Diversity-disturbance Relationship Research Articles

Anti-seasonal flooding drives substantial alterations in riparian plant diversity and niche characteristics in a unique hydro-fluctuation zone.

Human-induced disturbances such as dam construction and regulation have led to widespread alterations in hydrological processes and thus substantially influence plant characteristics in the hydro-fluctuation zones (HFZs). To reveal utilization of limited resources and mechanisms of inter-specific competition and species co-existence of plant communities based on niche breadth and overlap under the different HFZs of the Three Gorges Reservoir (TGR) in China, we conducted a field investigation with 368 quadrats on the effects of hydrological alterations on plant diversity and niche characteristics. The results showed anti-seasonal flooding precipitated the gradual disappearance of the original diverse niches, resulting in the reduction of plant species richness and functional diversity and more obvious competition among plant species with similar resource requirements. Annuals, perennials and shrubs accounted for 71.23%, 27.39% and 1.37%, respectively, suggesting that annuals and flood-tolerant riparian herbs were favored under such novel flooding conditions. A consistent increase in species number, Shannon-Wiener diversity index and Simpson dominance index with altitude was inconsistent with hump-shaped diversity-disturbance relationship of the intermediate disturbance hypothesis, while the opposite trend was observed for the Pielou evenness index. This species distribution pattern might be caused by several synergetic attributes (e.g., the submergence depth, plant tolerant capacity to flooding, life form, dispersal mode and inter-specific competition). Vegetation types shifted from xerophytes to mesophytes and eventually to hygrophytes with the increasing flooding time in the HFZs. Hydrological alterations proved to be the paramount driver of vegetation distribution in the different HFZs. The niche analysis provided the first insights on the mechanisms of resource utilization and inter-specific competition, of which annuals could germinate quickly after soil drainage to achieve the greatest competitive advantages and occupy a larger niche space than other plants. Vegetation was still in the early stage of primary succession in the novel riparian forests. Therefore, vegetation restoration strategies should be biased towards herbaceous plants, due to annuals with better environmental adaptability, supplemented by shrubs and small trees. To establish a complete reference system for vegetation restoration, natural vegetation monitory plots in the different succession stages should be established in the different HFZs of the TGR, and their environmental conditions, community structures and inter-specific relationships further analyzed.

Disturbance-diversity relationships of microbial communities change based on growth substrate.

Disturbance events can impact ecological community dynamics. Understanding how communities respond to disturbances and how those responses can vary is a challenge in microbial ecology. In this study, we grew a previously enriched specialized microbial community on either cellulose or glucose as a sole carbon source and subjected them to one of five different disturbance regimes of varying frequencies ranging from low to high. Using 16S rRNA gene amplicon sequencing, we show that the community structure is largely driven by substrate, but disturbance frequency affects community composition and successional dynamics. When grown on cellulose, bacteria in the genera Cellvibrio, Lacunisphaera, and Asticcacaulis are the most abundant microbes. However, Lacunisphaera is only abundant in the lower disturbance frequency treatments, while Asticcacaulis is more abundant in the highest disturbance frequency treatment. When grown on glucose, the most abundant microbes are two Pseudomonas sequence variants and a Cohnella sequence variant that is only abundant in the highest disturbance frequency treatment. Communities grown on cellulose exhibited a greater range of diversity (1.95-7.33 Hill 1 diversity) that peaks at the intermediate disturbance frequency treatment or one disturbance every 3 days. Communities grown on glucose, however, ranged from 1.63 to 5.19 Hill 1 diversity with peak diversity at the greatest disturbance frequency treatment. These results demonstrate that the dynamics of a microbial community can vary depending on substrate and the disturbance frequency and may potentially explain the variety of diversity-disturbance relationships observed in microbial systems.IMPORTANCEA generalizable diversity-disturbance relationship (DDR) of microbial communities remains a contentious topic. Various microbial systems have different DDRs. Rather than finding support or refuting specific DDRs, we investigated the underlying factors that lead to different DDRs. In this study, we measured a cellulose-enriched microbial community's response to a range of disturbance frequencies from high to low, across two different substrates: cellulose and glucose. We demonstrate that the community displays a unimodal DDR when grown on cellulose and a monotonically increasing DDR when grown on glucose. Our findings suggest that the same community can display different DDRs. These results suggest that the range of DDRs we observe across different microbial systems may be due to the nutritional resources microbial communities can access and the interactions between bacteria and their environment.

In Memoriam : Edward O. Wilson (1929-2021): It All Started with Ants.

