Affect Plant Species Richness Research Articles

Ecological conditions affect plant species richness of beech forests along an altitudinal gradient

Spatial heterogeneity of biodiversity is linked to environmental gradients across landscapes. In forest vegetation, trees essentially affect the forest understorey flora, not only by forming the microecological conditions but also by their competitiveness. Here, we investigate the changes in taxonomic diversity along the altitudinal gradient in forests dominated by the strongly competitive, European beech (Fagus sylvatica), considering different ecological drivers important for vegetation diversity. The response of plant species diversity to changing characteristics is mostly non-linear or has an increasing trend. This reflects the specific environments of the studied region of Western Carpathians (Central Europe), where a high variability of geological substrates and the occurrence of base-rich rocks at higher altitudes increase species diversity. The beech cover, the geological substrate and the orography were found to be the most significant factors determining the diversity patterns. The increasing trend towards higher altitudes is significant on carbonate bedrock regardless of any other studied factors. The geological conditions multiply the number of species almost twice on carbonates compared to the communities on silicates. The Veľká Fatra Mts and Slovenský raj Mts are the plant diversity hotspots of beech forests in the Western Carpathians.

Effects of long‐term mowing on species diversity, biomass and composition of plant community in a semi‐arid grassland in northeastern China

AbstractQuestionsHow does long‐term, moderate‐intensity mowing affect plant community diversity, biomass, and composition in semi‐arid grasslands? And what are the underlying mechanisms that drive differences in plant community structure and functions between mowed and unmowed grasslands?LocationA semi‐arid grassland in Jilin Province, northeastern China.MethodsIn a 30‐year experiment, we investigated long‐term effects of mowing on the grassland ecosystem in northeastern China by comparing plant community diversity, biomass, and composition between mowed and unmowed permanent plots across six grasslands.ResultsLong‐term mowing did not affect plant species richness, but increased species evenness by 29%. Mowing had little effects on above‐ground (shoot) and below‐ground (root) biomass. The unchanged above‐ground biomass in the mowed areas was attributed to a trade‐off between biomass of grasses and forbs: mowing significantly increased forb biomass by 50%, but reduced grass biomass by 20% and the grass to forb biomass ratio by 46%. Mowing also reduced biomass of the dominant Leymus chinensis grass by 24% and litter biomass by 56%. Regression analyses indicated that the species evenness was negatively affected by L. chinensis biomass and litter biomass.ConclusionsThis study demonstrates that, although long‐term moderate mowing has limited effects on plant species richness and plant biomass, such a managed strategy can greatly reduce the dominance of grass species and allow the growth of forb species, leading to a higher species evenness in the plant community. This win–win situation promotes forage production for farmers while increasing plant diversity for conservation, suggesting that current mowing tactics can be a sustainable management practice in this ecosystem. However, the continuous loss of plant litter because of mowing may have negative influences on soil nutrient availability in the long run. From this perspective, a short‐term halt in mowing or fertilization that allows plants and soil nutrients to recover could represent a more optimal protocol for management.

Land-use history, fire regime, and large-mammal herbivory affect deer-preferred plant diversity in longleaf pine woodlands

Since ecological systems often experience multiple disturbances, understanding changes in important ecological interactions, such as plant-herbivore interactions, may require studies capable of disentangling the unique and interactive effects of multiple forms of disturbance. For example, understanding how mammalian herbivores affect plant communities may require understanding how widespread past disturbances, such as agricultural land use, interact with contemporary disturbances, such as prescribed fire. We tested if past agricultural land use and contemporary fire regime modified the effects of mammalian herbivory on focal plant communities by measuring the richness of plant species preferred by deer at 26 longleaf pine woodlands containing paired open and large-mammal exclusion plots. Land-use history significantly affected the community composition of deer-preferred species, and focal species were 8.19 times more likely to be present in post-agricultural sites than non-agricultural sites. Large-mammal herbivory only affected plant species richness in woodlands with low fire frequencies wherein focal species were on average 2.23 times more likely to be present in exclusion plots than open plots. These results suggest that past and present disturbances can mediate contemporary plant-animal interactions and may explain spatial patterns in the intensity of large-mammal herbivory. Our findings also suggest that common management practices, such as use of prescribed burns, may indirectly promote plant species richness by reducing deer herbivory.

