Functional diversity of herbaceous species under different fire frequencies in Brazilian savannas

Fire plays an important role in determining the structure of savannas, so that frequent fires tend to favor the herbaceous species in savannas. Functional diversity is a measure of the range of the species functional traits and is thought to be related to many functions and properties like community stability, nutrient cycling, and productivity, for example. We examined if functional diversity of herbaceous species was different along a fire gradient, and if observed functional diversity differed from a random assembly of species traits. We sampled three sites with different fire frequencies. According to the intermediate disturbance hypothesis, we expected the functional diversity to be higher in the intermediate fire frequency site. Subsequently, we excluded rare species from the analyses – species with less than ten individuals in each site – to verify their contribution to the observed patterns of functional diversity. We expected to find no difference with the exclusion of rare species. Functional diversity values, either considering all species or excluding the rare ones, were not different among sites, suggesting that (1) frequent fires may not affect overall functioning of the herbaceous community in savannas and (2) rare herbaceous species seem to present lower functional complementarity. According to the neutral theory, most of the herbaceous species may have evolved stochastically in niches more similar than competitive exclusion could allow. Also, functional diversity was not different from what one may expect by chance in any of the three sites. We also calculated the floristic similarity among sites. When we considered qualitative data, we found floristic differences between the sites of high and intermediate as well as between high and low fire frequency sites, but there was no difference between the intermediate and low fire frequency situation. Floristic similarities were different among all the three sites when the abundances of species were considered. Consequently, different fire conditions may change species composition without affecting the functional diversity of the herbaceous layer of savannas, corroborating the idea that savannas are more stable in functional than in floristic terms.     

The intermediate disturbance hypothesis does not explain fire and diversity pattern in fynbos

The intermediate disturbance hypothesis is a widely accepted generalization regarding patterns of species diversity, but may not hold true where fire is the disturbance. In the Mediterranean-climate shrublands of South Africa, called fynbos, fire is the most importance disturbance and a controlling factor in community dynamics. The intermediate disturbance hypothesis states that diversity will be highest at sites that have had an intermediate frequency of disturbance and will be lower at sites that have experienced very high or very low disturbance frequencies. Measures of diversity are sensitive to scale; therefore, we compared species richness for three fire regimes in South African mountain fynbos to test the intermediate disturbance hypothesis over different spatial scales from 1 m² to 0.1 hectares. Species diversity response to fire frequency was highly scale-dependent, but the relationship between species diversity and disturbance frequency was opposite that predicted by the intermediate disturbance hypothesis. At the largest spatial scales, species diversity was highest at the least frequently burned sites (40 years between fires) and lowest at the sites of moderate (15 to 26 years between fires) and high fire frequency (alternating four and six year fire cycle). Community heterogeneity, measured both as the slope of the species-area curve for a site and as the mean dissimilarity in species composition among subplots within a site, correlated with species diversity at the largest spatial scales. Community heterogeneity was highest at the least frequently burned sites and lowest at the sites that experienced an intermediate fire frequency.     

Influence of grazing and fire on breeding birds and perennial plants in Cyprus scrub and forest systems

Grazing and browsing by sheep and goats has been an important anthropogenic influence on ecosystems in the Mediterranean region for centuries. This influence has changed significantly in recent decades, with a general shift from range grazing to the penning of animals. The intermediate disturbance hypothesis (IDH) proposes that perturbation - including anthropogenic disturbance - is the norm for Mediterranean ecosystems, and that higher species diversity is found under conditions of continuous, moderate disturbance. Here we test the IDH as it relates to grazing of scrub and open forest habitats in Cyprus, while also testing for the effects of fire. We carried out surveys of breeding birds and vegetation at 48 study sites in scrub and open woodland across Cyprus. We estimated relative grazing pressure (past and present) and fire history at these sites, and tested for associations between these factors, breeding bird species richness and perennial vegetation diversity. We found moderate current grazing reduces density of lower and middle level scrub, and a higher level of diversity of perennial vegetation at moderately over intensively grazed sites. We found that moderate historical grazing pressure had a positive influence on richness of breeding bird species in lower scrub habitat such as phrygana, but that this effect was weaker the taller and denser the habitat. By contrast, current grazing pressure had a negative effect on species richness in lower scrub, but species richness in grazed habitats was higher in denser, taller scrub habitat such as garrigue and maquis. Our study suggests grazing plays an important role in maintaining habitat heterogeneity but the impact on avian species richness depends on the density and height of the vegetation.     

