Functionally reversible impacts of disturbances on lake food webs linked to spatial and seasonal dependencies.

BackgroundSpecies recovery after disturbances depends on the strength and duration of disturbance, on the species traits and on the biotic interactions with other species. In order to understand these complex relationships, it is essential to understand mechanistically the transient dynamics of interacting species during and after disturbances. We combined microcosm experiments with simulation modelling and studied the transient recovery dynamics of a simple microbial food web under pulse and press disturbances and under different predator couplings to an alternative resource.ResultsOur results reveal that although the disturbances affected predator and prey populations by the same mortality, predator populations suffered for a longer time. The resulting diminished predation stress caused a temporary phase of high prey population sizes (i.e. prey release) during and even after disturbances. Increasing duration and strength of disturbances significantly slowed down the recovery time of the predator prolonging the phase of prey release. However, the additional coupling of the predator to an alternative resource allowed the predator to recover faster after the disturbances thus shortening the phase of prey release.ConclusionsOur findings are not limited to the studied system and can be used to understand the dynamic response and recovery potential of many natural predator–prey or host–pathogen systems. They can be applied, for instance, in epidemiological and conservational contexts to regulate prey release or to avoid extinction risk of the top trophic levels under different types of disturbances.

Disturbance regimes determine communities' structure and functioning. Nonetheless, little effort has been undertaken to understand interactions of press and pulse disturbances. In this context, leaf-shredding macroinvertebrates can be chronically exposed to wastewater treatment plant effluents (i.e., press disturbance) before experiencing pesticide exposure following agricultural runoff (i.e., pulse disturbance). It is assumed that wastewater pre-exposure alters animals' sensitivity to pesticides. To test this hypothesis, we exposed model-populations of the shredder Gammarus fossarum to wastewater at three field-relevant dilution levels (i.e., 0%, 50%, and 100%). After 2, 4, and 6 weeks, survival, leaf consumption, dry weight, and energy reserves were monitored. Additionally, animals were assessed for their sensitivity toward the neonicotinoid insecticide thiacloprid using their feeding rate as response variable. Both wastewater treatments reduced gammarids' survival, leaf consumption, dry weight, and energy reserves. Moreover, both wastewater pre-exposure scenarios increased animals' sensitivity toward thiacloprid by up to 2.5 times compared to the control. Our results thus demonstrate that press disturbance as posed by wastewater pre-exposure can enhance susceptibility of key players in ecosystem functioning to further (pulse) disturbances. Therefore, applying mitigation measures such as advanced treatment technologies seems sensible to support functional integrity in the multiple-stress situation.

Ecological stability encompasses multiple dimensions of functional and compositional responses to environmental change. Though no single stability dimension used in isolation can fully reflect the overall response to environmental change, a common vulnerability assessment that integrates simultaneously across multiple stability components is highly desirable for ecological risk assessment.We develop both functional and compositional counterparts of a novel, integrative metric of overall ecological vulnerability (OEV). We test the framework with data from a modularized experiment replicated in five lakes over two seasons, examining functional and compositional responses to both pulse and press disturbances across three trophic groups. OEV is measured as the area under the curve integrated over the entire observation period, with the curve delimiting the difference between the disturbance treatment and undisturbed parallel controls, expressed either as the log response ratio of biomass (functional OEV) or community dissimilarity index (compositional OEV).Both, functional and compositional OEV correlated negatively with functional and compositional ‘resistance’, ‘temporal stability’ and ‘final/extent of recovery’ following both pulse and press disturbances, though less so with ‘resilience’ following a pulse disturbance. We also found a positive correlation between functional and compositional OEV, which reveals the potential to also evaluate the intricate linkage between biodiversity and functional change.Our findings demonstrate that OEV comprises a robust framework to: (a) capture simultaneously multiple functional and compositional stability components, and (b) quantify the functional consequences of biodiversity change. Our results provide the basis for an overarching framework for quantifying the overall vulnerability of ecosystems to environmental change, opening new possibilities for ecological risk assessment and management.Synthesis. Ecological stability comprises multiple dimensions that together encapsulate how ecosystems respond to environmental change. Considering these multiple aspects of stability simultaneously often poses a problem in environmental assessments, which frequently require overarching indicators of risk or vulnerability. While an analysis of multiple dimensions allows for deeper exploration of mechanisms, here we develop and test a new univariate indicator that integrates stability aspects under a broad range of disturbance regimes. Using a modularized experiment in Swedish lakes, we show that this integrative measure captures multiple stability dimensions reflecting compositional and functional vulnerability and their relationships between them.

