Effects of productivity and disturbance on species richness: a neutral model

Productivity and disturbance are major determinants of species diversity, and results from theoretical models predict that species richness should peak at intermediate levels of both factors. Such "unimodal" responses have been documented in many field and laboratory studies and have usually been attributed to differences among species in competitive ability and/or trade-offs between competitive ability and tolerance to disturbance. Here we show that most documented patterns of disturbance-richness and productivity-richness relationships, as well as the observed interactions between the two factors, can be explained by a simple neutral model where all species are ecologically identical and lack trade-offs in species characteristics. This finding suggests that current neutral theories can be extended to explain patterns of species responses to productivity and disturbance.     

Grain-dependent relationships between plant productivity and invertebrate species richness and biomass in calcareous grasslands

The relationships among productivity, species richness and consumer biomass are of fundamental importance for understanding determinants of biodiversity. These relationships may depend on grain size. We examined the relationships between productivity (above-ground phytomass) and plant species richness and between productivity and species richness and biomass of gastropods and grasshoppers using sampling units of different sizes (0.5, 2.75 and 23 m²) in nutrient-poor, calcareous grasslands in north-western Switzerland in two successive years. Species richness of forbs had a unimodal relationship with productivity in sampling units of 0.5 m² and was negatively correlated with productivity at the other two plot sizes in one year. In the other year, forb species richness tended to decrease with productivity in sampling units of 23 m². No similar relationship was found for grasses. Gastropod biomass had a unimodal relationship with productivity at 0.5 m² in the first year. Grasshopper species richness was correlated with forb species richness at plot sizes of 2.75 and 23 m². This study demonstrates that patterns detected between productivity and diversity and between productivity and biomass of consumers depend on the grain size used in the investigation and vary among years.Die Zusammenhänge zwischen Produktivität, Artenreichtum und Biomasse von Konsumenten sind wichtig, um zu verstehen, was Biodiversität beeinflußt. Diese Zusammenhänge können von der Größe der Untersuchungsfläche abhängig sein. Wir untersuchten während zwei aufeinanderfolgenden Jahren die Zusammenhänge zwischen Produktivität (oberirdische Pflanzenbiomasse) und Artenreichtum von Gefäßpflanzen, sowie zwischen Produktivität und Artenreichtum und Biomasse von Schnecken und Heuschrecken bezüglich dreier räumlicher Skalen (0,5, 2,75 und 23 m²) in Kalkmagerrasen in der Nordwestschweiz. Der Zusammenhang zwischen dem Artenreichtum von Kräutern und der Produktivität war unimodal in Flächeneinheiten von 0,5 m² und negativ in Flächeneinheiten von 2,75 und 23 m² im ersten Jahr und war tendenziell negativ in Flächeneinheiten von 23 m² im zweiten Jahr, während kein solcher Zusammenhang bei Gräsern gefunden wurde. Der Zusammenhang zwischen Produktivität und Biomasse von Schnecken war unimodal in Flächeneinheiten von 0,5 m² im ersten Jahr. Außerdem bestand ein Zusammenhang zwischen dem Artenreichtum von Kräutern und Heuschrecken in Flächeneinheiten von 2,75 und 23 m². Diese Arbeit zeigt, daß Zusammenhänge zwischen Produktivität und Diversität sowie zwischen Produktivität und Biomasse von Konsumenten von der Größe der Untersuchungsfläche abhängen und zwischen Jahren variieren.     

