Disentangling structural and functional responses of native versus alien communities by canonical ordination analyses and variation partitioning with multiple matrices.

Freshwaters are under accelerated human pressure, and mollusk communities are among its most sensitive, threatened, and valuable components. To the best of our knowledge, the overall effects of damming, environment, space, time, and invasive alien mollusk species, on structural and functional responses of native mollusk communities were not yet compared. Using historical information and recent data from a river, we aimed to investigate and disentangle these effects and evaluate the differences in structural and functional responses of natives and alien invasives to the same predictors. Variation partitioning showed that alien species were as important predictors as were environmental factors and time in explaining species composition of native freshwater mollusk communities. Aliens were more independent of environmental conditions than natives and responded to different drivers, partially explaining their invasion success. The increased abundance of some alien gastropods was positively related to taxonomic diversity, while certain alien bivalves were negatively associated with the functional diversity of native communities. We introduce a cumulative variation partitioning with multiple response (native and alien) and predictor matrices, along with a diagram to show their relations, advocating for a conceptual shift in future community ecology, from "variables to matrices" and from "multivariate analyses to multi-matrix statistical modeling".

Stream bacterial diversity peaks at intermediate freshwater salinity and varies by salt type.

Anthropogenic freshwater salinization is an emerging and widespread water quality stressor that increases salt concentrations of freshwater, where specific upland land-uses produce distinct ionic profiles. In-situ studies find salinization in disturbed landscapes is correlated with declines in stream bacterial diversity, but cannot isolate the effects of salinization from multiple co-occurring stressors. By manipulating salt concentration and type in controlled microcosm studies, we identified direct and complex effects of freshwater salinization on bacterial diversity in the absence of other stressors common in field studies using chloride salts. Changes in both salt concentration and cation produced distinct bacterial communities. Bacterial richness, or the total number of amplicon sequence variants (ASVs) detected, increased at conductivities as low as 350 µS cm-1, which is opposite the observations from field studies. Richness remained elevated at conductivities as high as 1500 µS cm-1 in communities exposed to a mixture of Ca, Mg, and K chloride salts, but decreased in communities exposed to NaCl, revealing a classic subsidy-stress response. Exposure to different chloride salts at the same conductivity resulted in distinct bacterial community structure, further supporting that salt type modulates responses of bacterial communities to freshwater salinization. Community variability peaked at 125-350 µS cm-1 and was more similar at lower and upper conductivities suggesting possible shifts in deterministic vs. stochastic assembly mechanisms across freshwater salinity gradients. Based on these results, we hypothesize that modest freshwater salinization (125-350 µS cm-1) lessens hypo-osmotic stress, reducing the importance of salinity as an environmental filter at intermediate freshwater ranges but effects of higher salinities at the upper freshwater range differ based on salt type. Our results also support previous findings that ~300 µS cm-1 is a biological effect concentration and effective salt management strategies may need to consider variable effects of different salt types associated with land-use.

The potential impacts of invasions on native symbionts.

Symbionts, including parasites, pathogens, and mutualists, can play important roles in determining whether or not invasions by host species will be successful. Loss of enemies from the native habitat, such as parasites and pathogens, can allow for higher invader fitness in the invaded habitat. The presence of mutualists (e.g., pollinators, seed dispersers, mycorrhizae, and rhizobial bacteria) in the invaded habitat can facilitate invasion success. Although there has been a great deal of research focusing on how invading hosts may benefit from enemy losses or mutualist gains, far less attention has focused on how native symbiont populations and communities respond to invasion by non-indigenous hosts and symbionts. In this paper, we present a conceptual framework examining how symbionts such as parasites, pathogens, commensals, and mutualists can influence invader success and whether these native symbionts will benefit or decline during invasion. The first major factor in this framework is the competence of the invading host relative to the native hosts. Low- or non-competent hosts that support few if any native symbionts could cause declines in native symbiont taxa. Competent invading hosts could potentially support native parasites, pathogens, commensals, and mutualists, especially if there is a closely related or similar host in the invaded range. These symbionts could inhibit or facilitate invasion or have no discernible effect on the invading host. An understanding of how native symbionts interact with competent versus non-competent invading hosts as well as various invading symbionts is critical to our understanding of invasion success, its consequences for invaded communities and how native symbionts in these communities will fare in the face of invasion.

