Intermediate Disturbance Regimes Research Articles

Do Mesocarnivores Respond to the Seasonality in Management Practices in an Agroforestry Landscape?

In the Mediterranean, we find a mosaic of natural and cultural landscapes, where a variety of forest management practices created intermediate disturbance regimes that potentially increased biodiversity values. Nonetheless, it is essential to understand the species’ long-term response to the dynamic management in agroecosystems, since the species tolerance to disturbance can change throughout the life cycle. Mammalian carnivores can be sensitive to human disturbance and are an essential part of ecosystems due to their regulatory and community structuring effects. We investigated the spatial response of five mesocarnivores species to spatially- and temporally- varying management practices in an agroforestry landscape. More specifically, we assessed the mesocarnivores’ temporal changes in space use by implementing multi-season occupancy models in a Bayesian framework, using seasonal camera-trapping surveys for a 2-year period. All species had a weak response of local extinction to forestry management and livestock grazing pressure. For forest-dwelling species, occupancy was higher where productivity of perennial vegetation was high, while colonization between seasons was positively associated with vegetation cover. For habitat generalist species, we found that occupancy in the wet season increased with the distance to cattle exclusion plots. Most of these plots are pine stands which are subject to forestry interventions during winter. During the 2-year period we found seasonal fluctuations in occupancy for all species, with an overall slight decrease for three mesocarnivore species, while for the two forest-dwelling species there was an increase in occupancy between years. The weak species response to management practices supports the importance of traditional management for upholding a diverse mesocarnivore community in agroforestry systems but could also reflect these species’ ecological plasticity and resilience to disturbance.

Impacts of soil disturbance on plant diversity in a dry grassland

Dry grasslands are not only amongst the most species-rich habitats in Europe but also amongst the most threatened. Threats include habitat destruction as well as modification to the historic disturbance regime under which native plants have evolved (e.g. too high or too low levels of disturbance). The aim of this study is to examine the effects of soil disturbance intensity on species composition and diversity in a dry grassland in Mols Bjerge National Park in Denmark. We recorded vascular plant species inside and just outside patches of bare sand and at the transition between bare sand and dense vegetation outside the patches. We found that species richness was highest in the dense vegetation, intermediate in the transition zone and lowest in bare sand areas. However, an analysis of plant traits showed that the number of small annual species was highest in the transition zone. High abundance of small annual species may therefore indicate intermediate disturbance regimes. Moreover, literature indicates that many threatened species are adapted to such habitats, which suggests that dry grasslands should be managed to maintain areas with intermediate disturbance intensities to improve conditions for many threatened species. High abundance of small annual species thus indicates that favourable management has been achieved.

Cumulative Impacts of Land Cover Change and Dams on the Land–Water Interface of the Tocantins River

Riparian vegetation performs important ecosystems services, improving water quality, mitigating erosion, and maintaining regional plant and animal biodiversity. Regular annual flooding maintains riparian forests through an intermediate disturbance regime. In response, seasonally flooded vegetation has developed adaptations for seed dispersal and gas transfer to survive and reproduce while undergoing periods of flooding. In the Amazon, a dam building boom threatens the integrity of riparian vegetation by moving riparian corridors into dry-adapted ecosystems and reducing downstream flooding of riparian areas. Additionally, the region is undergoing intense development pressure resulting in the conversion of native riparian vegetation into agriculture. In this study, we measure how the installation of six large dams on the Tocantins River, coupled with land cover change from native forest and savanna to cattle pasture, has changed the land–water interface of this region. Using land cover data provided by MapBiomas, we quantified land cover change from 1985 to 2018 and measured changes in the riparian areas of the still free-flowing areas of the Tocantins River, riparian areas surrounding reservoirs, and in-stream vegetation dynamics. We found that deforestation in the riparian areas of the Tocantins River downstream of the dams is occurring at a higher rate than deforestation in the watershed. Additionally, reservoir filling resulted in creating hundreds of square kilometers of new riparian areas, pushing the riparian zone away from forest-dominated ecosystems into savanna-dominated areas. The quantity of in-stream vegetation throughout the study was dynamic and initially increased after damming before declining for the last decade of the study. Changes to native land cover in riparian areas of the Tocantins River threaten the integrity of ecosystem services provided by riparian vegetation and are likely to lead to further degradation of these areas.