Impact of livestock grazing intensity on plant diversity of montane grassland in the northern Drakensberg, South Africa

Communal livestock grazing is expected to impact botanical composition and plant diversity of Drakensberg montane grasslands. Accordingly, a grazing gradient extending outward from kraals, and fence-line contrasts between communal rangeland and protected areas (Golden Gate Highlands National Park and Royal Natal National Park), were studied in the QwaQwa region of South Africa. Presence of a grazing gradient was confirmed by a decrease in the amount of cattle dung with increasing distance from kraals that was independent of altitude, solar radiation, slope, or soil physico-chemical properties. Canonical correspondence analysis confirmed that grazing intensity had a significant influence on botanical composition although most compositional variation was unexplained. Higher grazing intensities tended to negatively affect plant species richness and diversity, and the cover of perennial graminoids, but benefitted the abundance of dwarf shrubs and alien invasive species. Similar effects were apparent across each of the fence line contrasts. Despite a long history of livestock grazing, communal rangeland supported >320 indigenous species, illustrating some contribution to biodiversity conservation, but being compromised near kraals. An improved approach for study of the impacts of livestock grazing on plant diversity is suggested that would overcome the difficulties of assessing rare and threatened species using a community approach.

Elevated precipitation alters the community structure of spring ephemerals by changing dominant species density in Central Asia.

Global climate change is one of the most pressing conservation challenges; in particular, changes in precipitation regimes have already substantially influenced terrestrial ecosystems. However, the mechanisms influencing precipitation changes on individual plants and the plant communities in desert grasslands have yet to be fully elucidated. We therefore examine the influence of increased precipitation on plant community compositions in the Gurbantunggut Desert, Xinjiang, northwestern China, from 2005 to 2009. We found that growth of all plant species and the community productivities increased markedly with enhanced water input. Cover of ephemeral synusia also significantly increased due to increased precipitation, implying that the role of the ephemeral community for stabilization of sand dunes was strengthened by increased precipitation. The response of plant community compositions to increased precipitation was primarily reflected as changes in plant density, while increased precipitation did not affect plant species richness and the diversity index. Dominant species drove the response of plant density to increasing precipitation during the five‐year study period. However, the relative responses of rare species were stronger than those of the dominant species, thereby potentially driving species turnover with long‐term increased precipitation. This finding improved our understanding of how increased precipitation drives the changes in plant community composition in desert grasslands and will help to better predict changes in the community composition of ephemerals under future global climate change scenarios.

Effects of organic farming on plant and butterfly functional diversity in mosaic landscapes

Organic farming can benefit plants and butterflies in terms of species richness and abundance, in particular in homogeneous landscapes. Nevertheless, whether organic farming can benefit functional diversity of these two organism groups is not well understood. Organic farming could benefit functional diversity by counteracting simplification and homogenisation of biotic communities caused by earlier agricultural intensification, and therefore contribute to communities more resilient to environmental or land use changes. We analysed species richness and four functional diversity indices (functional richness, evenness, divergence and dispersion) of plants and butterflies on 15 pairs of organic/conventional farms in North-West Spain, whilst accounting for independent and joint effects of landscape context. To better understand links between functional diversity and taxonomic species assemblages, we applied an indicator species analysis to determine whether any particular plant or butterfly species were significantly more abundant on organic or conventional farms. Both butterfly species and functional richness were higher on organic than on conventional farms. In addition, when the farms were surrounded by low proportions of agriculture, butterfly functional evenness was higher on organic farms. In contrast, organic farming did not affect plant species richness or functional diversity. Nevertheless, plants were affected by landscape openness and field size, so that plant functional richness decreased with increasing landscape openness while functional evenness showed a complex relationship to the interaction between field size and landscape openness. Indicator analyses revealed that three plant species, but no individual butterfly species, were significantly related to organic farming. In conclusion, organic farms provide higher-quality habitat for butterflies than conventional farms, either by promoting specific grassland plants benefiting butterflies, or more generally by providing a wider niche space, fostering functionally diverse butterfly communities.

