Effects of woody plant species richness on mammal species richness in southern Africa

Aim To determine the relationship between the species richness of woody plants and that of mammals after accounting for the effect of environmental variables. Location Southern Africa, including Namibia, South Africa, Lesotho, Swaziland, Botswana, Zimbabwe, and part of Mozambique. Methods We used a comprehensive dataset including the species richness of mammals and of woody plants and environmental variables for 118 quadrats (each of 25,000 km²) across southern Africa, and used structural equation models (SEMs) and spatial regressions to examine the relationship between the species richness of woody plants and of mammal trophic guilds (herbivores, insectivores, carni/omnivores) and habitat guilds (aquatic/fossorial, ground‐living, climbers, aerial), after controlling for environment. We compared the results of SEMs with those of single‐predictor regressions (without controlling for environment) and of spatial regressions (controlling for both environment and residual spatial autocorrelation). Results The geographical variation of mammal species richness in southern Africa was strongly and positively related to that of woody plant species richness, and this relationship held for most mammal guilds even when the influence of environment and spatial autocorrelation had been accounted for. However, the effect of woody plant species richness on the richness of aquatic/fossorial species almost disappeared after controlling for environment, suggesting that the congruence in species richness patterns between these two groups results from similar responses to the same environmental variables. For many mammal guilds, the relative role of environmental predictors as measured by standardized partial regression coefficients changed depending on whether non‐spatial single‐predictor regressions, non‐spatial SEMs, or spatial regressions were used. Main conclusions Woody plants are important determinants of the species richness of most mammal guilds in southern Africa, even when controlling for environment and residual spatial autocorrelation. Environmental correlates with animal species richness as measured by simple correlations or single‐predictor regressions might not always reflect direct effects; they might, at least to some degree, result from indirect effects via woody plants. Interpretations of the strength of the effect of environmental variables on mammal species richness in southern Africa depend largely on whether spatial or non‐spatial models are used. We therefore stress the need for caution when interpreting environmental ‘effects’ on broad‐scale patterns of species richness if spatial and non‐spatial methods yield contrasting results.     

Reintroduction of large herbivores restored plant species richness in abandoned dry temperate grassland

Naturalistic grazing by large herbivores is an increasingly practiced way of managing habitats with conservational value. It has the potential to restore and enhance biodiversity, creating self-sustainable environments vital for organisms requiring regular disturbances to moderate and/or reverse successional changes. European bison, Exmoor pony, and Tauros cattle were introduced in 2015 to a former military training area in Milovice, Czech Republic. The prevailing vegetation type is a forest-steppe savanna with Bromus erectus-dominated xeric grasslands mixed with deciduous shrubs and trees. After the cessation of military use, the area was abandoned which led to successional changes, including the dominance of tall grasses, litter accumulation, and bush encroachment. In 2017–2021, we monitored grassland vegetation in 30 grazed permanent plots (2?×?2 m) and 5 control plots representative of ungrazed, abandoned vegetation adjacent to the grazed areas. Naturalistic grazing increased species richness and the cover of forbs, while the cover of grasses and legumes was minimally affected. Grazing increased functional diversity of plant community, promoted a compositional change to small statured species and an increased incidence of red-list species. Seven years of continuous grazing increased the conservation value of this forest-steppe vegetation, a habitat type rapidly declining in Europe.

     

Effects of plant species richness on stand structure and productivity

Aims Aboveground biomass production commonly increases with species richness in plant biodiversity experiments. Little is known about the direct mechanisms that cause this result. We tested if by occupying different heights and depths above and below ground, and by optimizing the vertical distribution of leaf nitrogen, species in mixtures can contribute to increased resource uptake and, thus, increased productivity of the community in comparison with monocultures.Methods We grew 24 grassland plant species, grouped into four nonoverlapping species pools, in monoculture and 3- and 6-species mixture in spatially heterogeneous and uniform soil nutrient conditions. Layered harvests of above- and belowground biomass, as well as leaf nitrogen and light measurements, were taken to assess vertical canopy and root space structure.Important findings The distribution of leaf mass was shifted toward greater heights and light absorption was correspondingly enhanced in mixtures. However, only some mixtures had leaf nitrogen concentration profiles predicted to optimize whole-community carbon gain, whereas in other mixtures species seemed to behave more 'selfish'. Nevertheless, even in these communities, biomass production increased with species richness. The distribution of root biomass below ground did not change from monocultures to three- and six-species mixtures and there was also no indication that mixtures were better than monocultures at extracting heterogeneously as compared to homogeneously distributed soil resources. We conclude that positive biodiversity effect on aboveground biomass production cannot easily be explained by a single or few common mechanisms of differential space use. Rather, it seems that mechanisms vary with the particular set of species combined in a community.     

