The interactive effects of grazing ungulates and aboveground production on grassland diversity

The variable and nonlinear relationships between plant species richness (SR) and aboveground production (NAP) among terrestrial ecosystems indicate that the energetic capacity of ecosystems interacts with other environmental factors to control diversity. One contributing factor determining plant diversity is herbivory; but few studies have effectively examined the interaction of herbivores and NAP on SR. The objective of this study was to investigate how NAP and herds of native migrating ungulates determine plant SR in grasslands of Yellowstone National Park. Plant SR at peak aboveground biomass was compared inside and outside ungulate exclosures at two spatial scales, 1.0 m² (“local”) and 100 m² (“community”), in ten variable grasslands. NAP also was determined inside and outside exclosures. The relationship between SR and NAP was unimodal for grazed and ungrazed grassland at both spatial scales. Grazers increased local SR, independent of NAP. In contrast, herbivore effects on community SR ranged from no effect among low-productive grassland to an increasingly positive influence as NAP increased. In addition, ungulates reduced beta diversity (the contribution to community SR attributed to variability among local patches) at dry, low-productive and wet, high-productive sites. These results suggest that the size of the pool of species available to colonize grassland is an important factor controlling the response of grassland SR to herbivory, particularly from low- to intermediate-productive grassland.     

Effects of additional N on herbaceous species of desertified steppe in arid regions of China: a four-year field study

Human-induced nitrogen (N) enrichment is impacting the structure and function of grassland ecosystems, although few studies have been conducted in arid and semi-arid ecosystems under field conditions. Here, a 4-year multi-level N addition experiment under ambient precipitation in a typical temperate desert steppe in arid regions of China was designed to evaluate the impact of N enrichment on the herbaceous vegetation community in desert ecosystems. The results showed that species richness declined in response to N addition during all 4 years, while aboveground biomass increased in the relatively wet year (2007) and decreased in the relatively dry years (2008–2010) according to N addition level, as compared to the control. Plant community composition differed among plant functional groups (PFGs) based on added N. Perennial grasses (PG) benefited more compared with annuals and perennial forbs (PF) from N enrichment in terms of biomass production under conditions of high precipitation in 2007. In relatively dry years, biomass production of all PFGs was strictly dependent on inter- and intra-annual precipitation, which led PG to dominate in 2008 (with a dry spring) and PF to dominate in 2009 and 2010. Our results demonstrate that the impact of N enrichment on the herbaceous vegetation community in desert steppes may be strongly dependent on natural precipitation patterns characterized by shifts in plant community composition, particularly in terms of biomass production.     

Contrasting relationships between precipitation and species richness in space and time

Future changes in precipitation regimes are likely to impact species richness in water-limited plant communities. Regional, spatial relationships between precipitation and richness could offer information about how altered rainfall will impact local communities, assuming that processes driving the regional relationship are also dominant at fine spatial and short temporal scales. To test this assumption, we compared spatial and temporal relationships between precipitation and both species richness and species turnover in central North American grasslands. Across a broad geographic gradient, mean plant species richness in 1-m² plots increased significantly with mean annual precipitation. In contrast, over a 36-yr period at one mixed-grass prairie in the center of the regional gradient, single-year precipitation and richness were poorly correlated, and consecutive wet years had little effect on richness. Instead, richness increased most in wet years that followed dry years. Geographically dispersed sites receiving different levels of mean annual precipitation displayed strong differences in species composition, whereas temporal variation in precipitation at one site was not related to compositional dissimilarity, indicating that species turnover plays a key role in generating the regional relationship. Analyses of individual species' presence-absence suggest that the lagged temporal responses reflect environmental germination cues more than resource competition. These complex cues may dampen the initial impact of altered precipitation on diversity, but over the long term, turnover in species composition should lead to changes in richness, as in the regional, spatial relationship. How quickly this long-term response develops may depend on the colonization rates of species better adapted to the altered rainfall regime.     