Disturbances are important determinants of diversity, and the combination of their aspects (e.g., disturbance intensity, frequency) can result in complex diversity patterns. Here, we leverage an important approach to classifying disturbances in terms of temporal span to understand the implications for species coexistence: pulse disturbances are acute and discrete events, while press disturbances occur continuously through time. We incorporate the resultant mortality rates into a common framework involving disturbance frequency and intensity. Press disturbances can be encoded into models in two distinct ways, and we show that the appropriateness of each depends on the type of data available. Using this framework, we compare the effects of pulse versus press disturbance on both asymptotic and transient dynamics of a two-species Lotka-Volterra competition model to understand how they engage with equalizing mechanisms of coexistence. We show that press and pulse disturbances differ in transient behavior, though their asymptotic diversity patterns are similar. Our work shows that these differences depend on how the underlying disturbance aspects interact and that the two ways of characterizing press disturbances can lead to contrasting interpretations of disturbance-diversity relationships. Our work demonstrates how theoretical modeling can strategically guide and help the interpretation of empirical work.

Species may reach novel habitats through dispersal but nevertheless be restricted in their establishment at different life history stages by local conditions. However, disturbances may alter local conditions within a habitat and hence change establishment outcomes. We investigated the establishment of two non-native weeds of pastures and roadsides, Carduus nutans and Carduus acanthoides, in three contiguous habitats (abandoned pasture, forest edge, and forest) under three disturbance treatments: no disturbance, initial disturbance only (pulse disturbance; i.e., temporary), and initial disturbance with weeding (press disturbance; i.e., sustained). For both species, establishment was not limited by seedling emergence in any habitat, although emergence in the pasture and forest edge relied heavily on disturbance. Combined emergence and survival in the pasture and forest edge were also significantly increased by disturbance, but disturbance had no effect in the forest, where seedlings were unable to ...

Tidal freshwater wetlands are critical for removing or sequestering watershed‐derived nitrogen loads before they reach the coast, where they can lead to eutrophication. However, rising seas and increasing climate variability will alter important physicochemical parameters that control nitrogen generation (e.g., nitrogen fixation) and removal processes (e.g., denitrification) in these habitats. Furthermore, the frequency and timing of these changes could vary from short, finite pulses during a storm or drought to long‐term presses from sea level rise, which may differentially affect biogeochemical cycling. We used intact core mesocosms to examine how microbial community structure and nitrogen cycling changed in response to increased temperature and salinity under pulse and press disturbances. We found that net N 2 flux rates, defined as the balance between nitrogen fixation, which adds nitrogen, and denitrification, which removes it, did not directionally change in response to stressor pulse or press. Instead, it became more variable under both disturbance regimes, underscoring the importance of both denitrification and nitrogen fixation in these systems. Nitrous oxide production rates, however, decreased and became more stable over time in the press scenario but remained highly variable in the pulse scenario. Under both pulse and press disturbance, both the overall and the active component of the microbial community changed, particularly in response to the salinity treatment. Although there was an overall community shift, core members of the microbiome capable of denitrification and nitrogen fixation persisted. Both pulses and presses of temperature and salinity changed the microbial communities of tidal freshwater wetlands, but a combination of microbial resistance and functional redundancy appears to allow important N cycling processes to persist. These findings provide valuable knowledge on the functional and structural potential of the nitrogen cycling microbial communities in tidal freshwater wetlands when facing future climate variability.

