Intermediate Disturbance Hypothesis Research Articles

Disturbance and diversity: Lichen species richness decreases with increasing anthropogenic disturbance

Anthropogenic disturbance is rapidly increasing through habitat degradation, development, and deforestation. Gaps remain in understanding the effects of this disturbance on diverse and ecologically important organisms such as lichens. In North America, studies have focused on epiphytic macrolichens and catastrophic disturbance, largely ignoring microlichens and less severe disturbances. The Intermediate Disturbance Hypothesis predicts these moderate disturbances will lead to higher species richness. Here we examine the effects of anthropogenic disturbance in the form of land management on overall lichen richness, including microlichens, and on the species richness of eight lichen functional groups. The study draws on a comprehensive data set of 872 species, in 208 one-hectare plots throughout the Southern Appalachian Mountains, a global biodiversity hotspot in eastern North America. A habitat quality index based on an established forestry metric was used as a proxy for anthropogenic disturbance and was quantified using a 10-part score including categories such as percent native tree canopy cover and degree of fragmentation. Linear models were used to compare habitat quality scores to overall species richness and to species richness of functional groups. Rather than following the Intermediate Disturbance Hypothesis, all groups uniformly follow a negative linear relationship: as disturbance increases, species richness decreases. This pattern held even for widely variable functional groups such as morphotype. Effective conservation of lichen richness should prioritize the maintenance of existing older, less-disturbed stands within large, contiguously forested areas.

Nutrient enrichment decreased the Cross-Taxon congruence across bacteria, fungi, and zoobenthos in sediment

Eutrophication is a major problem currently affecting sediment biodiversity in different kingdoms. Surveys at different spatial scales have suggested that nutrient enrichment drove the congruence in biodiversity across different kingdoms. In this study, we examined the diversity indexes of bacteria, fungi, and zoobenthos in Dongting Lake, which is undergoing different trophic levels. The species richness and Shannon index of bacteria, fungi, and zoobenthos were all consistent with the intermediate disturbance hypothesis along the content of TN and TP, with the hysteresis of fungal group. Significant cross-taxon congruence was found in species richness between bacteria and zoobenthos, and Shannon index between bacteria and fungi. Also, the strength of congruence in biodiversity among bacteria, fungi and zoobenthos would decrease along nutrient enrichment. Multi- and single-kingdom networks found that bacterial group contained more nodes and edges compared to fungal and zoobenthos groups, and bacterial removal led to the lowest natural connectivity. And highest positive interaction proportion and lowest absolute value of natural connectivity slope were observed in zoobenthos group. These results highlighted the importance of bacteria as stabilizer in sediment ecosystem, and the most sensitive feature of zoobenthos to nutrient enrichment. Taken together, our results confirmed the cross-taxon congruence between multi-kingdoms, and the effectiveness of bacteria as a potential bioindicator of ecological change.

Grazing led to an increase in the root: shoot ratio and a shallow root system in an alpine meadow of the Tibetan plateau

Grazing is a main land use of natural grasslands in the world, which has both positive and negative impact on plant community structure and ecosystem functioning. However, the effects of long-term grazing management on the plant–soil system, in particular above- and belowground community characteristics, are still not well understood in alpine meadow community. In this study, we investigated the vegetation, roots, and soil properties under three management types (16 years of fencing since 2004-2020, moderate grazing and heavy grazing managements) in an alpine meadow on the Tibetan Plateau. The results showed that, compared with moderate grazing meadows, long-term fencing increased plant community cover, above- and belowground biomass, proportion of grass and litter but reduced forbs and soil bulk density, which caused the increases in soil organic carbon, total nitrogen and water content and the decreases in soil pH. However, heavy grazing led to opposite changes in proportion of grass, community biomass and soil physicochemical properties. The maximum of species richness and plant density appeared in moderate grazing meadows, supporting the intermediate disturbance hypothesis, and it can maintain above- and belowground biomass and soil physicochemical properties at medium level. Grazing increased the root: shoot ratio and caused root system shallow, which is consistent with the optimal partitioning hypothesis. Overall, our study suggested that moderate grazing is a more reasonable grazing management for sustainable development in alpine meadows of Tibetan Plateau, fencing could be an effective management strategy for vegetation restoration as well as for nutrient sequestration in degraded grasslands, but long-term fencing dose not benefit for biodiversity maintenance.