Mammal species richness and habitat use in rainforest and abandoned agricultural fields in Chiapas, Mexico

1. We evaluated the potential of small, isolated cornfields embedded in a large forest matrix as an appropriate compromise between conservation and sustainable development.
2. We examined the relationship between habitat structure and diversity with overall mammal species richness and abundance of six common small mammals in four 6-year-old abandoned cornfields (oldfields) and four continuous rainforest sites in the most speciose region in Mexico, the Selva Lacandona, Chiapas.
3. Species richness did not differ between forests and oldfields, indicating that forest-dependent mammals penetrate oldfields of size 0·9–2·9 ha, presumably because forest cover and resources are nearby. Even species which strongly selected forest habitat, such as H. desmarestianus and D. marsupialis , were present in oldfields, and only the latter showed a significant decline in the number of captures as a function of distance from forest edge.
4. The Lacandon Indian use of the land for agriculture in the form of small (> 3 ha) agricultural plots embedded in a large forest matrix increases spatial heterogeneity and promotes mammal diversity.
5. Those small plots (in agricultural production terms) are intermediate in size when the whole continuum of the local natural and anthropogenic disturbance sizes are considered; more common are the extensive clearings for agriculture or cattle ranching, whereas the most common forest disturbance is treefall gaps, at least 20 times smaller than oldfields.
6. This form of land use should be considered as an important, appropriate productive element when decisions are made about managing reserves and pursuing sustainable development.     

Scale-dependent effects of land use on plant species richness of mountain grassland in the European Alps

Traditionally managed mountain grasslands in the Alps are species‐rich ecosystems that developed during centuries of livestock grazing. However, changes in land use including fertilisation of well accessible pastures and gradual abandonment of remote sites are increasingly threatening this diversity. In five regions of the Swiss and French Alps we assessed the relationship between land use, soil resource availability, cover of the unpalatable species Veratrum album, species richness and vegetation composition of mountain grasslands across four spatial scales ranging from 1 to 1000 m². Mean species richness and the increase in the number of species with increasing area were lower in intensively grazed, fertilised pastures than in traditional pastures or in abandoned pastures. Species composition of abandoned pastures differed from that of the other management types. Plant species richness was influenced by different factors at different spatial scales. At the 1 m² scale, plant species richness was negatively related to soil nitrate and influenced by the cover of V. album, depending on land use: species richness and cover of V. album were negatively correlated in abandoned pastures, but positively correlated in fertilised grasslands. At the 1000 m² scale, a negative effect of fertilization on richness was evident. These results indicate that at small scales species richness in mountain grasslands is determined by competition for light, which should be more important if nutrient availability is high, and by positive and negative interactions with unpalatable plants. In contrast, species richness at the large scale appears to be mainly influenced by land use. This result emphasizes the importance of studying such inter‐relationships at multiple scales. Our study further suggests that the maintenance of the traditional land use scheme is crucial for the conservation of plant species richness of mountain pastures as both intensification and abandonment changed species composition and reduced plant species diversity.     