Adaptation of wild boar (Sus scrofa) activity in a human-dominated landscape

BackgroundWild boars (Sus scrofa L.) are globally widely distributed, and their populations have increased in Europe during recent decades. Encounters between humans and wild boars are rare because of the predominantly nocturnal lifestyle of the latter, and wild boar management by hunting is a challenging task. Animal activity patterns are important for understanding the behaviour of a species. However, knowledge of detailed temporal patterns and an understanding of the drivers of wild boar activity at a fine temporal scale are lacking. Of special relevance for human–wild boar interactions (e.g., encounters, conflicts, and management) is the question of whether nocturnal activity depends on anthropogenic factors and, particularly, how local hunting regimes may affect activity patterns. We used GPS telemetry and acceleration measurements to shed light on this part of wild boar behaviour, observing 34 animals in Central Europe. Animals were tracked along a gradient of hunting pressure from hunting-free areas to areas with low or high hunting pressure. Fitted generalised additive models allowed predicting the probability of active behaviour under differing disturbance regimes precisely to day of year and time of day.ResultsThe wild boars were predominantly nocturnal, with peak activity at approximately midnight. However, the data showed increased activity during daylight for wild boars that used no-hunting zones or reduced-hunting zones. Large areas with low disturbance levels promoted activity during daylight more than smaller areas with an intermediate disturbance regime. High air temperatures and locations within forests reduced the probability of active behaviour, whereas proximity to tracks used for forestry or agriculture was accompanied by a higher probability of activity.ConclusionsWe conclude that wild boars flexibly adjust their activity to their local environmental conditions, considering disturbances at the scale of long-term home ranges as well as actual small-scale landscape quality. Entire wild boar home ranges should be covered in the delineation of reserves intending to stimulate activity during daylight.

Architecture of Mabea fistulifera Mart. (Euphorbiaceae), a Neotropical semideciduous tree: development and variations in crown allometry between environments

Tree crown growth reflects many aspects of the species’ life history. It was postulated that the potential for a pioneer species to invade a deforested area, or maintain a viable population in a forest under intermediate disturbance regime, is related to the tree trunk growth rates, patterns of crown construction, as well as the ability to regenerate broken branches. We investigated if Mabea fistulifera Mart. (Euphorbiaceae) crown architecture and tree allometry varies in two contrasted environments in a secondary fragment of Atlantic rainforest: forest edge and interior. First, we described the development of crown architecture by following the growth of the crown in 15 young individuals, with special attention to growth and branching models and to the morphologic differentiation of axes. Secondly, allometric differences in crown construction patterns between environments were studied by quantifying 10 traits in 20 individuals from each environment. Nozeran architectural model was found, a process of parenchymatization of the terminal meristem and a tier of branches form distally followed by an orthotropic shoot forming the next relay axis. This results in modular sympodial branching and polyaxial morphology of branches, allowing this tree species to occupy different environments by quick expansion wherever light availability is higher, towards the upper canopy (forest interior) or towards one side (forest edge). Crown asymmetry occurred towards border sunlit spaces or in response to inner canopy openings caused by hilly topography. Crown structure and growth patterns here found are suitable for a pioneer species adapted to intermediate disturbance regime.

Predator refuges for conservation biological control in an intermediately disturbed system: the rise and fall of a simple solution

Summary Managed systems harvested at intermediate time‐scales have advantages over annual short‐cycled systems in maintaining top‐down control of insect herbivores, and the flexible harvest regimes in these systems provide opportunities for habitat management that can stabilize predator–prey population dynamics across harvests – resulting in reduced risk of pest outbreaks. In a large‐scale field experiment, we explored whether retaining refuges, that is preserving parts of the stand to reduce predator mortality, could reduce the risk of pest insect outbreaks in willow short‐rotation coppice. Population densities of three omnivorous predator species and three outbreaking herbivorous leaf beetle species were monitored over four years after coppice (stem harvest) in eight stands with refuges (treatment) and eight stands without refuges (control). Predation pressure was estimated in years three and four. Contrary to our predictions, leaf beetle densities were higher in stands with refuges and predator densities were higher in stands without refuges. Leaf beetle egg mortality increased with total predator density, but did not differ between stands with and without refuges. These unexpected results can be attributed to interactions between dispersal and patch age. The altered phenology of coppiced stems may have triggered leaf beetle aggregation in refuges and migration from stands without refuges. A behavioural response to resource concentration in retained old patches likely transformed the predator refuge from a ‘source’ to a ‘sink’. Synthesis and applications. This study shows that retaining refuges in willow short‐rotation coppice to facilitate predator population recovery after harvest can come at the cost of more attractive herbivore habitats – and thus increased pest problems. We conclude that crop refuges in systems with intermediate disturbance regimes pose new challenges for conservation biological control, in particular the need to consider how patch age affects dispersal and recolonization of both pest and predators.