Systems theory, thermodynamics and life: Integrated thinking across ecology, organization and biological evolution.

In this paper we explore the relevance and integration of system theory and thermodynamics in terms of the Earth system. It is proposed that together, these fields explain the evolution, organization, functionality and directionality of life on Earth. We begin by summarizing historical and current thinking on the definition of life itself. We then investigate the evidence for a single unit of life. Given that any definition of life and its levels of organization are intertwined, we explore how the Earth system is structured and functions from an energetic perspective, by outlining relevant thermodynamic theory relating to molecular, metabolic, cellular, individual, population, species, ecosystem and biome organization. We next investigate the fundamental relationships between systems theory and thermodynamics in terms of the Earth system, examining the key characteristics of self-assembly, self-organization (including autonomy), emergence, non-linearity, feedback and sub-optimality. Finally, we examine the relevance of systems theory and thermodynamics with reference to two specific aspects: the tempo and directionality of evolution and the directional and predictable process of ecological succession. We discuss the importance of the entropic drive in understanding altruism, multicellularity, mutualistic and antagonistic relationships and how maximum entropy production theory may explain patterns thought to evidence the intermediate disturbance hypothesis.

Relationships between plant species richness and grazing intensity in a semiarid ecosystem.

Plant species richness is an important property of ecosystems that is altered by grazing. In a semiarid environment, we tested the hypotheses that (1) small-scale herbaceous plant species richness declines linearly with increasing grazing intensity by large ungulates, (2) precipitation and percent sand interact with grazing intensity, and (3) response of herbaceous plant species richness to increasing intensity of ungulate grazing varies with patch productivity. During January-March 2012, we randomly allocated 50, 1.5-m × 1.5-m grazing exclosures within each of six 2500 ha study sites across South Texas, USA. We counted the number of herbaceous plant species and harvested vegetation in 0.25-m2 plots within exclosures (ungrazed control plots) and in the grazed area outside the exclosures (grazed treatment plots) during October-November 2012-2019. We estimated percent use (grazing intensity) based on the difference in herbaceous plant standing crop between control plots and treatment plots. We selected the negative binomial regression model that best explained the relationship between grazing intensity and herbaceous plant species richness using the Schwarz-Bayesian information criterion. After accounting for the positive effect of precipitation and percent sand on herbaceous plant species richness, species richness/0.25 m2 increased slightly from 0% to 30% grazing intensity and then declined with increasing grazing intensity. Linear and quadratic responses of herbaceous plant species richness to increasing grazing intensity were greater for the least productive patches (<15.7 g/0.25 m2) than for productive patches (≥15.7 g/0.25 m2). Our results followed the pattern predicted by the intermediate disturbance hypothesis model for the effect of grazing intensity on small-scale herbaceous plant species richness.

The vertical distribution of modern pollen in the southeastern edge of the Tibetan Plateau, China

The tropical mountainous area of the southeastern Tibetan Plateau (TP) is a hotspot of ecological environment research because of its rich biodiversity. However, limited knowledge regarding modern pollen rain within this area has impeded further investigation of eco-environmental evolution using the pollen record. We collected 62 modern pollen samples from lowland tropics to the subalpine region (740–3550 m a.s.l.) in Medog County, located in the southeastern TP, and performed pollen analysis. Cluster analysis, redundancy analysis, variance partitioning analysis, and boosted regression trees were employed to investigate the relationship between pollen and vegetation/climate. Additionally, Hill numbers (N0, N1, N2) and specificity–occupancy plots were utilized to explore plant diversity. The results indicate that modern pollen rain in Medog reflects well the current characteristics of the vegetation, encompassing both primary and secondary features. Pollen of Cyclobalanopsis, Tsuga and Abies, which are dominant components of the primary vegetation above 2100 m a.s.l., can serve as indicator species for paleovegetation reconstructions. Mean annual precipitation is the most important climatic determinant affecting pollen distribution. Pollen indices reveal that the plant diversity in this study area has a bimodal pattern of ‘relatively high–low–highest–low’ with increasing altitude. The plant diversity above 2100 m a.s.l. is mainly decided by primary vegetation, whereas below 2100 m a.s.l. it can be explained by the intermediate disturbance hypothesis. Furthermore, it is advisable to exercise caution when dealing with arboreal pollen evenness in open mountainous environments due to the increased potential pollen source area caused by valley wind.

