Distinguishing between direct and indirect effects of predators in complex ecosystems

Colonisation after disturbance on artificial structures: The influence of timing and grazing

Artificial structures are poor surrogates of natural rocky shores, meaning they generally support depauperate assemblages. These differences may result from a combination of recruitment processes, biotic interactions, and structuring by environmental factors. In this study, plots were cleared on two seawalls and two natural shores at two separate timepoints - in August 2020 (summer) and February 2021 (winter) - and monitored over one year to determine the influence of timing of disturbance on recruitment and succession. Additional plots were cleared at one of the seawalls at a single timepoint in August 2020, and exclusion cages were installed to determine the influence of grazing pressure on colonisation; these were monitored for 18 months. Disturbance during winter resulted in higher concentrations of all biofilm components up to 3 months, but did not impact benthic community composition beyond this point. Grazer exclusion on artificial structures increased biofilm concentrations and influenced community composition in comparison to plots on artificial structures without exclusion, while communities on natural surfaces differed in terms of species composition to those on artificial plots at 12 months. We conclude that the timing of routine maintenance works on artificial structures may impact initial biofilm abundances. Furthermore, while grazing pressure does influence community structure on artificial structures, this alone is not sufficient to explain biological differences between artificial structures and natural shores.

Climate change negates positive CO2 effects on marine species biomass and productivity by altering the strength and direction of trophic interactions

One of the biggest challenges in more accurately forecasting the effects of climate change on future food web dynamics relates to how climate change affects multi-trophic species interactions, particularly when multiple interacting stressors are considered. Using a dynamic food web model, we investigate the individual and combined effect of ocean warming and acidification on changes in trophic interaction strengths (both direct and indirect) and the consequent effects on biomass structure of food web functional groups. To do this, we mimicked a species-rich multi-trophic-level temperate shallow-water rocky reef food web and integrated empirical data from mesocosm experiments on altered species interactions under warming and acidification, into food-web models. We show that a low number of strong temperature-driven changes in direct trophic interactions (feeding and competition) will largely determine the magnitude of biomass change (either increase or decrease) of high-order consumers, with increasing consumer biomass suppressing that of prey species. Ocean acidification, in contrast, alters a large number of weak indirect interactions (e.g. cascading effects of increased or decreased abundances of other groups), enabling a large increase in consumer and prey biomass. The positive effects of ocean acidification are driven by boosted primary productivity, with energy flowing up to higher trophic levels. We show that warming is a much stronger driver of positive as well as negative modifications of species biomass compared to ocean acidification. Warming affects a much smaller number of existing trophic interactions, though, with direct consumer-resource effects being more important than indirect effects. We conclude that the functional role of consumers in future food webs will be largely regulated by alterations in the strength of direct trophic interactions under ocean warming, with ensuing effects on the biomass structure of marine food webs.

A guideline to frame stressor effects in freshwater ecosystems

Environmental policies fall short in protecting freshwater ecosystems, which are heavily threatened by human pressures and their associated stressors. One reason is that stressor effects depend on the context in which they occur and it is difficult to extrapolate patterns to predict the effect of stressors without these being contextualized in a general frame. This study aims at improving existing decision-making frameworks such as the DPSIR approach (Driver-Pressure-State-Impact-Response) or ERA (Environmental Risk Assessment) in the context of stressors. Here, we delve into stressor-impact relationships in freshwater ecosystems and develop a guideline which includes key characteristics such as stressor type, stressor duration, location, the natural levels of environmental variables to which each ecosystem is used to, among others. This guideline is intended to be useful in a wide range of ecosystem conditions and stressors. Incorporating these guidelines may favor the comparability of scientific results and may lead to a substantial advancement in the efficacy of diagnosis and predictive approaches of impacts.