Forest disturbance accelerates thermophilization of understory plant communities

Summary Climate change is likely to shift plant communities towards species from warmer regions, a process termed ‘thermophilization’. In forests, canopy disturbances such as fire may hasten this process by increasing temperature and moisture stress in the understory, yet little is known about the mechanisms that might drive such shifts, or the consequences of these processes for plant diversity. We sampled understory vegetation across a gradient of disturbance severity from a large‐scale natural experiment created by the factorial combination of forest thinning and wildfire in California. Using information on evolutionary history and functional traits, we tested the hypothesis that disturbance severity should increase community dominance by species with southern‐xeric biogeographic affinities. We also analysed how climatic productivity mediates the effect of disturbance severity, and quantified the functional trait response to disturbance, to investigate potential mechanisms behind thermophilization. The proportion of north‐temperate flora decreased, while the proportion of southern‐xeric flora increased, with greater disturbance severity and less canopy closure. Disturbance caused a greater reduction of north‐temperate flora in productive (wetter) forests, while functional trait analyses suggested that species colonizing after severe disturbance may be adapted to increased water stress. Forests with intermediate disturbance severity, where abundances of northern and southern species were most equitable, had the highest stand‐scale understory diversity. Synthesis. Canopy disturbance is likely to accelerate plant community shifts towards species from warmer regions, via its effects on understory microclimate at small scales. Understory diversity can be enhanced by intermediate disturbance regimes that promote the coexistence of species with different biogeographic affinities.

Long‐term effects of salinity and disturbance regime on active and dormant crustacean communities

In a large‐scale and long‐term outdoor mesocosm (400‐liter) experiment, we studied the interacting effects of salinity and disturbance (hydrological) regime on the active and dormant crustacean communities of Mediterranean temporary wetlands (Camargue, southern France). Sixty‐four mesocosms, inoculated with a regional species pool (mixed dormant egg banks), were exposed to a full factorial treatment combination of four salinity levels and four disturbance regimes during three consecutive years. Both in the active and dormant community component, considerable shifts in community composition occurred because of direct and probably also second‐order effects of the treatments. All large branchiopod species had low long‐term salinity tolerances and showed species‐specific preferences for disturbance regimes according to their life cycle strategy. The highest salinity (5) was not limiting for cladocerans and ostracods, which thrived in the absence of the competitively stronger, predatory, and bioturbating large branchiopods. Copepods were negatively associated with salinity and coped better with the imposed biotic pressure. Zooplankton diversity and density peaked in intermediate‐disturbance regimes, probably because only specialized species survived the high‐disturbance regimes, whereas at low‐disturbance frequencies high densities of predatory Triops controlled zooplankton communities. Although crustacean dormant egg banks can temporarily buffer against unfavorable conditions, persisting bad conditions may lead to their exhaustion within 4 to 10 yr. Predicted aridification (leading to more intense disturbance regimes) may result in the loss of late successional species (chydorids and ostracods), whereas salinization may wipe out sensitive freshwater species such as large branchiopods.

Patterns of tree diversity in response to logging in Algonquin Provincial Park

The acceleration of human disturbance on natural systems necessitates the understanding of the effects of disturbance on species diversity, in order to mitigate its impacts. The Intermediate Disturbance Hypothesis (IDH) provides a general framework for predicting how these effects will unfold, centered on the premise that diversity will be maximized at intermediate levels of disturbance, or after an intermediate amount of time has passed since the last disturbance. We investigated the impact of disturbance by logging in Algonquin Provincial Park, ON, Canada, on tree species diversity. We sampled 7 sites, with known times since the last logging event, ranging between 4 and 149 years. We found that species richness and diversity indeed peaked at an intermediate disturbance regime of approximately 80 years since logging. We also found that this maximum diversity was caused by a combination of early, pioneer, post-pioneer, sub-climax, and climax species. Our experiment shows that IDH is applicable to the mixed forests of Algonquin, and therefore, provided that sites are logged after a suitable amount of time has passed, it is possible to harvest trees and maintain diversity.