The carbon budget of Canadian forests: A sensitivity analysis of changes in disturbance regimes, growth rates, and decomposition rates

Moderate severity disturbances, those that do not result in stand replacement, play an essential role in ecosystem dynamics. Despite the prevalence of moderate severity disturbances and the significant impacts they impose on forest functioning, little is known about their effects on forest canopy structure and how these effects differ over time across a range of disturbance severities and disturbance types. Using longitudinal data from the National Ecological Observatory Network project, we assessed the effects of three moderate severity press disturbances (beech bark disease, hemlock woolly adelgid and emerald ash borer, which are characterized by continuous disturbance and sustained mortality) and three moderate severity pulse disturbances (spring cankerworm moth, spongy moth and ground fire, which are associated with discrete and relatively short mortalities) on temperate forest canopy structure in eastern US. We studied (1) how light detection and ranging (LiDAR)‐derived metrics of canopy structure change in response to disturbance and (2) whether initial canopy complexity offsets impact of disturbances on canopy structure over time. We used a mixed‐effects modelling framework which included a non‐linear term for time to represent changes in canopy structure caused by disturbance, and interactions between time and both disturbance intensity and initial canopy complexity. We discovered that high intensity of both press and pulse disturbances inhibited canopy height growth while low intensity pulse disturbances facilitated it. In addition, high intensity pulse disturbances facilitated increases in the complexity of the canopy over time. Concerning the impact of initial canopy complexity, we found that the initial canopy complexity of disturbed plots altered the effects of moderate disturbances, indicating potential resilience effects. Synthesis . This study used repeated measurements of LiDAR data to examine the effects of moderate disturbances on various dimensions of forest canopy structure, including height, openness, density and complexity. Our study indicates that both press and pulse disturbances can inhibit canopy height growth over time. However, while the impact of press disturbances on other dimensions of canopy structure could not be clearly detected, likely because of compensatory growth, the impact of pulse disturbances over time was more readily apparent using multi‐temporal LiDAR data. Furthermore, our findings suggest that canopy complexity might help to mitigate the impact of moderate disturbances on canopy structures over time. Overall, our research highlights the usefulness of multi‐temporal LiDAR data for assessing the structural changes in forest canopies caused by moderate severity disturbances.

As climate change intensifies, there is a pressing concern regarding how ecological communities respond to disturbances occurring at different intensities and time scales. We explored how the type of disturbance influences the dynamics of a marine community. A pulse disturbance is an abrupt, high-magnitude shift in conditions that can cause immediate and significant impacts to an ecological community. Alternatively, press disturbances are long-term, multi-generational pressures acting on communities over time. The Northeast US Continental Shelf Large Marine Ecosystem (NES LME) is one of the fastest-warming regions in the world and has experienced historic overfishing. Assemblage shifts in the NES LME have previously been characterized; however, these were prior to an unprecedented pulse disturbance marine heatwave (MHW) event in 2012 followed by punctuated MHWs over the last decade. We quantified community change across the NES LME using a community trajectory analysis, a multivariate tool that utilizes geometric analyses and comparisons of community trajectories, to quantify shifts in dynamic beta diversity. We hypothesized that the pulse MHWs would strongly influence ecosystem structure; however, no significant impact was detected. Our analysis indicates that the NES LME continues to tropicalize. However, it was not the pulse MHW events that seemed to drive change but rather ecosystem overfishing and rising temperatures. We quantified beta diversity over time in marine communities undergoing abrupt environmental changes and press disturbances. When expanded globally, this analysis can compare how variable disturbance pressures may result in different manifestations of beta diversity change within marine assemblages.

Soil microorganisms play an integral role in the regulation of carbon (C) cycling. In high-latitude ecosystems, climate warming is leading to higher plant productivity, shrub expansion and faster nutrient cycling; all of which increase resource availability to soil microorganisms. To understand how a legacy of enhanced resource availability affects the functional traits of microbial communities, and their feedbacks to further environment change, we collected soils from a field-experiment in a subarctic dry heath, where the consequences of climate warming were simulated by adding birch litter or inorganic N as either chronic additions during three years or as a single extreme addition. Soils were then re-exposed to the same resource or a modified resource environment in the laboratory and were monitored for 2 months. We hypothesized that a history of resource input would affect microbial functional profiles, which could result in two possibilities: 1) soil microbes exposed to a historical resource input would perform better when presented with the same resource, because the communities would be specialized to use the added resource, or 2) soil microbes would perform better when presented with a new resource, because the added resource would relieve the nutrient limitation induced by the previous resource input. We also hypothesized that with the same resource, a chronic and long-term input (i.e., a press disturbance) would select for K-strategists (i.e., fungi), while a sudden and large input (i.e., a pulse disturbance) would select for r-strategists (i.e., bacteria). We observed that bacteria in soils exposed to a history of N input showed a stronger growth response to new litter addition, while fungi in soils with a history of litter input showed a stronger growth response to both new litter and new N additions. When presented with new litter, the increase of fungal growth in soil from the extreme litter field-treatment was lower than in the chronic litter field-treatment, demonstrating that a pulse disturbance could weaken the stimulation of fungal growth. When presented with new litter, the increases of bacterial growth did not differ between the chronic N field-treatment and the extreme N field-treatment, suggesting that bacterial responses were not favoured by a press disturbance. We conclude that the enhanced resource availabilities expected in warming arctic soils will generate a positive microbial feedback to climate change.

