (>60% disparity in mean mass). Thus, exposure to B. marinus eggs reduced the number of O. ornatus metamorphs, but increased their body sizes. If the increased size at metamorphosis more than compensates for the reduced survival, the effective reproductive output of native anurans may be increased rather than decreased by the invasive toad. Minor interspecific differences in the seasonal timing of oviposition thus have the potential to massively alter the impact of invasive cane toads on native anurans.     

Indirect predator effects influence behaviour but not morphology of juvenile coral reef Ambon damselfish Pomacentrus amboinensis

A 6-week laboratory experiment exposed juvenile Ambon damselfish Pomacentrus amboinensis to visual and chemical cues of either a predator, a herbivore or a null control (sea water) and found no effect of predator cues on prey morphology (proportion of ocellus to eye diameter, body depth, standard length and fin area). Nonetheless, behaviour was significantly affected by predator presence, with prey less active and taking half as many feeding strikes when exposed to predators compared to fish from the null control. The presence of a herbivore also affected prey behaviour similar to that of the predator, suggesting that the presence of a non-predator may have important effects on development.     

Habitat complexity modifies the impact of piscivores on a coral reef fish population

Patterns in juvenile mortality rates can have a profound affect on the distribution and abundance of adult individuals, and may be the result of a number of interacting factors. Field observations at Lizard Island (Great Barrier Reef, Australia) showed that for a coral reef damselfish, Pomacentrus moluccensis, juvenile mortality (over 1 year) varied between 20 and almost 100% among sites. Correlative data showed that juvenile mortality increased as a function of initial densities (recruitment), predator densities and the availability of preferred coral substrata. A multiple regression showed that these three variables together did not explain significantly more variation in mortality than the single factor showing the strongest relationship. This appeared to be because recruitment, predator densities and preferred coral substrata were all highly correlated, suggesting that one, two or all of these factors may be influencing juvenile mortality rates. One hypothesis was that density-dependent mortality in juveniles was the result of an interaction between predators (which appear to aggregate at high-recruitment sites) and the availability of preferred substrata (predator refuges). We tested this hypothesis by using both laboratory and field experiments to see whether fish predation could significantly alter survivorship of this damselfish, and whether this impact was dependent upon the coral substratum. The laboratory experiment was designed to test the effects of three common predators (Pseudochromis fuscus, Cephalopholis boenak and Thalassoma lunare) and three different coral substrata that varied in their complexity (Pocillopora damicornis, Acropora nasuta and A. nobilis) on the survival of juvenile Pomacentrus moluccensis. There was a significant interaction between predator species and microhabitat in determining survival. Pseudochromis fuscus and C. boenak were both significantly better at capturing juvenile damselfish than T. lunare. Juvenile survivorship was significantly better when they were given the more complex corals, Pocillopora damicornis and A. nasuta, compared with those given the open-structured species A. nobilis. This pattern reflects habitat selection in the field. Predators differed in their strike rates and the proportion of strikes that were successful, but all exhibited greater success at prey capture where A. nobilis was provided as shelter. The interaction between the effect of predator species and microhabitat structure on damselfish survival was tested in the field for a cohort of juvenile Pomacentrus moluccensis. We examined juvenile survival in the presence and absence of two predators that co-occur on natural patch reefs (C. boenak and Pseudochromis fuscus). The experimental patch reefs we used for this purpose were constructed from both high complexity (Pocillopora damicornis) and low complexity (A. nobilis) coral substrata. Both juveniles and predators were translocated to reefs at natural densities. The effects of predation were clearly dependent upon the microhabitat. Reefs of the high-complexity coral with predators supported the same high numbers of Pomacentrus moluccensis as the reefs with no resident predators. However, damselfish abundance was significantly lower on low-complexity reefs with resident predators, relative to the other treatments. Background rates of loss were high, even on preferred coral in the absence of the manipulated predator, suggesting that transient predators may be even more important than the residents. We suggest that adult abundances in this species were strongly influenced by the densities of different predators and the availability of preferred refuges. Received: 3 April 1997 / Accepted: 26 August 1997     

Inoculation history affects community composition in experimental freshwater bacterioplankton communities