The ecological impacts of multiple environmental stressors on coastal biofilm bacteria

Ecological communities are increasingly exposed to multiple interacting stressors. For example, warming directly affects the physiology of organisms, eutrophication stimulates the base of the food web, and harvesting larger organisms for human consumption dampens top-down control. These stressors often combine in the natural environment with unpredictable results. Bacterial communities in coastal ecosystems underpin marine food webs and provide many important ecosystem services (e.g. nutrient cycling and carbon fixation). Yet, how microbial communities will respond to a changing climate remains uncertain. Thus, we used marine mesocosms to examine the impacts of warming, nutrient enrichment, and altered top-predator population size structure (common shore crab) on coastal microbial biofilm communities in a crossed experimental design. Warming increased bacterial α-diversity (18% increase in species richness and 67% increase in evenness), but this was countered by a decrease in α-diversity with nutrient enrichment (14% and 21% decrease for species richness and evenness, respectively). Thus, we show some effects of these stressors could cancel each other out under climate change scenarios. Warming and top-predator population size structure both affected bacterial biofilm community composition, with warming increasing the abundance of bacteria capable of increased mineralization of dissolved and particulate organic matter, such as Flavobacteriia, Sphingobacteriia, and Cytophagia. However, the community shifts observed with warming depended on top-predator population size structure, with Sphingobacteriia increasing with smaller crabs and Cytophagia increasing with larger crabs. These changes could alter the balance between mineralization and carbon sequestration in coastal ecosystems, leading to a positive feedback loop between warming and CO₂ production. Our results highlight the potential for warming to disrupt microbial communities and biogeochemical cycling in coastal ecosystems, and the importance of studying these effects in combination with other environmental stressors.

Multitrophic diversity sustains ecological complexity by dampening top-down control of a shallow marine benthic food web

Biodiversity is typically considered as a one-dimensional metric (e.g., species richness), yet the consequences of species loss may be different depending on where extinctions occur in the food web. Here, I used a manipulative field experiment in a temperate subtidal marine system to explore the implications of diversity loss at multiple trophic levels for ecosystem functioning and food web structure. The four manipulated predators included the small painted goby and common prawn, which are also fed on by the larger black goby and shore crab. Antagonistic interactions between the manipulated predators (e.g., intraguild predation, intimidation, interference competition) limited their negative effects on the rest of the food web. Top-down control was so suppressed at the highest level of multitrophic diversity that the resulting food webs were as complex and productive as those containing no manipulated predators. Negative interactions between the predators weakened as multitrophic diversity decreased, however, resulting in stronger consumption of lower trophic levels and a simpler food web with lower rates of two key ecosystem processes: primary production and decomposition. These results show how indirect interactions between predators on multiple trophic levels help to promote the complexity and functioning of natural systems.

Individual species provide multifaceted contributions to the stability of ecosystems

Exploration of the relationship between species diversity and ecological stability has occupied a prominent place in ecological research for decades. Yet, a key component of this puzzle-the contributions of individual species to the overall stability of ecosystems-remains largely unknown. Here, we show that individual species simultaneously stabilize and destabilize ecosystems along different dimensions of stability, and also that their contributions to functional (biomass) and compositional stability are largely independent. By simulating experimentally the extinction of three consumer species (the limpet Patella, the periwinkle Littorina and the topshell Gibbula) from a coastal rocky shore, we found that the capacity to predict the combined contribution of species to stability from the sum of their individual contributions varied among stability dimensions. This implies that the nature of the diversity-stability relationship depends upon the dimension of stability under consideration, and may be additive, synergistic or antagonistic. We conclude that, although the profoundly multifaceted and context-dependent consequences of species loss pose a significant challenge, the predictability of cumulative species contributions to some dimensions of stability provide a way forward for ecologists trying to conserve ecosystems and manage their stability under global change.