Dynamic tree aggregation patterns in a species‐poor temperate woodland disturbed by fire

Abstract. Question: How does fire affect the aggregation patterns of trees in a species‐poor oak woodland? Location: East‐central Minnesota, USA. Methods: More than 10 000 trees with DBH > 2 cm (comprising more than 11 000 stems) were monitored in a 16‐ha grid on an annual basis from 1995‐ 2001 in a species‐poor temperate woodland. Different portions of the grid experienced different frequencies of controlled burns. Aggregation indices were calculated for individual species and individual size classes within species. A community‐wide aggregation index was also calculated for different burn units. Spatial data were managed, and many of the aggregation indices calculated using a GIS ArcInfo? (ESRI). Results: Fire initially increased clumping, although repeated fires reduced it, a finding that suggests a corollary to the intermediate‐disturbance hypothesis, the corollary stating that intermediate levels of disturbance are expected to maximize community‐wide patterns of aggregation. Analyses also showed that all species are aggregated at small scales, that the degree of aggregation of a stem type (species or size) declines with distance from individual stems, that the degree of aggregation of large stems is usually less than that of small stems, and that rare species are more aggregated than common species. Findings from this study are consistent with those from similar studies in other temperate and tropical forests, woodlands, and savannas. Conclusion: The spatial patterns of trees in this woodland are dynamic, continually changing in response to the relative strengths of the often opposing forces of competition, which tends to reduce clumping, and disturbance, which, at low and intermediate frequencies, tends to increase it.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Genotypic diversity of a dominant C4 grass across a long-term fire frequency gradient

Aims and Methods Diversity-disturbance research has focused on community diversity, but disturbance frequency could impact diversity within species as well, with important consequences for community diversity and ecosystem function. We examined patterns of genetic diversity of a dominant grass species, Andropogon gerardii, in native North American tallgrass prairie sites located in eastern Kansas that have been subjected to a gradient of fire frequency treatments (burned every 1, 2, 4 or 20 years) since the 1970s. In addition, we were able to assess the relationships between genetic diversity of A. gerardii, species diversity and productivity across this range of fire frequencies.Important findings We found no significant relationships between genetic diversity of A. gerardii at the local scale (1 m 2 plot level) and disturbance frequency (burned 2 to 32 times over a 38-year period). However, at the site level (i.e. across all plots sampled within a site, ~100 m 2) there were differences in genotype richness and composition, as well as genomic dissimilarity among individuals of A. gerardii. Genotype richness was greatest for the site burned at an intermediate (4-year) frequency and lowest for the infrequently (20-year) burned site. In addition, genotypes found in the frequently burned sites were more similar from each other than expected by random chance than those found in the infrequently burned sites. Genotype composition of A. gerardii was not significantly different between the frequently burned sites (annual vs. 2 year) but did differ between frequently burned and infrequently burned sites (1 and 2 year vs. 4 and 20 year, etc.). Together, these results suggest site-level ecological sorting of genotypes in intact prairie across a broad gradient of disturbance frequencies, likely driven by alterations in environmental conditions. Frequent fire promotes the abundance of dominant grass species, reduces plant community diversity and impacts ecosystem processes such as productivity. Our study suggests that genetic diversity within dominant grass species also may be affected by disturbance frequency, which could have important implications for how species are able to respond to disturbance.     

不同母质温带草地植物群落多样性对人为干扰的响应

本研究以赛罕乌拉保护区2种不同土壤母质(黄土和沙母质)与3种利用方式(围栏打草、季节性放牧和自由放牧)草地为对象,基于群落组成调查数据,计算人为干扰指数与物种多样性指数,研究草地植物群落物种多样性与草地退化程度间的关系。结果表明: 土壤母质与人为利用方式的差异使草地处于不同的退化状态,黄土与沙母质类型草地退化程度均随人为利用强度的增加而上升,且当人为利用方式相同时,黄土母质草地人为干扰指数(均值1.21)比沙母质草地(均值1.48)低。各样地物种多样性指数总体随土壤母质的砂质化和人为利用强度的增加而下降,其中Margalef丰富度指数为1.57～4.27,Shannon多样性指数为1.16～2.39,Simpson优势度指数为0.76～0.87,Pielou均匀度指数为0.71～0.80。随着人为干扰指数增加,丰富度指数、多样性指数和优势度指数均下降,而均匀度指数有上升趋势。过度放牧对2种土壤母质草地均会造成严重威胁,黄土与沙母质草地最适宜的利用方式分别为围封打草和季节性放牧。在今后生物多样性保护的实施过程中,需兼顾草地不同土壤母质与人为利用方式的影响,针对不同土壤母质条件规划不同的草地利用方式,实现因地制宜的草地恢复与管理。     

Assessing disturbance-sensitivity and generalism in mammals: Corroborating a hump-shaped relationship using a hemerobiotic approach