Strong effects of a mutualism on freshwater community structure.

Numerous mutualisms have been described from terrestrial and marine communities and many of these mutualisms have significant effects on community structure and function. In contrast, there are far fewer examples of mutualisms from freshwater habitats and there is no evidence that any mutualism has community-wide or ecosystem-level consequences. Northern hemisphere crayfish are host to a variety of ectosymbiotic worms called branchiobdellidans. The association between some of these "crayfish worms" and their hosts is a mutualism. The outcome of the association is context dependent and can be influenced by host size, symbiont number, and the environment. Here we document in two experiments that the mutualism between crayfish and these worms alters the effect of crayfish on stream community structure and sediment deposition, an important ecosystem variable. We enclosed crayfish stocked with 0 worms and intermediate (3-6) and high worm densities (12) in cages in streams in Boone, North Carolina and Clemson, South Carolina, United States. At both locations, there was a negative relationship between initial worm density and final macroinvertebrate abundance. There was a significant effect of worm treatment on macroinvertebrate community structure in both the Boone and Clemson experiments. In Boone, there were effects on both overall macroinvertebrate abundance and community composition, whereas in Clemson, changes to community structure were primarily driven by changes in total abundance. There was a negative relationship between benthic sediment volume and initial worm density in both experiments, primarily later in the experiments, though these effects were influenced by sediment deposition rates. Our results are the first to demonstrate strong effects of a mutualism on freshwater communities. Both members of this mutualism are found throughout the northern hemisphere, so similar impacts may occur in many other waterways. Given that various species in addition to crayfish function as keystone species and ecosystem engineers in freshwater systems throughout the world, mutualisms involving these strongly interacting species may be as important to the structure and functioning of freshwater systems as comparable mutualisms in marine and terrestrial systems.

Host specificity and microhabitat preference of symbiotic copepods (Cyclopoida: Clausiididae) associated with ghost shrimps (Decapoda: Callichiridae, Callianideidae).

We examined the host specificity of two ectosymbiotic Clausidium Kossman, 1874 copepods (Cyclopoida: Clausiididae) on two co-occurrence species of host ghost shrimps. Our results revealed that both species of symbiotic copepod demonstrated extremely high host specificity. Moreover, within a single host shrimp species, each symbiont species displayed strong spatial patterns in microhabitat selection on their hosts' bodies. Clausidium persiaensis Sepahvand & Kihara, 2017, was only found on the host Callianidea typa Milne Edwards, 1837 and almost exclusively within the host shrimp gill chamber, while C. iranensis Sepahvand, Kihara, & Boxshall, 2019 was only found on the host Neocallichirus jousseaumei (Nobili, 1904) and showed extremely strong preferences for the chelae and anterior walking legs. We also found that while the number of symbionts tends to increase with the host size, the two host species differed in the degree of symbiont infestation, with large C. typa hosting approximately 7× as many symbionts as the similarly sized N. jousseaumeia. The mechanisms resulting in the observed differences in infestation levels and microhabitat preferences of clausidium copepods among their hosts, including differences in physiology, burrowing pattern, and host grooming behavior should be further investigated.

Ectosymbionts alter spontaneous responses to the Earth's magnetic field in a crustacean.