End-user involvement to improve predictions and management of populations with complex dynamics and multiple drivers

Sustainable management of wildlife populations can be aided by building models that both identify current drivers of natural dynamics and provide near-term predictions of future states. We employed a Strategic Foresight Protocol (SFP) involving stakeholders to decide the purpose and structure of a dynamic state-space model for the population dynamics of the Willow Ptarmigan, a popular game species in Norway. Based on local knowledge of stakeholders, it was decided that the model should include food web interactions and climatic drivers to provide explanatory predictions. Modeling confirmed observations from stakeholders that climate change impacts Ptarmigan populations negatively through intensified outbreaks of insect defoliators and later onset of winter. Stakeholders also decided that the model should provide anticipatory predictions. The ability to forecast population density ahead of the harvest season was valued by the stakeholders as it provides the management extra time to consider appropriate harvest regulations and communicate with hunters prior to the hunting season. Overall, exploring potential drivers and predicting short-term future states, facilitate collaborative learning and refined data collection, monitoring designs, and management priorities. Our experience from adapting a SFP to a management target with inherently complex dynamics and drivers of environmental change, is that an open, flexible, and iterative process, rather than a rigid step-wise protocol, facilitates rapid learning, trust, and legitimacy.

Novel co-occurrence of functionally redundant consumers induced by range expansion alters community structure

Ongoing climate change is shifting the geographic distributions of some species, potentially imposing rapid changes in local community structure and ecosystem functioning. Besides changes in population-level interspecific interactions, such range shifts may also cause changes in functional structure within the host assemblages, which can result in losses or gains in ecosystem functions. Because consumer-resource dynamics are central to community regulation, functional reorganization driven by introduction of new consumer species can have large consequences on ecosystem functions. Here we experimentally examine the extent to which the recent poleward range expansion of the intertidal grazer limpet Scurria viridula along the coast of Chile has altered the role of the resident congeneric limpet S. zebrina, and whether the net collective impacts, and functional structure, of the entire herbivore guild have been modified by the introduction of this new member. We examined the functional role of Scurria species in controlling ephemeral algal cover, bare rock availability, and species richness and diversity, and compared the effects in the region of range overlap against their respective "native" abutted ranges. Experiments showed depression of per capita effects of the range-expanded species within the region of overlap, suggesting environmental conditions negatively affect individual performance. In contrast, effects of S. zebrina were commonly invariant at its range edge. When comparing single species versus polycultures, effects on bare rock cover were altered by the presence of the other Scurria species, suggesting competition between Scurria species. Importantly, although the magnitude of S. viridula effects at the range overlap was reduced, its addition to the herbivore guild seems to complement and intensify the role of the guild in reducing green algal cover, species richness and increasing bare space provision. Our study thus highlights that range expansion of an herbivore can modify the functional guild structure in the recipient community. It also highlights the complexity of predicting how functional structure may change in the face of natural or human-induced range expansions. There is a need for more field-based examination of regional functional compensation, complementarity, or inhibition before we can construct a conceptual framework to anticipate the consequences of species range expansions.

Co-occurring predators increase biotic resistance against an invasive prey

The presence of multiple predators can lead to variation in predator behavior and ultimately altered risk for shared prey. This concept has seldom been accounted for in studies that consider predator-driven biotic resistance from native marine predators against invasive prey. This study compared the prey selection of whelks and rock lobsters when co-occurring and when foraging in isolation. When in isolation, both predators preferred the native mussel Choromytilus meridionalis, regardless of the abundance of alternative prey. However, when co-occurring, predation risk for all prey species, including the invasive mussel Semimytilus algosus, increased. This was largely driven by greater variation in prey selection by rock lobsters in the presence of whelks. This indicates that predatory efforts from co-occurring predators can result in stronger predation pressure on invasive prey than would be recognized if predators were assessed in isolation.

