Disturbance-driven changes in the variability of ecological patterns and processes

Understanding how disturbance shapes the dynamics of ecological systems is of fundamental importance in ecology. One emerging approach to revealing and appreciating disturbance effects involves examining disturbance-driven changes in the variability of ecological responses. Variability is rarely employed as a response variable to assess the influence of disturbance, but recent studies indicate that it can be an extremely sensitive metric, capturing differences obscured by averaging and conveying important ecological information about underlying causal processes. In this paper, we present a conceptual model to understand and predict the effects of disturbance on variability. The model estimates qualitative changes in variability by considering disturbance extent, frequency and intensity, as well as ecosystem recovery, and thereby captures not only the immediate effects of disturbance but also those that arise over time due to the biotic response to an event. We evaluate how well the model performs by comparing predictions with empirical results from studies examining a wide variety of disturbances and ecosystems, and discuss factors that may modify or even confound predictions. We include a concise guide to characterizing and detecting changes in variability, highlighting the most common and easily applied methods and conclude by describing several future directions for research. By considering variability as a response to disturbance, we gain another metric of fundamental system behaviour, an improved ability to identify organizing features of ecosystems and a better understanding of the predictability of disturbance-driven change – all critical aspects of assessing ecosystem response to disturbance.     

Towards a mechanistic understanding of variation in aquatic food chain length

Ecologists have long sought to understand variation in food chain length (FCL) among natural ecosystems. Various drivers of FCL, including ecosystem size, resource productivity and disturbance, have been hypothesised. However, when results are aggregated across existing empirical studies from aquatic ecosystems, we observe mixed FCL responses to these drivers. To understand this variability, we develop a unified competition-colonisation framework for complex food webs incorporating all of these drivers. With competition-colonisation tradeoffs among basal species, our model predicts that increasing ecosystem size generally results in a monotonic increase in FCL, while FCL displays non-linear, oscillatory responses to resource productivity or disturbance in large ecosystems featuring little disturbance or high productivity. Interestingly, such complex responses mirror patterns in empirical data. Therefore, this study offers a novel mechanistic explanation for observed variations in aquatic FCL driven by multiple environmental factors.     

Multiscale responses of soil stability and invasive plants to removal of non-native grazers from an arid conservation reserve

Disturbances and ecosystem recovery from disturbance both involve numerous processes that operate on multiple spatial and temporal scales. Few studies have investigated how gradients of disturbance intensity and ecosystem responses are distributed across multiple spatial resolutions and also how this relationship changes through time during recovery. We investigated how cover of non-native species and soil-aggregate stability (a measure of vulnerability to erosion by water) in surface and subsurface soils varied spatially during grazing by burros and cattle and whether patterns in these variables changed after grazer removal from Mojave National Preserve, California, USA. We compared distance from water and number of ungulate defecations — metrics of longer-term and recent grazing intensity, respectively, — as predictors of our response variables. We used information-theoretic analyses to compare hierarchical linear models that accounted for important covariates and allowed for interannual variation in the disturbance–response relationship at local and landscape scales. Soil stability was greater under perennial vegetation than in bare interspaces, and surface soil stability decreased with increasing numbers of ungulate defecations. Stability of surface samples was more affected by time since removal of grazers than was stability of subsurface samples, and subsurface soil stability in bare spaces was not related to grazing intensity, time since removal, or any of our other predictors. In the high rainfall year (2003) after cattle had been removed for 1–2 years, cover of all non-native plants averaged nine times higher than in the low-rainfall year (2002). Given the heterogeneity in distribution of large-herbivore impacts that we observed at several resolutions, hierarchical analyses provided a more complete understanding of the spatial and temporal complexities of disturbance and recovery processes in arid ecosystems.     