Magnetic sensing is used to structure every-day, non-migratory behaviours in many animals. We show that crayfish exhibit robust spontaneous magnetic alignment responses. These magnetic behaviours are altered by interactions with Branchiobdellidan worms, which are obligate ectosymbionts. Branchiobdellidan worms have previously been shown to have positive effects on host growth when present at moderate densities, and negative effects at relatively high densities. Here we show that crayfish with moderate densities of symbionts aligned bimodally along the magnetic northeast-southwest axis, similar to passive magnetic alignment responses observed across a range of stationary vertebrates. In contrast, crayfish with high symbiont densities failed to exhibit consistent alignment relative to the magnetic field. Crayfish without symbionts shifted exhibited quadramodal magnetic alignment and were more active. These behavioural changes suggest a change in the organization of spatial behaviour with increasing ectosymbiont densities. We propose that the increased activity and a switch to quadramodal magnetic alignment may be associated with the use of systematic search strategies. Such a strategy could increase contact-rates with conspecifics in order to replenish the beneficial ectosymbionts that only disperse between hosts during direct contact. Our results demonstrate that crayfish perceive and respond to magnetic fields, and that symbionts influence magnetically structured spatial behaviour of their hosts.

Species Pool Functional Diversity Plays a Hidden Role in Generating ß-Diversity.

Functional trait diversity is used as a way to infer mechanistic processes that drive community assembly. While functional diversity within communities is often viewed as a response variable, here we present and test a framework for how functional diversity among taxa in the regional species pool drives the assembly of communities among habitats. We predicted that species pool functional diversity should work with environmental heterogeneity to drive ß-diversity. We tested these predictions by modeling empirical patterns in invertebrate communities from 570 streams in 52 watersheds. Our analysis of the field data provided strong support for the inclusion of both functional diversity and environmental heterogeneity in the models, and our predictions were supported when the community was analyzed all together. However, analyses within individual functional feeding guilds revealed strong context dependency in the relative importance of functional diversity, γ-richness, and environmental heterogeneity to ß-diversity. We interpret the results to mean that functional diversity can play an important role in driving ß-diversity; however, within guilds the nature of interspecific interactions and species pool size complicate the relationship. Future research should test this conceptual model across different ecosystems and in experimental settings using metacommunity mesocosms to enhance our understanding of the role that functional variation plays in generating spatial biodiversity patterns.

Metacommunity theory meets restoration: isolation may mediate how ecological communities respond to stream restoration.

An often-cited benefit of river restoration is an increase in biodiversity or shift in composition to more desirable taxa. Yet, hard manipulations of habitat structure often fail to elicit a significant response in terms of biodiversity patterns. In contrast to conventional wisdom, the dispersal of organisms may have as large an influence on biodiversity patterns as environmental conditions. This influence of dispersal may be particularly influential in river networks that are linear branching, or dendritic, and thus constrain most dispersal to the river corridor. As such, some locations in river networks, such as isolated headwaters, are expected to respond less to environmental factors and less by dispersal than more well-connected downstream reaches. We applied this metacommunity framework to study how restoration drives biodiversity patterns in river networks. By comparing assemblage structure in headwater vs. more well-connected mainstem sites, we learned that headwater restoration efforts supported higher biodiversity and exhibited more stable ecological communities compared with adjacent, unrestored reaches. Such differences were not evident in mainstem reaches. Consistent with theory and mounting empirical evidence, we attribute this finding to a relatively higher influence of dispersal-driven factors on assemblage structure in more well-connected, higher order reaches. An implication of this work is that, if biodiversity is to be a goal of restoration activity, such local manipulations of habitat should elicit a more profound response in small, isolated streams than in larger downstream reaches. These results offer another significant finding supporting the notion that restoration activity cannot proceed in isolation of larger-scale, catchment-level degradation.

Manipulation of local environment produces different diversity outcomes depending on location within a river network.

Ecologists have long been interested in mechanisms governing community composition and assembly. Spatial connectivity is one potential mechanism that can have a large influence on community processes. In accordance with network metrics such as closeness and betweenness, headwater streams are more isolated than mainstem streams. Theory and observational studies predict that community structure in isolated locations of dispersal networks should respond more strongly to manipulations of local conditions, whereas well-connected communities subject to high levels of dispersal should not respond strongly to local manipulations. We experimentally investigated this prediction by manipulating habitat complexity in headwaters and mainstem streams while monitoring macroinvertebrate communities through time. As predicted, the manipulation of local habitat had a stronger influence in headwaters than mainstreams. Both taxon richness and community similarity showed strong responses to alterations in habitat complexity in headwaters, but not in mainstem streams. These findings support the hypothesis that location within a dispersal network affects the relative importance of local and regional factors in structuring the local communities within a spatially structured metacommunity. In addition, our results suggest that conservation strategies need to account for the possibility that the relative importance of local and regional drivers of community composition and assembly can vary spatially.