Herbaceous plant richness and vegetation cover in Mediterranean grasslands and shrublands

Different types of relationship between herbaceous species richness and several parameters indicating abundance of plant material (herbaceous, woody plants, litter and bare ground cover) are presented. The data were obtained from 50 sites along a 300 km strip running from E to W within Spain and Portugal. Each site was representative of the silvo-pastoral landscape of the Mediterranean type ecosystems of the Iberian peninsula, and contained two neighboring patches, one of grassland and the other of shrubland. 3,600 20 × 20 cm subplots were randomly located (72 per site, 36 per patch) crossing the boundary grassland/shrubland. This approach allowed us to analyze the richness-occupation relationship of the space from different points of view: among and within the sites, and among and within the grassland and shrubland plant communities. We found a unimodal relationship between richness-cover similar to the one generally accepted between richness and biomass. Our results show that the dependence of this relationship varies depending on the spatial scale of the analysis and on the type of data used. When the whole region is taken into account, significant unimodal relationships are found between richness and herbaceous cover, litter and bare ground, and a negative linear relationship with woody plant cover. Within the sites there are mainly linear or non-significant relationships. But the results also depend on the type of communities analyzed. In pastures, the unimodal relationship represents the combination of positive and negative linear responses for low and high cover values, respectively. The value for herbaceous cover in which maximum richness occurs is around 60%. In shrublands, this value for cover also corresponds to maximum species richness, although the possibilities of reaching it are limited by other variables, such as woody plant cover. This implies that, on not considering variability at local scale, the relationship is linear and positive. This paper shows the existence of a common model related to herbaceous cover, but this model has multiple controlling factors that act differently in each type of community.     

What determines disturbance‐productivity‐diversity relationships? The effect of scale,species and environment on richness patterns in an Australian woodland

Much of the observed variation in relationships between diversity and disturbance or productivity may be attributed to scale, species characteristics, or environment. We used exclusion fences to create gradients of grazing (by native and introduced herbivores), cover, and standing crop in temperate Eucalypt woodlands. We investigated patterns of native, exotic, and total plant species richness at two scales (1 m² and 625 m²). Richness patterns were similar at both scales, though species richness at 1m² was more strongly affected by our grazing treatments. Season and rainfall explained more variation in richness than did surrogate measures of productivity or disturbance by herbivores. The richness-herbivory relationship depended strongly on rainfall, season, and species origin, and altering these factors produced the entire range of observed diversity-disturbance relationships. Richness-biomass and richness-cover relationships were consistently hump-shaped, and related to species origin with native richness negatively related and exotic richness positively related. The ability of weedy annuals to pre-empt space after death may have contributed to the observed unimodal responses.     

Environmental and plant community determinants of species loss following nitrogen enrichment

Global energy use and food production have increased nitrogen inputs to ecosystems worldwide, impacting plant community diversity, composition, and function. Previous studies show considerable variation across terrestrial herbaceous ecosystems in the magnitude of species loss following nitrogen (N) enrichment. What controls this variation remains unknown. We present results from 23 N-addition experiments across North America, representing a range of climatic, soil and plant community properties, to determine conditions that lead to greater diversity decline. Species loss in these communities ranged from 0 to 65% of control richness. Using hierarchical structural equation modelling, we found greater species loss in communities with a lower soil cation exchange capacity, colder regional temperature, and larger production increase following N addition, independent of initial species richness, plant productivity, and the relative abundance of most plant functional groups. Our results indicate sensitivity to N addition is co-determined by environmental conditions and production responsiveness, which overwhelm the effects of initial community structure and composition.     

Altitude effects on spatial components of vascular plant diversity in a subarctic mountain tundra

Environmental gradients are caused by gradual changes in abiotic factors, which affect species abundances and distributions, and are important for the spatial distribution of biodiversity. One prominent environmental gradient is the altitude gradient. Understanding ecological processes associated with altitude gradients may help us to understand the possible effects climate change could have on species communities. We quantified vegetation cover, species richness, species evenness, beta diversity, and spatial patterns of community structure of vascular plants along altitude gradients in a subarctic mountain tundra in northern Sweden. Vascular plant cover and plant species richness showed unimodal relationships with altitude. However, species evenness did not change with altitude, suggesting that no individual species became dominant when species richness declined. Beta diversity also showed a unimodal relationship with altitude, but only for an intermediate spatial scale of 1 km. A lack of relationships with altitude for either patch or landscape scales suggests that any altitude effects on plant spatial heterogeneity occurred on scales larger than individual patches but were not effective across the whole landscape. We observed both nested and modular patterns of community structures, but only the modular patterns corresponded with altitude. Our observations point to biotic regulations of plant communities at high altitudes, but we found both scale dependencies and inconsistent magnitude of the effects of altitude on different diversity components. We urge for further studies evaluating how different factors influence plant communities in high altitude and high latitude environments, as well as studies identifying scale and context dependencies in any such influences.     