Archaea are better adapted to antimony stress than their bacterial counterparts in Xikuangshan groundwater

Archaea are important ecological components of microbial communities in various environments, but are currently poorly investigated in antimony (Sb) contaminated groundwater particularly on their ecological differences in comparison with bacteria. To address this issue, groundwater samples were collected from Xikuangshan aquifer along an Sb gradient and subjected to 16S rRNA gene amplicon sequencing and bioinformatic analysis. The results demonstrated that bacterial communities were more susceptibly affected by elevated Sb concentration than their archaeal counterparts, and the positive stimulation of Sb concentration on bacterial diversity coincided with the intermediate disturbance hypothesis. Overall, the balance of environmental variables (Sb, pH, and EC), competitive interactions, and stochastic events jointly regulated bacterial and archaeal communities. Linear fitting analysis revealed that Sb significantly drove the deterministic process (heterogeneous selection) of bacterial communities, whereas stochastic process (dispersal limitation) contributed more to archaeal community assembly. In contract, the assembly of Sb-resistant bacteria and archaea was dominated by the stochastic process (undominated), which implied the important role of diversification and drift instead of selection. Compared with Sb-resistant microorganisms, bacterial and archaeal communities showed lower niche width, which may result from the constraints of Sb concentration and competitive interaction. Moreover, Sb-resistant archaea had a higher niche than that of Sb-resistant bacteria via investing on flexible metabolic pathways such as organic metabolism, ammonia oxidation; and carbon fixation to enhance their competitiveness. Our results offered new insights into the ecological adaptation mechanisms of bacteria and archaea in Sb-contaminated groundwater.

Age matters: variations in parasitoid diversity along a successional gradient in a dry semi-deciduous tropical forest.

Parasitoids are an important group of insects because their species number is among the highest. Multiple studies have addressed the relationships between forest successional age and insect diversity by focusing on herbivorous organisms, but changes in diversity of parasitoids are still poorly known. This work analyses the diversity of parasitoids in tropical forests representing three successional stages. A total of 30 traps were placed, ten in each forest successional stages. We estimated true diversity of Ichneumonidae species and guilds and explored the relationship between their diversity and the abundance of plant species using an Indicator Species Analysis; the relationship between parasitoid species and plant richness and abundance was tested using a Redundancy Analysis. A total of 1522 individuals and 168 morpho-species were captured in four months. Species richness showed no differences; however, parasitoid abundance was higher in young forest, while intermediate forest had the highest true diversity values (1D) with 71.6 effective species. According to insect guilds, richness, abundance, and diversity were similar in the three vegetation successional stages. This finding may be explained based on the intermediate disturbance hypothesis, which postulates that moderate disturbance levels favor the highest diversity. In conclusion, successional age matters, i.e., diversity is the highest in intermediate stages, while the old forests harbors guilds unique to that successional stage, such as parasitoids of melitophagous larvae of bees. Other successional stages were characterized by a single species of parasitoid, belonging to the genera Eiphosoma and Anomalon, which may indicate altered and preserved forests, respectively.

Long‐term dynamics of species diversity in natural secondary forest stands after severe windthrow damage

AbstractSpecies diversity encompasses both richness and evenness, but the interrelationship between these two aspects remains poorly understood. The long‐term dynamics of species diversity were examined in three natural secondary forests in Hokkaido, northern Japan, using permanent plot data gathered over 65–66 years after severe windthrow damage. The relationships among species diversity, species richness, and evenness were analyzed in the context of stand dynamics. Temporal trends in species diversity were unimodal‐shaped in three permanent plots, consistent with the pattern predicted by the intermediate disturbance hypothesis. Species richness and evenness increased following the windthrow, but decreased 27–37 years later in all three plots. In two plots, species diversity was closely associated with species richness and evenness at the early and late successional stages, respectively; in the other plot, species diversity was significantly related to both species richness and evenness in the later stage. The former two plots and the latter one plots differed markedly in terms of the extents of fatal tree damage caused by windthrow. Thus, windthrow severity affected the relationships among diversity, richness, and evenness as succession proceeded in these secondary forests. The tree density, species richness, evenness, and species diversity in all three plots decreased when the basal areas exceeded 18–24 m2/ha, indicating that increased competition among trees affected the temporal trends in these metrics. In conclusion, after a severe disturbance, the relationships among species diversity, richness, and evenness in secondary forests change with succession.