Modelling BPA effects on three-spined stickleback population dynamics in mesocosms to improve the understanding of population effects

Bisphenol A (BPA), a well-known endocrine-disrupting chemical, is ubiquitously present in the aquatic environment. Its impacts at the population level on three-spined sticklebacks (Gasterosteus aculeatus) have been studied in artificial streams with low-dose BPA concentrations. The causes explaining the observed effects remained unclear. Here, we used an individual-based model coupled to a Dynamic Energy Budget model to (i) assess the potential of modelling to predict impacts at the population level using individual level laboratory ecotoxicological endpoints and (ii) provide insight on the mechanisms of BPA toxicity in these mesocosms. To do that, both direct and indirect effects of BPA on three-spined sticklebacks were incorporated in the model. Indeed, direct BPA effects on fish have been identified based on literature data whereas indirect effects on sticklebacks have been taken into account using sampling data of their prey from the exposed artificial streams. Results of the modelling showed that direct BPA effects on fish (impacts on gonad formation, growth, male reproductive behavior, eggs and larvae survival) mainly explained the three-spined stickleback population structure in the mesocosms, but indirect effects were not negligible. Hence, this study showed the potential of modelling in risk assessment to predict the impacts on fish population viability from behavioral and physiological effects measured on organisms.

When Do Ecosystem Services Depend on Rare Species?

Conservation aims to preserve species and ecosystem services. If rare species contribute little to ecosystem services, yet are those most in need of preservation, tradeoffs may exist for these contrasting objectives. However, little attention has focused on identifying how, when, and where rare species contribute to ecosystem services and at what scales. Here, we review distinct ways that ecosystem services can positively depend on the presence, abundance, disproportionate contribution or, counterintuitively, the scarcity of rare species. By contrast, ecosystem services are less likely to depend on rare species that do not have a unique role in any service or become abundant enough to contribute substantially. We propose a research agenda to identify when rare species may contribute significantly to services.

Combined effects of warming and nutrients on marine communities are moderated by predators and vary across functional groups

Warming, nutrient enrichment and biodiversity modification are among the most pervasive components of human-induced global environmental change. We know little about their cumulative effects on ecosystems; however, even though this knowledge is fundamental to predicting and managing their consequences in a changing world. Here, we show that shifts in predator species composition can moderate both the individual and combined effects of warming and nutrient enrichment in marine systems. However, all three aspects of global change also acted independently to alter different functional groups in our flow-through marine rock-pool mesocosms. Specifically, warming reduced macroalgal biomass and assemblage productivity, whereas enrichment led to increased abundance of meso-invertebrate consumers, and loss of predator species led to increased gastropod grazer biomass. This disparity in responses, both across trophic levels (macroalgae and intermediate consumers), and between detecting additive effects on aggregate measures of ecosystem functioning, yet interactive effects on community composition, illustrates that our forecasting ability depends strongly on the level of ecological complexity incorporated within global change experiments. We conclude that biodiversity change-and loss of predator species in particular-plays a critical and overarching role in determining how ecological communities respond to stressors.

Water-quality impacts in semi-arid regions: can natural 'green filters' mitigate adverse effects on fish assemblages?

The effective aridity in riparian areas is increasing from climate change and from human water consumption, which exacerbates the impacts of effluents from wastewater-treatment plants and from catchment run-off in rivers. The potential of natural riparian areas to act as 'green filters' has long been recognized, but the possible ecological benefits of natural riparian areas over large-scale environmental gradients on fish have not been explored in detail. Using an extensive data-set from northeastern Spain (99,700 km², 15 catchments, 530 sites), ours is the first study to ask whether natural riparian vegetation can mitigate the effects of pollution on fish in rivers experiencing water scarcity. We used multimodel inference to explore the additive and interactive effects of riparian vegetation with nutrient pollution and water conductivity, which are among the world's worst river stressors, on multiple fish guilds, including widely distributed species and highly invasive alien fish species. Most models (54%) supported the additive effects of water-quality factors on fish, after having accounted for the influence of geography and hydrological alterations. Although many fewer models (7%) included riparian vegetation as an important predictor, riparian vegetation modulated the forms of the associations between fish and pollution. The relationship of nutrient pollution with native and alien fish richness changed from negative to positive with greater riparian structure or species richness. However, we found the opposite effect for the mean body size of sedentary fish, and only positive additive effects of riparian richness for the probability of occurrence of pelagic fish. Ammonium and nitrite concentrations adversely affected fish in these rivers up to 10 years after the enforcement of the implementation of the Water Framework Directive by the European Union. High conductivity also much affects fish, having negatives associations with migratory, pelagic, invertivorous and native fish, and positive associations with sedentary, benthic, omnivorous and alien fish. Therefore, the current status of natural riparian areas is unlikely to fully mitigate water-quality impacts on fish. The conservation of freshwater resources in semi-arid regions, such as north-eastern Spain, requires improved waste-water treatments and better agriculture practices.