Scales of spatiotemporal variability in macroinvertebrate abundance and diversity in monsoonal streams: detecting environmental change

1. We quantified spatial and temporal variability in benthic macroinvertebrate species richness, diversity and abundance in six unpolluted streams in monsoonal Hong Kong at different scales using a nested sampling design. The spatial scales were regions, stream sites and stream sections within sites; temporal scales were years (1997–99), seasons (dry versus wet seasons) and days within seasons. 2. Spatiotemporal variability in total abundance and species richness was greater during the wet season, especially at small scales, and tended to obscure site‐ and region‐scale differences, which were more conspicuous during the dry season. Total abundance and richness were greater in the dry season, reflecting the effects of spate‐induced disturbance during the wet season. Species diversity showed little variation at the seasonal scale, but variability at the site scale was apparent during both seasons. 3. Despite marked variations in monsoonal rainfall, inter‐year differences in macroinvertebrate richness and abundance at the site scale during the wet season were minor. Inter‐year differences were only evident during the dry season when streams were at base flow and biotic interactions may structure assemblages. 4. Small‐scale patchiness within riffles was the dominant spatial scale of variation in macroinvertebrate richness, total abundance and densities of common species, although site or region was important for some species. The proportion of total variance contributed by small‐scale spatial variability increased during the dry season, whereas temporal variability associated with days was greater during the wet season. 5. The observed patterns of spatiotemporal variation have implications for detection of environmental change or biomonitoring using macroinvertebrate indicators in streams in monsoonal regions. Sampling should be confined to the dry season or, in cases where more resources are available, make use of data from both dry and wet seasons. Sampling in more than one dry season is required to avoid the potentially confounding effects of inter‐year variation, although variability at that scale was relatively small.     

Species pools and the "hump-back" model of plant species diversity: an empirical analysis at a relevant spatial scale

We investigated the relative roles of productivity, the species pool, and spatial habitat structure in determining local species richness (alpha diversity) of plant communities within a single, well-defined landscape unit, at spatial and ecological scales where the relationship between community productivity and species diversity often assumes a unimodal or "hump-back" form. At high levels of productivity, the decrease-phase of the unimodal model of the diversity-productivity relationship is typically explained as the dynamic outcome of increased competitive exclusion, but it may also be the passive consequence of a small pool of species possessing attributes necessary to competitively survive in high-fertility environments. We conducted statistical analyses of previously collected data to determine whether variations in local richness in the herbaceous vegetation of a Slovakian mountain valley were best explained by habitat productivity itself (which presumably leads to more intense competition) or by the sizes of the relevant community species pools. We also used measures of spatial habitat structure to investigate the extent to which habitat patchiness influenced patterns of species diversity. In the study system, both community biomass and size of the species pools contributed significantly to local species richness, but the positive effect of the species pools was about twice as important as the negative effect of biomass. The combined area of related associations (alliance area), association perimeter, and habitat patch geometry were all closely related to species pool size.     

Diversity-dependent stability under mowing and nutrient addition: evidence from a 7-year grassland experiment

Anthropogenic perturbations may affect biodiversity and ecological stability as well as their relationships. However, diversity-stability patterns and associated mechanisms under human disturbances have rarely been explored. We conducted a 7-year field experiment examining the effects of mowing and nutrient addition on the diversity and temporal stability of herbaceous plant communities in a temperate steppe in northern China. Mowing increased population and community stability, whereas nutrient addition had the opposite effects. Stability exhibited positive relationships with species richness at population, functional group and community levels. Treatments did not alter these positive diversity-stability relationships, which were associated with the stabilising effect of species richness on component populations, species asynchrony and portfolio effects. Despite the difficulty of pinpointing causal mechanisms of diversity-stability patterns observed in nature, our results suggest that diversity may still be a useful predictor of the stability of ecosystems confronted with anthropogenic disturbances.     