Making sense of metacommunities: dispelling the mythology of a metacommunity typology.

Metacommunity ecology has rapidly become a dominant framework through which ecologists understand the natural world. Unfortunately, persistent misunderstandings regarding metacommunity theory and the methods for evaluating hypotheses based on the theory are common in the ecological literature. Since its beginnings, four major paradigms-species sorting, mass effects, neutrality, and patch dynamics-have been associated with metacommunity ecology. The Big 4 have been misconstrued to represent the complete set of metacommunity dynamics. As a result, many investigators attempt to evaluate community assembly processes as strictly belonging to one of the Big 4 types, rather than embracing the full scope of metacommunity theory. The Big 4 were never intended to represent the entire spectrum of metacommunity dynamics and were rather examples of historical paradigms that fit within the new framework. We argue that perpetuation of the Big 4 typology hurts community ecology and we encourage researchers to embrace the full inference space of metacommunity theory. A related, but distinct issue is that the technique of variation partitioning is often used to evaluate the dynamics of metacommunities. This methodology has produced its own set of misunderstandings, some of which are directly a product of the Big 4 typology and others which are simply the product of poor study design or statistical artefacts. However, variation partitioning is a potentially powerful technique when used appropriately and we identify several strategies for successful utilization of variation partitioning.

Compensatory dynamics stabilize aggregate community properties in response to multiple types of perturbations.

Compensatory dynamics are an important suite of mechanisms that can stabilize community and ecosystem attributes in systems subject to environmental fluctuations. However, few experimental investigations of compensatory dynamics have addressed these mechanisms in systems of real-world complexity, and existing evidence relies heavily on correlative analyses, retrospective examination, and experiments in simple systems. We investigated the potential for compensatory dynamics to stabilize plankton communities in plankton mesocosm systems of real-world complexity. We employed four types of perturbations including two types of nutrient pulses, shading, and acidification. To quantify how communities responded to these perturbations, we used a measure of community-wide synchrony combined with spectral analysis that allowed us to assess timescale-specific community dynamics, for example, whether dynamics were synchronous at some timescales but compensatory at others. The 150-d experiment produced 32-point time series of all zooplankton taxa in the mesocosms. We then used those time series to evaluate total zooplankton biomass as an aggregate property and to evaluate community dynamics. For three of our four perturbation types, total zooplankton biomass was significantly less variable in systems with environmental variation than in constant environments. For the same three perturbation types, community-wide synchrony was much lower in fluctuating environments than in the constant environment, particularly at longer timescales (periods ≈ 60 d). Additionally, there were strong negative correlations between population temporal variances and the level of community-wide synchrony. Taken together, these results strongly imply that compensatory interactions between species stabilized total biomass in response to perturbations. Diversity did not differ significantly across either treatments or perturbation types, thus ruling out several classes of mechanisms driven by changes in diversity. We also used several pieces of secondary evidence to evaluate the particular mechanism behind compensatory responses since a wide variety of mechanisms are hypothesized to produce compensatory dynamics. We concluded that fluctuation dependent endogenous cycles that occur as a consequence of consumer-resource interactions in competitive communities were the most likely explanation for the compensatory dynamics observed in our experiment. As with our previous work, scale-dependent dynamics were also a key to understanding compensatory dynamics in these experimental communities.

Ontogenetic shifts in a freshwater cleaning symbiosis: consequences for hosts and their symbionts.