Habitat with small inter-structural spaces promotes mussel survival and reef generation

Spatially complex habitats provide refuge for prey and mediate many predator-prey interactions. Increasing anthropogenic pressures are eroding such habitats, reducing their complexity and potentially altering ecosystem stability on a global scale. Yet, we have only a rudimentary understanding of how structurally complex habitats create ecological refuges for most ecosystems. Better informed management decisions require an understanding of the mechanisms underpinning the provision of physical refuge and this may be linked to prey size, predator size and predator identity in priority habitats. We tested each of these factors empirically in a model biogenic reef system. Specifically, we tested whether mortality rates of blue mussels (Mytilus edulis) of different sizes differed among: (i) different forms of reef structural distribution (represented as 'clumped', 'patchy' and 'sparse'); (ii) predator species identity (shore crab, Carcinus maenas and starfish, Asterias rubens); and (iii) predator size. The survival rate of small mussels was greatest in the clumped experimental habitat and larger predators generally consumed more prey regardless of the structural organisation of treatment. Small mussels were protected from larger A. rubens but not from larger C. maenas in the clumped habitats. The distribution pattern of structural objects, therefore, may be considered a useful proxy for reef complexity when assessing predator-prey interactions, and optimal organisations should be considered based on both prey and predator sizes. These findings are essential to understand ecological processes underpinning predation rates in structurally complex habitats and to inform future restoration and ecological engineering practices.

The Importance of Marine Predators in the Provisioning of Ecosystem Services by Coastal Plant Communities

Food web theory predicts that current global declines in marine predators could generate unwanted consequences for many marine ecosystems. In coastal plant communities (kelp, seagrass, mangroves, and salt marsh), several studies have documented the far-reaching effects of changing predator populations. Across coastal ecosystems, the loss of marine predators appears to negatively affect coastal plant communities and the ecosystem services they provide. Here, we discuss some of the documented and suspected effects of predators on coastal protection, carbon sequestration, and the stability and resilience of coastal plant communities. In addition, we present a meta-analysis to assess the strength and direction of trophic cascades in kelp forests, seagrasses, salt marshes, and mangroves. We demonstrate that the strength and direction of trophic cascades varied across ecosystem types, with predators having a large positive effect on plants in salt marshes, a moderate positive effect on plants in kelp and mangroves, and no effect on plants in seagrasses. Our analysis also identified that there is a paucity of literature on trophic cascades for all four coastal plant systems, but especially seagrass and mangroves. Our results demonstrate the crucial role of predators in maintaining coastal ecosystem services, but also highlights the need for further research before large-scale generalizations about the prevalence, direction, and strength of trophic cascade in coastal plant communities can be made.

Transition probabilities help identify putative drivers of community change in complex systems