In Memoriam : Edward O. Wilson (1929-2021): It All Started with Ants.

The effect of natural disturbances on forest biodiversity: an ecological synthesis.

Disturbances alter biodiversity via their specific characteristics, including severity and extent in the landscape, which act at different temporal and spatial scales. Biodiversity response to disturbance also depends on the community characteristics and habitat requirements of species. Untangling the mechanistic interplay of these factors has guided disturbance ecology for decades, generating mixed scientific evidence of biodiversity responses to disturbance. Understanding the impact of natural disturbances on biodiversity is increasingly important due to human-induced changes in natural disturbance regimes. In many areas, major natural forest disturbances, such as wildfires, windstorms, and insect outbreaks, are becoming more frequent, intense, severe, and widespread due to climate change and land-use change. Conversely, the suppression of natural disturbances threatens disturbance-dependent biota. Using a meta-analytic approach, we analysed a global data set (with most sampling concentrated in temperate and boreal secondary forests) of species assemblages of 26 taxonomic groups, including plants, animals, and fungi collected from forests affected by wildfires, windstorms, and insect outbreaks. The overall effect of natural disturbances on α-diversity did not differ significantly from zero, but some taxonomic groups responded positively to disturbance, while others tended to respond negatively. Disturbance was beneficial for taxonomic groups preferring conditions associated with open canopies (e.g. hymenopterans and hoverflies), whereas ground-dwelling groups and/or groups typically associated with shady conditions (e.g. epigeic lichens and mycorrhizal fungi) were more likely to be negatively impacted by disturbance. Across all taxonomic groups, the highest α-diversity in disturbed forest patches occurred under moderate disturbance severity, i.e. with approximately 55% of trees killed by disturbance. We further extended our meta-analysis by applying a unified diversity concept based on Hill numbers to estimate α-diversity changes in different taxonomic groups across a gradient of disturbance severity measured at the stand scale and incorporating other disturbance features. We found that disturbance severity negatively affected diversity for Hill number q= 0 but not for q= 1 and q= 2, indicating that diversity-disturbance relationships are shaped by species relative abundances. Our synthesis of α-diversity was extended by a synthesis of disturbance-induced change in species assemblages, and revealed that disturbance changes the β-diversity of multiple taxonomic groups, including some groups that were not affected at the α-diversity level (birds and woody plants). Finally, we used mixed rarefaction/extrapolation to estimate biodiversity change as a function of the proportion of forests that were disturbed, i.e. the disturbance extent measured at the landscape scale. The comparison of intact and naturally disturbed forests revealed that both types of forests provide habitat for unique species assemblages, whereas species diversity in the mixture of disturbed and undisturbed forests peaked at intermediate values of disturbance extent in the simulated landscape. Hence, the relationship between α-diversity and disturbance severity in disturbed forest stands was strikingly similar to the relationship between species richness and disturbance extent in a landscape consisting of both disturbed and undisturbed forest habitats. This result suggests that both moderate disturbance severity and moderate disturbance extent support the highest levels of biodiversity in contemporary forest landscapes.

Competition-colonization dynamics and multimodality in diversity-disturbance relationships.

Disturbance has long been recognized as a critical driver of species diversity in community ecology. Recently, it has been found that the well-known intermediate disturbance hypothesis, which predicts a unimodal diversity-disturbance relationship (DDR), fails to describe numerous experimental observations, as empirical DDRs are diverse. Consequently, the precise form of the DDR remains a topic of debate. Here we develop a simple yet comprehensive metacommunity framework that can account for complex competition patterns. Using both numerical simulations and analytical arguments, we show that strongly multimodal DDRs arise naturally, and this multimodality is quite robust to changing parameters or relaxing the assumption of a strict competitive hierarchy. Having multimodality as a robust property of DDRs in competition models suggests that much of the noise observed in empirical DDRs could be a critical signature of the underlying competitive dynamics.

Fire disturbance effects on plant taxonomic and functional β-diversity mediated by topographic exposure.