Increased rainfall and nitrogen alter colonization and extinction during postgrazing steppe succession

AbstractQuestionSteppes may be sensitive to rainfall and nitrogen (N) pulses; however, their potential effects on local colonization and extinction during postgrazing steppe succession have not been thoroughly elucidated to date. We addressed how increased rainfall and N influenced plant species richness, gain, loss, and turnover in an ungrazed steppe.LocationNortheastern Inner Mongolia, China.MethodsWe performed a five‐growing‐season field experiment, in which an ungrazed steppe was subject to nine different combinations of rainfall and N. The three rainfall levels were ambient amount of rainfall, a 14% increase in amount of rainfall, and a 28% increase in amount of rainfall; the three N levels were no N addition, an addition of 5 g N m−2 year−1, and an addition of 10 g N m−2 year−1. We determined successional changes at the taxonomic and functional levels by surveying plant species in the first and fifth growing seasons.Results(a) Increased rainfall and N addition did not affect plant species richness but did influence plant species loss, gain, and turnover. (b) At the taxonomic level, species loss, gain, and turnover decreased monotonically with increasing rainfall; N addition decreased species loss and turnover but did not influence species gain; the slope of the species loss–gain relation was approximately 1.0. (c) At the plant functional group level, increased rainfall favored C3 plants by sustaining species gain and decreasing species loss; N addition favored C4 plants by increasing the species gain and decreasing the species loss.ConclusionIncreased rainfall and simulated N deposition could induce rapid colonization and extinction during postgrazing steppe succession, but maintain plant diversity homeostasis by equally affecting species colonization and extinction. Future rainfall enrichment and N deposition might strongly influence the relative abundance of different plant functional groups, thereby affecting successional changes under global change.

Wild bee and floral diversity co‐vary in response to the direct and indirect impacts of land use

AbstractLoss of habitat area and diversity poses a threat to communities of wild pollinators and flowering plants in agricultural landscapes. Pollinators, such as wild bees, and insect‐pollinated plants are two groups of organisms that closely interact. Nevertheless, it is still not clear how species richness and functional diversity, in terms of pollination‐relevant traits, of these two groups influence each other and how they respond to land use change. In the present study, we used data from 24 agricultural landscapes in seven European countries to investigate the effect of landscape composition and habitat richness on species richness and functional diversity of wild bees and insect‐pollinated plants. We characterized the relationships between the diversity of bees and flowering plants and identified indirect effects of landscape on bees and plants mediated by these relationships. We found that increasing cover of arable land negatively affected flowering plant species richness, while increasing habitat richness positively affected the species richness and functional diversity of bees. In contrast, the functional diversity of insect‐pollinated plants (when corrected for species richness) was unaffected by landscape composition, and habitat richness showed little relation to bee functional diversity. We additionally found that bee species richness positively affected plant species richness and that bee functional diversity was positively affected by both species richness and functional diversity of plants. The relationships between flowering plant and bee diversity were modulated by indirect effects of landscape characteristics on the biotic communities. In conclusion, our findings demonstrate that landscape properties affect plant and bee communities in both direct and indirect ways. The interconnection between the diversities of wild bees and insect‐pollinated plants increases the risk for parallel declines, extinctions, and functional depletion. Our study highlights the necessity of considering the interplay between interacting species groups when assessing the response of entire communities to land use changes.

Mammal burrowing in discrete landscape patches further increases soil and vegetation heterogeneity in an arid environment

Burrowing mammal disturbances often create heterogeneity within landscapes. Aardvark (Orycteropus afer) are extensive burrowers in sub-Saharan ecosystems and play an important role in structuring communities in arid environments. The burrowing activities of aardvark are often associated with heuweltjies (nutrient-rich mounds differing in soil and vegetation characteristics from surrounding areas) which contribute strongly to landscape heterogeneity. This study determined the impact of aardvark burrowing and heuweltjies on soil and vegetation characteristics. Data were collected at four microsites: 1) at each burrow entrance on a heuweltjie, 2) on the burrowed heuweltjie (i.e. heuweltjie with an aardvark burrow), 3) on the nearest unburrowed heuweltjie, and 4) on the adjacent matrix. Aardvark burrowing and heuweltjies both impacted the soil and vegetation characteristics. Despite having more moderate thermal regimes than other microsites, burrow entrances were largely unvegetated. Although aardvark burrowing did not affect plant species richness on heuweltjies, it decreased vegetation cover. Vegetation composition differed between heuweltjies and the matrix, and this dissimilarity was increased further by aardvark burrowing. As a result, the combined effect of burrowing mammals and heuweltjies increases landscape heterogeneity. This emphasises the important ecosystem engineering role that aardvark have in arid environments, even where considerable abiotic heterogeneity already exists.