Nutrient stoichiometry and land use rather than species richness determine plant functional diversity

Plant functional traits reflect individual and community ecological strategies. They allow the detection of directional changes in community dynamics and ecosystemic processes, being an additional tool to assess biodiversity than species richness. Analysis of functional patterns in plant communities provides mechanistic insight into biodiversity alterations due to anthropogenic activity. Although studies have consi‐dered of either anthropogenic management or nutrient availability on functional traits in temperate grasslands, studies combining effects of both drivers are scarce. Here, we assessed the impacts of management intensity (fertilization, mowing, grazing), nutrient stoichiometry (C, N, P, K), and vegetation composition on community‐weighted means (CWMs) and functional diversity (Rao's Q) from seven plant traits in 150 grasslands in three regions in Germany, using data of 6 years. Land use and nutrient stoichiometry accounted for larger proportions of model variance of CWM and Rao's Q than species richness and productivity. Grazing affected all analyzed trait groups; fertilization and mowing only impacted generative traits. Grazing was clearly associated with nutrient retention strategies, that is, investing in durable structures and production of fewer, less variable seed. Phenological variability was increased. Fertilization and mowing decreased seed number/mass variability, indicating competition‐related effects. Impacts of nutrient stoichiometry on trait syndromes varied. Nutrient limitation (large N:P, C:N ratios) promoted species with conservative strategies, that is, investment in durable plant structures rather than fast growth, fewer seed, and delayed flowering onset. In contrast to seed mass, leaf‐economics variability was reduced under P shortage. Species diversity was positively associated with the variability of generative traits. Synthesis. Here, land use, nutrient availability, species richness, and plant functional strategies have been shown to interact complexly, driving community composition, and vegetation responses to management intensity. We suggest that deeper understanding of underlying mechanisms shaping community assembly and biodiversity will require analyzing all these parameters.     

Recovery of soil productivity with forest succession on abandoned agricultural land

Much of the primary forest in the eastern United States that was converted to farmland between 1600 and 1900 has reverted back to second growth forest as a result of agriculture abandonment. This reversion back to forest gives soil productivity a chance to recover, though the rates of recovery are not well understood. Understanding the legacy effects of past disturbances like agriculture can provide important insights to support ecological restoration efforts on disturbed soils. Our goal with this study was to further understand the effects of forest development on soil productivity after agriculture abandonment. We used a chronosequence approach to examine soil properties over a 60‐year temporal scale of forest development on abandoned agricultural lands in Saratoga and Rensselaer Counties in New York, U.S.A. We measured soil properties within this chronosequence to test the hypothesis that there would be measurable recoveries of soil physical properties and fertility over time. We observed rapid recovery of physical properties (lower bulk density and higher macroporosity) of surface soils within 5–10 years after agricultural abandonment. However, we found a legacy effect of agricultural compaction still evident in subsoils, with soil strength measurements indicating that past agricultural practices still limited root growth 55–60 years after abandonment. Soil percent organic matter and mineralizable nitrogen (N) both increased with forest development, but biomass accumulation may be slowed by limited root growth in the subsoil due to high strength. We recommend assessing subsoil physical properties when developing ecological restoration plans for agricultural lands.     

Dependence of broad-scale geographical variation in fleshy-fruited plant species richness on disperser bird species richness

Aim We analysed the interdependence of avian frugivore‐ and fruited plant‐species richness at the scale of major river basins across Europe, taking into account several environmental factors along different spatial gradients. Location Continental Europe and the British Isles. Methods We focused on wintering birds and autumn/winter fruiting plants, and used major river basins as geographical units and Structural Equation Modelling as the principal analytical tool. Results The statistical influence of disperser species richness on fleshy‐fruited plant species richness is roughly double that of the reverse. Broad‐scale variation in frugivore richness is more dependent on environmental factors than on fruited plant richness. However, the influence of disperser richness on plant richness is four times higher than the influence of environmental factors. Environmental influences on both birds and plants are greater than purely spatial influences. Main conclusions Our results are interpreted as indicating that biotic dispersal of fruits strongly affects broad‐scale geographical trends of fleshy‐fruited plant species richness, whereas richness of fruited plants moderately affects frugivore richness.     

Habitat patchiness and plant species richness

The pattern of woody species richness decline with a decrease in woody vegetation cover was studied within a tallgrass prairie. The decline in species richness is highly non-linear, with a well-defined threshold below which species richness collapses. This relationship can be understood after considering information on how landscape structure changes with woody vegetation cover, and how species richness is related to landscape structure.     