The Impact of Flood Frequency on the Heterogeneity of Floodplain Surface Soil Properties

Floodplain soils are subject to quasi-periodic flood disturbances. This flooding serves to enrich floodplain soils, increasing their fertility and often making them ideal locations for agriculture. However, what is less well understood is how the frequency of flooding impacts on soil fertility and the diversity of soil character. This study investigates how flood frequency influences the heterogeneity (assessed using 26 physical and geochemical soil properties) of floodplain soils in a semi-arid floodplain wetland system in New South Wales, Australia. The study includes an investigation of soil properties across four flood frequency (or disturbance frequency) categories ranging from frequent through to infrequent flood disturbance. Thirty samples were collected from each zone and the physical and geochemical soil data were analyzed using a suite of univariate and multivariate statistical tests. The results show that sites subject to an intermediate level of flood disturbance have a greater level of diversity in soil properties than those sites subject to frequent flood disturbances. These results reflect those of the Intermediate Disturbance Hypothesis, an ecological theory that posits the highest biological diversity will also be found in intermediately disturbed environments and suggests that there might be physical habitat drivers of biological diversity in intermediately disturbed floodplains.

Moderate Grazing Promotes Arthropod Species Diversity in an Alpine Meadow.

Livestock grazing is an important tool used in grassland land management practices. Studies have substantially addressed the effect of grazing on plant species diversity, revealing that moderate grazing increases plant species diversity. However, few studies have dealt with the relationship between grazing and arthropod species diversity, which remains unclear. Here, we hypothesize that moderate grazing promotes arthropod species diversity because arthropods are directly or indirectly dependent on plant diversity. In this study, we conducted a two-year plant and arthropod survey from 2020 to 2021 at four levels of grazing intensity, i.e., nongrazing (as a control), light grazing, moderate grazing, and heavy grazing, of the long-term grazing experiment starting in 2016. The data show that plant species diversity peaked in the moderate grazing treatment, and herbivore species diversity was positively correlated with plant species diversity (and hence peaked in the moderate grazing treatment). Moderate grazing promoted parasitoid species diversity, which was positively correlated with herbivore species diversity. However, predator species diversity did not significantly differ among the four treatments. In addition, saprophage species diversity decreased, whereas coprophages increased with increasing grazing levels, such that species richness (but not species diversity of detritivores statistically) was highest in the moderate grazing treatment. Consequently, the species diversity of arthropods as a whole peaked at the moderate grazing level, a phenomenology that is consistent with the intermediate disturbance hypothesis. Considering that moderate grazing has been found to increase plant species diversity, facilitate soil carbon accumulation, and prevent soil erosion, we suggest that moderate grazing would maximize multi-functional ecosystem services.

Individual response traits of understory plants vary along linked-press and compounded-pulse disturbance gradients in northern temperate and boreal forests

Community-weighted means (CWM) and functional dispersion (FDis) of response traits of woody and herbaceous understory plants in northern temperate and boreal forest communities were studied to determine how these traits respond to linked-press (i.e., climate, historic fire regimes, and silviculture systems) and compounded-pulse (i.e., harvest, site preparation, and release treatments) disturbances. We used fifth year post-harvesting disturbance data from the NEBIE plot network (NEBIE), a large-plot field experiment in northern temperate and boreal forests in Ontario, Canada. To analyze treatment effects, a randomized complete block design was assumed. Treatments were applied in a semi-operational manner to two-hectare experimental units. Individual response traits were affected by linked-press and compounded-pulse disturbances in unique ways, with logarithmic (stress-gradient hypothesis) or hump-shaped (intermediate disturbance hypothesis) patterns typically observed. Some traits (i.e., lateral spread of clones, seed weight) did not differ between disturbance types. Results suggest that all interactions between linked-press and compounded-pulse disturbances typically exert non-linear effects on CWM and FDis of individual plant response traits for both woody and herbaceous understory plants in boreal and temperate forests. Specific responses varied with trait, understory layer, and type of disturbance.