Animal fitness is influenced by diverse assemblages of internal and external symbionts. These assemblages often change throughout host ontogeny, but the mechanisms that underlie these changes and their consequences for host fitness are seldom revealed. Here we examine a cleaning symbiosis between crayfish and an assemblage of ectosymbiotic branchiobdellidan worms to uncover what mechanisms drive changes in symbiont composition during host ontogeny and the consequences of these changes for both the host and symbionts. In surveys of a North Carolina river, the dominant worm species shifted from Cambarincola philadelphicus to Cambarincola ingens as crayfish (Cambarus bartonii) increased in size. We demonstrate that this shift is a function of host regulation by small crayfish and exclusion by a dominant symbiont on large crayfish. In a controlled lab experiment, small crayfish often removed their symbionts but C. ingens was removed at a higher rate than C. philadelphicus. In contrast, C. ingens had higher survivorship and reproduction than C. philadelphicus on large crayfish. We also measured the effect of each worm species on crayfish growth through ontogeny; neither worm species had an effect on small crayfish but both species had similar positive effects on the growth of large crayfish relative to controls. Evidence from another experiment suggested that intraguild predation by C. ingens caused a decline in C. philadelphicus on large crayfish. We have shown that shifts in partner fitness are a function of host size and that these shifts can involve the succession of symbionts. Further, our results suggest that changes in the outcome of symbioses can remain robust throughout host ontogeny despite interactive mechanisms that lead to shifts in symbiont community structure.

The rules for symbiont community assembly change along a mutualism-parasitism continuum.

Symbiont community assembly is driven by host-symbiont and symbiont-symbiont interactions. The effects that symbionts exert on their hosts are often context-dependent, and existing theoretical frameworks of symbiont community assembly do not consider the implications of variable outcomes to assembly processes. We hypothesized that symbiont-symbiont interactions become increasingly important along a parasitism/mutualism continuum because; (i) negative outcomes favour host resistance which in turn reduces symbiont colonization and subsequently reduce symbiont-symbiont interactions, whereas (ii) positive host outcomes favour tolerance and consequently higher symbiont colonization rates, leading to stronger interactions among symbionts. We found support for this hypothesis in the cleaning symbiosis between crayfish and ectosymbiotic branchiobdellidan worms. The symbiosis between crayfish and their worms can shift from parasitism/commensalism to mutualism as crayfish age. Here, field surveys identified changes in worm density, diversity and composition that were concomitant to changing symbiosis outcomes. We conducted several laboratory experiments and behavioural assays to relate patterns from the field to their likely causal processes. Young crayfish typically hosted only two relatively small worm species. Older crayfish hosted two additional larger species. In laboratory experiments, young crayfish exhibited a directed grooming response to all worm species, but were unable to remove small species. Conversely, adult crayfish did not exhibit grooming responses to any worm species. Relaxed grooming allowed the colonization of large worm species and initiated symbiont-symbiont intraguild predation that reduced the abundance and altered the behaviour of small worm species. Thus, the dominant processes of symbiont community assembly shifted from host resistance to symbiont-symbiont interactions through host ontogeny and a concomitant transition towards mutualism. This work shows that host resistance can have a prevailing influence over symbiont community assembly when symbiosis is disadvantageous to the host. However, when symbiosis is advantageous and resistance is relaxed, symbiont colonization rate and consequently abundance and diversity increases and interactions among symbionts become increasingly important to symbiont community assembly.

A symbiont's dispersal strategy: condition-dependent dispersal underlies predictable variation in direct transmission among hosts.