An assessment of natural and human disturbance effects on Mexican ecosystems: current trends and research gaps

Mexico harbors more than 10% of the planet’s endemic species. However, the integrity and biodiversity of many ecosystems is experiencing rapid transformation under the influence of a wide array of human and natural disturbances. In order to disentangle the effects of human and natural disturbance regimes at different spatial and temporal scales, we selected six terrestrial (temperate montane forests, montane cloud forests, tropical rain forests, tropical semi-deciduous forests, tropical dry forests, and deserts) and four aquatic (coral reefs, mangrove forests, kelp forests and saline lakes) ecosystems. We used semi-quantitative statistical methods to assess (1) the most important agents of disturbance affecting the ecosystems, (2) the vulnerability of each ecosystem to anthropogenic and natural disturbance, and (3) the differences in ecosystem disturbance regimes and their resilience. Our analysis indicates a significant variation in ecological responses, recovery capacity, and resilience among ecosystems. The constant and widespread presence of human impacts on both terrestrial and aquatic ecosystems is reflected either in reduced area coverage for most systems, or reduced productivity and biodiversity, particularly in the case of fragile ecosystems (e.g., rain forests, coral reefs). In all cases, the interaction between historical human impacts and episodic high intensity natural disturbance (e.g., hurricanes, fires) has triggered a reduction in species diversity and induced significant changes in habitat distribution or species dominance. The lack of monitoring programs assessing before/after effects of major disturbances in Mexico is one of the major limitations to quantifying the commonalities and differences of disturbance effects on ecosystem properties.     

A Framework to Assess Biogeochemical Response to Ecosystem Disturbance Using Nutrient Partitioning Ratios

Disturbances affect almost all terrestrial ecosystems, but it has been difficult to identify general principles regarding these influences. To improve our understanding of the long-term consequences of disturbance on terrestrial ecosystems, we present a conceptual framework that analyzes disturbances by their biogeochemical impacts. We posit that the ratio of soil and plant nutrient stocks in mature ecosystems represents a characteristic site property. Focusing on nitrogen (N), we hypothesize that this partitioning ratio (soil N: plant N) will undergo a predictable trajectory after disturbance. We investigate the nature of this partitioning ratio with three approaches: (1) nutrient stock data from forested ecosystems in North America, (2) a process-based ecosystem model, and (3) conceptual shifts in site nutrient availability with altered disturbance frequency. Partitioning ratios could be applied to a variety of ecosystems and successional states, allowing for improved temporal scaling of disturbance events. The generally short-term empirical evidence for recovery trajectories of nutrient stocks and partitioning ratios suggests two areas for future research. First, we need to recognize and quantify how disturbance effects can be accreting or depleting, depending on whether their net effect is to increase or decrease ecosystem nutrient stocks. Second, we need to test how altered disturbance frequencies from the present state may be constructive or destructive in their effects on biogeochemical cycling and nutrient availability. Long-term studies, with repeated sampling of soils and vegetation, will be essential in further developing this framework of biogeochemical response to disturbance.     

Resilience and regime change in a southern Rocky Mountain ecosystem during the past 17?000 years

Paleoecological records indicate that subalpine forests in western North America have been resilient in response to multiple influences, including severe droughts, insect outbreaks, and widely varying fire regimes, over many millennia. One hypothesis for explaining this ecosystem resilience centers on the disruption of forest dynamics by frequent disturbance and climatic variability, and the resulting development of non-steady-state regimes dominated by early-successional conifers with broad climatic tolerances, such as lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.). To evaluate this hypothesis, we independently reconstructed the vegetation, fire, and effective-moisture histories of a small, forested watershed at 2890 m elevation in southeastern Wyoming, USA, using sedimentary pollen and charcoal counts in conjunction with sedimentary lake-level indicators. The data indicate that prominent vegetation shifts (from sagebrush steppe to spruce–fir parkland at ca. 10.7 ka and spruce–fir parkland to pine-dominated forest at ca. 8.5 ka [ka stands for thousands of years before the present, defined as AD 1950]) coincided with changes in effective moisture. However, after lodgepole pine forests established at ca. 8.5 ka, similar hydroclimatic changes did not produce detectable vegetation responses. Fire history data show that other aspects of the ecosystem were responsive to changes in effective moisture at centennial timescales with prolonged fire-free episodes coinciding with periods of low effective moisture ca. 7.2–5.6 and 3.7–1.6 ka. Throughout our record, the ratio of ecosystem perturbation time (i.e., fire frequency and changes in effective moisture) to recovery time (assuming 200–600-year successional processes) falls within estimates of the ratio for non-steady state ecosystems. Frequent perturbations, therefore, may have prevented this ecosystem from reaching compositional equilibrium with the varied climatic conditions over the past 8.5 ka. Equilibrium states could have included more abundant spruce (Picea spp.) and fir (Abies spp.) than presently observed based on brief increases in pollen abundances of these taxa during prolonged dry, fire-free intervals. Our results show that, although current climate changes favor widespread disturbance in Rocky Mountain forests, the composition of these ecosystems could be highly resilient and recover through successional dynamics over the next few decades to centuries.     

