Disturbance Gradient Research Articles

Intense mycorrhizal root colonization in a human-modified landscape of the Caatinga dry forest

Arbuscular mycorrhizal fungi (AMF) symbioses are thought to help plants to cope with harsh environments and to affect not only plant fitness, but also ecological organization from population to ecosystem level. Here, we investigated to what extent this association was present, and what the major environmental drivers were in a human-modified landscape of the Caatinga dry forest in north-eastern Brazil. AMF spore density in the soil and the frequency of AMF root colonization were examined at the species and forest-stand level for the nine most abundant and widespread woody plant species in 18 forest plots across gradients of chronic anthropogenic disturbance, rainfall and soil attributes. Soil spore density was low (1.0 ± 0.6 spore/g soil) across all conditions. However, AMF were present across all species, and colonization frequencies were high (50%, on average) throughout the entire environmental gradients. At species level, AMF colonization frequency only responded to environmental variables (disturbance, rainfall, soil available phosphorus, soil organic matter and soil pH) in one species (Cnidoscolus pubescens). At forest-stand level, colonization frequency responded positively to both soil pH and rainfall, but not to chronic anthropogenic disturbance. Our results suggest AMF symbiosis is widespread among the plant species and ecological conditions in the Caatinga. Moreover, this symbiosis, which in this case was mediated by rainfall, is likely affecting the resilience of the Caatinga dry forest as it is associated with the most abundant and biomass-relevant forest species and in the harshest conditions for plant survival and reproduction (i.e. nutrient-poor soils and dry habitats).

Biotic homogenization of wetland vegetation in the conterminous United States driven by Phalaris arundinacea and anthropogenic disturbance

Biotic homogenization (BH), the process by which β-diversity erodes, is suspected to be severe among plant communities due to widespread anthropogenic disturbances. However, few studies have directly linked anthropogenic disturbance with patterns of BH. The US National Wetlands Condition Assessment (NWCA) provides an opportunity to investigate patterns of β-diversity related to anthropogenic disturbances. Our objectives were to compare β-diversity between highly, intermediately and less-disturbed herbaceous emergent wetlands across the temperate region of the conterminous US, and to identify species with greatest influence on β-diversity patterns within disturbance categories. Using species occurrence and abundance data, average distances to group centroids in ordination space were used to assess wetland β-diversity across a disturbance gradient. Species contributions to β-diversity were calculated per disturbance category. Compared to the least and intermediately disturbed wetlands, highly disturbed wetlands had significantly lower β-diversity based on species abundances across the study area. When using only species occurrences, highly disturbed wetlands had significantly lower β-diversity than intermediately disturbed wetlands. No significant differences were found amongst disturbance categories at the within-site scale. The invasive grass Phalaris arundinacea had the greatest influence on β-diversity, especially in the most disturbed wetlands. Our results demonstrate that anthropogenic disturbances have homogenized regional botanical diversity. This homogenization is a function of disturbance intensity, which, after a threshold is crossed creates environmental conditions that lead to the local dominance of a single, widespread invader. This process is repeated across the region, scaling up to homogenize the flora of the entire region.

Community recovery dynamics in yellow perch microbiome after gradual and constant metallic perturbations

BackgroundThe eco-evolutionary processes ruling post-disturbance microbial assembly remain poorly studied, particularly in host-microbiome systems. The community recovery depends not only on the type, duration, intensity, and gradient of disturbance, but also on the initial community structure, phylogenetic composition, legacy, and habitat (soil, water, host). In this study, yellow perch (Perca flavescens) juveniles were exposed over 90 days to constant and gradual sublethal doses of cadmium chloride. Afterward, the exposure of aquaria tank system to cadmium was ceased for 60 days. The skin, gut and water tank microbiomes in control and treatment groups, were characterized before, during and after the cadmium exposure using 16s rDNA libraries and high throughput sequencing technology (Illumina, Miseq).ResultsOur data exhibited long-term bioaccumulation of cadmium salts in the liver even after two months since ceasing the exposure. The gradient of cadmium disturbance had differential effects on the perch microbiota recovery, including increases in evenness, taxonomic composition shifts, as well as functional and phylogenetic divergence. The perch microbiome reached an alternative stable state in the skin and nearly complete recovery trajectories in the gut communities. The recovery of skin communities showed a significant proliferation of opportunistic fish pathogens (i.e., Flavobacterium). Our findings provide evidence that neutral processes were a much more significant contributor to microbial community turnover in control treatments than in those treated with cadmium, suggesting the role of selective processes in driving community recovery.ConclusionsThe short-term metallic disturbance of fish development has important long-term implications for host health. The recovery of microbial communities after metallic exposure depends on the magnitude of exposure (constant, gradual), and the nature of the ecological niche (water, skin, and gut). The skin and gut microbiota of fish exposed to constant concentrations of cadmium (CC) were closer to the control negative than those exposed to the gradual concentrations (CV). Overall, our results show that the microbial assembly during the community recovery were both orchestrated by neutral and deterministic processes.8KR8qv6CEcKmimx9at5UakVideo Abtract.