Hemeroby is an indicator widely used in plant ecology to evaluate the level of disturbance of the optimal habitat of a species. Hemeroby can be scaled on a range of ten point scores, higher scores of hemeroby meaning higher level of disturbance. In this study we applied two indicators of sensitivity to disturbance based on the concept of hemeroby: HS_i (mean hemeroby score), calculated on the habitat types where each species occurs, and the recently proposed HH_i (mean hemerobiotic entropy), i.e. the level of generalism of a species with respect to the range of levels of disturbance where the species occurs. Both indices are based on the position and range of species along a gradient of disturbance, from pristine to completely human-made habitats. From a recent regional atlas of mammals, it was possible to calculate the ecological preferences for a large number of habitat types for 36 common mammal species for Latium (Central Italy). From the occurrences of mammals in each habitat, we calculated the HS_i (here rescaled: HS_rescaled) and HH_i indices. The relationship between habitat-related disturbance (HS_rescaled) vs. generalism (HH_i) of species showed a hump-shaped relationship peaking at intermediate levels of HS_rescaled, suggesting that generalism is maximum at intermediate levels of disturbance and corroborating analogous results obtained for birds. Alien mammal species exhibited higher averaged values of HS_i when compared to autochthonous species, supporting the evidence regarding the close relationship between alien species and more disturbed ecosystems. The application of the two indices to mammals could open new perspectives in conservation and management of species inhabiting a wide range of differently disturbed habitat types.     

Dynamics of cryptogamic soil crusts in a derived grassland in south‐eastern Australia

We examined the dynamics of cryptogamic soil crusts in a derived (disclimax) grassland near Orange in south‐eastern Australia. Changes in the cover of cryptogamic crusts and floristics and abundance of the constituent species were measured on four treatments with two levels each of grazing and cultivation. Twenty‐two lichens, mosses and liverworts were found at the study site and, of these, 13 were collected in the quadrats. Three moss species (Barbula calycina, Eccremidium arcuatum and Bryum pachytheca) and one lichen species (Cladonia tessalata) accounted for 67% of total cover‐abundance scores. Generally, cover‐abundance was significantly higher in the unvegetated microsites than in the vegetated microsites. Species richness was not significantly different between the four grazing‐cultivation treatments but, on average, there were significantly more species in the unvegetated microsites (mean = 3.2 species) than in the vegetated microsites (0.54 species). Grazing and cultivation resulted in significantly greater cover of bare ground and consequently significantly greater crust cover. Averaged across all treatments, approximately half of the area of unvegetated soil was occupied by cryptogams. Overall, the results indicate that lichens and bryophytes are important components of derived temperate grasslands, surviving in even densely vegetated swards. This study suggests that strategies which disturb the soil surface (e.g. grazing and cultivation) will stimulate the abundance and cover of soil crust organisms by increasing the availability of unvegetated microsites.     

Soft bottom species richness and diversity as a function of depth and iceberg scour in Arctic glacial Kongsfjorden (Svalbard)

Macrozoobenthic soft-sediment communities of central Arctic Kongsfjorden inhabiting six depth zones between 5 and 30 m were sampled using SCUBA-diving during June–August 2003 and analysed comparatively. About 63 taxa were found, nine of which had not been reported for Kongsfjorden and four for Svalbard. Suspension feeding or surface and sub-surface detritivorous polychaetes and deposit-feeding amphipods were dominant. Only 11 of the 63 taxa (45 species and additional 18 families not further identified) inhabited the complete depth range. Biomass ranged from 3.5 to 25.0 g ash free dry mass m⁻² and mean Shannon diversity (Log e) was 2.06. Similarity clustering from abundance and biomass data showed a significant difference between the shallow station (5 m) and the rest. The latter formed two sub-groups (10–20 and 25–30 m). Depth is irrevocably correlated with ice-scouring. Thus the differences in diversity together with the predicted iceberg scour intensity support the ‘intermediate disturbance hypothesis’ indicating that habitats impacted by moderate iceberg scouring enable higher diversity. In contrast, biotopes frequently affected only host pioneer communities, while mature, less diverse assemblages dominate depths of low impact.     

Gap formation and species diversity in Japanese beech forests: a test of the intermediate disturbance hypothesis on a geographic scale

To evaluate whether the intermediate-disturbance hypothesis applies on regional scales, the relationship between the species diversity and gap formation regime of beech forests was examined. The mean gap size and the variation of gap sizes showed no correlation with species diversity. The mean windstorm interval varied widely, but geographical trends, such as latitudinal gradient, were not observed. However, locations that sustained an intermediate frequency of disturbance had the highest species diversity. Although a latitudinal gradient of disturbance was not apparent, the intermediate-disturbance hypothesis was partly supported on a geographic scale. The most predictable model for species diversity was a multiple regression model composed of two factors, the windstorm interval and the cumulative temperature of the growing season. The fact that the temperature was of greater importance than the disturbance interval indicates that the most important factor in predicting forest species diversity is the amount of available energy on a geographic scale.     