How plot shape and spatial arrangement affect plant species richness counts: implications for sampling design and rarefaction analyses

AbstractQuestionsHow does the spatial configuration of sampling units influence recorded plant species richness values at small spatial scales? What are the consequences of these findings for sampling methodology and rarefaction analyses?LocationSix semi‐natural grasslands in Western Eurasia (France, Germany, Bulgaria, Hungary, Italy, Turkey).MethodsIn each site we established six blocks of 40 cm × 280 cm, subdivided into 5 cm × 5 cm micro‐quadrats, on which we recorded vascular plant species presence with the rooted (all sites) and shoot (four sites) presence method. Data of these micro‐quadrats were then combined to achieve larger sampling units of 0.01, 0.04 and 0.16 m² grain size with six different spatial configurations (square, 4:1 rectangle, 16:1 rectangle, three variants of discontiguous randomly placed micro‐quadrats). The effect of the spatial configurations on species richness was quantified as relative richness compared to the mean richness of the square of the same surface area.ResultsSquare sampling units had significantly lower species richness than other spatial configurations in all countries. For 4:1 and 16:1 rectangles, the increase of rooted richness was on average about 2% and 8%, respectively. In contrast, the average richness increase for discontiguous configurations was 7%, 17% and 40%. In general, increases were higher with shoot presence than with rooted presence. Overall, the patterns of richness increase were highly consistent across six countries, three grain sizes and two recording methods.ConclusionsOur findings suggest that the shape of sampling units has negligible effects on species richness values when the length–width ratio is up to 4:1, and the effects remain small even for more elongated contiguous configurations. In contrast, results from discontiguous sampling units are not directly comparable with those of contiguous sampling units, and are strongly confounded by spatial extent. This is particularly problematic for rarefaction studies where spatial extent is often not controlled for. We suggest that the concept of effective area is a useful tool to report effects of spatial configuration on richness values, and introduce species–extent relationships (SERs) to describe richness increases of different spatial configurations of sampling units.

Recreational use of urban and suburban forests affects plant diversity in a Western Siberian city

The recreational use of urban forests is a highly valued service. However, strong recreational pressure may contribute to the loss of biodiversity in forests. This study assessed the frequency of visitors and their characteristics in 14 urban and 11 suburban forests in the rapidly expanding city of Pavlodar, Kazakhstan, in Western Siberia. Furthermore, the effects of recreation disturbance (trampling, damage to ground vegetation and damage to trees and shrubs) and other human-mediated disturbances (waste deposits, soil disturbance, etc.) on both the vegetation and plant characteristics of urban and suburban forests were quantified. In Pavlodar, urban forests are poorly managed, motivating the people to spend their sparetime in the more distant suburban forests. Urban and suburban forests did not differ in visitor frequency during the summer season (July–September). However, the two forest types differed in the age structure and group size of visitors as well as in the activities of visitors. Urban forests were more frequently visited by younger people for walking, sports, sitting and talking, and playing with children, while suburban forests were often visited by older people for picnicking, fishing and gathering mushrooms. In urban forests, total plant species richness was reduced by recreation disturbance. Urban forests also harboured a large proportion of alien plant species (0.42; in suburban forests 0.24). Neither recreation disturbance nor other human-mediated disturbance affected plant species richness in suburban forests, while both disturbance types enhanced the colonization success of alien species. Alterations in plant life forms can be considered as an indicator of changes in ecosystem function. In suburban forests, recreation disturbance caused a decline of geophytes and an increase of therophytes. We recommended various management actions to improve the recreation value of degraded urban forests in this Western Siberian city.