El Niño impacts on human‐modified tropical forests: Consequences for dung beetle diversity and associated ecological processes

AbstractOur knowledge of how tropical forest biodiversity and functioning respond to anthropogenic and climate‐associated stressors is limited. Research exploring El Niño impacts are scarce or based on single post‐disturbance assessments, and few studies assess forests previously affected by anthropogenic disturbance. Focusing on dung beetles and associated ecological functions, we assessed (a) the ecological effects of a strong El Niño, (b) if post‐El Niño beetle responses were influenced by previous forest disturbance, and (c) how these responses compare between forests impacted only by drought and those affected by both drought and fires. We sampled 30 Amazonian forest plots distributed across a gradient of human disturbance in 2010, 2016, and 2017—approximately 5 years before, and 3–6 and 15–18 months after the 2015–16 El Niño. We found 14,451 beetles from 98 species and quantified the beetle‐mediated dispersal of >8,600 seed mimics and the removal of c. 30 kg of dung. All dung beetle responses (species richness, abundance, biomass, compositional similarity to pre‐El Niño condition, and rates of dung removal and seed dispersal) declined after the 2015–16 El Niño, but the greatest immediate losses (i.e., in 2016) were observed within fire‐affected forests. Previous forest disturbance also influenced post‐El Niño dung beetle species richness, abundance, and species composition. We demonstrate that dung beetles and their ecological functions are negatively affected by climate‐associated disturbances in human‐modified Amazonian forests and suggest that the interaction between local anthropogenic and climate‐related stressors merits further investigation.Abstract in Portuguese is available with online material.

Intraspecific variation in tree lizard escape behaviour in relation to habitat and temperature

Abstract Optimal escape theory has proven useful for understanding the dynamics of antipredator behaviour in animals; however, approaches are often limited to single-population studies. We studied how the escape behaviour of tree lizards (Urosaurus ornatus) varied across a disturbance gradient. We also considered how sex, body temperature, and perch temperature affected their escape decisions. Both sexes exhibited similar response patterns; however, lizards in the most-disturbed habitat, as well as cooler (body or perch temperature) lizards, initiated escape earlier (but did not flee further) than other animals. Increased wariness as indicated by earlier escape suggests that frequently-disturbed, more-open localities may be stressful habitats for species like U. ornatus. In addition, because cooler temperatures limit locomotor performance capacity, escape decisions should also depend on a species’ thermal ecology. Overall, we stress the importance of multi-population approaches for capturing the variety of ways species adaptively respond to the threat of predation across habitat gradients.

Influence of soil tillage on natural regulation of the cabbage root fly Delia radicum in brassicaceous crops

Ground dwelling predators provide regulation services of several insect pests. Enhancing these services may be a step toward integrated crop protection. Many studies have shown that soil tillage is deleterious to ground dwelling predators but pest regulation processes and services have rarely been measured. We performed an experiment to study whether simplifying soil tillage before the establishment of spring broccoli enhanced ground dwelling predator populations and the control they provide on Delia radicum. The direct effect of tillage on arthropods was assessed by comparing their emergence rates in plots differing in soil tillage management. The natural regulation service was assessed by comparing a control and an exclusion treatment in which predators were removed. The effect of soil tillage on carabids, spiders and staphylinids did not match the gradient of disturbance induced by tillage treatments. Tillage did not appear to affect the predators that likely contribute to D. radicum regulation. Consistently, the number of pests suppressed and the root injuries were unaffected by tillage treatments. The main deleterious effect of soil tillage was on the emergence of those carabid species that overwinter partly as larvae, suggesting that spring tillage could affect pest control in the following crops.