Explaining grassland biomass – the contribution of climate, species and functional diversity depends on fertilization and mowing frequency

Summary1. Grassland ecosystems are often used to generate biomass in temperate regions of the world. It is well known that biomass is influenced by climate and biodiversity, but the relative importance of these two factors in relation to management has not been widely studied. To recommend management treatments maximizing biomass yields we aim to quantify the relative effects of climate species and functional diversity on biomass in differently managed grasslands.2. We studied the development of biomass yields over the last 37 years on a grassland site in Germany, with mowing at five frequencies (one to eight times per year), each with and without fertilization. We measured plant species richness (SR) and functional richness (FR) (the diversity of functional species properties) using presence–absence indices. We also measured species evenness (SE), functional evenness and functional divergence (FD) using abundance weighted indices. Climate was included as the mean temperature and sum of precipitation during the growing period. By relating biomass to the above‐mentioned climatic and biodiversity parameters, we extracted the contribution of these to biomass yields.3. Biomass changed over time for all treatments and was maximal at intermediate mowing frequencies. Temporal changes in biomass were partly explained by climate and different aspects of biodiversity, although this differed significantly between treatments. The relative importance of precipitation was highest at high mowing frequencies; the contribution of temperature was highest on less disturbed, unfertilized plots. FR and SR influenced biomass changes in the most intensive disturbance regimes on unfertilized and fertilized plots respectively. FD was most important on intensively disturbed, fertilized plots. SE influenced biomass at low mowing frequencies.4. Synthesis and applications. Climate, species and functional diversity influence annual grassland biomass yields but their importance depends on nutrient status and management frequency. Our results indicate that management treatments with intermediate disturbance regimes will maximize biomass yields in temperate environments. This recommendation may become even more important in the context of climate change: at intermediate mowing frequencies the influence of climatic variables on biomass is less important by comparison to different aspects of biodiversity.

A hierarchical, multi-scale, management-responsive model of Mediterranean vegetation dynamics

Ecosystems of the Mediterranean basin are characterized by a heterogeneous and dynamic landscape mosaic of vegetation formations. This landscape has been shaped over millenia by disturbances associated with agropastoral land use: clearing, grazing and burning, and by regeneration processes of the natural woody vegetation. The ability to predict the effects of management decisions on the structure and composition of the vegetation is essential for present and future land management. To improve this ability we developed a hierarchical multi-scale, management-responsive model of vegetation dynamics. The model was initially developed at two hierarchically nested spatial scales: “cell” (1 m 2) and “site” (400 m 2). The cell model was conceptually derived from a “States and transitions” framework. Vegetation state in a cell was defined by the identity, age and height of the dominant plant functional type. Biological realism was enhanced by defining transition probabilities not as constants but as functions of variables that represent biological processes: reproduction, colonization, expansion, replacement, aging, mortality, grazing, regrowth after fire, and dispersal. Up-scaling to the site was essentially done by averaging: vegetation state of a site was defined by frequencies of cells in different states. However, there was a feedback from site to cell by seed production and dispersal functions. The model was parameterized and implemented for a study area in Mediterranean woody vegetation (garrigue) in Israel. The biological components and processes represented in the model are sufficiently general to allow adaptation to similar vegetation in other Mediterranean regions. A series of simulation experiments over 100 years was carried out under different management scenarios defined by combinations of initial vegetation cover (cleared and control plots), cattle and goat grazing intensity and fire frequency. The results highlight the following trends: • In most scenarios, initial cover has an effect even after 100 years. • In the absence of any disturbance, vegetation becomes dominated by the two tallest woody functional types. • Under extremely intense multiple disturbances (fire + goat grazing), the vegetation becomes dominated by Herbaceous plants. • Scenarios of intermediate disturbance regimes often result in heterogeneous vegetation composition. • Under most of the management scenarios examined, vegetation composition had not reached a steady state (or steady cycle of fire-regrowth) even after 100 years. The model can be a useful tool for land managers by eliminating obviously undesirable scenarios and focusing on a limited set of desirable or acceptable scenarios.