Direct horizontal transmission of pathogenic and mutualistic symbionts has profound consequences for host and symbiont fitness alike. While the importance of contact rates for transmission is widely recognized, the processes that underlie variation in transmission during contact are rarely considered. Here, we took a symbiont's perspective of transmission as a form of dispersal and adopted the concept of condition-dependent dispersal strategies from the study of free-living organisms to understand and predict variation in transmission in the cleaning symbiosis between crayfish and ectosymbiotic branchiobdellidan worms. Field study showed that symbiont reproductive success was correlated with host size and competition among worms for microhabitats. Laboratory experiments demonstrated high variability in transmission among host contacts. Moreover, symbionts were more likely to disperse when host size and competition for microhabitat created a fitness environment below a discrete minimum threshold. A predictive model based on a condition-dependent symbiont dispersal strategy correctly predicted transmission in 95% of experimental host encounters and the exact magnitude of transmission in 67%, both significantly better than predictions that assumed a fixed transmission rate. Our work provides a dispersal-based understanding of symbiont transmission and suggests adaptive symbiont dispersal strategies can explain variation in transmission dynamics and complex patterns of host infection.

Multiple diversity-stability mechanisms enhance population and community stability in aquatic food webs.

Biodiversity has been shown to increase the temporal stability of community and ecosystem attributes through multiple mechanisms, but these same mechanisms make less consistent predictions about the effects of richness on population stability. The overall effects of biodiversity on population and community stability will therefore depend on the dominant mechanisms that are likely to vary with the nature of biodiversity loss and the degree of environmental variability. We conducted a mesocosm experiment in which we generated a gradient in zooplankton species richness by directly manipulating dominant species and by allowing/preventing immigration from a metacommunity. The mesocosms were maintained under either constant or variable nutrient environments. Population, community, and ecosystem data were collected for five months. We found that zooplankton population and community stability is enhanced in species-rich communities in both constant and variable environments. Species richness increased primarily through the addition of species with low abundance. The communities that were connected to a metacommunity via immigration were the most diverse and the most stable, indicating the importance of both metacommunity dynamics and rare species for stability. We found little evidence for selection effects or overyielding as stabilizing forces. We did find support for asynchronous dynamics and statistical averaging, both of which predict destabilizing effects at the population level. We also found support for weak interactions, which predicts that both populations and communities will become more stable as richness increases. In order to understand the effects of biodiversity loss on stability, we will need to understand when different stabilizing mechanisms tend to operate but also how multiple mechanisms interact.

Preventing overexploitation in a mutualism: partner regulation in the crayfish-branchiobdellid symbiosis.

For a symbiosis to be a mutualism, benefits received must exceed costs incurred for both partners. Partners can prevent costly overexploitation through behaviors that moderate interactions with the other symbiont. In a symbiosis between crayfish and branchiobdellidan annelids, the worms can increase crayfish survival and growth by removing fouling material from the gills. However, overexploitation by the worms is possible and results in damage to host gills. We used behavioral observations to assess the degree to which two species of crayfish (Cambarus chasmodactylus and Orconectes cristavarius) use grooming to moderate their interaction with branchiobdellids. We found that grooming could effectively reduce worm numbers, and the proportion of total grooming directed at worms differed between crayfish species and as a function of worm number. O. cristavarius increased grooming in response to the addition of a single worm, while C. chasmodactylus only increased grooming in response to ten worms. These differences in the number of worms that trigger grooming behavior reflect differences between crayfish species in field settings. We also assessed whether antibacterial compounds in circulating crayfish hemolymph could limit bacterial gill fouling. O. cristavarius hemolymph inhibited some test bacteria more effectively than C. chasmodactylus did. Differences in the antibacterial properties of crayfish hemolymph may therefore help explain differences in both worm-directed grooming and worm loads in the field. We conclude that crayfish can use grooming to reduce worm numbers, which could lower the potential for gill damage, and that the level of grooming varies between crayfish species.

The fine line between mutualism and parasitism: complex effects in a cleaning symbiosis demonstrated by multiple field experiments.