Carbon in Chinese grasslands: meta-analysis and theory of grazing effects

Globally, livestock grazing is an important management factor influencing soil degradation, soil health and carbon (C) stocks of grassland ecosystems. However, the effects of grassland types, grazing intensity and grazing duration on C stocks are unclear across large geographic scales. To provide a more comprehensive assessment of how grazing drives ecosystem C stocks in grasslands, we compiled and analyzed data from 306 studies featuring four grassland types across China: desert steppes, typical steppes, meadow steppes and alpine steppes. Light grazing was the best management practice for desert steppes (< 2 sheep ha−1) and typical steppes (3 to 4 sheep ha−1), whereas medium grazing pressure was optimal for meadow steppes (5 to 6 sheep ha−1) and alpine steppes (7 to 8 sheep ha−1) leading to the highest ecosystem C stocks under grazing. Plant biomass (desert steppes) and soil C stocks (meadow steppes) increased under light or medium grazing, confirming the ‘intermediate disturbance hypothesis’. Heavy grazing decreased all C stocks regardless of grassland ecosystem types, approximately 1.4 Mg ha−1 per year for the whole ecosystem. The regrowth and regeneration of grasslands in response to grazing intensity (i.e., grazing optimization) depended on grassland types and grazing duration. In conclusion, grassland grazing is a double-edged sword. On the one hand, proper management (light or medium grazing) can maintain and even increase C stocks above- and belowground, and increase the harvested livestock products from grasslands. On the other hand, human-induced overgrazing can lead to rapid degradation of vegetation and soils, resulting in significant carbon loss and requiring long-term recovery. Grazing regimes (i.e., intensity and duration applied) must consider specific grassland characteristics to ensure stable productivity rates and optimal impacts on ecosystem C stocks.Graphical

Experimental disturbance and productivity gradients drive community diversity in aquatic mesocosms.

Combined effects of disturbance and productivity on ecological diversity have been considered for decades as the dynamic equilibrium model (DEM) but are rarely tested together. Instead, most studies focus on either the intermediate disturbance hypothesis or sometimes the intermediate productivity hypothesis. In addition, most analyses of disturbance and productivity effects have relied on nonexperimental patterns, limited sample sizes, inaccurate proxies for productivity, and/or simple measures of diversity. The DEM operates at regional and local scales; here, we conducted a year-long experiment at local scales using submersed aquatic vegetation in outdoor mesocosms with a factorial combination of physical disturbance and productivity treatments. We evaluated diversity in several ways, directly measured productivity, and compared alternative hypotheses using model selection. The DEM was supported for effective diversity; both productivity and disturbance effects were clear, though productivity effects were stronger. Other diversity measures for the simple communities in the mesocosms did not clearly reflect treatments. The DEM is a valuable general framework for understanding disturbance and productivity effects on ecological systems and is made more general by minor conceptual adjustments here.

Interactions between metabolism and growth can determine the co-existence of Staphylococcus aureus and Pseudomonas aeruginosa.

Most bacteria exist and interact within polymicrobial communities. These interactions produce unique compounds, increase virulence and augment antibiotic resistance. One community associated with negative healthcare outcomes consists of Pseudomonas aeruginosa and Staphylococcus aureus. When co-cultured, virulence factors secreted by P. aeruginosa reduce metabolism and growth in S. aureus. When grown in vitro, this allows P. aeruginosa to drive S. aureus toward extinction. However, when found in vivo, both species can co-exist. Previous work has noted that this may be due to altered gene expression or mutations. However, little is known about how the growth environment could influence the co-existence of both species. Using a combination of mathematical modeling and experimentation, we show that changes to bacterial growth and metabolism caused by differences in the growth environment can determine the final population composition. We found that changing the carbon source in growth media affects the ratio of ATP to growth rate for both species, a metric we call absolute growth. We found that as a growth environment increases the absolute growth for one species, that species will increasingly dominate the co-culture. This is due to interactions between growth, metabolism, and metabolism-altering virulence factors produced by P. aeruginosa. Finally, we show that the relationship between absolute growth and the final population composition can be perturbed by altering the spatial structure in the community. Our results demonstrate that differences in growth environment can account for conflicting observations regarding the co-existence of these bacterial species in the literature, provides support for the intermediate disturbance hypothesis, and may offer a novel mechanism to manipulate polymicrobial populations.