Priority effects occur when the arrival order of species or genotypes has a lasting effect on community or population structure. For freshwater bacteria , priority effects have been shown experimentally among individual species, but no experiments have been performed using complex natural communities. We investigated experimentally whether a foreign bacterioplankton community influences the community assembly trajectory when inoculated prior to the local community, whether inoculation time lag affects priority effects, and how the individual OTUs responded to time lag. Two bacterioplankton communities from dissimilar ponds were inoculated into one of the natural media with a time lag of 0, 12, 36 or 60 h, giving advantage in time to the foreign community . All three time lags resulted in priority effects, as the final community composition of these treatments differed significantly from that of the treatment with no time lag, but compositional shifts were not linear to inoculation time lag. The responses of individual OTUs to time lag were highly diverse and not predictable based on their immigration history or relative abundance in the inocula or control. The observed impact and complexity of priority effects in multispecies systems emphasize the importance of this process in structuring both natural and industrial bacterial communities.     

Are numbers enough? Colonizer phenotype and abundance interact to affect population dynamics

1. Ecologists have long recognized that the number of colonizers entering a population can be a major driver of population dynamics, but still struggle to explain why the importance of colonizer supply varies so dramatically. While there are indications that differences in the phenotype among dispersing individuals could also be important to populations, the role of phenotypic variation relative to the number of individuals, and the extent to which they interact, remains unknown. 2. We simultaneously manipulated the phenotype (dispersal duration) and abundance of settlers of a marine bryozoan and measured subsequent population structure in the field. 3. Increases in the number of colonizing individuals increased the subsequent recruitment and biomass of populations, regardless of colonizer phenotype. However, the relationship between colonizer abundance and the subsequent reproductive yield of the population was strongly reduced in populations containing individuals that had long dispersal durations. 4. The interactive effects of colonizer phenotype and abundance on the reproductive yield of populations occurred because longer dispersal durations decreased the proportion of individuals that reproduced. In fact, populations established from a few individuals with short dispersal durations had similar reproductive yield to populations c. 30 times larger established from individuals with long dispersal durations. 5. Interactions between colonizer phenotype and abundance have important implications for predicting population dynamics beyond those previously provided by numerical abundance or recruit phenotype alone.     

Attack intensity by two species of territorial damselfish (Pomacentridae) as estimates of competitive overlap with two species of wrasse (Labridae)

The intensity of interspecific territorial defense should be based upon the degree of competitive overlap. We tested this relationship in two territorial Caribbean damselfish (dusky, Stegastes adustus, and longfin, S. diencaeous) with intruders being the bluehead wrasse (Thalassoma bifascatus) and the slippery dick wrasse (Halichoeres bivittatus). Based on food habits, the slippery dick and the bluehead wrasse should have the same degree of competitive overlap to the two damselfish species. We also predicted that the larger slippery dick wrasse intruder would receive more aggression than the smaller bluehead wrasse intruder. Neither damselfish species distinguished between the two wrasses suggesting that they were ecologically equivalent. We also tested size differences within both species of wrasse and found that size had no influence on the aggression in the dusky damselfish. The longfin also did not show a size preference for the bluehead but did prefer to attack the larger slippery dick wrasse. In spite of the similarities between the dusky and the longfin damselfish and the similar food habits of the bluehead and the slippery dick wrasse, our results suggest that using intensity of interspecific territorial defense alone may not be an adequate measure of competitive overlap.     

Initial colonization,community assembly and ecosystem function: fungal colonist traits and litter biochemistry mediate decay rate