Disturbance-mediated consumer assemblages determine fish community structure and moderate top-down influences through bottom-up constraints.

Disturbance is a strong structuring force that can influence the strength of species interactions at all trophic levels, but controls on the contributions to community structure of top-down and bottom-up processes across such gradients remain poorly understood. Changes in the composition of predator and consumer assemblages, and their associated traits, across gradients of environmental harshness (e.g. flooding), are likely to be a particularly important influence on the strength of top-down control and may drive bottom-up constraints. We examined how consumers with particular traits, and the predators that consumed them, varied across a gradient of stream flooding disturbance and used experiments to assess the predation impact on those contrasting consumer communities (ultimately quantifying how flood disturbance altered the strength of top-down control). Consumer community composition and mobility were strongly related to flood disturbance; the biomass and drift of protected primary consumers (i.e. those with morphological defences) decreased with increasing flood disturbance. Predatory fish species had different disturbance niches, and path analysis identified that both direct flood disturbance effects and indirect bottom-up constraints of flood disturbance on consumers influenced predatory fish composition and biomass. Fish generally fed most effectively on consumer types associated with their particular niche, but all fish were strongly size-selective when feeding on protected consumers. Although protected consumers did not grow large enough to escape predation, an in situ experiment showed protected consumers were at a reduced risk of predation as disturbance increased compared to unprotected consumers. Overall, top-down control declined with flood disturbance, but the effect depended on consumer traits. Predatory fish were only capable of exerting top-down control on protected consumers in benign habitats but impacted unprotected consumers across a larger range of the disturbance gradient. Collectively our findings suggest that a shift towards a more disturbed state will probably result in reduced predator impacts and a weakening of top-down control. Moreover, predicted increases in the frequency and intensity of climatic events causing disturbance, such as flooding, are likely to result in a community shift that disproportionately impacts protected consumers and the predators that utilize them as prey through the subsequent bottom-up constraints.

Impact of disturbance on vegetation, biomass and carbon stock in tropical forests of Tripura, Northeast India

In this study, the impact of anthropogenic disturbances on vegetation, soil characteristics, biomass and carbon stock was studied in tropical mixed deciduous forests of Tripura, North-east India. The forests stands were selected based on the presumed disturbance index and categorized in three distinct categories viz. Least Disturbed Forests (LDF), Moderately Disturbed Forests (MDF) and Highly Disturbed Forests (HDF). The study revealed that species diversity is much higher in LDF followed by MDF and HDF sites. Few species like Anogeissus accuminata, Lannea coromandelica, Schima wallichii, and Syzygium cumini are common in all the three categories of forests. The Shannon–Wiener diversity index ranged from 1.32 to 2.51 in all the stands. The phytosociological parameters varied along the disturbance gradient in different stands. The maximum tree density was 198.57 trees ha−1 in LDF, 176.67 trees ha−1 in MDF and least in HDF (111.67 trees ha−1). Soil Organic Carbon (SOC %) recorded maximum in LDF (1.41–1.67%) and minimum in HDF (0.91–1.36%). Tree biomass showed a significant relation with soil organic carbon stock (r = 0.998, p < 0.05). Furthermore, the present study suggests the need for quantification of forest disturbance and biomass which will improve carbon budget estimates and lead to better parameterization of forest carbon cycle models.

Quantifying spatiotemporal rhythm of stream metabolism along human disturbance gradients

This study aimed at investigating the effects of the urban wastewater treatment plant (WTP) discharges on the metabolism of Büyüksu Stream (Bolu, Turkey), and modelling the metabolism components as a function of measured environmental variables. Dissolved oxygen (DO) and water temperatures (Tw) were measured to estimate montly stream metabolism in the four reaches: Before and after discharges of the WTP, and the headwaters of Abant Creek and Mudurnu Creek feeding Büyüksu Stream. The DO and Tw measurements were performed for 17 months between August 2015 and December 2016. Metabolism components of community respiration (Rc), gross primary production (GPP) and net ecosystem metabolism (NEM) were estimated by using the two-station method. According to naturality gradient (reach disturbance gradients: before and after discharges, and headwaters of the creeks), mean metabolism components were compared by performing the one-way analysis of variance. The comparison results showed that the WTP discharges increased the average Rc from −30.6 g O2 m−2 day−1 to −130.9 g O2 m−2 day−1, and had no significant impact on the average GPP, statistically (15.6 and 9.1 g O2 m−2 day−1 before and after discharges, respectively). Validations of the multiple non-linear regression models of NEM and Rc gave coefficients of determination of 74.9% and 66.6%, respectively.