Past and present disturbances influence biodiversity value of subtropical grassland ecological networks

The effect of transforming natural habitat to commercial land uses can be mitigated by implementing ecological networks (ENs) in production landscapes. However, EN ability to conserve biodiversity is potentially influenced by past and present disturbances, and spatial configuration of habitat patches. Here, we investigated how plant assemblages of a subtropical EN in a timber production landscape in South Africa respond to disturbance history (i.e. natural vs. recovering after removal of Pinus spp.), mowing in firebreaks (i.e. 30 m wide grassy strips intended to stop spread of fires in plantations), and three design parameters: corridor width, distance to protected area boundary, and context i.e. proportion eucalypt compartments within 500 m radius. Reference sites were in the adjacent iSimangaliso Wetland Park, a protected area (PA). Past disturbances and the current practice of mowing in firebreaks significantly influenced plant species composition, but not plant species richness. Plant dominance was greatest in mowed firebreaks, but similar between natural and recovering grassland. None of the design variables affected plant species richness, or composition. However, dominance was significantly and negatively correlated with corridor width, and positively correlated with context. Hence, sites are ordered in terms of their biodiversity value from greatest to lowest: natural areas > recovering areas > firebreaks. Based on these results, natural grassland in wide, well-connected corridors should have greatest conservation priority. We recommend that past disturbances, and current management practices should be taken into account and integrated into the design of future ENs. Such an approach is necessary for effective biodiversity conservation in production landscapes.

Native and Alien Plant Species Richness Response to Soil Nitrogen and Phosphorus in Temperate Floodplain and Swamp Forests

Soil nitrogen and phosphorus are commonly limiting elements affecting plant species richness in temperate zones. Our species richness-ecological study was performed in alder-dominated forests representing temperate floodplains (streamside alder forests of Alnion incanae alliance) and swamp forests (alder carrs of Alnion glutinosae alliance) in the Western Carpathians. Species richness (i.e., the number of vascular plants in a vegetation plot) was analyzed separately for native and alien vascular plants in 240 vegetation plots across the study area covering Slovakia, northern Hungary and southern Poland. The relationship between the species richness of each plant group and total soil nitrogen content, plant-available phosphorus and carbon to nitrogen (C/N) ratio was analyzed by generalized linear mixed models (GLMM) with Poisson error distribution and log-link function. The number of recorded native and alien species was 17–84 (average 45.4) and 0–9 (average 1.5) species per plot, respectively. The GLMMs were statistically significant (p ˂ 0.001) for both plant groups, but the total explained variation was higher for native (14%) than alien plants (9%). The richness of native species was negatively affected by the total soil nitrogen content and plant-available phosphorus, whereas the C/N ratio showed a positive impact. The alien richness was predicted only by the total soil nitrogen content showing a negative effect.

Landscape structure effects on forest plant diversity at local scale: Exploring the role of spatial extent

Since landscape attributes show different patterns at different spatial extents, it is fundamental to identify how the relation between landscape structure and plant species diversity at local scale varies with scale. Then, it is fundamental to assess the appropriate extent at which landscape factors affect plant species richness at the local scale. To investigate this relation, data on plant species richness of forest communities at plot scale were extracted from a large data set and landscape metrics were calculated around the same plots for a range of extents (250–3000m). Then, multiple regression models and variance partitioning techniques were applied to assess the amount of variance explained by the landscape metrics on plant species richness for a range of extents. In general, we found that increasing extent of the surrounding landscape analyzed, improved the strength of relationship between the landscape metrics and the properties of plant communities at plot scale. The medium-large extent was most informative as it combined a decent total variance explained with high variance explained by the pure fractions of complexity, fragmentation and disturbance and the minimum of collinearity. In conclusion, we found that it is possible and beneficial to identify a specific extent, where the redundancy in the predictor variables is minimized and the explanatory power of the pure fractions (or single groups) maximized, when examining landscape structure effects on local plant species richness.