Although the diversity–disturbance relationship has been extensively studied, the differences in responses of taxonomic vs. functional diversity to natural disturbances (i.e., fire) call for an improved understanding of this relationship. Here, we investigated how fire disturbance influenced plant taxonomic and functional diversity in Golestan National Park, in northeastern Iran. We evaluated the response of α‐ and β‐plant diversity considering both taxonomic and functional diversity and different β‐diversity components (i.e., turnover and nestedness) as a function of fire regime, topographic exposure, and their interactive effect. We considered different indices of functional diversity including functional richness, functional evenness, functional divergence, functional dispersion, Rao's quadratic entropy, and community‐weighted mean (CWM). Functional diversity indices were computed using four leaf traits related to species growth strategy and fire response including leaf thickness and leaf length, specific leaf area (SLA) and leaf dry matter content (LDMC).Taxonomic and functional diversity had contrasting response to fire disturbance. Fire significantly decreased taxonomic α‐diversity similarly in both north and south exposures. β‐diversity increased in south exposures but decreased in north exposures. Fire decreased functional richness, increased CWM of SLA, and decreased CWM of LDMC. In contrast, abundance‐weighted metrics of functional diversity (functional evenness, functional divergence, functional dispersion, Rao's quadratic entropy) were not impacted by fire disturbance. Finally, the main contributors to heterogeneity were driven by a fire × exposure interaction, suggesting that fire disturbance interacts with topographic exposure.Our results suggest that taxonomic and functional α‐ and β‐diversity have contrasting responses to fire illustrating the need to consider both dimensions to understand how disturbance impacts plant communities. At large spatial scale, species turnover and nestedness appear as essential parameters to maintain species‐rich communities in response to fire disturbance.

Environmental fluctuations reshape an unexpected diversity-disturbance relationship in a microbial community.

Environmental disturbances have long been theorized to play a significant role in shaping the diversity and composition of ecosystems. However, an inability to specify the characteristics of a disturbance experimentally has produced an inconsistent picture of diversity-disturbance relationships (DDRs). Here, using a high-throughput programmable culture system, we subjected a soil-derived bacterial community to dilution disturbance profiles with different intensities (mean dilution rates), applied either constantly or with fluctuations of different frequencies. We observed an unexpected U-shaped relationship between community diversity and disturbance intensity in the absence of fluctuations. Adding fluctuations increased community diversity and erased the U-shape. All our results are well-captured by a Monod consumer resource model, which also explains how U-shaped DDRs emerge via a novel 'niche flip' mechanism. Broadly, our combined experimental and modeling framework demonstrates how distinct features of an environmental disturbance can interact in complex ways to govern ecosystem assembly and offers strategies for reshaping the composition of microbiomes.

Latitudinal variation in the diversity–disturbance relationship demonstrates the context dependence of disturbance impacts

AbstractAimThe shape of the diversity–disturbance relationship is context dependent, but the mechanisms driving this context dependence are unclear due to limited standardized empirical assessments across different environmental and ecological settings. At seven sites and over 20° of latitude, spanning both temperate and tropical systems, we measured community response to a fine‐scale experimental disturbance gradient in communities of benthic invertebrates.LocationEast coast of Australia (16.48–36.43° S).Time periodSixty‐six weeks (August 2012–November 2013).Major taxa studiedBenthic marine invertebrates.MethodsCommunities were grown on acrylic tiles and subjected to four experimental disturbance events over a 66‐week period. At the end of this period, the effect of increasing disturbance extent (from 0% to 100% community clearance) on measures of alpha and beta diversity were assessed.ResultsIn the tropics, richness and diversity increased with disturbance because the creation of free space allowed for the colonization of new recruits. In temperate communities, increasing disturbance led to a monotonic loss in species richness and a more hump‐shaped relationship in evenness and diversity. In the tropics, compositional change with increasing disturbance was driven by changes in species identities and abundances as the cover of rare species increased. In temperate regions, however, differences in species abundances were primarily responsible for compositional changes, reflecting the shift to algal dominance in conditions of high disturbance.Main conclusionsThe shape of the diversity–disturbance relationship changed over latitude, indicating that the concept of “intermediate” disturbance differs amongst these communities. Faster rates of increases in bare space, loss of species richness and a shift to algal dominance suggest that, in these systems, higher‐latitude communities are more susceptible to negative outcomes of high levels of disturbance than lower‐latitude communities. We discuss how differences in community dynamics, including competition, regional diversity and availability of propagules, help to explain this pattern.

Forests affected by frequent and intense typhoons challenge the intermediate disturbance hypothesis

AbstractTropical cyclones (hurricanes and typhoons) are extreme disturbances that have a significant impact on ecosystem structure and processes. The intermediate disturbance hypothesis (IDH) generalizes disturbance–diversity relationships but its validity is hotly debated. The IDH has been challenged both theoretically and with results from experimental studies; however, few studies have empirically tested the proposed mechanisms of IDH using results from the actual ecosystems. In this commentary, based on empirical observations from studies on the interactions between tropical cyclones and forest dynamics, we outlined seven possible outcomes of gap dynamics and tree diversity as a result of different frequency and intensity combinations. We argue that the lack of distinction and differentiation between disturbance intensity and severity, and the overlooked role of tree mortality, seedling recruitment, and tree adaptations limit the applicability of IDH in predicting the disturbance–diversity relationship. In the era of climate change characterized with more frequent climate extremes and natural disturbances, we should move beyond the generalizations and directly address the processes leading to the observed disturbance–diversity relationships to make reliable predictions.Abstract in Chinese is available with online material.