The effect of colonization and competition processes on the relation between disturbance and diversity in plant communities

Many theoretical and field studies have emphasized the impact of disturbance in the dynamics and diversity of sessile organism communities. This view is best reflected by the Intermediate Disturbance Hypothesis (IDH), which states that a maximum of diversity is found in ecosystems or communities experiencing intermediate disturbance regimes or at an intermediate stage of development since the last major disturbance event. Although theoretical models based on competitive interactions tend to validate this hypothesis, a recent meta-analysis of field experiments revealed that the mono-modal relationship between disturbance and diversity might not be a general pattern. In this article, we investigate the relationship between disturbance and diversity through the study of patch models, combining two types of competitive interactions: with or without competitive hierarchy, with two mechanisms influencing colonization: negative frequency dependence in colonization rates and immigration. These combinations led to various disturbance–diversity patterns. In the model without competitive hierarchy (founder effect model), a decreasing relationship appeared to be the rule as mentioned in previous studies. In the model with competitive hierarchy, the IDH pattern was obtained for low frequency dependence and low immigration. Nevertheless, high negative frequency dependence in colonization rates led to a decreasing relationship between disturbance and diversity. In contrast, high immigration led to an increasing relationship. The coexistence window (the range of disturbance intensity allowing coexistence) was the widest for intermediate immigration rates. For random species assemblages, patterns with multiple peaks were also possible. These results highlight the fact that the mono-modal IDH pattern should not be considered a rule. Competition and colonization mechanisms have a profound impact on the relationship between disturbance and diversity.

Channel pattern and river-floodplain dynamics in forested mountain river systems

Channel pattern effectively stratifies the dynamics of rivers and floodplains in forested mountain river systems of the Pacific Northwest, USA. Straight channels are least dynamic, with relatively slow floodplain turnover and floodplains dominated by old surfaces. Braided channels are most dynamic, with floodplain turnover as low as 25 years and predominantly young floodplain surfaces. Island-braided and meandering channels have intermediate dynamics, with moderately frequent disturbances (erosion of floodplain patches) maintaining a mix of old and young surfaces. Return intervals for the erosion of floodplains increase in the order of braided, island-braided, meandering, and straight (8, 33, 60, and 89 years, respectively). A threshold for the lateral migration of a channel occurs at a bankfull width of 15–20 m. The most likely mechanism underlying this threshold is that larger channels are deep enough to erode below the rooting zone of bank vegetation. Above this threshold, channels not confined between valley walls exhibit channel patterns distinguishable by slope and discharge, and slope–discharge domains can be used to predict channel patterns. Meandering and braided patterns are most consistently identified by the model, and prediction errors are largely associated with indistinct transitions among channel patterns that are adjacent in plots of slope against discharge. Locations of straight channels are difficult to identify accurately with the current model. The predicted spatial distribution of channel patterns reflects a downstream decline in channel slope, which is likely correlated with a declining ratio of bed load to suspended load. Ecological theory suggests that biological diversity should be highest where the intermediate disturbance regime of island-braided channels sustains high diversity of habitat and successional states through time.

Invasibility: the local mechanism driving community assembly and species diversity

Recent theoretical developments involving community assembly on the one hand, and invasion biology on the other, suggest a gradual convergence in thought between what have been two largely separate theoretical initiatives. The term ‘‘invasibility’’ emerged in the field of invasion ecology to describe the susceptibility of environments to invasion by species from other regions of the world. Although Elton did not use the term ‘‘invasibility’’ in his pioneering book (1958), he did employ the concept, referring to an ecosystem’s ‘‘vulnerability to invasion’’. Given its original definition, the concept of invasibility has been limited in its scope and use, with rather little application to the larger field of community ecology. However, our assessment and usage of the concept (Davis et al. 2000, 2001) has prompted us to consider invasibility as a more general condition of all environments. This expanded perspective of invasibility has caused us to reconsider some fundamental questions and issues regarding community assembly and species diversity as well as recent discussions involving the notion of metacommunities (Leibold and Miller 2004, Leibold et al. 2004). By metacommunity, we mean a set of local communities that are linked by dispersal of multiple, and potentially interacting, species (Leibold et al. 2004). Recent theoretical efforts to characterize community assembly processes have reemphasized understanding the importance of interactions between local and regional processes (Levine 2000, Hubbell 2001, Tilman 2004, Foster and Dickson 2004, Jiang and Morin 2004, Steiner and Leibold 2004, Leibold et al. 2004). There is general agreement that the diversity of the regional species pool and the extent of dispersal of the species from this pool throughout the region are the principal regional processes involved. However, investigators have emphasized the importance of different local conditions and processes, including productivity (Jiang and Morin 2004, Steiner and Leibold 2004), demographic stochasticity (Tilman 2004), ecosystem size (Fukami 2004), biotic limitation of diversity (Tilman 2004), and even extent of tree lean, the latter which affects colonization success of epiphyes (Snall et al. 2005). We propose that the notion of invasibility can serve as a unifying concept in these discussions and thereby can facilitate current efforts to develop a more comprehensive and realistic theory of community assembly and metacommunity dynamics.