喀斯特关键带不同干扰梯度下土壤-岩石界面对土壤有机质水解酶活性的影响

喀斯特关键带不同干扰梯度下土壤-岩石界面对土壤有机质水解酶活性的影响

Assessment of Thalassia hemprichii seagrass metrics for biomonitoring of environmental status

Seagrass has long been known to be very sensitive to environmental changes, especially caused by human activities and has been used as a bioindicator for environmental condition of ecosystems. This research aimed to study 19 Thalassia hemprichii metrics (10 measured and 9 derived metrics) at two organizational levels (individual and population), to explore and confirm these metrics for development of a multimetric index of environmental quality. Seagrass meadows was selected along a gradient of an anthropogenic disturbance at Kepulauan Seribu (inhabited and uninhabited locations, fixed effect, namedly Status), 4 sites for each location (random effect nested in status) within intertidal and subtidal zones (random effect across Sites). We also briefly described about social-ecological system of seagrass being studied using a qualitative network model, for an understanding of interaction that affects the exploited seagrass ecosystems. The significance of variability between states, sites, and zones were examined using linear mixed effect model followed by exploratory factor analysis in the confirmatory factor analysis (CFA) framework (E/CFA strategy) to explore and confirm adequacy of the metrics as indicators for two-factor organizational levels (individual and population). Based on the analysis, leaf surface area, leaf wide, leaf area index, density, rhizome diameter, and Thalassia hemprichii cover differed significantly at the scale of interest and represented two levels of organizational levels (individual and population).

Impact Angle Constraint Dual-Layer Adaptive Guidance Law With Actuator Faults

To intercept maneuvering target considering impact angle constraint and actuator faults, a novel three-dimensional (3D) guidance law is proposed in this paper. To guarantee the interception, the multi-variable super-twisting-algorithm-like (STA-like) is adopted in the proposed guidance law, so as to drive the line-of-sight (LOS) angle to the desired value and its rate to zero in finite time in both pitch and azimuth directions. However, it is usually a difficult task for STA-like to select the proper design parameters, and the necessary disturbance gradient for STA-like is also not clearly known, owing to realistic actuator faults and the independence between missile and target. Moreover, the actuator faults in this paper are formulated as disturbances in the control scheme, and the necessary disturbance gradient for STA-like is not clearly known as well. To solve these problems, a multi-variable dual-layer adaptive scheme is employed to adjust the control gains and guarantees its precision. The theoretical analysis and numerical simulation results demonstrate the effectiveness of the proposed guidance law. The combination of STA-like and adaptive theory in the presented guidance law for the first time can guarantee the successful interception and can generate precise and robust control signal simultaneously with impact angle constraint and actuator faults consideration.

Fear of the dark? Contrasting impacts of humans versus lynx on diel activity of roe deer across Europe.

Humans, as super predators, can have strong effects on wildlife behaviour, including profound modifications of diel activity patterns. Subsequent to the return of large carnivores to human-modified ecosystems, many prey species have adjusted their spatial behaviour to the contrasting landscapes of fear generated by both their natural predators and anthropogenic pressures. The effects of predation risk on temporal shifts in diel activity of prey, however, remain largely unexplored in human-dominated landscapes. We investigated the influence of the density of lynx Lynx lynx, a nocturnal predator, on the diel activity patterns of their main prey, the roe deer Capreolus capreolus, across a gradient of human disturbance and hunting at the European scale. Based on 11 million activity records from 431 individually GPS-monitored roe deer in 12 populations within the EURODEER network (http://eurodeer.org), we investigated how lynx predation risk in combination with both lethal and non-lethal human activities affected the diurnality of deer. We demonstrated marked plasticity in roe deer diel activity patterns in response to spatio-temporal variations in risk, mostly due to human activities. In particular, roe deer decreased their level of diurnality by a factor of 1.37 when the background level of general human disturbance was high. Hunting exacerbated this effect, as during the hunting season deer switched most of their activity to night-time and, to a lesser extent, to dawn, although this pattern varied noticeably in relation to lynx density. Indeed, in the presence of lynx, their main natural predator, roe deer were relatively more diurnal. Overall, our results revealed a strong influence of human activities and the presence of lynx on diel shifts in roe deer activity. In the context of the recovery of large carnivores across Europe, we provide important insights about the effects of predators on the behavioural responses of their prey in human-dominated ecosystems. Modifications in the temporal partitioning of ungulate activity as a response to human activities may facilitate human-wildlife coexistence, but likely also have knock-on effects for predator-prey interactions, with cascading effects on ecosystem functioning.