Models of underlying autotrophic biomass dynamics fit to daily river ecosystem productivity estimates improve understanding of ecosystem disturbance and resilience

Directly observing autotrophic biomass at ecologically relevant frequencies is difficult in many ecosystems, hampering our ability to predict productivity through time. Since disturbances can impart distinct reductions in river productivity through time by modifying underlying standing stocks of biomass, mechanistic models fit to productivity time series can infer underlying biomass dynamics. We incorporated biomass dynamics into a river ecosystem productivity model for six rivers to identify disturbance flow thresholds and understand the resilience of primary producers. The magnitude of flood necessary to disturb biomass and thereby reduce ecosystem productivity was consistently lower than the more commonly used disturbance flow threshold of the flood magnitude necessary to mobilize river bed sediment. The estimated daily maximum percent increase in biomass (a proxy for resilience) ranged from 5% to 42% across rivers. Our latent biomass model improves understanding of disturbance thresholds and recovery patterns of autotrophic biomass within river ecosystems.     

Effects of disturbance intensity and frequency on bacterial community composition and function

Disturbances influence community structure and ecosystem functioning. Bacteria are key players in ecosystems and it is therefore crucial to understand the effect of disturbances on bacterial communities and how they respond to them, both compositionally and functionally. The main aim of this study was to test the effect of differences in disturbance strength on bacterial communities. For this, we implemented two independent short-term experiments with dialysis bags containing natural bacterial communities, which were transplanted between ambient and 'disturbed' incubation tanks, manipulating either the intensity or the frequency of a salinity disturbance. We followed changes in community composition by terminal restriction fragment analysis (T-RFLP) and measured various community functions (bacterial production, carbon substrate utilization profiles and rates) directly after and after a short period of recovery under ambient conditions. Increases in disturbance strength resulted in gradually stronger changes in bacterial community composition and functions. In the disturbance intensity experiment, the sensitivity to the disturbance and the ability of recovery differed between different functions. In the disturbance frequency experiment, effects on the different functions were more consistent and recovery was not observed. Moreover, in case of the intensity experiment, there was also a time lag in the responses of community composition and functions, with functional responses being faster than compositional ones. To summarize, our study shows that disturbance strength has the potential to change the functional performance and composition of bacterial communities. It further highlights that the overall effects, rates of recovery and the degree of congruence in the response patterns of community composition and functioning along disturbance gradients depend on the type of function and the character of the disturbance.     

Riparian Restoration in the Western United States: Oveview and Perspecive

This historical and conceptual overview of riparian ecosystem restoration discusses how riparian ecosystems have been defined, describes the hydrologic, geomorphic, and biotic processes that create and maintain riparian ecosystems of the western USA, identifies the main types of anthropogenic desturbances occurring in these ecosystems, and provides an overview of restoration methods for each disturbance type. We suggest that riparian ecosystems consist of two zones: Zone I occupies the active floodplain and is frequently inundated and Zone II extends from the active floodplain to the valley wall. Successful restoration depends n understanding the physical and biological processes that influence natural riparian ecosystems and the types of disturbance that have degraded riparian areas. Thus we recommend adopting a process-based approach for riparian restoration. Disturbances to riparian ecosystems in the western USA result from streamflow modifications by dams, reservoirs, and diversions; stream channelization; direct modification of the riparian ecosystem; and watershed disturbances. Four topics should be addressed to advance the state of science for restoration of riparian ecosys-tems: (1) interdisciplinary approaches, (2) a unified framework, (3) a better understanding of fundamental riparian ecosystem processes, and (4) restoration po-tential more closely related to disturbance type. Three issues should be considered regarding the cause of the degraded environment: (1) the location of the causative disturbance with respect to the degraded riparian area, (2) whether the disturbance is ongoing or can be elim-inated, and (3) whether or not recovery will occur nat-urally if the disturbance is removed.     