Regularity of species richness relationships to patch size and shape

This study aims to assess the degree of regularity in the effect of patch size and patch shape on plant species richness across a macroscale region, and to evaluate the implications for nature conservation.
Our study area covers south-eastern Norway and contains 16 agricultural landscapes with 2162 patches.
To analyse regularity a local linear mixed model (LLMM) was applied. This procedure estimates the richness trends due to shared effects of size and shape, and simultaneously provides the landscape-specific random effect. The latter is a direct estimate of the degree of irregularity between the landscapes, conditioned on specific values of size and shape.
The results show a positive interaction between the shape and size of patches, which is repeated for all landscapes. The shape of the patches produces more regular patterns in species richness than the size of patches. This we attribute to effects of dispersal and distance to neighbouring patches of different environmentally conditioned species pools. Large and complex patches have shorter average distance to neighbouring patches (of different types) than large simple-shaped (circular) patches have. We attribute the higher species richness of the former, given a similar area, to a higher number of species dispersed from the outside into the more complex plot. For small patches, however, the distance to the edge is short relative to normal dispersal distances, for patches of all shapes. This explains why the positive effect of shape complexity on species richness is stronger for large patches. This interpretation is supported by a strong spatial correlation conditioned on the most complex patches.
Theories of dynamics in biodiversity in patchy landscapes must consider shape as a regulator at the same level as size, and both shape and size of patches should be simultaneously taken into account for management planning.     

放牧干扰下高寒草甸物种和生活型丰富度与地上及地下生物量的关系

明晰放牧干扰下高寒草甸植物丰富度与生物量的相关关系,为草地植物不同生长时期生物量的预测提供依据。设置6个放牧强度样地,连续3a放牧,2014年进行3个季节(6月、8月、10月)的植物丰富度和地上、地下生物量调查,对比分析放牧干扰下物种和生活型丰富度(生活型的种类)分别与地上、地下生物量的相关关系。结果表明:(1)物种和生活型丰富度与地上生物量均受放牧强度的显著影响,物种丰富度仅在8月与放牧强度显著负相关,生活型丰富度在10月随放牧强度单峰变化,地上生物量在不同季节均与放牧强度显著负相关,而地下生物量与放牧强度无关。(2)物种丰富度与地上和地下生物量均受季节的显著影响,物种丰富度和地上生物量仅在低强度放牧区随季节呈单峰变化,地下生物量在中等强度放牧区随季节呈单峰变化;生活型丰富度与季节无关。(3)放牧干扰前物种和生活型丰富度与地上和地下生物量均显著正相关。3a放牧后仅在8月,物种丰富度只与地上生物量显著正相关,生活型丰富度与地上和地下生物量均显著正相关。(4)对于不同放牧强度,物种丰富度仅在低强度放牧区与地上生物量显著正相关,而生活型丰富度在所有放牧强度区均与地上生物量显著正相关。综上所述,放牧干扰扰乱了高寒草甸丰富度与生物量之间的关系,尤其影响了物种丰富度与地下生物量之间的相关关系。生活型丰富度与地上生物量之间的显著关系不受放牧强度干扰,使生活型丰富度在预测生物量方面表现出优势。     

Differentiation between responses of primary productivity and phosphorus exploitation to species richness

Several studies have shown that ecosystem functioning increases with increasing species richness. Most of these studies examined the effects of species richness on primary productivity. The underlying mechanism that explains this pattern is usually the selection effect. The higher the diversity in plant communities the higher the chance in including a very productive species that dominates the community, or a legume species that brings N into the soil. Less attention has been given so far to the effects of species richness on phosphorus exploitation. The aim of this work was to investigate the effect of species richness on aboveground primary productivity and P accumulation in a plant diversity experiment. For this reason, 14 grassland plant species were grown in containers as monocultures and in mixtures of 2-, 3-, 4-, 8-, 11- and 14-species combinations. Results show that the aboveground phytomass and total P increased with increasing species richness. Complementarity effects, probably through partitioning of resources, were most apparent in the highest levels of species richness, and were observed to be greater for total P in comparison to phytomass. Selection effects generally were greater for phytomass than for total P; they were significantly positive at the 2- to 8-species combinations but close to 0 or negative in the highest levels of species richness. The increases in phytomass and total P at the highest levels of species richness appeared to be caused by the increased performance of intermediate-productive species. Responsible Editor: Tibor Kalapos.     