Neglected but Potent Dry Forest Players: Ecological Role and Ecosystem Service Provision of Biological Soil Crusts in the Human-Modified Caatinga

Biological soil crusts (biocrusts) have been recognized as key ecological players in arid and semiarid regions at both local and global scales. They are important biodiversity components, provide critical ecosystem services, and strongly influence soil-plant relationships, and successional trajectories via facilitative, competitive, and edaphic engineering effects. Despite these important ecological roles, very little is known about biocrusts in seasonally dry tropical forests. Here we present a first baseline study on biocrust cover and ecosystem service provision in a human-modified landscape of the Brazilian Caatinga, South America's largest tropical dry forest. More specifically, we explored (1) across a network of 34 0.1 ha permanent plots the impact of disturbance, soil, precipitation, and vegetation-related parameters on biocrust cover in different stages of forest regeneration, and (2) the effect of disturbance on species composition, growth and soil organic carbon sequestration comparing early and late successional communities in two case study sites at opposite ends of the disturbance gradient. Our findings revealed that biocrusts are a conspicuous component of the Caatinga ecosystem with at least 50 different taxa of cyanobacteria, algae, lichens and bryophytes (cyanobacteria and bryophytes dominating) covering nearly 10% of the total land surface and doubling soil organic carbon content relative to bare topsoil. High litter cover, high disturbance by goats, and low soil compaction were the leading drivers for reduced biocrust cover, while precipitation was not associated Second-growth forests supported anequally spaced biocrust cover, while in old-growth-forests biocrust cover was patchy. Disturbance reduced biocrust growth by two thirds and carbon sequestration by half. In synthesis, biocrusts increase soil organic carbon (SOC) in dry forests and as they double the SOC content in disturbed areas, may be capable of counterbalancing disturbance-induced soil degradation in this ecosystem. As they fix and fertilize depauperated soils, they may play a substantial role in vegetation regeneration in the human-modified Caatinga, and may have an extended ecological role due to the ever-increasing human encroachment on natural landscapes. Even though biocrusts benefit from human presence in dry forests, high levels of anthropogenic disturbance could threaten biocrust-provided ecosystem services, and call for further, in-depth studies to elucidate the underlying mechanisms.

Imaging spectroscopy reveals the effects of topography and logging on the leaf chemistry of tropical forest canopy trees.

Logging, pervasive across the lowland tropics, affects millions of hectares of forest, yet its influence on nutrient cycling remains poorly understood. One hypothesis is that logging influences phosphorus (P) cycling, because this scarce nutrient is removed in extracted timber and eroded soil, leading to shifts in ecosystem functioning and community composition. However, testing this is challenging because P varies within landscapes as a function of geology, topography and climate. Superimposed upon these trends are compositional changes in logged forests, with species with more acquisitive traits, characterized by higher foliar P concentrations, more dominant. It is difficult to resolve these patterns using traditional field approaches alone. Here, we use airborne light detection and ranging‐guided hyperspectral imagery to map foliar nutrient (i.e. P, nitrogen [N]) concentrations, calibrated using field measured traits, over 400 km2 of northeastern Borneo, including a landscape‐level disturbance gradient spanning old‐growth to repeatedly logged forests. The maps reveal that canopy foliar P and N concentrations decrease with elevation. These relationships were not identified using traditional field measurements of leaf and soil nutrients. After controlling for topography, canopy foliar nutrient concentrations were lower in logged forest than in old‐growth areas, reflecting decreased nutrient availability. However, foliar nutrient concentrations and specific leaf area were greatest in relatively short patches in logged areas, reflecting a shift in composition to pioneer species with acquisitive traits. N:P ratio increased in logged forest, suggesting reduced soil P availability through disturbance. Through the first landscape scale assessment of how functional leaf traits change in response to logging, we find that differences from old‐growth forest become more pronounced as logged forests increase in stature over time, suggesting exacerbated phosphorus limitation as forests recover.