Interactive Effects of Nitrogen Deposition and Grazing on Plant Species Composition in a Serpentine Grassland

The interaction of resource availability and disturbance can strongly affect plant species richness and the spread of exotic plants. Several ecological theories posit that disturbance mediates the richness-reducing effects of increased competition as resource levels rise. In the low-nutrient serpentine grasslands of the San Francisco Bay Area, the fertilizing effects of atmospheric nitrogen (N) deposition may threaten native species by promoting nitrophilic exotic grasses. Attempts to mitigate these N deposition effects have focused on cattle grazing as a strategy to reduce exotic grass cover. We simulated realistic N deposition increases with low-level fertilization, manipulated grazing with fencing, and monitored grazing intensity using camera traps in a 4 yr factorial experiment to assess the effects of grazing and N deposition on several measures of native and exotic species dynamics in California's largest serpentine grassland. Our results suggest that native species diversity may increase slightly under low-level N deposition with moderate grazing in this system. However, grazing may not be effective at limiting exotic cover as N accumulates in the future. Examination of treatment trajectories using principal response curves indicated that responses to grazing might be determined more by functional group (forb or grass) than origin (native or exotic). Grazing intensity varied dramatically within the single stocking rate used to manage this ecosystem. Given this variation and the contrasting effects of grazing on different functional groups, more targeted management may be required to improve conservation outcomes.

Short‐term harvesting of biomass from conservation grasslands maintains plant diversity

AbstractHigh yields are a priority in managing biomass for renewable energy, but the environmental impacts of various feedstocks and production systems should be equally considered. Mixed‐species, perennial grasslands enrolled in conservation programs are being considered as a source of biomass for renewable energy. Conservation grasslands are crucial in sustaining native biodiversity throughout the US Upper Midwest, and the effects of biomass harvest on biodiversity are largely unknown. We measured the effect of late‐season biomass harvest on plant community composition in conservation grasslands in three regions of Minnesota, USA from 2009 to 2012. Temporal trends in plant species composition within harvested grasslands were compared to unharvested grasslands using mixed effects models. A before‐after control‐impact approach using effect sizes was applied to focus on pre‐ and postharvest conditions. Production‐scale biomass harvest did not affect plant species richness, species or functional group diversity, nor change the relative abundance of the main plant functional groups. Differences in the relative abundances of plant functional groups were observed across locations; and at some locations, changed through time. The proportion of non‐native species remained constant, while the proportion of noxious weeds decreased through time in both harvested and unharvested grasslands at the central location. Ordination revealed patterns in species composition due to location, but not due to harvest treatment. Therefore, habitat and bioenergy characteristics related to grassland plant communities are not expected to change due to short‐term or intermittent late‐season biomass harvest.

Scale‐dependent effects of grazing and topographic heterogeneity on plant species richness in a Dutch salt marsh ecosystem

AbstractQuestionFor over three decades, low‐intensity grazing has been used to maintain or increase plant species richness in European natural areas, but the effects are highly variable. Thus far, good predictors of whether grazing will have positive effects on plant species richness are limited. How does the interplay between low‐intensity grazing and topographic heterogeneity affect plant species richness at different spatial scales?LocationLong‐term grazed and ungrazed salt marshes of the Dutch Wadden Sea island of Schiermonnikoog.MethodsWe selected ten plots of 2200 m2 in grazed and ungrazed areas of our study sites, and recorded and compared plant species richness in 0.1, 1, 10, 100 and 1000 m2 subplots. Topographic heterogeneity was quantified at the plot scale using the standard deviation of the elevation derived from a high‐resolution (5 m × 5 m) digital elevation model. We calculated species–area relationships to analyse our data.ResultsWe found that large‐scale topographic heterogeneity (based on the whole plot of 2200 m2) positively affects plant species richness at all scales (even at the smallest 0.1‐m2 scale), and that grazing has a positive additive effect at the small scales (0.1 and 10 m2). While grazing also had a positive effect on species richness at larger scales (1000 m2), the strength of the effect was dependent on the topographic heterogeneity at that scale. The effectiveness of grazing for increased plant species richness was highest at low topographic heterogeneity, and lowest at intermediate topographic heterogeneity. Effects of intermediate heterogeneity were probably counterbalanced by the effects of grazing.ConclusionsOur results suggest that the variation in elevation is an important predictor of whether low‐intensity grazing has positive effects on plant species richness or not. Grazing appears most beneficial at low topographic heterogeneity, but whether these findings hold for other grazed ecosystems will depend on several factors, most importantly, the relationship between topographic and abiotic heterogeneity. Results of our study are highly relevant for the application of low‐intensity grazing as tool for conservation management in salt marshes and other natural areas.