Sarcosaprophagous Muscid Fly (Diptera: Muscidae) Assemblages Along an Anthropized Gradient in the Humid Chaco Ecoregion, Chaco Province, Argentina.

Deforestation, agriculture, farmyard animal husbandry, and urbanization are known to be the main causes of biodiversity loss and habitat fragmentation. The present study evaluated the role of anthropization in modulating Muscidae (Diptera) assemblages in the Humid Chaco ecoregion of Argentina, by testing the biotic homogenization and intermediate disturbance hypotheses. The study focused on natural, rural, and urban habitats in San Lorenzo Department, Chaco Province, where sarcosaprophagous muscid flies were surveyed. A total of 1,343 muscid flies were captured and identified to 7 genera and 24 species and morphospecies. We observed the effect of anthropization on the structuring of the assemblages and the presence of exotic species associated with human activity resulting from biotic homogenization. The highest abundance was recorded in the urban habitat, while the highest species richness and diversity (Shannon-Wienner and Hill's numbers) were found in the rural habitat, supporting the intermediate disturbance hypothesis. Several species are classified as indicators of habitat as well as according to their index of synanthropy. Our results provide valuable information about the use of sarcosaprophagous muscids as indicators of disturbance of natural habitats and about possible health risks related to this family of Calyptratae flies previously unsurveyed in northeastern Argentina. This information could be used in the ecological, agronomy, sanitary, and forensic fields.

Revisiting Vegetation Gradient Analysis and the Intermediate Disturbance Hypothesis for the Interpretation of Riverine Geomorphic Patterns

Human effects on the water economy of the river systems are currently well documented at the worldwide scale, impacting a range of ecosystem services. In this perspective article, we discuss the findings of recent papers that under different intensities of human disturbance have coupled the analyses of riverine geomorphological and plant community patterns. The discussion is carried out within the historical framework of past and current methods of sampling and analysing the river geomorphology and the plant communities along cross-sectional profiles. The research has been conducted along three major gravel-bed rivers of the south-eastern Italian Alps: Brenta, Piave, and Tagliamento. The collated and summarised results here demonstrate the existence of a strong relationship between the woody species variance that can be explained by geomorphologic patterns and human disturbance intensity. The less disturbed river has an intermediate value of species variance that can be explained by geomorphology, the intermediate-disturbed river has the highest value, and the highly disturbed river has the lowest value. Then, we proposed an interpretation key and an adaptation of the intermediate disturbance hypothesis, which reads as: “in rivers, the greatest influence of geomorphic properties on vegetation occurs in the moderate or middle ranges of a human disturbance gradient”. We argue that the “influence of the geomorphic properties on vegetation” is assessed through the species constrained variance through an ordination analysis, such as that which is explained here. The most recent collection techniques based on field survey and remote sensing are making it increasingly easy and accurate to study of the trends of geomorphic and plant community variables throughout time and space. Thus, we encourage that researchers should check whether and how our observation is conserved through different groups of taxa and intensities of natural and human disturbance.