火干扰对森林生态系统土壤呼吸组分的影响研究进展

土壤呼吸是陆地生态系统与大气碳交换的主要方式,主要分为自养呼吸和异养呼吸。土壤呼吸不仅是森林生态系统碳循环过程的关键环节,也是森林生态系统能量流动和物质循环的重要生态过程。火作为森林生态系统中一个重要的生态因子,可以在短时间内对土壤呼吸组分造成巨大的影响。火干扰对土壤呼吸组分的影响与火烧强度、火烧频率、火烧持续时间以及火后恢复等因子有关,通过影响植被的根系与组成、微生物群落数量与结构,凋落物的数量以及生态系统的环境和小气候等,进而对土壤呼吸产生影响。火干扰对土壤呼吸影响整体表现为火烧后土壤呼吸速率下降,在几个月至几年内恢复到火烧前水平,之后火继续对土壤呼吸产生影响长达数年至数十年。通过描述火烧强度、火烧频率以及火后恢复时间,阐述火干扰对土壤呼吸组分的直接影响,以及通过火后环境对土壤呼吸组分产生的间接影响,来揭示火干扰对森林生态系统土壤呼吸组分的影响。同时针对火干扰对土壤呼吸组分的影响进行以下3个方面的研究展望：（1）火后产生的黑碳对土壤呼吸组分的影响;（2）火后植被恢复对土壤呼吸组分产生的影响;（3）火后土壤呼吸组分的长期变化规律。     

Altered dynamics of forest recovery under a changing climate

Forest regeneration following disturbance is a key ecological process, influencing forest structure and function, species assemblages, and ecosystem–climate interactions. Climate change may alter forest recovery dynamics or even prevent recovery, triggering feedbacks to the climate system, altering regional biodiversity, and affecting the ecosystem services provided by forests. Multiple lines of evidence – including global‐scale patterns in forest recovery dynamics; forest responses to experimental manipulation of CO₂, temperature, and precipitation; forest responses to the climate change that has already occurred; ecological theory; and ecosystem and earth system models – all indicate that the dynamics of forest recovery are sensitive to climate. However, synthetic understanding of how atmospheric CO₂ and climate shape trajectories of forest recovery is lacking. Here, we review these separate lines of evidence, which together demonstrate that the dynamics of forest recovery are being impacted by increasing atmospheric CO₂ and changing climate. Rates of forest recovery generally increase with CO₂, temperature, and water availability. Drought reduces growth and live biomass in forests of all ages, having a particularly strong effect on seedling recruitment and survival. Responses of individual trees and whole‐forest ecosystems to CO₂ and climate manipulations often vary by age, implying that forests of different ages will respond differently to climate change. Furthermore, species within a community typically exhibit differential responses to CO₂ and climate, and altered community dynamics can have important consequences for ecosystem function. Age‐ and species‐dependent responses provide a mechanism by which climate change may push some forests past critical thresholds such that they fail to recover to their previous state following disturbance. Altered dynamics of forest recovery will result in positive and negative feedbacks to climate change. Future research on this topic and corresponding improvements to earth system models will be a key to understanding the future of forests and their feedbacks to the climate system.     

The greenhouse gas value of ecosystems

As society faces the urgent need to mitigate climate change, it is critical to understand how various ecosystems contribute to the climate, and to express these contributions in terms that are meaningful to policymakers, economists, land managers, and other nonscience interest holders. Efforts to mitigate climate change call for quantification of the full greenhouse gas (GHG) effects of land use decisions, yet we lack an appropriate metric of the full GHG implications of maintaining a given ecosystem over a multiple year time frame. Here, we propose the concept of greenhouse gas value (GHGV) of ecosystems, which accounts for potential GHG release upon clearing of stored organic matter, annual GHG flux, and probable GHG exchanges resulting from disturbance. It treats these ecosystem–atmosphere exchanges in a time‐sensitive manner, thereby providing an appropriate framework for computing of the GHG consequences of any land use decision. To illustrate this concept, we provide estimates of the GHGV of various biome types (based on data compiled from the literature), disturbance regimes, and decisions on the treatment of time. We show that natural ecosystems generally have high GHGV's, whereas managed ecosystems generally have lower or negative GHGV's; that GHGV decreases with increasing probability of disturbance, and that decisions on the treatment of time can be important, affecting some ecosystem types more strongly than others. In addition, we show how GHGV may be used to quantify the full GHG effects of land‐use or land‐cover change in a thorough and rigorous manner. Finally, we provide comparisons of GHGV to other major paradigms for valuing the GHG contributions of ecosystems, showing that – for many purposes –GHGV is the most appropriate method of quantifying the GHG services of ecosystems.     