Habitat fragmentation in a Mediterranean-type forest alters resident and propagule mycorrhizal fungal communities

AimsAnthropogenic activities disturb forests and their associated mycorrhizal fungi. The combination of climate change and habitat fragmentation are linked to increased incidence of a canker disease in a Mediterranean-type forest tree in Western Australia. As changes in communities of mycorrhizal fungi could predispose these Mediterranean-type forest trees to decline, we investigated how two aspects of mycorrhizal fungal community structure, soil propagules and resident communities on mature trees, respond to habitat fragmentation. MethodsRoots were collected from a forest tree (Corymbia calophylla) across a disturbance gradient. Soil collected from the same disturbance gradient was used in a glasshouse bioassay with C. calophylla as the bioassay host (i.e., soil propagule community). After four months, we harvested the seedlings and collected roots. DNA was extracted from the field roots (resident community) and glasshouse roots (propagule community), amplified with fungal specific primers, labelled with specific barcodes and subjected to 454 pyrosequencing. ResultsMycorrhizal fungal community composition overlapped substantially between the soil propagule and resident communities. However, the resident community had greater mycorrhizal fungal richness than the soil propagule community. Habitat fragmentation had a similar effect on each community structure: communities along highly fragmented areas had different community compositions than communities in a healthy forest. ConclusionWith the increased mortality of C. calophylla forest trees in recent years along edge habitats, understanding the effects habitat fragmentation has on communities of mycorrhizal fungi will further elucidate host-mutualist interactions in these forest ecosystems. The changes in community composition of mycorrhizal fungal species in both propagule and resident pools can have cascading effects on future tree establishment and health by predisposing forest trees to other abiotic or biotic factors.

Are primary forests irreplaceable for sustaining Neotropical landscapes' biodiversity and functioning? Contributions for restoration using ecological indicators

AbstractChanges in biodiversity and ecosystem services are usually evaluated using taxonomic indicators or, to a lesser extent, functional indicators. The present work adds new insights on the links between taxonomic indicators and functional indicators within a disturbance gradient in the Brazilian Amazon. Ground dwelling beetles' functional groups were used to complement taxonomic diversity in indicating disturbance and/or ecosystem restoration. Significant correlations were found between taxonomic and functional indicators. Our results show that increasingly disturbed ecosystems are related with reductions in functional redundancy with probable detrimental effects on the future landscape resilience. Moreover, results highlight that even species‐rich Neotropical forest landscapes may be highly vulnerable to the removal of keystone diversity. Remarkably, secondary forests support equivalent functional diversity and, partially, taxonomic diversity to primary forests. Overall, disturbance indicators supported on complementary measures such as composition, species diversity and functionality were sensitive to the ecosystems' gradient and could be used for assessing the extent of landscape change but also to guide restoration and management practices in Neotropical forest landscapes.

Disturbances by the European ground squirrel enhance diversity and spatial heterogeneity of plant communities in temperate grassland

Fossorial rodents are recognized as diversity drivers in grassland ecosystems and are therefore considered ecosystem engineers and keystone species. However, there is a lack of evidence regarding this function for species in temperate grasslands, especially one of the most threatened, the European ground squirrel. We examined the effect of the European ground squirrel on plant species composition and diversity along the disturbance gradient mediated by their different population density and subsequently different disturbance intensity. We evaluated the effect of ground squirrel disturbance on plant diversity patterns in two plant communities with different species richness to determine whether the same pattern exists in contrasting habitats. In each plant community, we established transect of 25 × 250 m composed of 10 quadrats with different disturbance intensities of the ground squirrels. Vascular plant species were recorded in 320 plots of 1 m2. The distribution of individual species along the ground squirrel-mediated disturbance gradient was analysed using redundancy analysis. Diversity measures were calculated and modelled as a function of disturbances using generalized additive models. We observed significant compositional changes in plant communities accompanied by a reduction in dominant graminoid cover and a non-decreasing trend in forb cover along the disturbance gradient in both types of communities. We found that increasing disturbance activity leads to an increase in diversity at coarse spatial scale (625 m2) and spatial heterogeneity in species composition of both species-poor and species-rich plant community. The fine-scale (1 m2) diversity increased significantly only in species-poor community. Our results demonstrate that the European ground squirrel can be deservedly labelled as an important ecosystem engineer and keystone species promoting the diversity and heterogeneity of European temperate grasslands.