HARVESTING DISRUPTS BIOLOGICAL CONTROL OF HERBIVORES IN A SHORT-ROTATION COPPICE SYSTEM

Disturbances such as harvesting often interfere with the ecological processes that lead to the biological control of insect pests. For willows, grown as short-rotation coppice crops harvested every third to fifth year, it has been suggested that high plant quality in the resprouting shoots after harvesting may explain observed high densities of herbivorous insects, especially leaf beetles (Coleoptera: Chrysomelidae), in the plantations. In this study, we show that generalist predators may be important as regulators of leaf beetle populations. All three leaf beetle species, which were studied for five years in 12 plantations, showed a negative correlation between the population growth rate from spring to fall and the abundance of the most common generalist predator Orthotylus marginalis (Heteroptera: Miridae). For the most abundant leaf beetle, Phratora vulgatissima, we also found a significant positive correlation between its population growth rate and egg survival, indicating an overall effect of predation on herbivore population growth. Harvesting, which takes place during the winter, had a negative effect on the abundance of leaf beetles and predators. However, the first year after harvesting, all three leaf beetle species regained this loss with a very high population growth rate. A reason for the better ability of the herbivores to recover from the disturbance may be that they, unlike the predators, mainly overwinter outside the plantations. All three leaf beetles peaked in density three years after harvesting whereas the density of generalist natural enemies increased or leveled off during the five-year period after harvesting. We conclude that predation by generalist predators is potentially important for population control of leaf beetles in willow coppice, but that the intermediate disturbance regime of around five years between harvests, appears to be too short to avoid disruption of biological control. Alternatives for more efficient biological control in short-rotation coppice systems may be a longer period between harvests that enables the predators to fully respond numerically, to leave natural enemies refuges at harvest, or to harvest adjacent plantations asynchronously.

Moving from pattern to process: coexistence mechanisms under intermediate disturbance regimes

AbstractCoexistence mechanisms that require environmental variation to operate contribute importantly to the maintenance of biodiversity. One famous hypothesis of diversity maintenance under disturbance is the intermediate disturbance hypothesis (IDH). The IDH proposes patterns of peaked diversity under intermediate disturbance regimes, based on a tension between competitively superior species and species which can rapidly colonize following disturbance. We review the literature, and describe recent research that suggests that more than one underlying mechanism can generate this unimodal diversity pattern in disturbed environments. Several exciting emerging research areas are identified, including interactions between disturbance types, operation of the IDH in multi‐trophic systems, and changes in disturbance regimes. However, empirical work is still focussed on describing the IDH pattern, with little emphasis on identifying its mechanistic basis. We discuss how to extend methods for identifying different coexistence mechanisms, developed in the theoretical literature, to experimental research. In an attempt to operationalize these various ideas we outline a hypothetical IDH research programme. A solid understanding of the life history attributes of the component species and their responses to disturbance will facilitate identification of the coexistence mechanism(s) underlying the IDH pattern, and provide a framework by which empirical and theoretical results can be more fully integrated.

THE INTERMEDIATE DISTURBANCE HYPOTHESIS: PATCH DYNAMICS AND MECHANISMS OF SPECIES COEXISTENCE

The intermediate disturbance hypothesis (IDH) has been used for several decades as an explanation for the coexistence of species in ecological communities. It is intuitively simple, but deceptively so. We show, via discussion and examples, that the IDH is not one mechanism of coexistence, but rather summarizes a set of similar phenomena that can arise from the action of several different coexistence mechanisms. These underlying mechanisms are defined by the various ways in which species differ in their response to disturbance-induced spatial and temporal variability in resources and environmental conditions. As an example, the original specification of the IDH required patchy disturbances for coexistence. However, because the underlying mechanisms of coexistence can also operate at the within-patch scale, patchy disturbances are not a necessary requirement for coexistence under intermediate-disturbance regimes. These conclusions are illustrated through the analysis of three models: a spatial within-patch model, a spatial between-patch model, and a purely temporal model. All three generate similar patterns of coexistence under intermediate disturbance, yet underlying that coexistence lie at least two quite-distinct mechanisms of species coexistence: the storage effect and relative nonlinearity. The results from our analyses suggest that, as a promoter of species coexistence, the IDH is both broader in scope and richer in detail than has previously been recognized.