Local native plant diversity responds to habitat loss and fragmentation over different time spans and spatial scales

Separating the threats that habitat loss and habitat fragmentation pose to biodiversity is challenging because these processes usually occur simultaneously. Additionally, their importance may be underestimated due to time-delayed extinction. In central Texas savannas, woody plant encroachment reduces the amount of habitat available to herbaceous species while fragmenting remaining habitat. We examined the relationships between present species richness and present and past habitat amount and fragmentation (measured as fractal dimension) using a series of aerial photographs taken over nearly 60 years. We show that woody plant encroachment, a common phenomenon in savannas worldwide, reduces the diversity of herbaceous vegetation through both habitat loss and fragmentation. Habitat loss has the strongest impact on species richness over short time spans and small spatial scales in these savannas. Habitat fragmentation, however, has the strongest impact over longer time spans and larger spatial scales and generates long-term extinction debts. We also demonstrate that examining habitat loss and habitat fragmentation across different time periods and at different spatial scales is essential for understanding their joint and individual effects on plant community composition.     

Productivity-diversity relationships in lake plankton communities

One of the most intriguing environmental gradients connected with variation in diversity is ecosystem productivity. The role of diversity in ecosystems is pivotal, because species richness can be both a cause and a consequence of primary production. However, the mechanisms behind the varying productivity-diversity relationships (PDR) remain poorly understood. Moreover, large-scale studies on PDR across taxa are urgently needed. Here, we examined the relationships between resource supply and phyto-, bacterio-, and zooplankton richness in 100 small boreal lakes. We studied the PDR locally within the drainage systems and regionally across the systems. Second, we studied the relationships between resource availability, species richness, biomass and resource ratio (N:P) in phytoplankton communities using Structural Equation Modeling (SEM) for testing the multivariate hypothesis of PDR. At the local scale, the PDR showed variable patterns ranging from positive linear and unimodal to negative linear relationships for all planktonic groups. At the regional scale, PDRs were significantly linear and positive for phyto- and zooplankton. Phytoplankton richness and the amount of chlorophyll a showed a positive linear relationship indicating that communities consisting of higher number of species were able to produce higher levels of biomass. According to the SEM, phytoplankton biomass was largely related to resource availability, yet there was a pathway via community richness. Finally, we found that species richness at all trophic levels was correlated with several environmental factors, and was also related to richness at the other trophic levels. This study showed that the PDRs in freshwaters show scale-dependency. We also documented that the PDR complies with the multivariate model showing that plant biomass is not mirroring merely the resource availability, but is also influenced by richness. This highlights the need for conserving diversity in order to maintain ecosystem processes in freshwaters.     

Plant species richness and diversity along an altitudinal gradient in the Sierra Nevada, Mexico

This article presents an analysis of plant species richness and diversity and its association with climatic and soil variables along a 1300‐m elevation gradient on the Cerro Tláloc Mountain in the northern Sierra Nevada in Mexico. Two 1000‐m² tree sampling plots were created at each of 21 selected sampling sites, as well as two 250‐m² plots for shrubs and six 9‐m² plots for herbaceous plants. Species richness and diversity were estimated for each plant life form, and beta diversity between sites was estimated along the gradient. The relationship between species richness and diversity and environmental variables was modelled using simple linear correlation and regression trees. Species richness and diversity showed a unimodal pattern with a bias towards high values in the lower half of the elevation gradient under study. This response was consistent for all three life forms. Beta diversity increased steadily along the elevation gradient, being lower between contiguous sites at intermediate elevations and high – the species replacement rate was nearly 100%– between sites at the extremes of the gradient. Few species were adapted to the full spectrum of environmental variation along the elevation gradient studied. The regression tree suggests that differences in species richness are mainly influenced by elevation (temperature and humidity) and soil variables, namely A₂ permanent wilting point, organic matter and horizon field capacity and A₁ horizon Mg²⁺.     

Does ecological filtering across a productivity gradient explain variation in species pool‐richness relationships?

Concerns about the impact of species diversity loss has heightened the importance of determining what limits local diversity and the relative roles of the available species pool and local interactions. Recent theory suggests that local diversity may exhibit linear or asymptotic responses to regional variation in species pools depending resource supply rates. We tested this idea by augmenting species pools in three sites along a natural productivity gradient (a surrogate for resource supply) in a mid-successional old-field. Augmentation of the species pool, enhanced local diversity more at medium than low productivity sites in this field and seed addition had no effect at high productivity, suggesting that diversity was saturated in high resource areas of this field. These results suggest that resource supply rates may mediate species pool-richness relationships. Species additions into cleared plots demonstrated that the observed differences in species recruitment along this gradient were largely driven by abiotic filtering at low and competitive filtering at high productivity. Furthermore, we observed that augmentation of the species pool shifted the productivity–diversity relationship from a negative linear to unimodal suggesting that species pools may influence reported productivity–diversity patterns.     