Staged invasions across disparate grasslands: effects of seed provenance,consumers and disturbance on productivity and species richness

Exotic plant invasions are thought to alter productivity and species richness, yet these patterns are typically correlative. Few studies have experimentally invaded sites and asked how addition of novel species influences ecosystem function and community structure and examined the role of competitors and/or consumers in mediating these patterns. We invaded disturbed and undisturbed subplots in and out of rodent exclosures with seeds of native or exotic species in grasslands in Montana, California and Germany. Seed addition enhanced aboveground biomass and species richness compared with no‐seeds‐added controls, with exotics having disproportionate effects on productivity compared with natives. Disturbance enhanced the effects of seed addition on productivity and species richness, whereas rodents reduced productivity, but only in Germany and California. Our results demonstrate that experimental introduction of novel species can alter ecosystem function and community structure, but that local filters such as competition and herbivory influence the magnitude of these impacts.     

Scale,disturbance and productivity control the native‐exotic richness relationship

The relationship between native and exotic species richness may be highly context‐dependent. Spatial scale, including both plot size (grain) and study area (extent), is likely to influence this relationship, as are environmental conditions such as resource availability and disturbance intensity. We used experimental manipulations of soil fertility and disturbance in a California coastal grassland to directly examine the sensitivity of the native–exotic richness relationship (NERR) to these factors across five grain sizes and two spatial extents. The slope of the NERR was a function of grain size, extent and treatment. Over small spatial extents, native and exotic richness were usually uncorrelated. Across a larger extent, NERRs were negative in control plots, neutral in disturbance plots, and positive in plots with experimentally reduced soil fertility. These patterns were strongest for small grain sizes. We verify the importance of spatial grain in determining the NERR, and emphasize the role of spatial extent.     

Functional group dominance and not productivity drives species richness

Background: There is a lack of consensus about the productivity–richness relationship, with several recent studies suggesting that it is not productivity but other factors that are the important drivers that determine species richness.

Aims: Here, we examine the relationship between productivity, functional group dominance and plant species richness at the plot scale in Tibetan Plateau meadows. These alpine meadows are ideal to examine the species productivity-richness relationship because they have a very high species richness, a large gradient in productivity, and can be dominated by either graminoids (grasses and sedges) or forbs.

Methods: We measured plant species richness and above-ground biomass along a natural gradient of functional group abundance in 44 plots distributed across five natural, winter-grazed but otherwise undisturbed sites in the eastern part of the Qing-Hai Tibetan Plateau, in Gansu province, China in 2008.

Results: Graminoid abundance (i.e. graminoid biomass as percent of the total above-ground biomass) explained 39% of plot differences in species richness while neither productivity nor the biomass of the three most abundant plant species, either individually or combined, were a significant predictor of species richness.

Conclusions: Our results show that within these alpine meadows, a shift from graminoid to forb dominance, rather than the individual dominant species or productivity itself, is strongly correlated with species richness. Thus, differences in functional group abundance can be a strong driver of observed plant species richness patterns.     

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.     

Beetle species richness along the forest productivity gradient in northern Finland

The occurrence and habitat associations of the majority of invertebrate groups in boreal forests are poorly known, even though these groups represent perhaps over 99% of the animal species diversity in the forests. We studied the beetle (Coleoptera) fauna of four forest site types in northern Finland: in spruce mires, herb rich, mesic and sub-xeric forests. We sampled beetles in 32 study sites with five window and five pitfall traps in each. We describe the species abundance and diversity patterns within and among forest types and relate these patterns to structural components of the forests. The volume of decaying wood varied from 14 to 93 m³ ha⁻¹ among sampling sites. The total beetle catch consisted of 100 333 individuals and 435 species. The beetle species richness did not vary according to site fertility but the number of specimens increased with increasing fertility in heath forest sites. The richness of beetle species correlated only weakly with any of the stand structure characteristics at the stand level. Nevertheless, the detrended correspondence analysis (DCA) indicated that different beetle assemblages are characteristic of different forest types. The high level of beta-diversity in beetles among forest types indicates that focusing exclusively on, for example, key-biotopes (presumed biodiversity hotspots) when selecting areas to be set aside would result in a situation where a large proportion of species, even of the rare and threatened ones, is not included in this network of protected areas. This suggests that the complementary set of different forest types may be the best general strategy to maintain the overall beetle species diversity in boreal forests.     