While previous studies have investigated environmental and biotic variables influencing plankton community composition, comprehensive analyses across multiple trophic levels and broad spatial scales remain limited. Using a Joint Species Distribution Modelling framework, we identified key abiotic and biotic variables, including body size and fish presence that shape variation in plankton food webs across Canadian lakes. We analyzed the joint responses of plankton biomass to lake morphometry, water physico‐chemistry, and fish species presence, using data collected from 301 lakes spanning the four main national continental catchments. We also examined how the body size trait modulated plankton food web associations with these variables. Results showed that lake nutrient and ion status were the most important factors explaining variation in plankton community composition, particularly in the Arctic and Hudson continental basins, which exhibited large environmental gradients. Lake morphometry also played an important role, especially in shaping communities in large, shallow lakes. Plankton body sizes modulated some niche responses, but these effects varied across continental basins and plankton trophic levels. Little residual variation in the models indicated limited roles of plankton species interactions or unmeasured environmental variables. While fish presence only explained small amounts of variation, further studies should instead assess and incorporate fish biomass data. Our findings suggest that future national strategies for the study of Canadian freshwaters should combine a continental‐scale perspective (e.g., gradients across ecozones, climate regions, biogeographic zones) with regionally focused monitoring programs to better capture critical factors influencing different lake ecosystems.

Interactions between press and pulse disturbances can significantly impact benthic macroinvertebrate (crayfish) populations. Press disturbances such as land-use change can make crayfish more vulnerable to the detrimental effects of pulse disturbances, by changing the habitats available to crayfish. The impact of a pulse disturbance, a major flood (1 in 28 years return period), on crayfish (Paranephrops planifrons) was significantly greater in pasture than native forest streams. Population densities of crayfish in three forest and three pasture streams had been measured seasonally for 2 years prior to the flood and many crayfish had been marked with individually identifiable tags. Low numbers of marked crayfish were recaptured after the flood in the forest streams, but no marked crayfish were identified in any pasture stream. Crayfish densities in one pasture stream declined from an average of 5 m−2 prior to the flood to <1 m−2 soon after the flood and it took 3 years for the population to show evidence of recovery. Macrophytes and cobbles, the dominant habitats of crayfish in pasture streams, did not appear to provide stable refugia during the flood. Habitat stability was linked to the riparian zone in forest streams where undercut banks, tree roots, and pools were important habitats for crayfish. Frequent pulse disturbance could affect population persistence of refugia-dependent species when the pressure of land-use change affects the stability of habitats, but this may only be evident over long time scales.