Genetic diversity of Halodule wrightii is resistant to large scale dieback: a case study from the Indian River Lagoon

There is a reciprocal relationship between disturbance and diversity such that disturbance can result in changes to diversity that in turn impact a population’s response to disturbance. Seagrasses are systems that are experiencing increasing disturbances and loss, and it is therefore important to understand this diversity–disturbance relationship. In this study, we observe changes in diversity and population differentiation of shoal grass (Halodule wrightii) during a large scale dieoff in the Indian River Lagoon, Florida USA. While allelic richness and heterozygosity were relatively high and do not change over time, population differentiation increased (estimated using FST), indicating genetic drift. This maintenance of diversity is important since seagrasses with high genetic diversity have been shown to withstand several environmental disturbances (grazing, low light, and high temperature) better than meadows with low diversity. This small increase in differentiation was only detectable because of replicate collection over time, which is rare in the literature and in monitoring programs. However, it is important since it indicates potential dispersal limitation which could hinder large scale recovery. We recommend plant nurseries as management tools in this system to preserve existing diversity and to aid in large scale restoration.

How multiple and interacting disturbances shape tree diversity in European mountain landscapes

The relationship between disturbances and diversity remains uncertain, especially in forest landscapes where large spatial extents need to be considered, dynamics are slow, and disturbance interactions are common. To analyse the individual and combined effects of ungulate browsing and wind disturbance on tree species diversity at the landscape scale. We used a dynamic forest landscape model to simulate the effects of browsing and wind disturbances (i.e., frequency and windthrow size) on tree species diversity in four mountain landscapes in Central Europe. Using boosted regression trees, we analysed the relative importance of each disturbance type for diversity at different layers (i.e., regeneration versus overstorey, and all tree sizes), the shapes of the diversity–disturbance relationships and the combined effect of wind and browsing disturbances on diversity. Across all landscapes, browsing and windthrow were equally important for tree species diversity when considering all forest layers, but no consistent patterns could be observed for the regeneration and overstorey layer. The shape of the disturbance–diversity relationships differed between disturbance types. More frequent and severe windthrow events typically increased diversity almost linearly, while browsing showed a non-linear response with the highest diversity at intermediate browsing pressure. However, these relationships were not consistent across the four landscapes. Tree species diversity can be influenced by both browsing and windthrow disturbances. Forested landscapes are likely to experience multiple disturbances, and their relative influence on diversity needs to consider their different spatial and temporal scales.

Species turnover differentiates diversity–disturbance relationships between aboveground vegetation and soil seedbank

An understanding of relationships between species diversity and disturbance gradient is important to comprehend the role of disturbances in the structure of plant communities. Although some studies have demonstrated incongruence in diversity–disturbance relationships (hereafter DDRs) between aboveground vegetation and soil seedbank, the process that causes the difference remains unclear. This incongruence between the two DDRs could result in the decrease in the source of recovery of aboveground vegetation following disturbances being overlooked when only aboveground vegetation is surveyed. Here, we verified a process that species turnover across the disturbance gradient causes the incongruence. Based on a vegetation and seedbank survey, we examined DDRs and species turnover of aboveground vegetation and seedbank along disturbance duration (i.e., excluding years of ungulate grazing). The degree of species turnover was considerably greater in aboveground vegetation than in seedbank; thus, the degree of species turnover along a disturbance gradient caused the difference in DDR between aboveground vegetation and seedbank.