How important are species richness,species evenness and interspecific differences to productivity? A mathematical model

We present a theoretical model to quantify the influence of diversity on productivity and nutrient acquisition in plant communities during exponential growth. The model fractionates diversity into three components, namely species richness ( S ), species evenness ( E ) and the degree of difference between species ( D ). The influence of each of these components is assessed individually: S is varied by changing the number of species, E by changing their population size, and D by changing the range of species traits critical to productivity (specific nutrient uptake rate, Σ _r , or nutrient use efficiency, NUE ). D was quantified as the coefficient of variation of Σ _r or NUE . All three components of diversity enhance the biomass and nutrient stocks in the community, but the response patterns are different. Species richness has a saturating influence, whereas effects of E and D are linear and exponential, respectively. In all cases the non-linear dependence of productivity and nutrient acquisition on Σ _r and NUE during exponential growth was the single mechanism underlying these effects. This causes the presence of plants with extreme traits to promote productivity, and S , E and D all affect the abundance and/or intensity of these extremes. The model offers a framework to explain differences between experimental observations, and suggests a concept of diversity where S and E are structural components and D a qualitative or functional component, which modulates the influence of the two others. We propose to explicitly recognise D as an integral constituent of plant diversity in future studies.     

Different relationships between galling and non-galling herbivore richness and plant species richness: a meta-analysis

The plant richness hypothesis (PRH) is used to explain herbivorous insect richness based on the number of plant species, predicting a positive relationship. However, the influence of plant richness on insect distribution can become stronger with greater levels of specialization of herbivores. In this meta-analysis, I tested whether there is any difference in the correlation force recorded between studies that investigated endophagous versus exophagous herbivores, and galling versus non-galling guilds, in order to determine whether more specialized groups have a stronger relationship. Furthermore, I calculated whether effect sizes were homogeneous between galling studies carried out at local and regional scales, and between tropical and temperate regions. A total of 52 correlations were analyzed between plant species richness and herbivore species richness, with 18 correlations derived from galling herbivores and 34 from non-galling herbivores. The effect sizes were significant and positive in all studies, being higher for endophages than for exophages, and for galling than for non-galling studies. These results provide evidence that groups of insects with a higher level of host specialization and specificity (e.g., endophagous and galling) exhibit a greater dependence on plant richness. There was no difference in effect sizes for galling studies between the local and regional level or between tropical and temperate groups. Despite the large variability found for galling studies, effect sizes were consistent independently of climatic region and latitudinal variation. These results suggest that the PRH for galling insects can be generalized to most ecosystem and vegetation types.     

The relationship between species richness and productivity in metazoan parasite communities

Biodiversity is not distributed homogeneously in space, and it often covaries with productivity. The shape of the relationship between diversity and productivity, however, varies from a monotonic linear increase to a hump-shaped curve with maximum diversity values corresponding to intermediate productivity. The system studied and the spatial scale of study may affect this relationship. Parasite communities are useful models to test the productivity-diversity relationship because they consist of species belonging to a restricted set of higher taxa common to all host species. Using total parasite biovolume per host individual as a surrogate for community productivity, we tested the relationship between productivity and species richness among assemblages of metazoan parasites in 131 vertebrate host species. Across all host species, we found a linear relationship between total parasite biovolume and parasite species richness, with no trace of a hump-shaped curve. This result remained after corrections for the potential confounding effect of the number of host individuals examined per host species, host body mass, and phylogenetic relationships among host species. Although weaker, the linear relationship remained when the analyses were performed within the five vertebrate groups (fish, amphibians, reptiles, mammals and birds) instead of across all host species. These findings agree with the classic isolationist-interactive continuum of parasite communities that has become widely accepted in parasite ecology. They also suggest that parasite communities are not saturated with species, and that the addition of new species will result in increased total parasite biovolume per host. If the number of parasite species exploiting a host population is not regulated by processes arising from within the parasite community, external factors such as host characteristics may be the main determinants of parasite diversity.