Effects of an afforestation process on plant species richness: A retrogressive analysis

Effective conservation of biodiversity in the face of increasing human impacts and global environmental changes requires accurate measurement of key trends and alternative management actions at landscape scales. Past ecological conditions are certainly important key factors in determining the present species diversity patterns and the inclusion of such factors (e.g. by historical cartographic data) can dramatically improve the predictive power of ecological models. In this paper we applied a retrogressive approach with the aim of simulating secondary forest regrowth effects on plant species diversity using present field data and historic land-use maps. The field data from an extensive sample were here used to model the temporal species richness change among the forest areas in the last 60 years. In order to rebuild the past species pool matrix using present field data and historical land use map, we applied a nearest neighbour selection using spatial query. Species-based rarefaction curves were derived for the two dates (1954 and 2010); the two datasets have been interpolated using inverse distance weighted algorithm, obtaining two maps showing the distribution of plant species richness for the two dates.The results showed that the cessation of human pressure on semi-natural areas and the consequent forest recovery, resulted in a decrease of vascular plant as a woodland flora replaces the open habitats flora. This study also showed that secondary forest regrowth and its effect on plant species diversity may be revealed by a retrogressive analysis, which represent a valid support in case of high uncertainty or absence of historical data.     

Effect of plant species richness on invasibility of experimental plant communities

Invasion of unsown species to artificially created assemblages of grassland species was investigated in a 3-year field experiment. In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1–4, grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass decreased significantly with increasing biomass of sown species and their functional group richness, but not with number of species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional trait space could be a mechanism promoting the resistance to invasion. In Addition, species composition of invaders were significantly related to initial composition of sown residents.     

The influence of agricultural land-use intensity on bryophyte species richness

This study is a quantitative approach to the estimation of bryophyte species richness in relation to land-use intensity at three spatial scales in highly cultivated areas. A total of 460 randomly selected habitats and their various substrates within 29 study sites were investigated with regard to their land-use intensity and their bryophyte species richness in an agricultural region of eastern Austria. On bare soils (substrate-scale), low but regular disturbance increases bryophyte diversity in comparison to lower land-use intensity. However, more frequent disturbance (e.g. ploughing more than two times a year) dramatically reduces species richness at these sites, with more than 50% of these sites showing no bryophytes. The production of reproductive units (sporophytes and vegetative units) is highest at an intermediate disturbance regime. On the habitat, as well as on the landscape-scale, there is a significant increase in total bryophyte species number as well as in the number of threatened species with decreasing land-use intensity. This is mainly due to habitat and structural diversity, which increases with decreasing land-use intensity. There are significant correlations between landuse intensity, structural diversity and species richness at the habitat as well as on the landscape scale.     

Effects of sampling time,species richness and observer on the exhaustiveness of plant censuses

Question: How may sampling time affect exhaustiveness of vegetation censuses in interaction with observer effect and quadrat species richness? Location: French lowland forests. Methods: Two data sets comprised of 75 timed, one‐hour censuses of vascular plants carried out by five observers on 24 400‐m² forest quadrats were analysed using mixed‐effect models. Results: The level of exhaustiveness increased in a semi‐logarithmic way with sampling time and decreased with quadrat species richness. After one hour, 20 to 30% of the species remained undetected by single observers. This proportion varied among observers and the discrepancy increased with increasing sampling time. Fixing the sampling time may make richness estimates vary less between observers but the time limit should be at least 30 min to reduce the bias in exhaustiveness between rich and poor quadrats. Conclusions We advocate the use of sampling methods based on spatially or temporally‐replicated censuses and statistical analyses that correct for the lack of census exhaustiveness in vegetation studies.     