Intermediate disturbance promotes termite functional diversity in intensively managed Vietnamese coffee agroecosystems

The intensive agricultural practices used in coffee plantations have profound impacts on invertebrate biodiversity. We surveyed termite diversity in the central highlands of Vietnam and evaluated it relative to the in situ ecological conditions of the coffee farms. Two survey sites were established at farms that were in place for 1 month and 1, 3, 5, and 25 years at the time of sampling. In addition, two models were tested: the diversity–resource relationship and the diversity–disturbance relationship. Our results demonstrated that the loss of termite diversity due to land perturbation in newly cultivated coffee farms may be as high as 86% compared with the nearest forested site. Only two feeding groups of termite (Group II and Group III) were present in the study sites. Termite composition of young and old farms was similar, but they only shared 48% similarity and differed significantly from the composition of the 1, 3, and 5 year old coffee farms. Understory vegetation cover and moisture content were positively associated with the occurrence of Group II and III termites but negatively associated with soil bulk density. Termite species richness did not increase linearly with the increased biomass production (plant litter) that is characteristic of old coffee farms. In contrast, termite species richness and occurrence were related to the intensity of farm management. Farms subjected to an intermediate level of management intensity due to annual crop cultivation recorded the highest diversity. Our study highlights the importance of annual crop cultivation to enhance termite diversity.

Sharma, Lila Nath. 2016. The disturbance-diversity relationship: integrating biodiversity conservation and resource management in anthropogenic landscapes

Sharma, Lila Nath. 2016. The disturbance-diversity relationship: integrating biodiversity conservation and resource management in anthropogenic landscapes

Disturbance Regimes Predictably Alter Diversity in an Ecologically Complex Bacterial System.

ABSTRACTDiversity is often associated with the functional stability of ecological communities from microbes to macroorganisms. Understanding how diversity responds to environmental perturbations and the consequences of this relationship for ecosystem function are thus central challenges in microbial ecology. Unimodal diversity-disturbance relationships, in which maximum diversity occurs at intermediate levels of disturbance, have been predicted for ecosystems where life history tradeoffs separate organisms along a disturbance gradient. However, empirical support for such peaked relationships in macrosystems is mixed, and few studies have explored these relationships in microbial systems. Here we use complex microbial microcosm communities to systematically determine diversity-disturbance relationships over a range of disturbance regimes. We observed a reproducible switch between community states, which gave rise to transient diversity maxima when community states were forced to mix. Communities showed reduced compositional stability when diversity was highest. To further explore these dynamics, we formulated a simple model that reveals specific regimes under which diversity maxima are stable. Together, our results show how both unimodal and non-unimodal diversity-disturbance relationships can be observed as a system switches between two distinct microbial community states; this process likely occurs across a wide range of spatially and temporally heterogeneous microbial ecosystems.

Disturbance Increases Microbial Community Diversity and Production in Marine Sediments.

Disturbance strongly impacts patterns of community diversity, yet the shape of the diversity-disturbance relationship remains a matter of debate. The topic has been of interest in theoretical ecology for decades as it has practical implications for the understanding of ecosystem services in nature. One of these processes is the remineralization of organic matter by microorganisms in coastal marine sediments, which are periodically impacted by disturbances across the sediment-water interface. Here we set up an experiment to test the hypothesis that disturbance impacts microbial diversity and function during the anaerobic degradation of organic matter in coastal sediments. We show that during the first 3 weeks of the experiment, disturbance increased both microbial production, derived from the increase in microbial abundance, and diversity of the active fraction of the community. Both community diversity and phylogenetic diversity increased, which suggests that disturbance promoted the cohabitation of ecologically different microorganisms. Metagenome analysis also showed that disturbance increased the relative abundance of genes diagnostic of metabolism associated with the sequential anaerobic degradation of organic matter. However, community composition was not impacted in a systematic way and changed over time. In nature, we can hypothesize that moderate storm disturbances, which impact coastal sediments, promote diverse, and productive communities. These events, rather than altering the decomposition of organic matter, may increase the substrate turnover and, ultimately, remineralization rates.

Functional response of leaf- and planthoppers to modern fertilisation and irrigation of hay meadows