Plant diversity changes and succession along resource availability and disturbance gradients in Kamchatka

Changes in plant species richness across environmental and temporal gradients have often been explained by the intermediate disturbance hypothesis and a unimodal diversity–productivity relationship. We tested these predictions using two sets of mountain plant communities assembled along postglacial successional and snow depth (disturbance and stress) gradients in maritime Kamchatka. In each community, we counted the number of species in plots of increasing sizes (0.0025–100 m²) and analyzed them using species–area curves fitted by the Arrhenius power function and the Gleason logarithmic function. A comparison of successional communities along a 270-year-old moraine chronosequence behind the receding Koryto Glacier—representing gradients of increasing productivity and resource competition—confirmed the unimodal species richness pattern. The plant diversity peaked in a 60–80-year-old Salix–Alnus stand where light availability was sufficient to sustain a rich understory combining pioneer and late successional herbs. The closed Alnus canopy on older moraines caused a pronounced decrease in species richness for all plot sizes (interactive stage 80–120 years since deglaciation). A slight increase in species richness in the oldest assortative stages (120–270 years), when Alnus stands are mature, was found only at the smaller spatial scales. This reflects (i) the consolidation of clonal understory dominants and (ii) the absence of other woody species such as Betula ermanii whose invasion would eliminate Alnus and increase diversity at larger spatial scales. A comparative study of major mountain plant communities distributed above the Koryto Glacier foreland did not confirm the highest species richness at intermediate levels of disturbance and stress. Contrary to our expectation, the species richness was highest in alpine tundra and snowbed communities, which are subjected to severe winter frost and a short summer season, while less disturbed communities of subalpine meadows, heaths, and Betula ermanii woods were less species-rich. We attribute this pattern to differences in habitat area and species pool size.     

高寒草甸不同草地类型功能群多样性及组成对植物群落生产力的影响

对不同类型草地功能群多样性和组成与植物群落生产力之间的关系进行了探讨。结果表明：(1)在矮嵩草(Kobresia humlis)草甸和金露梅(Potentilla froticosa)灌丛中,豆科植物的作用比较明显,而其他功能群植物的作用较弱。(2)在藏嵩草(Kobresia tibetica)沼泽化草甸和小嵩草(K．pygmaca)草甸中,虽然杂类草、C3植物和莎草科植物功能群的生产力占群落初级生产力的比例较大,但二者在统计上没有显著性差异,这表明群落生产力除受物种多样性的影响外,也受物种本身特征和环境资源的影响,更主要的是受到功能群内物种密度和均匀度的影响,即功能群组成比功能群多样性更能说明对生态系统过程的影响。(3)不同类型草地群落植物功能群盖度与群落初级生产力呈显著的线性相关。(4)不同类型草地群落生产力与功能群内物种数的变化均表现为单峰曲线关系,即功能群内物种数处于中间水平时,群落生产力最高。     

Native remnants can be sources of plants and topsoil to restore dry and wet cerrado grasslands

Neotropical grasslands have undergone intensive degradation by land conversion or biological invasion, but their restoration is still challenging. Here, we integrated two approaches to (1) assess the resilience of pristine dry and wet cerrado grasslands after removal of plants and topsoil and (2) evaluate the effectiveness of different treatments based on the material extracted from pristine grasslands to restore degraded dry and wet grasslands after pine invasion. We used old‐growth cerrado grasslands in southeastern Brazil as donor ecosystems and assessed their resilience after the removal of all plants and the upper 5‐cm soil layer. To restore both wet and dry grasslands, we tested topsoil translocation, plant transplantation, direct seeding, topsoil translocation + direct seeding, and needle layer removal. Both wet and dry grasslands were resilient to plants and topsoil removal, as evidenced by their fast recovery. The major mechanisms promoting resilience were seed germination in the wet grasslands and resprouting from underground organs in the dry grasslands. Transplantation was the most successful treatment to restore vegetation cover, species richness, and composition in both wet and dry grasslands, especially for herbaceous species. Restoration of the herbaceous layer of cerrado grasslands can be successful using natural ecosystems as donor sites without impairing their resilience in the studied scale. Improving the resilience of degraded dry and wet cerrado grasslands depends on reestablishing the condition to seed germination in the wet grasslands and reintroducing species with the ability to resprout after disturbance in the dry grasslands, attributes that explained the quick recovery of the donor ecosystems.     