Mechanisms of trophic niche compression: Evidence from landscape disturbance.

Natural and anthropogenic disturbances commonly alter patterns of biodiversity and ecosystem functioning. However, how networks of interacting species respond to these changes remains poorly understood. We described aquatic food webs using invertebrate and fish community composition, functional traits and stable isotopes from twelve agricultural streams along a landscape disturbance gradient. We predicted that excessive deposition of fine inorganic sediment (sedimentation) associated with agricultural activities would negatively influence aquatic trophic diversity (e.g. reduced vertical and horizontal trophic niche breadths). We hypothesized that multiple mechanisms might cause trophic niche 'compression', as indicated by changes in realized trophic roles. Food-web properties based on consumer stable isotope data (δ13 C and δ15 N) showed that increasing sediment disturbance was associated with reduced trophic diversity. In particular, the aquatic invertebrate community occupied a smaller area in isotopic niche space along the sedimentation gradient that was best explained by a narrowing of the invertebrate community δ13 C range. Decreased niche partitioning, driven by increasing habitat homogeneity, environmental filtering and resource scarcity all seemingly lead to greater trophic equivalency caused by the collapse of the autochthonous food-web channel. Bayesian mixing-model analyses supported this contention with invertebrate consumers increasingly reliant on detritus along the sedimentation gradient, and predatory invertebrates relying more on the prey using these basal resources. The narrowing of the fish community δ13 C range along the sedimentation gradient contributed to an apparent 'trophic shift' towards terrestrial carbon, further indicating the loss of the autochthonous food-web channel. On the vertical trophic niche axis, fish became increasingly separated from aquatic invertebrates with an increase in their estimated trophic position. In combination, these responses were most likely mediated through reduced fish densities and a diminished reliance on aquatic prey. Although species losses remain a major threat to ecosystem integrity, the functional roles of biota that persist dictate how food webs and ecosystem functioning respond to environmental change. Sedimentation was associated with nonlinear reductions in trophic diversity which could affect the functioning and stability of aquatic ecosystems. Our study helps explain how multiple mechanisms may radically reshape food-web properties in response to this type of disturbance.

Interacciones de corales masivos con céspedes algales y otros organismos en arrecifes del Parque Nacional Natural Tayrona

In coral reefs, competition between sessile benthic organisms for access to space and other resources is a determining factor in community structuring. Recent coral mortality has favored the development of algal turfs, which are now competing with corals, often displacing them,although slowly. To determine the frequency of interactions between coral-turf and coral-other benthic categories, and how their distribution and results (coral wins or coral loses) are modulated by environmental factors, band photo transects were evaluated for 12 stations in the ParqueNacional Natural Tayrona, Santa Marta, Colombian Caribbean. It was found that algal turfs are the most frequent competitors of massive corals, and that the frequency of coral-turf interactions depends on the particular susceptibility of a coral species to the loss of tissue, and past disturbances that have affected the reef in which it lives. Furthermore, the results of these interactions depend on the ways the colonies are organized, meandroidand cerioid colonies being the most successful in competition against turfs, as their vertical growth constitutes an effective avoidance mechanism. By contrast, plocoid colonies are more prone to lose. In general, biological factors (the coral species in question and type of colonial organization) were determinants in the frequency and apparent result of coral-turf interactions, while environmental factors (disturbance gradient, depth, anddegree of wave exposure) appear not to play a predominant role. As a consequence, historical and future changes in coral species cover, and the nature of colony organization in the Santa Marta area, should reflect a combination of their susceptibility to deleterious agents and their capacityto counteract or evade competition, especially with algal turfs.