Meta‐analysis on pulse disturbances reveals differences in functional and compositional recovery across ecosystems

Most ecosystems are affected by anthropogenic or natural pulse disturbances, which alter the community composition and functioning for a limited period of time. Whether and how quickly communities recover from such pulses is central to our understanding of biodiversity dynamics and ecosystem organisation, but also to nature conservation and management. Here, we present a meta‐analysis of 508 (semi‐)natural field experiments globally distributed across marine, terrestrial and freshwater ecosystems. We found recovery to be significant yet incomplete. At the end of the experiments, disturbed treatments resembled controls again when considering abundance (94%), biomass (82%), and univariate diversity measures (88%). Most disturbed treatments did not further depart from control after the pulse, indicating that few studies showed novel trajectories induced by the pulse. Only multivariate community composition on average showed little recovery: disturbed species composition remained dissimilar to the control throughout most experiments. Still, when experiments revealed a higher compositional stability, they tended to also show higher functional stability. Recovery was more complete when systems had high resistance, whereas resilience and resistance were negatively correlated. The overall results were highly consistent across studies, but significant differences between ecosystems and organism groups appeared. Future research on disturbances should aim to understand these differences, but also fill obvious gaps in the empirical assessments for regions (especially the tropics), ecosystems and organisms. In summary, we provide general evidence that (semi‐)natural communities can recover from pulse disturbances, but compositional aspects are more vulnerable to long‐lasting effects of pulse disturbance than the emergent functions associated to them.     

Estimation of aboveground biomass using in situ hyperspectral measurements in five major grassland ecosystems on the Tibetan Plateau

Aims There are numerous grassland ecosystem types on the Tibetan Plateau. These include the alpine meadow and steppe and degraded alpine meadow and steppe. This study aimed at developing a method to estimate aboveground biomass (AGB) for these grasslands from hyperspectral data and to explore the feasibility of applying air/satellite-borne remote sensing techniques to AGB estimation at larger scales.Methods We carried out a field survey to collect hyperspectral reflectance and AGB for five major grassland ecosystems on the Tibetan Plateau and calculated seven narrow-band vegetation indices and the vegetation index based on universal pattern decomposition (VIUPD) from the spectra to estimate AGB. First, we investigated correlations between AGB and each of these vegetation indices to identify the best estimator of AGB for each ecosystem type. Next, we estimated AGB for the five pooled ecosystem types by developing models containing dummy variables. At last, we compared the predictions of simple regression models and the models containing dummy variables to seek an ecosystem type-independent model to improve prediction of AGB for these various grassland ecosystems from hyperspectral measurements.Important findings When we considered each ecosystem type separately, all eight vegetation indices provided good estimates of AGB, with the best predictor of AGB varying among different ecosystems. When AGB of all the five ecosystems was estimated together using a simple linear model, VIUPD showed the lowest prediction error among the eight vegetation indices. The regression models containing dummy variables predicted AGB with higher accuracy than the simple models, which could be attributed to the dummy variables accounting for the effects of ecosystem type on the relationship between AGB and vegetation index (VI). These results suggest that VIUPD is the best predictor of AGB among simple regression models. Moreover, both VIUPD and the soil-adjusted VI could provide accurate estimates of AGB with dummy variables integrated in regression models. Therefore, ground-based hyperspectral measurements are useful for estimating AGB, which indicates the potential of applying satellite/airborne remote sensing techniques to AGB estimation of these grasslands on the Tibetan Plateau.     