The effects of large herbivore grazing on meadow steppe plant and insect diversity

Summary The interactions between adjacent trophic levels are essential for ecosystem functioning and stability. Grazing by domestic herbivores is an essential interaction in grasslands, but little information is available on the nature of relationship between plant and insect diversity under grazing by large herbivores. We examined the effects of large herbivores on the relationship between plant and insect diversity with five grazing treatments (control, cattle, goat, sheep and a mixture of the three grazing types) across three plant diversity levels (low: 4–5 species, intermediate: 8–9 species and high: 15–17 species) in a meadow steppe. We found that the grazing treatments did not significantly affect plant species richness, but reduced plant biomass, plant height and cover. Grazers affected variation in plant height differently at different plant diversity levels, and this variation increased at the low plant diversity level and decreased at the high plant diversity level after grazing. A similar pattern was observed for insect species richness: grazing had a positive impact at the low plant diversity level, but had a negative impact at the high plant diversity level. In the absence of grazing, insect species richness was positively associated with plant species richness, but it decreased with increasing plant diversity in the grazing treatments. This was attributed to strong responses of insect species richness to plant height heterogeneity under grazing by large herbivores, implying that plant structural heterogeneity is more important than plant diversity in influencing insect diversity in grazed grasslands. Synthesis and applications. Grazing by large herbivores may reverse the positive relationship between plant diversity and insect diversity by modifying plant structural heterogeneity. Therefore, the spatial heterogeneity of vegetation structure should be given more attention in future work on plant–insect interactions. This study further highlights the importance of using large herbivore grazing in management actions, not only to maintain diversity but also to mediate trophic interactions in grasslands.

Factors affecting plant species richness in field boundaries in the Mediterranean region

Field boundaries are expected to support the maintenance of biodiversity in agroecosystems, since they provide the habitat for a range of plant species. However, plant diversity in field boundaries has decreased substantially in recent decades. This pattern is generally linked with the intensification of agricultural land use at field and landscape level. Therefore, we aimed to test the effect of farming management (field and boundary management), boundary structure (width and habitat assemblage considering the Mediterranean grassland element), and landscape heterogeneity on plant species richness of field boundaries. Plants were recorded along 30 field boundaries next to organic fields and 30 next to conventional fields located in 15 agrarian localities of NE Iberian Peninsula along a gradient of landscape complexity. A total of 517 plant species were identified in the 60 field boundaries. We recorded 162 species (31%) catalogued as rare, very rare or extremely rare in the flora of the Catalan Countries. Our results showed the importance of landscape heterogeneity, field management and habitat assemblage, since they were found to be the most influential variables for plant species richness; whereas boundary width and boundary management were seen to contribute less to explaining plant diversity. Accordingly, agri-environmental schemes should be designed to promote organic farming and maintain the structure of the landscape mosaic in order to benefit plant diversity in field boundaries in the Mediterranean region.

The impact of nesting cormorants on plant and arthropod diversity

Seabirds can strongly affect several major factors correlated with species diversity by concentrating marine nutrients on their nesting islands and by physically disturbing island vegetation. In this study, we investigated the effects of nesting cormorants on the abundance, species richness, and composition of plants and arthropods (Coleoptera, Heteroptera, Araneae, and Chironomidae) on islands in Stockholm archipelago, Sweden. Nesting cormorants negatively affected plant species richness and vegetation cover and that changed plant species composition. The effect of nesting cormorants on island arthropods varied between feeding groups and sampling methods. Most orders did not change in abundance or species richness but some, such as coleopterans and spiders changed in species composition. Herbivorous coleopterans were generally negatively affected by cormorants whereas fungivorous species and scavengers were generally positively affected. In structural equation modeling we found that the effect of cormorants was sometimes direct, such as on scavengers, but many effects on island consumers were mediated by changes in vegetation caused by cormorant presence. Overall, arthropod communities were highly dissimilar between cormorant and reference islands, and we therefore conclude that nesting cormorants not only affect the diversity of their nesting islands but also of the archipelago as a whole. The total diversity in the archipelago may increase through regional increased habitat heterogeneity and by adding species which are favored by seabirds (e.g. scavenging and fungivorous coleopterans).