Vegetation Structure and Soil Properties in Ecuadorian Páramo Grasslands with Different Histories of Burning and Grazing

High-altitude grasslands in the north Andes (páramos) are subject to frequent fires that are usually set by farmers to support traditional cattle-raising systems. Apparently, this practice has caused dramatic changes in the structure and composition of native vegetation, but the mechanisms, magnitude, and direction of these changes still need to be documented for a broader range of conditions. This paper describes the differences in selected soil properties, ground cover, grass tussock structure, and populations of giant Andean rosettes (Espeletia pycnophylla) in four páramo sites in northwestern Ecuador, characterized by contrasting patterns of burning and grazing. The only differences between the soil properties of the sites were the higher pH in the least disturbed site, and the high concentration of P in the more recently burned site. Along the sequence of least impacted to most impacted sites, we observed a decrease in grass tussock cover, and an increase in the amount of bare ground and number of fragments per tussock. In the most undisturbed site tussock coverage was extremely low due to the dominance of shrubs. The density of giant Andean rosettes was higher in the sites with intermediate disturbance regimes, while the mortality of adult stem rosettes was significantly higher at the more recently burned site. The significance of these differences is discussed in the context of the regeneration of páramo vegetation after burning and grazing disturbance.

The influence of agricultural land-use intensity on bryophyte species richness

This study is a quantitative approach to the estimation of bryophyte species richness in relation to land-use intensity at three spatial scales in highly cultivated areas. A total of 460 randomly selected habitats and their various substrates within 29 study sites were investigated with regard to their land-use intensity and their bryophyte species richness in an agricultural region of eastern Austria. On bare soils (substrate-scale), low but regular disturbance increases bryophyte diversity in comparison to lower land-use intensity. However, more frequent disturbance (e.g. ploughing more than two times a year) dramatically reduces species richness at these sites, with more than 50% of these sites showing no bryophytes. The production of reproductive units (sporophytes and vegetative units) is highest at an intermediate disturbance regime. On the habitat, as well as on the landscape-scale, there is a significant increase in total bryophyte species number as well as in the number of threatened species with decreasing land-use intensity. This is mainly due to habitat and structural diversity, which increases with decreasing land-use intensity. There are significant correlations between landuse intensity, structural diversity and species richness at the habitat as well as on the landscape scale.

Invasibility of Species‐Rich Communities in Riparian Zones

Invasibility of riparian plant communities was estimated by the percentage of alien species found along the Adour River (Southwest France) and along Lockout Creek, McKenzie River, and Willamette River (Central Cascades, Oregon, U.S.A.). At the patch scale, the invasibilities of riparian plant communities were compared between one exceptionally rich site of the Adour River and patches selected in the Hoh and Dungeness watersheds (Olympic Peninsula, Washington, U.S.A.). Alien species represented 24% of 1396 species for the Adour and 30% of 851 species for the McKenzie. They represented 24% of 148 species for the Hoh drainage and 28% of 200 species for the Dungeness drainage. Similar trends were found along the Adour River and along the McKenzie River for changes in total number of species per site and in percentages of alien species per site. These trends may be related to the intermediate disturbance regimes and to the physical structure of the riparian corridors. Climatic and human factors are also involved in these longitudinal changes. Positive linear relationships were found between the total number of species and the percentage of aliens observed in each site. At the patch scale, most of the sampled communities contained alien species. Although mature vegetative patches appeared to be invasible, young communities contained more alien species than older ones. For entire corridors, a positive linear relationship was found between total species richness and percentage of alien species in each patch type for the richest site of the Adour River. This may be partially explained by landscape features considered in a successional context. We suggest the use of empirical rules, and stress the importance of riparian systems for monitoring the conservation of local and regional species pools are suggested.