Context‐dependent resistance of freshwater invertebrate communities to drying

More freshwater ecosystems are drying in response to global change thereby posing serious threat to freshwater biota and functions. The production of desiccation‐resistant forms is an important adaptation that helps maintain biodiversity in temporary freshwaters by buffering communities from drying, but its potential to mitigate the negative effects of drying in freshwater ecosystems could vary greatly across regions and ecosystem types. We explored this context dependency by quantifying the potential contribution of desiccation‐resistance forms to invertebrate community recovery across levels of regional drying prevalence (defined as the occurrence of drying events in freshwaters in a given region) and ecosystem types (lentic, lotic) in temporary neotropical freshwaters. We first predicted that regional drying prevalence influences the selection of species with desiccation‐resistant forms from the regional species pools and thus increases the ability of communities to recover from drying. Second, we predicted lentic freshwaters harbor higher proportions of species with desiccation‐resistant forms compared to lotic, in response to contrasted hydrologic connectivity. To test these predictions, we used natural experiments to quantify the contribution of desiccation‐resistant forms to benthic invertebrate community recovery in nine intermittent streams and six geographically isolated temporary wetlands from three Bolivian regions differing in drying prevalence. The contribution of desiccation‐resistant forms to community recovery was highest where regional drying prevalence was high, suggesting the species pool was adapted to regional disturbance regimes. The contribution of desiccation‐resistant forms to community recovery was lower in streams than in wetlands, emphasizing the importance of hydrologic connectivity and associated recolonization processes from in‐stream refuges to recovery in lotic systems. In all regions, the majority of functional traits were present in desiccation‐resistant taxa indicating this adaptation may help maintain ecosystem functions by buffering communities from the loss of functional traits. Accounting for regional context and hydrologic connectivity in community recovery processes following drying can help refine predictions of freshwater biodiversity response to global change.     

A review of similarity between seed bank and standing vegetation across ecosystems

The relationship between above and belowground species composition has been researched in forests, grasslands, and wetlands to understand what mechanisms control community composition. I thoroughly reviewed 108 articles published between 1945 and 2006 that summarized and provided specific values on similarities between above and belowground communities to identify common trends among ecosystems. Using Sørenson's index of similarity, I found that standing vegetation and its associated seed bank was the least similar in forest ecosystems, most similar in grasslands, and of intermediate similarity in wetlands. I also discovered that species richness was not related to seed bank – vegetation similarity in any of the three ecosystems. Disturbances were a common mechanism driving community composition in all ecosystems, where similarity decreased with time since disturbance in forest and wetland ecosystems and increased with time since disturbance in grasslands. Knowing the relationships between seed bank and standing vegetation may help conservationists to manage against exotic species, plan for community responses to disturbances, restore diversity, and better understand the resilience of an ecosystem.     

Are ecosystems hormetic?

The phenomenon of hormesis has been observed mainly for the response of individual organisms to stress. A reasonable line of inquiry might explore the possibility of observing hormesis at other levels of ecological organization. This initial examination focuses on ecosystem hormesis. Explorations of hormetic responses of ecosystems to stress cannot be made independently of a fundamental concept of ecosystem. The scale‐dependence of ecosystem dynamics also influences whether an ecological disturbance is in reality a stressor. Ecosystem hormesis might be claimed if one or more components of an ecosystem exhibit hormesis. By this definition, ecosystem hormesis would be a trivial extension of hormesis observed for individual organisms. A non‐trivial extension of ecosystem hormesis would include the observation that integrated (i.e., holistic) measures of ecosystem structure or function displayed an hormetic response to an ecological stressor. Several such examples of ecosystem structural and functional hormesis are presented.     

Predicting Plant Diversity Response to Disturbance: Applicability of the Intermediate Disturbance Hypothesis and Mass Ratio Hypothesis

Predicting the relationships between disturbance, biodiversity and productivity of ecosystems continue to preoccupy ecologists and resource managers. Two hypotheses underpin many of the discussions. The Intermediate Disturbance Hypothesis (IDH), which proposes that biodiversity peaks at intermediate levels of disturbance, is often extended to predict that productivity follows the same response pattern. The Mass Ratio Hypothesis (MRH) proposes that the biological traits of the dominant species are the critical drivers of ecosystem function (e.g., productivity) and that these species increase in biomass rapidly after disturbance then stabilize. As a consequence, species diversity first peaks then declines after disturbance as a few species dominate the site. Both provide a conceptual link among disturbance, species diversity and productivity (an index of ecosystem function). We assessed the current state of empirical support for these two hypotheses with a literature survey and determined if their conformance is related to ecosystem type or site productivity. Conformance of IDH reported in past reviews (considering all ecosystems) ranged from 16 to 21%. This contrasts with our finding that in terrestrial ecosystems conformance to IDH was 46% (22 of 48 studies), 17% studies reported non-compliance, and 23% reported inconclusive results. Most studies explained their results with respect to IDH or MRH. Only two studies were specifically designed to test the validity of IDH or MRH. We conclude that (i) the IDH is mostly applicable to predict species diversity response to disturbance in upland sites of medium to high productivity and the MRH is applicable to organic sites of low productivity; (ii) there is a critical need for more studies specifically designed to test these hypotheses in natural ecosystems using common protocols; and (iii) enhanced understanding of these models will add value in refining management policies and in the selection of meaningful diversity indicators of sustainability.     