Effects of topographic variability on the scaling of plant species richness in gradient dominated landscapes

It is commonly assumed that variation in abiotic site conditions influences the number of niches, which in turn affects the potential species richness in an area. Based on theoretical considerations, abiotic variation is often used as an estimator of species richness at broad scales, while at finer landscape scales the diversity of habitat types is used. However, habitat estimators assume the landscape to be composed of discrete, homogeneous patches with sharp boundaries, and such a concept is hard to apply in gradient-dominated landscapes. The aim of this study was therefore to investigate the influence of topographic variability (TV) on species richness at the landscape level (gamma (γ) diversity) and on its components (alpha (α) and beta (β) diversity) at microsite and habitat group levels. Using floristic data from 12 "landscapes" of 1 km² we investigated the influence on diversity components of two simple and one complex measures of TV. While the standard deviation (SD) of altitude explained a high proportion of the variation in γ diversity (linear regression model, R²=0.63), the complex measure, SD of solar radiation explained it even better (R²=0.82). There were strong effects of TV on α and β diversity components at the microsite level, but only marginal increases of the diversity components at the habitat level. Further analyses revealed that the missing increase of the habitat level components was caused by differences between habitat groups and that only grassland diversity components increased significantly with TV. We conclude that TV at a landscape scale has strong effects on niche or microsite diversity and is an appropriate estimator of relative species richness in landscapes that are topographically heterogeneous and gradient dominated.     

Effects of the expansion by Hippophaë rhamnoides on plant species richness in coastal dunes

Question: Is the expansion of Hippophaë rhamnoides in coastal dunes associated with a decline in plant species richness, and is this decline best described by a hump‐backed relationship between species number and shrub cover? Location: Grey and yellow dunes on the East Frisian islands Spiekeroog and Norderney. Methods: Total plant species richness as well as the number of herbaceous and cryptogam species were determined in 2001 using plots of 16 m² size. We compared shrubland plots with varying cover of Hippophaë with neighbouring dune grassland plots without shrubs as reference sites. Soil samples were collected to determine the values of some important edaphic variables (pH, organic matter, nitrogen). Results: The shrubland plots with Hippophaë had or tended to have lower soil pH and C/N ratios and higher contents of organic matter and nitrogen than the grassland plots. Total species richness was marginally significantly related to the cover of Hippophaë in a hump‐backed manner on both islands. The pattern was more pronounced for mosses and lichens than for herbaceous species. For all species groups on Spiekeroog and for the herbaceous species on Norderney, the hump‐backed relationship was much improved when using the difference in species number between shrubland and grassland plot as a dependent variable. Relationships could be improved by including the soil parameters as co‐variables. Species richness was highest at moderate levels of shrub expansion, while it was much reduced in very dense shrubland. The decrease in species number is caused by the decline in grassland species typical of the open dunes, including some rare taxa. Conclusions: The expansion of Hippophaë rhamnoides is a serious threat to the plant species richness of open coastal dunes, and needs to be counteracted by management measures.     

Effects of plant species richness and evenness on soil microbial community diversity and function

Understanding the links between plant diversity and soil communities is critical to disentangling the mechanisms by which plant communities modulate ecosystem function. Experimental plant communities varying in species richness, evenness, and density were established using a response surface design and soil community properties including bacterial and archaeal abundance, richness, and evenness were measured. The potential to perform a representative soil ecosystem function, oxidation of ammonium to nitrite, was measured via archaeal and bacterial amoA genes. Structural equation modeling was used to explore the direct and indirect effects of the plant community on soil diversity and potential function. Plant communities influenced archaea and bacteria via different pathways. Species richness and evenness had significant direct effects on soil microbial community structure, but the mechanisms driving these effects did not include either root biomass or the pools of carbon and nitrogen available to the soil microbial community. Species richness had direct positive effects on archaeal amoA prevalence, but only indirect impacts on bacterial communities through modulation of plant evenness. Increased plant evenness increased bacterial abundance which in turn increased bacterial amoA abundance. These results suggest that plant community evenness may have a strong impact on some aspects of soil ecosystem function. We show that a more even plant community increased bacterial abundance, which then increased the potential for bacterial nitrification. A more even plant community also increased total dissolved nitrogen in the soil, which decreased the potential for archaeal nitrification. The role of plant evenness in structuring the soil community suggests mechanisms including complementarity in root exudate profiles or root foraging patterns.     

Spatial scale and the determinants of plant species richness

Traditional ecological theory has stressed the importance of competitive interactions in regulating species richness. Recent research has transcended this viewpoint by considering the role of stochastic processes, mosaic phenomena and nonequilibrium conditions in the regulation of richness. This growing body of work indicates that the determinants of plant species richness may vary predictably over different spatial scales.