Ecological theory and observational evidence suggest that symbiotic interactions such as cleaning symbioses can shift from mutualism to parasitism. However, field experimental evidence documenting these shifts has never been reported for a cleaning symbiosis. Here, we demonstrate shifts in a freshwater cleaning symbiosis in a system involving crayfish and branchiobdellid annelids. Branchiobdellids have been shown to benefit their hosts under some conditions by cleaning material from host crayfish's gill filaments. The system is uniquely suited as an experimental model for symbiosis due to ease of manipulation and ubiquity of the organisms. In three field experiments, we manipulated densities of worms on host crayfish and measured host growth in field enclosures. In all cases, the experiments revealed shifts from mutualism to parasitism: host crayfish growth was highest at intermediate densities of branchiobdellid symbionts, while high symbiont densities led to growth that was lower or not significantly different from 0-worm controls. Growth responses were consistent even though the three experiments involved different crayfish and worm species and were performed at different locations. Results also closely conformed to a previous laboratory experiment using the same system. The mechanism for these shifts appears to be that branchiobdellids switched from cleaning host gills at intermediate densities of worms to consuming host gill tissue at high densities. These outcomes clearly demonstrate shifts along a symbiosis continuum with the maximum benefits to the host at intermediate symbiont densities. At high symbiont densities, benefits to the host disappear, and there is some evidence for a weak parasitism. These are the first field experimental results to demonstrate such shifts in a cleaning symbiosis.

Habitat heterogeneity and activity of an omnivorous ecosystem engineer control stream community dynamics.

All communities vary through time. This variability originates from both intrinsic and extrinsic sources. Intrinsic sources are due to actions of organisms in a community, i.e., population dynamics and species interactions, while extrinsic variability is variability created by elements of habitat or environmental change. There is a growing appreciation that these two sources may interact, producing patterns of community variability that cannot be predicted or explained by focusing on a single source. We performed a field experiment that simultaneously manipulated trophic structure (intrinsic) and habitat heterogeneity (extrinsic) in order to examine the interaction between sources of variability in a South Carolina (USA) stream macroinvertebrate community. To manipulate trophic structure, we experimentally altered local abundances of crayfish which are keystone species and ecosystem engineers, while our manipulation of habitat was to alter stream substrate heterogeneity. We focused on two types of community variability as responses to our manipulations: aggregate variability (i.e., variability of summed species) and compositional variability (i.e., variability in relative abundances of species) by monitoring community composition through a 10-week experiment. We found that community dynamics shifted from patterns in variability indicative of synchrony (high aggregate variability + low compositional) to variability indicative of compensation (low aggregate variability + high compositional) along a gradient of increasing habitat heterogeneity. However, the shift in community dynamics only occurred when crayfish were present in the community. Supporting evidence from the experiment suggested that sediment engineering effects of crayfish acted as a community-wide perturbation in low-heterogeneity habitat creating synchronous dynamics. However, in high-heterogeneity enclosures, crayfish effects were moderated by refugia provided by a more complex substratum. The switch from synchronous to compensatory dynamics is significant since compensation stabilizes aggregate community properties and ecosystem functions while synchrony frequently exacerbates aggregate variability. Results from this experiment demonstrate that an interaction between intrinsic and extrinsic sources of variability can alter community dynamics and that such an alteration does not occur in the absence of an interaction.

ENVIRONMENTAL FLUCTUATIONS INDUCE SCALE-DEPENDENT COMPENSATION AND INCREASE STABILITY IN PLANKTON ECOSYSTEMS.

The temporal stability of aggregate community and ecosystem properties is influenced by the variability of component populations, the interactions among populations, and the influence of environmental fluctuations on populations. Environmental fluctuations that enhance population variability are generally expected to destabilize community and ecosystem properties, but this will depend on the degree to which populations are synchronized in their dynamics. Here we use seminatural experimental ponds to show that reduced synchrony among zooplankton taxa increases the temporal stability of zooplankton density, abundance, and ecosystem productivity in fluctuating environments. However, asynchrony only occurs at long timescales (∼80-day periods) and under recurring environmental perturbations. At shorter timescales (∼10-day periods) and in constant environments, synchronous dynamics dominate. Our findings support recent theory indicating that compensatory dynamics can stabilize communities and ecosystems. They further indicate that environmental fluctuations can enhance the likelihood of long-period asynchrony and thus stabilize community and ecosystem properties despite their short term destabilizing effects.