Stability of ecosystem properties in response to above-ground functional group richness and composition

While there has been a rapidly increasing research effort focused on understanding whether and how composition and richness of species and functional groups may determine ecosystem properties, much remains unknown about how these community attributes affect the dynamic properties of ecosystems. We conducted an experiment in 540 mini‐ecosystems in glasshouse conditions, using an experimental design previously shown to be appropriate for testing for functional group richness and composition effects in ecosystems. Artificial communities representing 12 different above‐ground community structures were assembled. These included treatments consisting of monoculture and two‐ and four‐species mixtures from a pool of four plant species; each plant species represented a different functional group. Additional treatments included two herbivore species, either singly or in mixture, and with or without top predators. These experimental units were then either subjected to an experimentally imposed disturbance (drought) for 40 d or left undisturbed. Community composition and drought both had important effects on plant productivity and biomass, and on several below‐ground chemical and biological properties, including those linked to the functioning of the decomposer subsystem. Many of these compositional effects were due to effects both of plant and of herbivore species. Plant functional group richness also exerted positive effects on plant biomass and productivity, but not on any of the below‐ground properties. Above‐ground composition also had important effects on the response of below‐ground properties to drought and thus influenced ecosystem stability (resistance); effects of composition on drought resistance of above‐ground plant response variables and soil chemical properties were weaker and less consistent. Despite the positive effects of plant functional group richness on some ecosystem properties, there was no effect of richness on the resistance of any of the ecosystem properties we considered. Although herbivores had detectable effects on the resistance of some ecosystem properties, there were no effects of the mixed herbivore species treatment on resistance relative to the single species herbivore treatments. Increasing above‐ground food chain length from zero to three trophic levels did not have any consistent effect on the stability of ecosystem properties. There was no evidence of either above‐ground composition or functional group richness affecting the recovery rate of ecosystem properties from drought and hence ecosystem resilience. Our data collectively point to the role of composition (identity of functional group), but not functional group richness, in determining the stability (resistance to disturbance) of ecosystem properties, and indicates that the nature of the above‐ground community can be an important determinant of the consistency of delivery of ecosystem services.     

Resilience of Zostera muelleri seagrass to small‐scale disturbances: the relative importance of asexual versus sexual recovery

Resilience is the ability of an ecosystem to recover from disturbance without loss of essential function. Seagrass ecosystems are key marine and estuarine habitats that are under threat from a variety of natural and anthropogenic disturbances. The ability of these ecosystems to recovery from disturbance will to a large extent depend on the internsity and scale of the disturbance, and the relative importance of sexual versus asexual reproduction within populations. Here, we investigated the resilience of Zostera muelleri seagrass (Syn. Zostera capricorni) to small‐scale disturbances at four locations in Lake Macquarie – Australia's largest coastal lake – and monitored recovery over a 65‐week period. Resilience of Z. muelleri varied significantly with disturbance intensity; Z. muelleri recovered rapidly (within 2 weeks) from low‐intensity disturbance (shoot loss), and rates of recovery appeared related to initial shoot length. Recovery via rhizome encroachment (asexual regeneration) from high‐intensity disturbance (loss of entire plant) varied among locations, ranging from 18‐35 weeks, whereas the ability to recover was apparently lost (at least within the time frame of this study) when recovery depended on sexual regeneration, suggesting that seeds do not provide a mechanism of recovery against intense small‐scale disturbances. The lack of sexual recruits into disturbed sites is surprising as our initial surveys of genotypic diversity (using nine polymorphic microsatellite loci) at these location indicate that populations are maintained by a mix of sexual and asexual reproduction (genotypic diversity [R] varied from 0.24 to 0.44), and populations consisted of a mosaic of genotypes with on average 3.6 unique multilocus genotypes per 300 mm diameter plot. We therefore conclude that Z. muelleri populations within Lake Macquarie rely on clonal growth to recover from small‐scale disturbances and that ongoing sexual recruitment by seeds into established seagrass beds (as opposed to bare areas arising from disturbance) must be the mechanism responsible for maintaining the observed mixed genetic composition of Z. muelleri seagrass meadows.