Characteristics of the soil arbuscular mycorrhizal fungal community along succession stages in tropical forest and its driving factors

Arbuscular mycorrhizal fungi play an important role in mediating plant-soil interactions across succession stages. However, AMF community dynamics which about the change of community composition and member activity remain unclear. To complete the gap knowledge about microbial community dynamics during restoration succession, soil AMF community composition was studied within a tropical forest ecosystem in the Ganshiling nature reserve using high throughput sequencing methods. The results revealed that soil AMF communities characteristics about speices diversity, species composition and microbial correlation network showed significant differences between shrubland (SC) and secondary forest ecosystems, but the same differences were not found between 40-year recovery secondary forest (SF40) and 60-year recovery secondary forest (SF60). Plant community dynamics were the key factor for regulating soil AMF communities among succession stages. An important biotic factor explaining variance in AMF community composition was root biomass. The correlation network analysis showed that although the nodes were similar among succession stages, the complexity of networks was significant higher in SF40 than in SC and SF60, suggesting that AMF communities were more active in SF40, which verified the hypothesis of intermediate disturbance hypothesis. This study provides new insights into AMF community dynamics and their driving factors across succession stages, as well as expanding knowledge of the ecological value of AMF for tropical forest restoration processes.

Channel abandonment alters trophic characteristics of highland rivers

The Qinghai-Tibet Plateau (QTP) serves as a collection of unique ecosystems featured as oligotrophic and hypometabolic conditions, and is particularly vulnerable to threats posed by anthropogenic and natural disturbances. Active fluvial processes on the QTP, especially the lateral migration of rivers and channel abandonment accompanied by variations in the hydrological connectivity, and changes in river geomorphology, intensively modify this highland river-floodplain system. However, little is known about how these processes alter the trophic characteristics of highland river system on earth. In this study, we conducted field investigations on a typical meandering river, the Quanji River (QR), located in the northeastern QTP by sampling macroinvertebrates, surveying trophic sources, and measuring a range of environmental conditions. Based on the collected data, we identified four biotopes for the QR through hierarchical clustering, established the representative food web for each biotope through the allometric diet breadth model, and estimated the biomass storage and flux within food web for each biotope through the biomass balance model. Our results show that the identified biotopes differed markedly in hydrological connectivity. Biotopes 1 and 2, which were in the main river channel represented the condition of high connectivity, Biotope 3 in the open channel represented the median connectivity, while Biotope 4 in the semi-open/closed channel represented the low connectivity. In contrast to the unimodal pattern commonly observed in lowland rivers, indices for the highland QR, including the taxa richness, trophic group richness, biomass flux, biomass transfer efficiency, and trophic stability demonstrated a single-valley response pattern to the hydrological connectivity. We argue that the intermediate disturbance hypothesis still works on highland river ecosystems, whereas the optimal disturbance occurs in the low connectivity rather than median connectivity. We attribute this skewness to the low resistance and resilience of highland macroinvertebrate community in the face of hydrodynamic disturbances brought by flood events, and the biological disturbances from the predation by endemic migratory fish (Gymnocypris przewalskii) in QR. This study reveals that lateral migration and channel abandonment play important roles in highland river systems in the QTP in the context of biological and energetic perspectives and suggests that management strategies for highland rivers should be made different from those of lowland rivers.

Native and invasive bunchgrasses have different responses to trail disturbance on California coastal prairies

Ecosystems often experience small-scale disturbances through recreational foot and bike traffic. The intermediate disturbance hypothesis predicts that frequent disturbances in grasslands can negatively impact native species and lead to invasion by disturbance-adapted, non-native species. Disturbance studies often examine intentional or unintended larger scale disturbances, but do not commonly focus on common small-scale disturbances such as those caused by trampling along trails. As such, we were interested in how trail disturbance affects the vegetative growth and reproductive output of in situ native (Danthonia californica Bol. and Stipa pulchra Hitchc.) and invasive (Dactylis glomerata L. and Holcus lanatus L.) perennial bunchgrasses commonly found on California coastal prairies. We measured the basal circumference and seed production of plants located on-trail and off-trail on five coastal prairies in Santa Cruz, CA, USA. We hypothesized that native grasses located on-trail would have lower growth and reproductive output and non-native grasses would be unaffected. We found that native and non-native plants had differences in growth and reproductive output on-trail and off-trail. S. pulchra had lower reproductive potential (via culm sterility) with trail disturbance, whereas both native grasses showed evidence of compensatory growth on-trail. Invasive non-native D. glomerata and H. lanatus growth was unaffected by trail disturbance, but H. lanatus had lower reproductive output on-trail. Results suggest that it may be prudent to restore trails with disturbance tolerant species like D. californica and use less tolerant species like S. pulchra further into the central habitat space.