AimClimate‐induced pulse (e.g., hurricanes) and press (e.g., global warming) disturbances represent threats to populations, communities, and the ecosystem services that they provide. We leveraged three decades of annual data on tropical gastropods to quantify the effects of major hurricanes, associated secondary succession, and global warming on abundance, biodiversity, and species composition.LocationLuquillo Mountains, Puerto Rico.MethodsGastropod abundance, biodiversity, and composition were estimated annually for each of 27 years in a tropical montane forest that experienced three major hurricanes (Hugo, Georges, and Maria). Generalized linear mixed‐effects, linear mixed‐effects, and linear models evaluated population‐ and community‐level responses to year, ambient temperature, understorey temperature, hurricane, and time since hurricane. Variation partitioning determined the unique and shared variation in biotic responses associated with temperature, disturbance, and succession.ResultsRather than declining, gastropod abundances generally increased through time, whereas the responses of biodiversity were weak and scale dependent. Hurricanes and associated secondary succession, rather than ambient atmospheric temperature, moulded long‐term trends in abundances and biodiversity.Main conclusionsGlobal warming over the past 30 years has not progressed sufficiently to elicit significant responses by gastropods in the Luquillo Mountains. Rather, effects from pulse disturbances (i.e., hurricanes) and secondary succession currently drive long‐term variation in abundance and biodiversity. Gastropods evince high resilience in this tropical ecosystem. Historical exposure to recurrent hurricanes likely imbued the fauna with broad niches that make them resistant to current levels of global warming. We predict that biotic resiliency will be challenged once changes in temperature exceed interannual and inter‐habitat differences that typify this hurricane‐mediated system, or combine with an increased frequency of hurricanes and droughts to alter associations among environmental characteristics that define the fundamental niches of species. Only then might significant declines in abundance or the appearance of novel communities characterize the gastropod fauna in the Luquillo Mountains.

Dams are known to impact river channels and ecosystems, both during their lifetime and in their decommissioning. In this study, we applied a before-after-control-impact design associated with two small dam removals to investigate abiotic and biotic recovery trajectories from both the elimination of the press disturbance associated with the presence of dams and the introduction of a pulse disturbance associated with removal of dams. The two case studies represent different geomorphic and ecological conditions that we expected to represent low and high sensitivities to the pulse disturbance of dam removal: the 4 m tall, gravel-filled Brownsville Dam on the wadeable Calapooia River and the 12.5 m tall, sand and gravel-filled Savage Rapids Dam on the largely non-wadeable Rogue River. We evaluated both geomorphic and ecological responses annually for two years post removal, and asked if functional traits of the macroinvertebrate assemblages provided more persistent signals of ecological disturbance than taxonomically defined assemblages over the period of study. Results indicate that: 1) the presence of the dams constituted a strong ecological press disturbance to the near-downstream reaches on both rivers, despite the fact that both rivers passed unregulated flow and sediment during the high flow season; 2) ecological recovery from this press disturbance occurred within the year following the restoration action of dam removal, whereas signals of geomorphic disturbance from the pulse of released sediment persisted two years post-removal, and 3) the strength of the press disturbance and the rapid ecological recovery were detected regardless of whether recovery was assessed by taxonomic or functional assemblages and for both case studies, in spite of their different geomorphic settings.

Microorganisms are critically important for the function of surface water ecosystems but are frequently subjected to anthropogenic disturbances at either acute (pulse) or long-term (press) scales. Response and recovery of microbial community composition and function following pulse disturbance is well-studied in controlled, laboratory scale experiments but is less well-understood in natural environments undergoing continual press disturbance. The objectives of this study were to determine the drivers of sediment microbial compositional and functional changes in freshwaters receiving continual press disturbance from agricultural land runoff and to evaluate the ability of the native microbial community to resist disturbance related changes as a proxy for freshwater ecosystem health. Freshwater sediments were collected seasonally over 1 year in Kewaunee County, Wisconsin, a region impacted by concentrated dairy cattle farming, manure fertilization, and associated agricultural runoff which together serve as a press disturbance. Using 16S rRNA gene amplicon sequencing, we found that sediments in locations strongly impacted by intensive agriculture contain significantly higher abundances (p < 0.01) of the genera Thiobacillus, Methylotenera, Crenotrhix, Nitrospira, and Rhodoferax compared to reference sediments, and functions including nitrate reduction, nitrite reduction, and nitrogen respiration are significantly higher (p < 0.05) at locations in close proximity to large farms. Nine species-level potential human pathogens were identified in riverine sediments including Acinetobacer lwoffi and Arcobacter skirrowii, two pathogens associated with the cattle microbiome. Microbial community composition at locations in close proximity to intensive agriculture was not resistant nor resilient to agricultural runoff disturbance within 5 months post-disturbance but did reach a new, stable microbial composition. From this data, we conclude that sediment microbial community composition is sensitive and shifts in response to chemical and microbial pollution from intensive agriculture, has a low capacity to resist infiltration by non-native, harmful bacteria and, overall, the natural buffering capacity of freshwater ecosystems is unable to fully resist the impacts from agricultural press disturbance.