In the drier regions of the European Alps, traditional land-use of montane and subalpine meadows, i.e. extensively managed with solid manure application and irrigation via water channels, is currently shifting towards application of slurry and aerial irrigation. The impact of these new practices upon biodiversity remains poorly understood and calls for quantitative assessments of their effects. Relying on a full block design, we tested the effects of six management treatments corresponding to an increasing gradient of farming intensification (fertilisation with slurry and/or irrigation with sprinklers) on Auchenorrhyncha (Hemiptera) communities occurring in the inner Swiss Alps. The experimental set up consisted of: control plots (no fertiliser, no irrigation; C-plots); plots that received only fertiliser (F-plots); plots that were only irrigated (I-plots); and plots that received low-, medium- and high-input of fertiliser and water (F+I 1/3-plots; F+I 2/3, F+I 3/3-plots; 3/3 corresponds here to the input level necessary for achieving maximum theoretical hay yield locally). After two years of experimental treatment (2012), plots that were only fertilised or only irrigated showed no change in the population sizes of Auchenorrhyncha, while plots that received low-, medium- and high-input of fertiliser and water harboured significantly higher abundances (1.9, 1.5 and 1.4 times higher, respectively), biomass (1.8, 1.6 and 1.8 times higher, respectively), as well as species richness (+27–30%, on average) than control plots. Abundances and species richness were also higher in plots with low-input of fertiliser and water compared to fertilised only plots. Monophagous and oligophagous species were most abundant in plots with low-input of fertiliser and water. Medium- and high-input treatments (F+I 2/3 and 3/3) increased the number of generalist (eurytopic) species, while only low-input treatment (F+I 1/3) boosted the more specialised (stenotopic) species. This provides support to the hump-shaped diversity-disturbance relationship and guidance for sustainable management of biodiversity-rich mountain hay meadows.

Multiple factors influence plant richness and diversity in the cold and dry boreal forest of southwest Yukon, Canada

The intermediate disturbance hypothesis (IDH) is regarded as an appropriate model for understanding plant diversity–disturbance relationships in the boreal region. However, concern has been raised over its applicability across such a broad region, in particular, in low productive areas of the region such as the cold, dry region of the southwest Yukon. We examined plant richness and diversity in relation to the IDH in the context of both disturbance and productivity gradients. We then tested whether environmental variation and resource quantity interacted with development stage to shape richness patterns in the region. We found that richness and diversity did not conform to the IDH but that environmental variation and resource quantity influenced richness and diversity patterns. Overall richness and diversity patterns were not influenced by development stage though plant functional group patterns were. Richness patterns varied between plant functional groups suggesting that divergent processes shape inter functional group richness within the same stands. Our findings highlight the influence of multiple factors governing patterns of plant richness and diversity in the cold dry boreal forests of the southwest Yukon.

Diversity–disturbance relationship in forest landscapes

Despite decades of research, there is an intense debate about the consistency of the hump-shaped pattern describing the relationship between diversity and disturbance as predicted by the intermediate disturbance hypothesis (IDH). Previous meta-analyses have not explicitly considered interactive effects of disturbance frequency and intensity of disturbance on plant species diversity in terrestrial landscapes. We conducted meta-analyses to test the applicability of IDH by simultaneously examining the relationship between species richness, disturbance frequency (quantified as time since last disturbance as originally proposed) and intensity of disturbance in forest landscapes. The effects of disturbance frequency, intensity, and their interaction on species richness was evaluated using a mixed-effects model. We found that species richness peaks at intermediate frequency after both high and intermediate disturbance intensities, but the richness-frequency relationship differed between intensity classes. Our study highlights the need to measure multiple disturbance components that could help reconcile conflicting empirical results on the effect of disturbance on plant species diversity.

Site productivity overrides competition in explaining the disturbance–diversity relationship in riparian forests

The intermediate-disturbance hypothesis (IDH) is frequently invoked to explain the high biodiversity of certain ecosystems and largely permeates forest management guidelines. Although this hypothesis assumes a trade-off between competitive ability and colonization capacities, the key role of competition is still controversial. Here we evaluate the contribution of competition in explaining species richness along a flooding-disturbance gradient in a riparian forest ecosystem, relative to other potential factors acting in combination with flooding disturbance, namely site productivity and water-mediated seed arrival.Along a flooding-disturbance gradient across three riparian forests of north France, we assessed: (1) the disturbance–diversity relationship for forest herbs, (2) the intensity of competition using a phytometer transplant experiment, (3) the above-ground plant biomass in the herb layer, phytometer performances, and light intensity at the herb layer level, and (4) the contribution of the hydrochorous seed rain using mat traps and the emergence method. We built several candidate models to compare the relative importance of competition, site productivity, and water seed deposit on forest herb species richness.We found a typical hump-backed relationship between flooding disturbance intensity/frequency and forest herb species richness. Competition intensity did not vary along this gradient and did not contribute to this relationship. Increasing site productivity (via soil resources and light availability) was associated with increasing forest herb species richness, irrespective of disturbance intensity/frequency. Species supply via the hydrochorous seed rain did not significantly contribute to the herb species richness despite a similar hump-backed pattern along the flooding gradient.The hump-backed relationship between species richness and disturbance intensity/frequency observed in riparian forests can be explained without referring to competition. Instead, site productivity appears a more important factor, which acts in combination with physical disturbance and the likely resulting environment heterogeneity.