Understanding the role of larger-scale processes in modulating the assembly, structure, and dynamics of communities is critical for forecasting the effects of climate-change and managing ecosystems. Developing this comprehensive perspective is difficult though, because species interactions are complex, interdependent, and dynamic through space and time. Typically, experiments focus on tractable subsets of interactions that will be most critical to investigate and explain shifts in communities, but qualitatively base these choices on experience, natural history, and theory. One quantitative approach to identify the putative forces regulating communities, without reducing system complexity, is estimating transition probabilities among species occupying space (i.e., multispecies Markov chain models). Although not mechanistic, these models estimate the relative frequency and importance of ecological pathways in community assembly and dynamics, and can serve as a framework to identify how pathways change across large scales and which are most important to investigate further. Here, we demonstrate this method in the Gulf of Maine (GOM) intertidal zone, where research has largely focused on the local-scale processes that influence communities, while the mechanisms responsible for more regional shifts in communities are less clear. Transition probabilities of faunal elements were quantified bimonthly for ~2.5 yr in local intertidal communities at three replicate sites in the southern, mid-coast, and northern GOM. Transitions related to mortality, colonization, and replacement by mussels, barnacles, red algae, and encrusting corallines differed regionally, suggesting specific pathways related to consumer pressure and recruitment vary across the GOM with shifting intertidal community structure. Combined with species abundance data and insights from previous research, we develop and evaluate the pathways by which communities likely change in the GOM. Species interactions in local communities can be complex, and this complexity should be incorporated into hypothesis building, experiments, theory, interpretations, and forecasts in ecology. Such a comprehensive approach will be critical to understand how regional shifts in local interactions can drive large-scale community change.

Loss of predator species, not intermediate consumers, triggers rapid and dramatic extinction cascades

Ecological networks are tightly interconnected, such that loss of a single species can trigger additional species extinctions. Theory predicts that such secondary extinctions are driven primarily by loss of species from intermediate or basal trophic levels. In contrast, most cases of secondary extinctions from natural systems have been attributed to loss of entire top trophic levels. Here, we show that loss of single predator species in isolation can, irrespective of their identity or the presence of other predators, trigger rapid secondary extinction cascades in natural communities far exceeding those generally predicted by theory. In contrast, we did not find any secondary extinctions caused by intermediate consumer loss. A food web model of our experimental system-a marine rocky shore community-could reproduce these results only when biologically likely and plausible nontrophic interactions, based on competition for space and predator-avoidance behaviour, were included. These findings call for a reassessment of the scale and nature of extinction cascades, particularly the inclusion of nontrophic interactions, in forecasts of the future of biodiversity.

Interspecies Interactions Reverse the Hazard of Antibiotics Exposure: A Plankton Community Study on Responses to Ciprofloxacin hydrochloride

The ecotoxicological effects of Ciprofloxacin hydrochloride (CIP) were tested on population densities of plankton assemblages consisting of two algae (Isochrysis galbana and Platymonas subcordiformis) and a rotifer (Brachionus plicatilis). The I. galbana showed a significant decrease in densities when concentrations of CIP were above 2.0 mg L⁻¹ in single-species tests, while P. subcordiformis and B. plicatilis were stable in densities when CIP were less than10.0 mg L⁻¹. The equilibrium densities of I. galbana in community test increased with CIP concentrations after falling to a trough at 5.0 mg L⁻¹, showed a completely different pattern of P. subcordiformis which decreased with CIP concentrations after reaching a peak at 30.0 mg L⁻¹. The observed beneficial effect was a result of interspecies interactions of trophic cascade that buffered for more severe direct effects of toxicants. The community test-based NOEC of CIP (2.0 mg L⁻¹), embodying the indirect effects, was different from the extrapolated one derived by single-species tests (0.5 mg L⁻¹), but all lacked confidence interval. A CIP threshold concentration of obvious relevance to ecological interaction was calculated with a simplified plankton ecological model, achieving a value of 1.26 mg L⁻¹ with a 95% bootstrapping confidence interval from 1.18 to 1.31 mg L⁻¹.

Apparent competition drives community-wide parasitism rates and changes in host abundance across ecosystem boundaries

Species have strong indirect effects on others, and predicting these effects is a central challenge in ecology. Prey species sharing an enemy (predator or parasitoid) can be linked by apparent competition, but it is unknown whether this process is strong enough to be a community-wide structuring mechanism that could be used to predict future states of diverse food webs. Whether species abundances are spatially coupled by enemy movement across different habitats is also untested. Here, using a field experiment, we show that predicted apparent competitive effects between species, mediated via shared parasitoids, can significantly explain future parasitism rates and herbivore abundances. These predictions are successful even across edges between natural and managed forests, following experimental reduction of herbivore densities by aerial spraying of insecticide over 20 hectares. This result shows that trophic indirect effects propagate across networks and habitats in important, predictable ways, with implications for landscape planning, invasion biology and biological control.