Priority effects are an important ecological force shaping biotic communities and ecosystem processes, in which the establishment of early colonists alters the colonization success of later‐arriving organisms via competitive exclusion and habitat modification. However, we do not understand which biotic and abiotic conditions lead to strong priority effects and lasting historical contingencies. Using saprotrophic fungi in a model leaf decomposition system, we investigated whether compositional and functional consequences of initial colonization were dependent on initial colonizer traits, resource availability or a combination thereof. To test these ideas, we factorially manipulated leaf litter biochemistry and initial fungal colonist identity, quantifying subsequent community composition, using neutral genetic markers, and community functional characteristics, including enzyme potential and leaf decay rates. During the first 3 months, initial colonist respiration rate and physiological capacity to degrade plant detritus were significant determinants of fungal community composition and leaf decay, indicating that rapid growth and lignolytic potential of early colonists contributed to altered trajectories of community assembly. Further, initial colonization on oak leaves generated increasingly divergent trajectories of fungal community composition and enzyme potential, indicating stronger initial colonizer effects on energy‐poor substrates. Together, these observations provide evidence that initial colonization effects, and subsequent consequences on litter decay, are dependent upon substrate biochemistry and physiological traits within a regional species pool. Because microbial decay of plant detritus is important to global C storage, our results demonstrate that understanding the mechanisms by which initial conditions alter priority effects during community assembly may be key to understanding the drivers of ecosystem‐level processes.     

Long‐term experiment manipulating community assembly results in favorable restoration outcomes for invaded prairies

Invasive species are a common problem in restoration projects. Manipulating soil fertility and species arrival order has the potential to lower their abundance and achieve higher abundances of seeded native species. In a 7‐year experiment in Missouri, United States, we tested how nutrient addition and the timing of arrival of the invasive legume Lespedeza cuneata and seeded native prairie grass and forb species influenced overall community composition. Treatments that involved early arrival of seeded forb and grass species and late arrival of L. cuneata were most successful at creating community structure that fulfilled our restoration goals, displaying high abundance of seeded native forb species, low abundances of L. cuneata, and non‐native species. There were few treatment interactions, with the exception that timing seeded native forbs and timing of L. cuneata arrival interactively influenced the abundance of seeded native forbs. This suggests that the individual treatments are supporting the restoration goals, such as creating a community with low abundance of L. cuneate or high abundance of native seeded species, without restricting each other. This study demonstrates the importance of priority effects in disturbed habitats prone to invasion, the lasting effects of initial seeding on long‐term community composition, and the potential for fertilization to positively benefit restoration of degraded grasslands.     

Priority Effects of Time of Arrival of Plant Functional Groups Override Sowing Interval or Density Effects: A Grassland Experiment

Priority effects occur when species that arrive first in a habitat significantly affect the establishment, growth, or reproduction of species arriving later and thus affect functioning of communities. However, we know little about how the timing of arrival of functionally different species may alter structure and function during assembly. Even less is known about how plant density might interact with initial assembly. In a greenhouse experiment legumes, grasses or forbs were sown a number of weeks before the other two plant functional types were sown (PFT) in combination with a sowing density treatment. Legumes, grasses or non-legume forbs were sown first at three different density levels followed by sowing of the remaining PFTs after three or six-weeks. We found that the order of arrival of different plant functional types had a much stronger influence on aboveground productivity than sowing density or interval between the sowing events. The sowing of legumes before the other PFTs produced the highest aboveground biomass. The larger sowing interval led to higher asymmetric competition, with highest dominance of the PFT sown first. It seems that legumes were better able to get a head-start and be productive before the later groups arrived, but that their traits allowed for better subsequent establishment of non-legume PFTs. Our study indicates that the manipulation of the order of arrival can create priority effects which favour functional groups of plants differently and thus induce different assembly routes and affect community composition and functioning.     

Priority effects in experimental populations of the cyanobacterium Microcystis

The arrival order of colonists in developing populations can have a lasting influence on community and population structure, a phenomenon referred to as priority effects. To explore whether such priority effects are important in determining strain composition of populations of the cyanobacterium Microcystis , four Microcystis strains, isolated from a single lake and differing in functional traits, were grown during 4 weeks in the laboratory in all possible pairwise combinations, with the two strains either inoculated at the same time or with a time lag of 1 week, in the presence or absence of grazing Daphnia magna . The relative abundance of strains in the mixtures was assessed using denaturing gradient gel electrophoresis, and the growth rate of each strain in the mixtures was determined for the last 2 weeks of the experiment. We observed strong effects of inoculation order on the final population structure, and these effects were influenced by grazing Daphnia . The priority effects were strain-specific and occurred in two directions: some of the strains grew slower while others grew faster when inoculated second compared with when inoculated first. Our results indicate that priority effects may have a profound impact on strain composition of Microcystis populations.