Biomass-dependent susceptibility to drought in experimental grassland communities

Earlier studies indicated that plant diversity influences community resistance in biomass when ecosystems are exposed to perturbations. This relationship remains controversial, however. Here we constructed grassland communities to test the relationships between species diversity and productivity under control and experimental drought conditions. Species richness was not correlated with biomass either under constant conditions or under drought conditions. However, communities with lower biomass production were more resistant to drought stress than those that were more productive. Our results also showed that ecosystem resistance to drought is a decreasing but nonlinear function of biomass. In contrast, species diversity had little and an equivocal effect on ecosystem resistance. From the results reported here, and the results of several previous studies, we suggest that high biomass systems exhibited a greater biomass reduction in response to drought than low biomass systems did, regardless of the relationship between plant diversity and community biomass production.     

Beneath the veil: plant growth form influences the strength of species richness–productivity relationships in forests

Aim Species richness has been observed to increase with productivity at large spatial scales, though the strength of this relationship varies among functional groups. In forests, canopy trees shade understorey plants, and for this reason we hypothesize that species richness of canopy trees will depend on macroclimate, while species richness of shorter growth forms will additionally be affected by shading from the canopy. In this study we test for differences in species richness–productivity relationships (SRPRs) among growth forms (canopy trees, shrubs, herbaceous species) in small forest plots. Location We analysed 231 plots ranging from 34.0° to 48.3° N latitude and from 75.0° to 124.2° W longitude in the United States. Methods We analysed data collected by the USDA Forest Inventory and Analysis program for plant species richness partitioned into different growth forms, in small plots. We used actual evapotranspiration as a macroclimatic estimate of regional productivity and calculated the area of light‐blocking tissue in the immediate area surrounding plots for an estimate of the intensity of local shading. We estimated and compared SRPRs for different partitions of the species richness dataset using generalized linear models and we incorporated the possible indirect effects of shading using a structural equation model. Results Canopy tree species richness increased strongly with regional productivity, while local shading primarily explained the variation in herbaceous plant richness. Shrub species richness was related to both regional productivity and local shading. Main conclusions The relationship between total forest plant species richness and productivity at large scales belies strong effects of local interactions. Counter to the pattern for overall richness, we found that understorey herbaceous plant species richness does not respond to regional productivity gradients, and instead is strongly influenced by canopy density, while shrub species richness is under multivariate control.     

Plant species richness–productivity relationships in a low-productive boreal region

Local species richness–productivity (SR–P) relationship is usually reported as unimodal if long productivity gradients are sampled. However, it tends to be monotonically increasing in low-productive environments due to the decreasing part of the SR–P curve being truncated. Previous work indicated that this can hold true for forest herb layers, because of an upper bound on productivity caused mainly by canopy shading. Here, we ask whether the same pattern exists in a region with an upper bound on productivity caused by a harsh climate. We sampled herbaceous vegetation of boreal forests and grasslands in a low-productive region of central Yakutia (NE Siberia) with dry and winter-cool continental climate. We collected data on species composition, herb-layer productivity (aboveground herbaceous biomass), soil chemistry and light availability. We applied regression models to discriminate between monotonically increasing, decreasing and unimodal responses of herb-layer species richness to measured variables and analysed trends in the species-pool size and beta diversity along the productivity gradient. Our expectation of the monotonically increasing SR–P relationship was confirmed for neither forest herb layers nor grasslands. In the forest herb layers, no relationship was detected. In grasslands, the relationship was unimodal with species richness decline starting at much lower productivity levels than in more productive temperate grasslands. Potential causes for this decline are either limitation of local species richness by the species pool, which contains few species adapted to more productive habitats, or competitive exclusion, which can become an important control of species richness under lower levels of productivity than is the case in temperate grasslands.