Species sharing a common enemy such as a parasitoid or predator can indirectly affect one another. Here, Frost et al. use quantitative food-web data from communities of caterpillar hosts to show experimentally that apparent competition is important in predicting food-web structure across habitats.

Positive and negative effects of mesograzers on early-colonizing species in an intertidal rocky-shore community

The ecological consequences of human‐driven overexploitation and loss of keystone consumers are still unclear. In intertidal rocky shores over the world, the decrease of keystone macrograzers has resulted in an increase in the dominance of herbivores with smaller body (i.e., “mesograzers”), which could potentially alter community assembly and structure. Here, we experimentally tested whether mesograzers affect the structure of rocky intertidal communities during the period of early colonization after the occurrence of a disturbance. A manipulative field experiment was conducted to exclude mesograzers (i.e., juvenile chitons, small snails, amphipods, and juvenile limpets) from experimental areas in an ecosystem characterized by the overexploitation of keystone macrograzers and predators. The results of multivariate analyses suggest that mesograzers had significant effects on intertidal community structure through negative and positive effects on species abundances. Mesograzers had negative effects on filamentous algae, but positive effects on opportunistic foliose algae and barnacles. Probably, mesograzers indirectly favored the colonization of barnacles and foliose algae by removing preemptive competitors, as previously shown for other meso‐ and macrograzer species. These results strongly support the idea that small herbivores exert a firm controlling effect on the assembly process of natural communities. Therefore, changes in functional roles of top‐down controllers might have significant implications for the structure of intertidal communities.

Single gene locus changes perturb complex microbial communities as much as apex predator loss

Many bacterial species are highly social, adaptively shaping their local environment through the production of secreted molecules. This can, in turn, alter interaction strengths among species and modify community composition. However, the relative importance of such behaviours in determining the structure of complex communities is unknown. Here we show that single-locus changes affecting biofilm formation phenotypes in Bacillus subtilis modify community structure to the same extent as loss of an apex predator and even to a greater extent than loss of B. subtilis itself. These results, from experimentally manipulated multitrophic microcosm assemblages, demonstrate that bacterial social traits are key modulators of the structure of their communities. Moreover, they show that intraspecific genetic variability can be as important as strong trophic interactions in determining community dynamics. Microevolution may therefore be as important as species extinctions in shaping the response of microbial communities to environmental change.

Some species of social bacteria can chemically modify their nutrient environments, which may influence community interactions. Here, McClean et al. show that changes at a single gene locus in a biofilm-forming bacteria can perturb community structure to the same extent as the loss of an apex predator.

The Effects of Petroleum Hydrocarbons on Algae Can Be Reversed in the Presence of a Primary Consumer

The ecotoxicological effects of a mixture of petroleum hydrocarbons were tested on densities of two algae, Platymonas helgolandica var. tsingtaoensis and Isochrysis galbana, and of a rotifer, Brachionus plicatilis, by single-species and customized community experiments. Test concentrations ranged from 0 to 100 mg L(-1), while five to seven treatments were assessed in triplicate within 1 month. A significant decrease in densities during single-species toxicity tests were found when concentrations of petroleum hydrocarbons were above 1.0 mg L(-1). However, equilibrium densities of algae in the customized community showed a different pattern, which increased with concentration and reached a peak at 20.0 mg L(-1). The community-based no observed effect concentration (NOEC; 1.0 mg L(-1)) was different from the NOEC derived by single-species toxic tests (0.25 mg L(-1)). This demonstrates that ecotoxicological effects on plankton as part of a community is significantly different from single-species toxicity tests owing to ecological interactions.