Assessing recovery of biological soil crusts across a latitudinal gradient in Western Europe

Biological soil crusts are ecologically important communities in areas where vascular plant coverage is low, and their presence is often vital in prevention of soil erosion. Despite recurrent threats to biological soil crusts across different environments, their recovery after disturbance has been little studied. We therefore established experiments across a latitudinal gradient in Europe, from Öland, Sweden in the north, to Gössenheim, Germany and Hochtor, Austria, to Almeria, Spain in the south, spanning over 20° latitude and 2,300 m in altitude, and including natural and semi‐natural sites. At each site 10 (1 × 1 m) paired plots were constructed where the biological soil crusts were either completely removed, or left intact. Over a 2‐year period (2012–2014) the plots were regularly sampled to assess functional group recovery (cyanobacteria, algae, lichens, bryophytes, vascular plants), soil stability, and chlorophyll, carbon, and nutrient contents. Cyanobacterial assemblages were examined by denaturing gradient gel electrophoresis, a technique used to detect DNA in environmental samples. The results indicated that recovery was site dependent, suggesting that physical and climatic parameters play a major role in biological soil crust recovery. This conclusion is supported by the results of the soil properties, which were found to differ between sites, although they did not show meaningful recovery over the study period. Although 2 years was insufficient for pronounced biological soil crust recovery, this study documents changes over the initial recovery period, suggests management practices for future projects, and recommends proxies for measuring recovery over time.     

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.     

Dune plant species diversity and function in two barrier island biogeomorphic systems

Barrier island dune systems exhibit strong geographic contrasts in theinteraction between extrinsic disturbance from storm overwash and intrinsicbiogeomorphic recovery processes. To examine how these interactions shape duneplant species diversity, I sampled species cover and topography alongfrequentlystorm-overwashed (South Core Banks, North Carolina) and infrequently overwashed(Sapelo Island, Georgia) barrier islands. The observed compositional anddiversity patterns were in agreement with a complex systems model in whichextrinsic overwash exposure is either reinforced (South Core Banks) or dampened(Sapelo Island) by intrinsic biogeomorphic controls of topography. A largespatial-scale regularity in the distributional pattern of along-shore speciesdiversity was correlated to primary foredune height on South Core. On Sapelo, afine-spatial scale differentiation of species diversity patterns was lessstrongly correlated to topographic metrics. There were no significantdifferences between islands in along-shore alpha diversity (Shannon-Weinerindex). However, Sapelo was more diverse given its smaller area and finer-scalehabitat heterogeneity. I posit that the relevancy of the IntermediateDisturbance Hypothesis is weak when examining diversity patterns along a shoredisturbance gradient. Intrinsic biogeomorphic processes decouple the directcause-and-effect relationship between disturbance and diversity, a basicassumption of IDH. I posit that the Dynamic Equilibrium Model may be a moregenerally applicable conceptual framework. DEM incorporates the interaction ofintrinsic and extrinsic processes that shape habitat heterogeneity, aprerequisite for understanding how complex systems interactions shape diversitypatterns.     

铜陵铜尾矿废弃地生物土壤结皮中的蓝藻多样性

生物土壤结皮是生态系统原生演替过程中的一个早期阶段,在铜陵铜尾矿废弃地自然生态恢复过程中生物土壤结皮在尾矿废弃地表面广泛分布.以生长在铜陵杨山冲和铜官山2处铜尾矿废弃地的生物土壤结皮为研究对象,运用常规培养方法和变性梯度凝胶电泳技术(PCR-DGGE)对不同群落生物土壤结皮中的蓝藻多样性及优势类群进行研究.结果表明2种研究方法所获得的蓝藻种类组成具有明显差异.显微观察结果表明常规培养试验中主要蓝藻类群为微囊藻属(Microcystis)、色球藻属(Chroococcus)、颤藻属(Oscillatoria)、念珠藻属(Nostoc)和浮鞘丝藻属(Planktolyngbya),其中优势种类主要为铜绿微囊藻(Microcystis aeruginosa)、断裂颤藻(Oscillatoria fracta)和细浮鞘丝藻(Planktolyngbya subtilis);提取样品中微生物总DNA,对蓝藻16SrRNA进行PCR-DGGE分析,回收DGGE图谱中24个条带进行测序分析,结果显示,所有序列与GenBank数据库中的近缘蓝藻的相似性系数均在93％以上,其中优势蓝藻类群主要隶属于微鞘藻属(Microcoleus)和细鞘丝藻属(Leptolyngbya),裸地(YL)处和木贼群落下尾矿表面(YM)的生物土壤结皮中优势蓝藻类群主要为微鞘藻属,而黄色真藓-藻类混合结皮(YT)和白茅群落( YB,TG)下的生物土壤结皮中的优势类群主要隶属于细鞘丝藻属.     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.